Ways in which Chemical Control Agents Affect Bacteria

The basis of chemotherapeutic control of bacteria is selective toxicity. Selective toxicity means

that the chemical being used should inhibit or kill the intended pathogen without seriously

harming the host. A broad spectrum agent is one generally effective against a variety of Gram-

positive and Gram-negative bacteria; a narrow spectrum agent generally works against just

Gram-positives, Gram-negatives, or only a few bacteria. Such agents may be cidal or static in

their action. A cidal agent kills the organism while a static agent inhibits the organism's growth

long enough for body defenses to remove it. There are two categories of antimicrobial

chemotherapeutic agents: antibiotics and synthetic drugs. Antibiotics are metabolic products of

one microorganism that inhibit or kill other microorganisms. Synthetic drugs are antimicrobial

drugs synthesized by chemical procedures in the laboratory. Many of today's antibiotics are

now actually semisynthetic and some are even made synthetically. We will now look at the

various ways in which our control agents affect bacteria altering their structures or interfering

with their cellular functions.

• Describe one way an antibiotic can inhibit peptidoglycan synthesis, state how that

ultimately kills the bacterium, and give an example of such an antibiotic.

• Describe one way an antibiotic can alter bacterial ribosomes, state how that ultimately

inhibits or kills the bacterium, and give an example of such an antibiotic.

• Describe one way an antibiotic can interfere with bacterial DNA synthesis, state how

that ultimately kills the bacterium, and give an example of such an antibiotic

Many Antibiotics inhibit Synthesis of Peptidoglycan and cause Osmotic Lysis

peptide

cross-link

cytoplasm

Figure 4.2.44.2.4.2.4: Action of Transpeptidase in Peptidoglycan Synthesis. (Step 1) Finally,

transpeptidase enzymes reform the peptide cross-links between the rows and layers of

peptidoglycan to make the wall strong.

Interference with this process results in the formation of a weak cell wall and osmotic lysis of

the bacterium. Agents that inhibit peptidoglycan synthesis include the penicillins (penicillin G,

methicillin, oxacillin, ampicillin, amoxicillin, ticarcillin, etc.), the cephalosporins (cephalothin,

cefazolin, cefoxitin, cefotaxime, cefaclor, cefoperazone, cefixime, ceftriaxone, cefuroxime,

etc.), the carbapenems (imipenem, metropenem), the monobactems (aztreonem), and the

carbacephems (loracarbef). Penicillins, monobactams, carbapenems, and cephalosporins are

known chemically as beta-lactam antibiotics because they all share a molecular structure called

a beta-lactam ring (see Figure 4.2.54.2.5). The glycopeptides (vancomycin, teichoplanin) and

lipopeptides (daptomycin) also inhibit peptidoglycan synthesis.

a. Beta lactam antibiotics such as penicillins and cephalosporins

Penicillins, cephalosporins, as well as other beta-lactam antibiotics (see Common Antibiotics),

bind to the transpeptidase enzymes (also called penicillin-binding proteins) responsible for

reforming the peptide cross-links between rows and layers of peptidoglycan of the cell wall as

new peptidoglycan monomers are added during bacterial cell growth. This binding blocks the

transpeptidase enzymes from cross-linking the sugar chains and results in a weak cell wall. In

addition, these antibiotics appear to interfere with the bacterial controls that keep autolysins in

check, with resulting degradation of the peptidoglycan and osmotic lysis of the

bacterium (Figure 4.2.64.2.6).

Flash animation illustrating how penicillins inhibit peptidoglycan

synthesis.

html5 version of animation for iPad showing how penicillins inhibit

the synthesis of peptidoglycan.

Flash animation showing how penicillins inhibit peptidoglycan

synthesis.

use, ASM MicrobeLibrary.

YouTube movie showing lysis of E. coli after exposure to a

penicillin #1

YouTube movie showing lysis of E. coli after exposure to a

penicillin #2

b. Glycopeptides

Glycopeptides such as vancomycin (see Common Antibiotics) and the lipoglycopeptide

teichoplanin bind to the D-Ala-D-Ala portion of the pentapeptides of the peptidoglycan

monomers and block the formation of gycosidic bonds between the sugars by the

transgycosidase enzymes, as well as the formation of the peptide cross-links by the

transpeptidase enzymes. This results in a weak cell wall and subsequent osmotic lysis of the

bacterium (see Figure 4.2.74.2.7).

Flash animation illustrating how vancomycins inhibit peptidoglycan

synthesis.

html5 version of animation for iPad illustrating how vancomycins

inhibit peptidoglycan synthesis.

Flash animation showing how vancomycin inhibit peptidoglycan

synthesis.

use, ASM MicrobeLibrary.

c. Bacitracin

Bacitracin (see Common Antibiotics) binds to the transport protein bactoprenol after it inserts

a peptidoglycan monomer into the growing cell wall. It subsequently prevents the

dephosphorylation of the bactoprenol after it releases the monomer it has transported across

the membrane. Bactoprenol molecules that have not lost the second phosphate group cannot

assemble new monomers and transport them across the cytoplasmic membrane. As a result, no

new monomers are inserted into the growing cell wall. As the autolysins continue to break the

peptide cross-links and new cross-links fail to form, the bacterium bursts from osmotic

lysis (see Figure 4.2.84.2.8).

Flash animation illustrating how bacitracin inhibit peptidoglycan

synthesis.

html5 version of animation for iPad illustrating how bacitracin

inhibit peptidoglycan synthesis.

Flash animation showing how bacitracin inhibit peptidoglycan

synthesis.

use, ASM MicrobeLibrary.

Concept map for How Antibiotics and

Chemical Agents Affect Bacterial Structures

and Function:

Agents that Inhibit Cell Wall Synthesis,

Alter the