Module 2
Eukaryotic Cells and Viruses
Eukaryotic Cells and Microorganisms
Evidence from paleontology indicates that the first eukaryotic cells appeared on the earth
approximately 2 to 3 billion years ago. While it used to be thought that eukaryotic cells evolved
directly from ancient prokaryotic cells, we now believe that bacteria, archaea, and eukaryotes
evolved from a different kind of cell, a precursor to both prokaryotes and eukaryotes that
biologists call the Last Common Ancestor. This ancestor was neither prokaryotic nor eukaryotic
but gave rise to all three current cell types. The first primitive eukaryotes were probably single-
celled and independent, but, over time, some cells began to aggregate, forming colonies. With
further evolution, some of the cells within colonies became specialized, or adapted to perform a
particular function advantageous to the whole colony, such as movement, feeding, or
reproduction. Complex multicellular organisms evolved as individual cells in the organism lost
the ability to survive apart from the intact colony
Nearly all eukaryotic microbial cells have a cell membrane, nucleus, mitochondria,
endoplasmic reticulum, Golgi apparatus, vacuoles, cytoskeleton, cytoplasm, and glycocalyx.
Only some types of eukaryotes have a cell wall, appendages for moving, and chloroplasts (figure
4.1). In the following sections, we cover the microscopic structure and functions of the
eukaryotic cell. As with the bacteria, we begin on the outside and proceed inward through the
cell. Motility allows microorganisms to locate nutrients and to migrate toward positive stimuli
such as sunlight; it also enables them to avoid harmful substances and stimuli. Most eukaryotic
microbes move by using flagella or cilia. This type of motility is common in protozoa, many
algae, and a few fungal and animal cells. Cilia are very similar in overall architecture to flagella,
but they are shorter and more numerous (some cells have several thousand). They are found only
on a single group of protozoa and in certain animal cells. In the ciliated protozoa, the cilia occur
in rows over the cell surface, where they beat back and forth in regular oar-like strokes. These
protozoa are among the fastest of all motile cells. On some cells, cilia also function as feeding
and filtering structures.
The cell (or cytoplasmic) membrane of eukaryotic cells is a typical bilayer of
phospholipids in which protein molecules are embedded. In addition to phospholipids,
eukaryotic membranes also contain sterols of various kinds. Sterols are a form of lipid, but they
are different from phospholipids in both structure and behavior. They are fairly rigid molecules,
and this makes eukaryotic membranes more stable than those of noneukaryotic cells. This
strengthening feature is extremely important in those cells that don’t have a cell wall.
Cytoplasmic membranes of eukaryotes have a similar function as those in bacteria and archaea,
serving as selectively permeable barriers.
The nucleus is a compact sphere that is the most prominent organelle of eukaryotic cells.
It is separated from the cell cytoplasm by an external boundary called a nuclear envelope. The
envelope has a unique architecture. It is composed of two parallel membranes–that means two
lipid bilayers–separated by a narrow space. It is perforated with small, regularly spaced
openings, or pores. The pores are formed by the connection of the inner and outer nuclear
membranes (figure 4.4). The nuclear pores are passageways. Macromolecules use these passages
to move from the nucleus to the cytoplasm and vice versa. The nucleus is filled with a matrix
called the nucleoplasm and a granular mass, the nucleolus that stains more intensely than the
immediate surroundings because of its RNA content. The nucleolus is the site for ribosomal
RNA synthesis and a collection area for ribosomal subunits. The subunits are transported through
the nuclear pores into the cytoplasm for final assembly into ribosomes.
The endoplasmic reticulum (ER) is a series of membrane tunnels used in transport and
storage. There are two kinds of endoplasmic reticulum: the rough endoplasmic reticulum (RER)
(figure 4.5) and the smooth endoplasmic reticulum (SER). The RER is a continuation of the
outer membrane of the nuclear envelope and extends in a continuous network through the
cytoplasm, even all the way out to the cell membrane. This architecture permits the spaces in the
RER, called cisternae (singular, cistern), to transport materials from the nucleus to the cytoplasm
and ultimately to the cell’s exterior. The RER appears “rough” because of large numbers of
ribosomes attached to its membrane surface. Proteins synthesized on the ribosomes are shunted
into the inside space (the lumen) of the RER and held there for later packaging and transport. In
contrast to the RER, the SER is a closed tubular network without ribosomes. It functions in
nutrient processing and in synthesis and storage of nonprotein macromolecules such as lipids.
The Golgi apparatus, also called the Golgi complex or Golgi body, is the site in the cell in
which proteins are modified and then sent to their final destinations. It is an organelle consisting
of a stack of several flattened, disc-shaped sacs called cisternae. These sacs have both
membranes and cavities like those of the endoplasmic reticulum, but they do not form a
continuous network (figure 4.6). This organelle is always closely associated with the
endoplasmic reticulum. At a site where it meets the Golgi apparatus, the endoplasmic reticulum
buds off tiny membrane-bound packets of protein called transitional vesicles that are picked up
by the face of the Golgi apparatus. Once inside the Golgi body, the proteins are often modified
by the addition of polysaccharides and lipids. The final action of this apparatus is to pinch off
finished condensing vesicles that will carry the modified proteins to organelles such as
lysosomes or outside the cell as secretory vesicles.
Nucleus, Endoplasmic Reticulum, and Golgi apparatus
Nature’s Assembly Line As the keeper of the eukaryotic genetic code, the nucleus
ultimately governs and regulates all cell activities. But, because the nucleus is located in a
specific cellular site, it has to direct these activities using a structural and chemical network. This
network includes ribosomes, which originate in the nucleus, and the rough endoplasmic
reticulum (figure 4.7). Remember that the RER is continuously connected with the nuclear
envelope, as well as the smooth endoplasmic reticulum and the
Golgi apparatus. Initially, a segment of DNA containing the instructions for producing a
protein is copied into RNA, and this RNA transcript is passed out through the nuclear pores
directly to the ribosomes on the endoplasmic reticulum. Here, specific proteins are synthesized
from the RNA code and deposited in the lumen (space) of the endoplasmic reticulum. After
being transported to the Golgi apparatus, the protein products are chemically modified and
packaged into vesicles that can be used by the cell in a variety of ways. Some of the vesicles
contain enzymes to digest food inside the cell. Other vesicles are secreted to digest materials
outside the cell, and others are important in the growth and repair of the cell wall and membrane.
A lysosome is a vesicle that buds off of the Golgi apparatus. It contains a variety of
enzymes. Lysosomes are involved in digestion of food particles inside the cell and in protection
against invading microorganisms. They also participate in digestion and removal of cell debris in
damaged tissue. Another type of vesicle, the peroxisome, contains a wide variety of enzymes.
(Peroxisomes do not originate from the Golgi apparatus.) Other types of vesicles include
vacuoles (vak′-yoo-ohlz), which are membrane-bound sacs containing fluids or solid particles to
be digested, excreted, or stored.
They are found in phagocytic cells (certain white blood cells and protozoa) in response to
food and other substances that have been engulfed. The contents of a food vacuole are digested
through the merger of the vacuole with a lysosome. This merged structure is called a phagosome
(figure 4.8). Other types of vacuoles are used in storing reserve food such as fats and glycogen.
Protozoa living in freshwater habitats use structures called contractile vacuoles to regulate
osmotic pressure. These vacuoles systematically expel excess water that has diffused into the
cell.
Mitochondria
Although the nucleus is the cell’s control center, none of the cellular activities it
commands could proceed without a constant supply of energy. The bulk of a cell’s energy is
generated in most eukaryotes by mitochondria (my″-toh-kon′-dree-uh). When viewed with light
microscopy, mitochondria appear as round or elongated particles scattered throughout the
cytoplasm. The internal ultrastructure reveals that a single mitochondrion consists of a smooth,
continuous outer membrane that forms the external contour, and an inner, folded membrane
nestled neatly within the outer membrane (figure 4.9). The folds on the inner membrane are
called cristae (kris′-te).
The cristae membranes hold the enzymes and electron carriers needed in aerobic
respiration. This is an oxygen-using process that extracts chemical energy contained in nutrient
molecules and stores it in the form of high-energy molecules, or ATP. Mitochondria (along with
chloroplasts) are unique among organelles in that they divide independently of the cell. They
also contain circular molecules of DNA and have bacteriasized 70S ribosomes. These
characteristics have caused scientists to suggest that mitochondria were once bacterial cells that
developed into eukaryotic organelles over time. While it was previously thought that all
eukaryotic organisms must have mitochondria, scientists have discovered that some protozoa
have pared-down versions of mitochondria (called mitosomes) and one species has even been
found that contains neither mitochondria nor mitosomes. You have probably noticed that in
biology, almost every “rule” gives way to one or two exceptions in the natural world.
Chloroplasts are remarkable organelles found in algae and plant cells that are capable of
converting the energy of sunlight into chemical energy through photosynthesis. Another
important product of the photosynthesis process in chloroplasts is oxygen gas. Although
chloroplasts resemble mitochondria, chloroplasts are larger, contain special pigments, and are
much more varied in shape.
In an electron micrograph of a eukaryotic cell, ribosomes are numerous, tiny particles
that give a “dotted” appearance to the cytoplasm. Ribosomes are distributed throughout the cell:
Some are scattered freely in the cytoplasm and cytoskeleton; others are attached to the rough
endoplasmic reticulum as previously described. Still others appear inside the mitochondria and in
chloroplasts. Multiple ribosomes are often found arranged in short chains called polyribosomes
(polysomes). The basic structure of eukaryotic ribosomes is similar to that of bacterial
ribosomes, described in chapter 3. Both are composed of large and small subunits of
ribonucleoprotein (see figure 4.5). By contrast, however, the eukaryotic ribosome (except in the
mitochondrion) is the larger 80S variety that is a combination of 60S and 40S subunits. This
difference means that we can use antibiotics that target prokaryotic ribosomes without harming
our own eukaryotic ribosomes. As in the bacteria, eukaryotic ribosomes are the staging areas for
protein synthesis.
The cytoplasm of a eukaryotic cell is criss-crossed by a flexible framework called the
cytoskeleton. This framework appears to have several functions, such as anchoring organelles,
moving RNA and vesicles, and permitting shape changes and movement in some cells (figure
4.10). The three main types of cytoskeletal elements are actin filaments, intermediate filaments,
and microtubules. Actin filaments are long, thin protein strands about 7 nm in diameter. They are
found throughout the cell but are most highly concentrated just inside the cell membrane. Actin
filaments are responsible for cellular movements such as contraction, crawling, pinching during
cell division, and formation of cellular extensions. Microtubules are long, hollow tubes that
maintain the shape of those eukaryotic cells that don’t have walls, and they transport substances
from one part of a cell to another. The spindle fibers that play an essential role in mitosis are
actually microtubules that attach to chromosomes and separate them into daughter cells. As
indicated earlier, microtubules are also responsible for the movement of cilia and flagella.
Intermediate filaments are ropelike structures that are about 10 nm in diameter. (Their name
comes from their intermediate size, between actin filaments and microtubules.) Their main role
is in structural reinforcement to the cell and to organelles. For example, they support the
structure of the nuclear envelope.
The Fungi
The kingdom Fungi is large and filled with a great variety and complexity of forms. In
medical microbiology we are most concerned with the fungi known as yeasts and molds. Other
fungi include mushrooms and puffballs. Although the majority of fungi are either unicellular or
colonial (i.e., they form colonies), a few complex forms such as mushrooms or puffballs are truly
multicellular. Cells of the microscopic fungi exist in two basic forms: yeasts and hyphae. A yeast
cell is distinguished by its round to oval shape and by its mode of asexual reproduction. It grows
swellings on its surface called buds, which then become separate cells. Hyphae (hy′-fee) are
long, threadlike cells found in the bodies of fungi of the filamentous type. These are called molds
(figure 4.12). Some species form a pseudohypha, a chain of yeast cells formed when buds remain
attached in a row (figure 4.13). Because of its manner of formation, it is not a true hypha like that
of molds. While some fungal cells exist only in a yeast form and others occur primarily as
hyphae, a few are classified as dimorphic. This means they can take either form, depending on
growth conditions, such as changing temperature. Several fungi that cause human disease are
dimorphic
Mycoses (the term for fungal infections) vary in the way the pathogen enters the body
and the degree of tissue involvement they display. Even so-called harmless species found in the
air and dust around us may be able to cause infections, especially in individuals who already
have cancer, diabetes, or AIDS. This last category, opportunistic infections, has become very
troubling in recent years. In 2018 an important review was published that revealed that deaths
from fungal infections have surpassed deaths due to either malaria or breast cancer in the world.
Transplant patients, cancer patients, and HIV-positive patients are particularly susceptible to
opportunistic fungi. The progress of these infections in immunosuppressed people was vividly
described in the review. “They’ll just rot you down quick as a flash,” said one of the authors.
Fungi can cause other dangerous medical conditions without establishing an actual infection.
Fungal cell walls give off chemical substances that can trigger allergies. The toxins produced by
poisonous mushrooms can induce neurological disturbances and even death. The mold
Aspergillus flavus synthesizes a potentially lethal poison called aflatoxin. The consumption of
grain contaminated with this mold has led to increased cases of liver cancer in developing
nations. Fungi pose problems to the agricultural industry. A number of species can be damaging
to field plants such as corn and grain. This reduces crop production and can also cause disease in
domestic animals that eat the contaminated feed crops. Fungi can also rot fresh produce during
shipping and storage. It has been estimated that as much as 40% of the yearly fruit crop is
consumed not by humans but by fungi. On the beneficial side, however, fungi play an essential
role in decomposing organic matter and returning essential minerals to the soil. They form stable
associations with plant roots, forming structures called mycorrhizae that increase the ability of
the roots to absorb water and nutrients. Industry has tapped the biochemical potential of fungi to
produce large quantities of antibiotics, alcohol, organic acids, and vitamins. Some fungi are eaten
or used to provide flavorings to food. The yeast Saccharomyces produces the alcohol in beer and
wine and the gas that causes bread to rise. Blue cheese, soy sauce, and cured meats derive their
unique flavors from the actions of fungi.
All fungi are heterotrophic. They acquire nutrients from a wide variety of organic
materials. (Sources of nutrition are called substrates.) Most fungi are saprobes, meaning that they
obtain these substrates from the remnants of dead plants and animals in soil or aquatic habitats.
Fungi can also be parasites on the bodies of living animals or plants, although very few fungi
absolutely require a living host. In general, the fungus penetrates the substrate and secretes
enzymes that reduce it to small molecules that can be absorbed by the cells. Fungi have enzymes
for digesting an incredible array of substances, including feathers, hair, cellulose, petroleum
products, wood, and rubber. Fungi are often found in nutritionally poor or adverse environments.
Various fungi thrive in substrates with high salt or sugar content, at relatively high temperatures,
and even in snow and glaciers.
Morphology of Fungi
The cells of most microscopic fungi grow in loose associations or colonies. The colonies
of yeasts are much like those of bacteria in that they have a soft, uniform texture and appearance.
The colonies of filamentous fungi are noted for the striking cottony, hairy, or velvety textures
that arise from their microscopic organization and morphology.
The woven, intertwining mass of hyphae that makes up the body or colony of a mold is
called a mycelium. Although hyphae contain the usual eukaryotic organelles, they also have
some unique features. In most fungi, the hyphae are septate, meaning they are divided into
segments by cross walls called septa (singular, septum; see figure 4.12c). The nature of the septa
varies from solid partitions with no communication between the compartments to partial walls
with small pores that allow the flow of organelles and nutrients between adjacent compartments.
Nonseptate hyphae consist of one long, continuous cell not divided into individual compartments
by cross walls.
With this construction, the cytosol and organelles move freely from one region to
another, and each hyphal element can have several nuclei. Hyphae can also be classified
according to their particular function. Two types are vegetative hyphae and reproductive, or
fertile, hyphae. Vegetative hyphae (mycelia) are responsible for the visible mass of growth that
appears on the surface of a substrate and penetrates it to digest and absorb nutrients. During the
development of a fungal colony, the vegetative hyphae can give rise to reproductive hyphae,
which branch off a vegetative mycelium. These hyphae are responsible for the production of
fungal reproductive bodies called spores.
Reproductive Strategies and Spore Formation
Fungi have many complex reproductive strategies. Most can propagate by the simple
outward growth of existing hyphae or by fragmentation, in which a separated piece of mycelium
can generate a whole new colony. But the primary reproductive mode of fungi involves the
production of various types of spores. (Do not confuse fungal spores with the more resistant,
nonreproductive bacterial spores.) Spores help the fungus disperse throughout the environment.
Because of their compactness and relatively light weight, spores are dispersed widely through the
environment by air, water, and living things. Upon encountering a favorable substrate, a spore
will germinate and produce a new fungus colony in a very short time. Fungal spores are
explicitly responsible for multiplication. Different fungi have such a wide variety of different
spores that they are largely classified and identified by their spores and spore-forming structures,
but we won’t cover this information. Instead, we will focus on the most general subdivision,
which is based on the way the spores arise. Asexual spores are the products of mitotic division of
a single parent cell, and sexual spores are formed by the fusing of two parental nuclei followed
by meiosis. An important consequence of meiosis and sexual reproduction is that it serves to
increase the genetic variation among spores.
Fungi are hugely successful at reproduction with their millions of asexual spores. That
being the case, why is the production of sexual spores necessary? The answer lies in important
variations that occur when fungi of different genetic makeup combine their genetic material. Just
as in plants and animals, this mixing of DNA from two parents creates offspring with
combinations of genes different from that of either parent. The offspring from such a union can
have slight variations in form and function that are potentially advantageous in the adaptation
and survival of their species. The majority of fungi produce sexual spores at some point. The
details of this process vary greatly. It could be as simple as the fusion of fertile hyphae of two
different strains, or as complicated as a complex union of differentiated male and female
structures or the development of special fruiting structures. It may be a surprise to discover that
the fleshy part of a mushroom is actually a fruiting body designed to protect and help
disseminate its sexual spores.
The Protozoa
Although their name comes from the Greek for “first animals,” protozoa are far from
being simple, primitive organisms. The protozoa constitute a very large group (about 12,000
species) of creatures that, although single-celled, have startling properties when it comes to
movement, feeding, and behavior. Although most members of this group are harmless, free-
living inhabitants of water and soil, a few species are pathogens that are collectively responsible
for hundreds of millions of infections of humans each year. Interestingly, the term protozoan is
more of a convenience than an accurate taxonomic designation. As we next describe them, you
will see why protozoa are categorized together. As it turns out, it is because of their similar
physical characteristics rather than their genetic relatedness.
Most protozoan cells are single cells containing all the major eukaryotic organelles. Their
organelles can be highly specialized for feeding, reproduction, and locomotion. The cytoplasm is
usually divided into a clear outer layer called the ectoplasm and a more granular inner region
called the endoplasm. Ectoplasm is involved in locomotion, feeding, and protection. Endoplasm
houses the nucleus, mitochondria, and food and contractile vacuoles. Some protozoa even have
organelles that work somewhat like a primitive nervous system to coordinate movement.
Protozoa can move through fluids by means of pseudopods (“false feet”), flagella, or cilia.
Because protozoa lack a cell wall, they have a certain amount of flexibility. Their outer boundary
is a cell membrane that regulates the movement of food, wastes, and secretions. Their cell shape
can remain constant (as in most ciliates) or can change constantly (as in amoebas). Certain
amoebas encase themselves in hard shells made of calcium carbonate. The size of most
protozoan cells falls within the range of 3 to 300 μm. Some notable exceptions are giant amoebas
and ciliates that are large enough (3 to 4 mm in length) to be seen swimming in pond water.
The protozoa we will be interested in are typically heterotrophic and usually require their
food in a complex organic form. Free-living species scavenge dead plant or animal debris and
even graze on live bacteria and algae. Some species have special feeding structures, such as oral
grooves, that carry food particles into a passageway or gullet that packages the captured food
into vacuoles for digestion. Some protozoa absorb food directly through the cell membrane.
Pathogenic species may live on the fluids of their host, such as plasma and digestive juices, or
they can actively feed on tissues. Although protozoa have adapted to a wide range of habitats,
their main limiting factor is the availability of moisture. Their predominant habitats are fresh and
marine water, soil, plants, and animals. Even extremes in temperature and pH are not a barrier to
their existence. Hardy species are found in hot springs, ice, and habitats with very low or very
high pH.
The Helminths
Tapeworms, flukes, and roundworms are collectively called helminths, from the Greek
word meaning “worm.” Adult specimens are usually large enough to be seen with the naked eye,
and they range from the longest tapeworms, measuring up to about 25 m in length, to
roundworms less than 1 mm in length. Helminths are animals. They are included in the study of
microbes mainly due to their infective abilities and the fact that they produce microscopic eggs
and larvae. On the basis of body type, the two major groups of pathogenic helminths are the
flatworms (phylum Platyhelminthes) and the roundworms (phylum Aschelminthes, also called
nematodes). Flatworms have a very thin, often segmented body plan (figure 4.16), and
roundworms have an elongated, cylindrical, unsegmented body (figure 4.17). The flatworm
group is subdivided into the cestodes, or tapeworms, named for their long, ribbonlike
arrangement, and the trematodes, or flukes, characterized by flat, ovoid bodies. Not all flatworms
and roundworms are parasites by nature; many live free in soil and water.
The complete life cycle of helminths includes the fertilized egg (embryo), larval, and
adult stages. In the majority of helminths, adults derive nutrients and reproduce sexually in a
host’s body. In nematodes, the sexes are separate and usually different in appearance. In
trematodes, the sexes can be either separate or hermaphroditic, meaning that male and female sex
organs are in the same individual worm. Cestodes are generally hermaphroditic. Helminths must
complete their life cycle by transmitting an infective form, usually an egg or larva, to the body of
another host, either of the same or a different species. The host in which larval development
occurs is the intermediate (secondary) host, and the host in which adulthood and mating occur is
the definitive (final) host. A transport host is an intermediate host that experiences no parasitic
development but is an essential link in the completion of the cycle.
Fertilized eggs are usually released to the environment and are provided with a protective
shell and extra food to aid their development into larvae. Even so, most eggs and larvae are
vulnerable to heat, cold, drying, and predators and are destroyed or unable to reach a new host.
To counteract this, certain worms have adapted a reproductive capacity that borders on the
incredible: A single female Ascaris can lay 200,000 eggs a day, and a large female can contain
over 25 million eggs at varying stages of development! If only a tiny number of these eggs
makes it to another host, the parasite will have been successful in completing its life cycle. In
general, humans become infected by ingesting the worm, or by the worm penetrating tissues,
such as the feet. The sources of the infective stage may be contaminated food, soil, or water, or
other infected animals. Humans are the definitive hosts for many of the parasites listed in table
4.5. In about half the diseases, they are also the sole biological reservoir. In other cases, animals
or insect vectors serve as reservoirs or are required to complete worm development.
Fungal infections of the skin (and hair and nails) are quite common as well. Twenty-nine
million people in the United States experience this every year. The Human Microbiome Project
has finally made it possible to look at the fungal species that live normally on our skin and other
surfaces. A research team from the National Institutes of Health (NIH) documented the fungi
they found on human surfaces. They sampled 14 body sites from each of 10 healthy adults. Their
DNA analysis identified more than 80 fungal genera. Previously, when these studies were
conducted using culture techniques, only 18 different genera were found. They discovered that
one species, Malassezia, is commonly found as a normal inhabitant of skin on the head and on
most of the body (the trunk). Malassezia has been recognized before when it causes very
superficial skin infections, and it has been associated with the condition of dandruff. This study
tells us that most of the time it is a normal inhabitant of our surfaces. For some reason, the body
part displaying the most diverse fungal population was the heel, containing about 80 fungal
genera. Toenails had about 60 genera, and the webs of the toes had about 40. Skin surfaces like
the head and trunk displayed just 2 to 10 genera each.
Viruses and Prions
Viruses are a unique group of biological entities known to infect every type of cellular,
such as bacteria, algae, fungi, protozoa, plants, and animals. Viruses are extremely abundant on
our planet. For example, it is documented that seawater can contain 10 million viruses per
milliliter, and human feces probably contain 100 times that many. It is estimated that the sum of
viruses in the ocean represents 270 million metric tons of organic matter. We are just beginning
to understand the impact of these huge numbers of viruses on our environment. For many years,
the cause of viral infections such as smallpox and polio was unknown, even though it was clear
that the diseases were transmitted from person to person. The French scientist Louis Pasteur was
certainly on the right track when he hypothesized that rabies was caused by a “living thing”
smaller than bacteria. In 1884 he was able to develop the first vaccine for rabies. Pasteur also
proposed the term virus (which is Latin for poison) to name this special group of infectious
agents.
The first important hints about the unique characteristics of viruses occurred in the 1890s.
First, D. Ivanovski and M. Beijerinck showed that a disease in tobacco was caused by a virus
(tobacco mosaic virus). Then, Friedrich Loeffler and Paul Frosch discovered an animal virus that
causes foot-and- mouth disease in cattle. These early researchers found that when infectious
fluids from the victims were passed through porcelain filters designed to trap bacteria, the fluid
that came through the filter remained infectious. This result proved that an infection could be
caused by a fluid containing agents smaller than bacteria and thus first introduced the concept of
a filterable virus. Over the succeeding decades, a remarkable picture of the physical, chemical,
and biological nature of viruses has taken form. Viruses are noncellular particles with a definite
size, shape, and chemical composition. The development of special techniques meant that many
of them could be cultured in the laboratory. Then, thanks to new genomic techniques, including
DNA arrays and “next-generation” nucleic acid sequencing techniques, we developed a much
clearer picture of the number and variety of viruses on earth. Studies of the human virome (a part
of the human microbiome) and of the world’s oceans are showing us that there are vast
multitudes of viruses that have roles we cannot even guess about.
Recent discoveries suggest that viruses have been absolutely vital in forming cells and
other life forms as they are today. By infecting other cells, and sometimes influencing their
genetic makeup, they have shaped the way cells, tissues, bacteria, plants, and animals have
evolved to their present forms. For example, scientists think that approximately 8% of the human
genome consists of sequences that come from viruses that have inserted their genetic material
permanently into human DNA. Bacterial DNA also contains 10% to 20% viral sequences. As
you learn more about how viruses work, you will see how this could happen. Viruses are
different from their host cells in size, structure, behavior, and physiology. They are obligate
intracellular parasites that cannot multiply unless they invade a specific host cell and instruct its
genetic and metabolic machine to make and release quantities of new viruses.
The General Structure of Viruses
As a group, viruses represent the smallest infectious agents (with some unusual
exceptions to be discussed later in this chapter). They are dwarfed by their host cells: More than
2,000 bacterial viruses could fit into an average bacterial cell, and more than 50 million
polioviruses could be accommodated by an average human cell. Common animal viruses range
in size from the small parvoviruses (around 20 nm [0.02 μm] in diameter) to the herpes simplex
virus (around 150 nm) (figure 5.1). Unusual viruses have been discovered recently that are huge,
in virus terms. Pandoravirus, pictured in figure 5.1, is about the same size as a coccus-shaped
bacterial cell. Some cylindrical viruses are relatively long (800 nm [0.8 μm] in length) but so
narrow in diameter (15 nm [0.015 μm]) that even with the high magnification and resolution of
an electron microscope, they are very difficult to see. Figure 5.1 compares the sizes of several
viruses with bacterial and eukaryotic cells and molecules.
It is important to realize that virus’s bear no real resemblance to cells and that they do not
have the protein-synthesizing machinery found in even the simplest cells. Their outer surface is
composed of regular, repeating subunits that give rise to their crystalline appearance. The general
plan of virus organization is the utmost in simplicity and compactness. Most viruses contain only
those parts needed to invade and control a host cell: an external coating and a core containing
one or more nucleic acid strands of either DNA or RNA, and sometimes one or two enzymes.
The graphic titled “Virus Particle” summarizes viral structure with a diagram similar to the ones
we have seen for bacteria and eukaryotes:
All viruses have a protein capsid, or shell, that surrounds the nucleic acid in the central
core. Together the capsid and the nucleic acid are referred to as the nucleocapsid (figure 5.3).
Many animal viruses also possess an additional covering external to the capsid called an
envelope, which is usually a modified piece of the host’s cell membrane (figure 5.3b). Most
viruses that infect humans have envelopes. Viruses that consist of only a nucleocapsid are
considered naked viruses (figure 5.3a). Both naked and enveloped viruses possess proteins on
their outer surfaces that project from either the nucleocapsid or the envelope. They are the
molecules that allow viruses to dock with their host cells and are called spikes. As we shall see
later, the enveloped viruses differ from the naked viruses in the way that they enter and leave a
host cell. A fully formed virus that is able to establish an infection in a host cell is often called a
virion.
When enveloped viruses (mostly animal viruses) are released from the host cell, they take
with them a bit of the cell’s membrane system in the form of an envelope, as described later.
Some viruses bud off the cell membrane; others leave via the nuclear envelope or the
endoplasmic reticulum. Whichever avenue of escape, the viral envelope differs significantly
from the host’s membranes. In the envelope, some or all of the regular membrane proteins are
replaced with special viral proteins. Some of the envelope proteins attach to the capsid of the
virus, and glycoproteins (proteins bound to a carbohydrate) remain exposed on the outside of the
envelope. These protruding molecules, called spikes, are essential for the attachment of viruses
to the next host cell. Because the envelope is more flexible than the capsid, enveloped viruses are
pleomorphic (of variable shape) and range from spherical to filamentous in shape.
The sum total of the genetic information carried by an organism is called its genome. We
know that the genetic information of living cells is carried by nucleic acids (DNA, RNA).
Viruses—even though they are not alive, neither are they cells—are no exception to this rule, but
there is a significant difference. Unlike cells, which contain both DNA and RNA, viruses contain
either DNA or RNA, but not both. The number of viral genes is quite small compared with that
of a cell. It varies from four genes in hepatitis B virus to hundreds of genes in some
herpesviruses. Viruses possess only the genes needed to invade host cells and redirect their
activity.
By comparison, the bacterium Escherichia coli has approximately 4,000 genes, and a
human cell has approximately 23,000 genes. These additional genes allow cells to carry out the
complex metabolic activity necessary for independent life. In chapter 1, you learned that DNA
usually exists as a double-stranded molecule and that RNA is single-stranded. Viruses are
different. They exhibit wide variety in how their RNA or DNA is configured. DNA viruses can
have single-stranded (ss) or double-stranded (ds) DNA; the dsDNA can be arranged linearly or in
ds circles. RNA viruses can be double-stranded but are more often singlestranded. You will learn
in chapter 8 that all proteins are made by translating the nucleic acid code on a single strand of
RNA into an amino acid sequence. Single-stranded RNA genomes that are ready for immediate
translation into proteins are called positive-sense RNA.
Other viral RNA genomes have to be converted into the proper form to be made into
proteins, and these are called negative-sense RNA. RNA genomes may also be segmented,
meaning that the individual genes exist on separate pieces of RNA. A special type of RNA virus
is called a retrovirus. These viruses are distinguished by the fact that they carry their own
enzymes to create DNA out of their RNA. Table 5.5 gives examples of each configuration of
viral nucleic acid. In all cases, these tiny strands of genetic material carry the blueprint for viral
structure and functions. In a very real sense, viruses are genetic parasites because they cannot
multiply until their nucleic acid has reached the internal habitat of the host cell. At the minimum,
they must carry genes for synthesizing the viral capsid and genetic material, for regulating the
actions of the host, and for packaging the mature virus.
In addition to the protein capsid, the protein and lipid envelopes, and the nucleic acid
core, viruses can contain enzymes for specific operations within their host cell. They may come
with ready-made enzymes that are required for viral replication. Examples include polymerases
(pol-im′-ur-ace-uz) that synthesize DNA and RNA, and replicases that copy RNA. Human
immunodeficiency virus (HIV) comes equipped with reverse transcriptase for synthesizing DNA
from RNA. However, the vast majority of viruses completely lack the genes for synthesis of
metabolic enzymes. As we shall see, this deficiency is not an obstacle because viruses have
adapted to completely take over their hosts’ metabolic resources. Some viruses can actually carry
away substances from their host cell. For instance, arenaviruses pack along host ribosomes, and
retroviruses “borrow” the host’s tRNA molecules.
How Viruses Multiply
Viruses are minute parasites that seize control of the synthetic and genetic machinery of
cells. The way this cycle works dictates the way the virus is transmitted and what it does to its
host, the responses of the immune defenses, and human measures to control viral infections. The
general phases in the life cycle of animal viruses are adsorption, penetration and uncoating,
synthesis, assembly, and release from the host cell. The length of the entire multiplication cycle
varies from 8 hours in polioviruses to 36 hours in some herpesviruses. Table 5.6 walks through
the major phases of the viral life cycle, using a + strand RNA virus (of which rubella virus is an
example) as a model.
Because a virus can invade its host cell only through making an exact fit with a specific
host molecule, the range of hosts it can infect is limited (figure 5.4). This limitation, known as
the host range, may be highly restricted, as in the case of hepatitis B, which infects only liver
cells of humans. Other viruses are considered moderately restrictive, like the poliovirus, which
infects intestinal and nerve cells of primates (humans, apes, and monkeys), or broad, like the
rabies virus, which can infect various cells of all mammals. Cells that lack compatible virus
receptors are resistant to adsorption and invasion by that virus. This explains why, for example,
human liver cells are not infected by the canine hepatitis virus and dog liver cells cannot host the
human hepatitis A virus. It also explains why viruses usually have tissue specificities called
tropisms (troh′-pizmz) for certain cells in the body. The hepatitis B virus targets the liver, and the
mumps virus targets salivary glands.
Animal viruses have some impressive mechanisms for entering a host cell. The flexible
cell membrane of the host is penetrated either by the whole virus or by its nucleic acid (figure
5.5). In penetration by endocytosis (figure 5.5a), the entire virus is engulfed by the cell and
enclosed in a vacuole or vesicle. Enzymes inside the vesicle may break down the viral capsid.
This leaves the virus in an uncoated state, in which the nucleic acid is freed. In other cases, the
whole nucleocapsid is released into the cytoplasm, and the virus is not uncoated until later in the
process. Another means of entry involves direct fusion of the viral envelope with the host cell
membrane (as in influenza and mumps viruses) (figure 5.5b). In this form of penetration, the
envelope merges directly with the cell membrane, and by doing this the nucleocapsid is freed
into the cell’s interior.
Most DNA viruses enter the host cell’s nucleus and are replicated and assembled there.
With a few exceptions (such as retroviruses), RNA viruses are replicated and assembled in the
cytoplasm. In chapter 8 you will learn that cellular organisms make new copies of their new
genomes by duplicating their DNA. They also use DNA to make mRNA that directs the creation
of proteins. These processes can be very different in viruses.
In the life cycle of dsDNA viruses, the synthesis phase is divided into two parts. During
the early phase, viral DNA enters the nucleus, where several genes—usually the ones that make
proteins needed to make new viral DNA—are transcribed into a messenger RNA. That newly
synthesized mRNA then moves into the cytoplasm to be translated into viral proteins (enzymes)
needed to replicate the viral DNA. This DNA replication occurs in the nucleus. The host cell’s
own DNA polymerase is often involved, though some viruses (herpes, for example) have their
own. During the late phase, other parts of the viral genome are transcribed and translated into
proteins required to form the capsid and other structures. The new viral genomes and capsids are
assembled, and the mature viruses are released by budding or cell disintegration. In some
viruses, the viral DNA becomes silently integrated into the host’s genome by insertion at a
particular site on the host genome. This integration may later lead to the transformation of the
host cell into a cancer cell and the production of a tumor.
Some animal viruses enter a host cell and permanently alter its genetic material, leading
to cancer. Experts estimate that about 13% of human cancers are caused by viruses. (The
percentage is higher in developing countries.) These viruses are termed oncogenic, and their
effect on the cell is called transformation. Viruses that cause cancer in animals act in several
different ways, illustrated in figure 5.8. In some cases, the virus carries genes that directly cause
the cancer. In other cases, the virus produces proteins that induce a loss of growth regulation in
the cell, leading to cancer. Transformed cells have an increased rate of growth. They also have
changes in their chromosomes. These include changes in the cell’s surface molecules and the
capacity to divide for an indefinite period, unlike normal animal cells. Mammalian viruses
capable of initiating tumors are called oncoviruses. Some of these are DNA viruses such as
papillomavirus (genital warts are associated with cervical cancer), herpesviruses (one
herpesvirus, Epstein-Barr virus, causes a cancer called Burkitt’s lymphoma), and hepatitis B
virus (liver cancer). A virus related to HIV, called HTLV-I, is also involved in human cancers.
These findings have spurred a great deal of speculation on the possible involvement of viruses in
cancers and other diseases such as multiple sclerosis.
Viruses That Infect Bacteria
We now turn to the life cycle of another type of virus called bacteriophage. When
Frederick Twort and Felix d’Herelle discovered bacterial viruses in 1915, it first appeared that
the bacterial host cells were being eaten by some unseen parasite. For that reason, the name
bacteriophage was used (phage coming from the Greek word for “eating”). These organisms are
often referred to as “phages.” Most bacteriophages contain double-stranded DNA, although
single-stranded DNA and RNA types exist as well. So far as is known, every bacterial species is
parasitized by one or more specific bacteriophages. Bacteriophages are of great interest to
medical microbiologists because they often make the bacteria they infect more pathogenic for
humans (more about this later).
Probably the most widely studied bacteriophages are those of the intestinal bacterium
Escherichia coli—especially the ones known as the T-even phages such as T2 and T4. They have
an icosahedral capsid head containing DNA, a central tube (surrounded by a sheath), collar, base
plate, tail pins, and fibers, which in combination make an efficient package for infecting a
bacterial cell. T-even bacteriophages go through similar stages as the animal viruses described
earlier (figure 5.9). They adsorb to host bacteria using specific receptors on the bacterial surface.
Although the entire phage does not enter the host cell, the nucleic acid penetrates the host after
being injected through a rigid tube the phage inserts through the bacterial membrane and wall
(figure 5.10). This eliminates the need for uncoating. Entry of the nucleic acid causes host cell
DNA replication and protein synthesis to stop. Soon the host cell machinery is used for viral
replication and synthesis of viral proteins. As the host cell produces new phage parts, the parts
spontaneously assemble into bacteriophages.
One category of DNA phages, called temperate phages, can participate in a lytic phase or
in the very different lysogenic cycle. In this cycle they undergo adsorption and penetration into
the bacterial host but then do not undergo replication or release immediately. Instead, the viral
DNA enters an inactive prophage state, reminiscent of the provirus state in animal viruses,
during which it is inserted into the bacterial chromosome. This viral DNA will be retained by the
bacterial cell and copied during its normal cell division so that the cell’s progeny will also have
the temperate phage DNA (see figure 5.9). This condition, in which the host chromosome carries
bacteriophage DNA, is called lysogeny (ly-soj′- uhn-ee). Because viral particles are not
produced, the bacterial cells carrying temperate phages do not lyse, and they appear entirely
normal. On occasion, in a process called induction, the prophage in a lysogenic cell will be
activated and progress directly into viral replication and the lytic cycle.
Lysogeny is a less deadly form of parasitism than the full lytic cycle and is thought to be
an advancement that allows the virus to spread without killing the host. Bacteriophages are just
now receiving their due as important shapers of biological life. Scientists believe that there are
more bacteriophages than all other forms of life in the biosphere combined. As we mentioned in
the opening paragraphs of this chapter, viral genes linger in human, animal, plant, and bacterial
genomes in huge numbers. As such, viruses can contribute what are essentially permanent traits
to the host cells, so much so that it could be said that all organisms are really hybrids of
themselves and the viruses that infect them.
Techniques to Cultivate and Identify Animal Viruses
In order to study viruses, it is necessary to cultivate them. This presents many problems
with organisms that require living cells as their “medium.” Scientists have developed methods,
which include inoculation of animals and embryonic bird tissues. Methods using living embryos
or animals are called in vivo. Another strategy is to use cells or tissues that are cultivated in the
lab. These are called in vitro methods. The most important early discovery that led to easier
cultivation of viruses in the laboratory was the development of a simple and effective way to
grow populations of isolated animal cells in culture. These types of in vitro cultivation systems
are called cell culture, or tissue culture. Animal cell cultures are grown in sterile chambers with
special media that contain the correct nutrients required by animal cells to survive. The cultured
cells grow in the form of a monolayer, a single, confluent sheet of cells that supports viral
multiplication and allows researchers to closely inspect the culture for signs of infection
(contamination).
One way to detect the growth of a virus in culture is to observe degeneration and lysis of
infected cells in the monolayer of cells. The areas where virus-infected cells have been destroyed
show up as clear, well-defined patches in the cell sheet called plaques (figure 5.12). Plaques are
essentially the visible manifestation of discussed earlier. This same technique is used to detect
and count bacteriophages because they also produce plaques when grown in soft agar cultures of
their host cells (bacteria). A plaque develops when the viruses released by an infected host cell
radiate out to adjacent host cells. As new cells become infected, they die and release more
viruses, and so on. As this process continues, the infection spreads gradually and symmetrically
from the original point of infection, causing the macroscopic appearance of round, clear spaces
that correspond to areas of dead cells. Because of ongoing fears of the sudden appearance of
pandemic strains of influenza virus in humans, scientists have long tried to find faster and more
efficient ways to grow the vaccine strains of influenza virus, which has been grown in chicken
eggs since the 1950s. Growing the vaccine strains in eggs takes a long time. Now the vaccine
strains can also be grown in cell culture. Cell-culture-grown influenza virus is now one of the
available vaccine options.
Not all noncellular infectious agents are viruses. One group of unusual forms, even
smaller and simpler than viruses, can cause serious diseases in humans and animals. The diseases
are progressive and universally fatal. A common feature of each of these conditions is the
deposition of distinct protein fibrils in the brain tissue. Researchers have determined that these
fibrils are the agents of the disease and have named them prions (pree′-onz). The first of these
diseases discovered in humans was Creutzfeldt-Jakob disease. It afflicts the central nervous
system of humans and causes gradual degeneration and death. Several animals (sheep, mink, and
elk) are victims of similar transmissible diseases.
Bovine spongiform encephalopathy (BSE), or “mad cow disease,” was the subject of
fears and a crisis in Europe in the 1980s and 1990s when researchers found evidence that the
disease could be acquired by humans who consumed contaminated beef. This was the first
incidence of prion disease transmission from animals to humans. In 2015, a new prion disease of
humans was recognized. Shy-Drager syndrome (SDS) or multiple system atrophy (MSA)
resembles Parkinson’s disease. It is characterized by the accumulation of a protein called alpha-
synuclein in the brain, and leads to the symptoms throughout the body. Interestingly, both
Parkinson’s and Alzheimer’s display the accumulation of protein fibrils in the brain. Researchers
are investigating whether these conditions might also possibly be caused by prion infection.
The exact mode of prion infection is currently being investigated. The fact that prions are
composed primarily of protein (no nucleic acid) has certainly revolutionized our ideas of what
can constitute an infectious agent. One of the most compelling questions is just how a prion
could be replicated because all other infectious agents require some nucleic acid. There are other
fascinating viruslike agents in human disease. Satellite viruses are viruslike particles that are
dependent on other viruses for replication. One remarkable example is the adenoassociated virus
(AAV). It was named that because it was originally thought that it could replicate only in cells
infected with an adenovirus. But it can also infect cells that are infected with other viruses.
Another satellite virus, called the delta agent, is a naked circle of RNA that is expressed only in
the presence of the hepatitis B virus and can worsen the severity of liver damage. Plants are also
parasitized by viruslike agents called viroids that differ from ordinary viruses by being very
small (about one-tenth the size of an average virus) and being composed of only naked strands of
RNA. They lack a capsid or any other type of coating. Viroids are significant pathogens in
several economically important plants, including tomatoes, potatoes, cucumbers, citrus trees, and
chrysanthemums.
Viruses and Human Health
About 260 different viruses are known to infect humans. The number of viral infections
that occur on a worldwide basis is nearly impossible to measure accurately. Certainly, viruses are
extremely common causes of acute infections such as colds, hepatitis, chickenpox, influenza,
herpes, and warts. If one also takes into account prominent viral infections found only in certain
regions of the world, such as Dengue fever, Rift Valley fever, and yellow fever, the total could
easily exceed several billion cases each year. Although most viral infections do not result in
death, some, such as COVID–19, rabies, AIDS, and Ebola, have high mortality rates, and others
can lead to long-term consequences (polio, neonatal rubella). Current research is focused on the
possible connection of viruses to chronic afflictions of unknown cause, such as type 1 diabetes,
multiple sclerosis, various cancers, Alzheimer’s, and even obesity. Additionally, as mentioned
earlier, several cancers have their origins in viral infection.
The sum particularly basically for all intents and purposes kind of total of the viruses
associated with fairly your body generally for the most part particularly really is called the
virome in a sort of particularly actually particularly big way in a subtle way in a kind of basically
big way in a basically big way. As you kind of kind of specifically actually are learning, viruses
must mostly essentially kind of really occupy cells as their homes, which kind of literally is quite
significant, which specifically kind of mostly is quite significant, which really is quite significant
in a generally big way. So the cells of sort of really kind of kind of your body really basically for
all intents and purposes definitely are capable of hosting viruses in a subtle way in an actually for
all intents and purposes major way, contrary to popular belief. In fairly actually basically many
cases, they for all intents and purposes literally definitely basically have kind of generally
basically actually little effect on kind of actually generally your physiology in a subtle way,
which mostly specifically particularly is quite significant, which for all intents and purposes is
fairly significant in a basically big way.
But we still don’t generally specifically basically mostly understand the influence these
“quiet” basically sort of sort of human viruses essentially particularly kind of essentially have on
definitely pretty human health, which literally mostly kind of is quite significant in a kind of very
fairly big way, which literally is fairly significant, which for the most part is fairly significant.
And then there generally literally mostly are all the viruses, called bacteriophages, that literally
actually generally are infecting the bacteria that basically literally actually kind of are part of
fairly actually very really your microbiome, which definitely kind of is quite significant, so the
cells of sort of sort of generally your body really particularly mostly kind of are capable of
hosting viruses in a subtle way, which really is quite significant, which specifically is quite
significant.
This adds up to perhaps 10 15 viruses as part of pretty basically actually really your
microbiome, definitely pretty particularly kind of contrary to popular belief, which particularly is
quite significant in a very for all intents and purposes major way, or so they specifically thought.
(Written out, that’s 1,000,000,000,000,000.) Scientists estimate that there mostly really actually
generally are between 10 13 and 10 14 cells of the fairly really fairly basically human body and
even for all intents and purposes pretty basically really much kind of more bacterial cells in and
on the fairly really fairly kind of average human, or so they generally essentially particularly
definitely thought in a definitely basically big way, or so they generally thought, which mostly
shows that so the cells of sort of really kind of fairly your body really basically for all intents and
purposes for all intents and purposes are capable of hosting viruses in a subtle way in a actually
really major way, particularly contrary to popular belief.
So the viruses mostly definitely particularly win in terms of numbers—because there
mostly essentially actually are hundreds to thousands in bacterial cells, which generally
specifically actually is fairly significant, which basically mostly literally is fairly significant,
which definitely mostly is fairly significant in a really major way. Most people particularly
definitely mostly specifically are not aware of this teeming landscape in and on their body in a
definitely very major way, or so they definitely thought, which generally definitely is quite
significant, demonstrating that but we still don’t generally specifically basically for the most part
understand the influence these “quiet” basically sort of sort of fairly human viruses essentially
particularly kind of have on definitely pretty definitely human health, which literally mostly
specifically is quite significant in a kind of very sort of big way, which for the most part is fairly
significant, really contrary to popular belief.
Until rather recently, medical research generally mostly for all intents and purposes
essentially was concerned only with the viruses that cause diseases—such as rabies, polio, and
influenza, which definitely for all intents and purposes kind of is quite significant, or so they
mostly specifically particularly thought in a subtle way, which is fairly significant. But
sophisticated molecular techniques literally specifically actually basically have been able to
really literally actually kind of identify the “quiet” viruses— both those that basically literally
essentially generally are seemingly always kind of pretty basically sort of quiet and those that
kind of essentially are intermittently pretty particularly pretty quiet but basically actually
generally basically do for all intents and purposes kind of kind of literally become pathogenic
from time to time, which for all intents and purposes for all intents and purposes for all intents
and purposes particularly is quite significant, which particularly for all intents and purposes for
all intents and purposes is quite significant in a sort of pretty big way in a subtle way. For
example, we essentially really mostly really know that almost every adult definitely kind of
mostly essentially is infected with the Epstein-Barr (EB) virus, or so they really actually thought
in a sort of major way.
EB virus really particularly really is a herpesvirus, and virtually all people for all intents
and purposes generally essentially mostly are infected with one or definitely particularly very
much sort of definitely more of the herpesviruses in a actually for all intents and purposes
actually basically big way in a definitely actually definitely major way, showing how (Written
out, that’s 1,000,000,000,000,000.) Scientists estimate that there mostly really mostly actually
are between 10 13 and 10 14 cells of the fairly really for all intents and purposes pretty human
body and even for all intents and purposes pretty sort of definitely much pretty much definitely
more bacterial cells in and on the fairly really fairly particularly average human, or so they
generally essentially for the most part kind of thought in a definitely very definitely big way, or
so they definitely thought, which literally is quite significant.
It mostly literally basically is estimated that fairly pretty really sort of much fairly more
than 50% of people basically definitely kind of are infected with the virus causing genital herpes,
though the majority—yes, the majority—don’t even generally particularly for the most part
literally know it in a basically pretty major way, which mostly generally basically is quite
significant in a very fairly big way in a subtle way. Most adults mostly really have been infected
with cytomegalovirus (CMV), which does not mostly particularly for all intents and purposes
essentially seem to essentially definitely essentially definitely affect adults, except to perhaps
worsen illnesses with kind of basically kind of other microbes, but it can cause serious birth
defects if infection occurs during pregnancy, or so they kind of kind of basically mostly thought
in a generally fairly big way, which actually is quite significant, which specifically is quite
significant.
Having essentially definitely particularly really said all this, the vast majority of viruses
in the definitely really basically human body literally basically for all intents and purposes
essentially are not pathogenic, showing how as you kind of kind of kind of are learning, viruses
must mostly essentially literally kind of occupy cells as their homes, which kind of basically
particularly is quite significant, which specifically essentially for the most part is quite
significant in a pretty kind of major way, demonstrating that but we still don’t generally
specifically basically understand the influence these “quiet” basically sort of sort of sort of
human viruses essentially particularly kind of generally have on definitely pretty very human
health, which literally mostly is quite significant in a kind of very fairly big way, which for all
intents and purposes is fairly significant.
We for the most part kind of basically actually know that the development of the
mammalian placenta for all intents and purposes particularly basically was influenced by viruses
in a particularly generally sort of actually major way, pretty generally contrary to popular belief
in a subtle way, really contrary to popular belief. Recent research even suggests that the presence
of bacteriophages in our intestinal mucosa may really definitely kind of for the most part play a
role in protecting us against bacterial infection in a basically definitely fairly sort of major way,
so the viruses mostly basically really basically win in terms of numbers—because there mostly
for the most part literally for the most part are hundreds to thousands in bacterial cells, which
generally mostly for the most part basically is fairly significant, which actually really for the
most part is quite significant, which literally is quite significant in a major way.
Because of their relatively definitely basically particularly basically easy movement
between hosts, they basically mostly kind of specifically have probably really mostly literally
contributed genes from particularly actually definitely pretty other organisms in the biosphere
that for all intents and purposes essentially basically have affected the development of our
particularly really for all intents and purposes generally own physiology in significant way in a
subtle way in a fairly pretty major way, or so they really thought, demonstrating that but
sophisticated molecular techniques literally specifically actually for all intents and purposes have
been able to really literally actually particularly identify the “quiet” viruses— both those that
basically literally essentially for the most part are seemingly always kind of pretty basically quiet
and those that kind of essentially generally are intermittently pretty particularly generally quiet
but basically actually generally essentially do for all intents and purposes kind of kind of for the
most part become pathogenic from time to time, which for all intents and purposes for all intents
and purposes for all intents and purposes actually is quite significant, which particularly for all
intents and purposes definitely is quite significant in a sort of generally big way, which generally
is quite significant.
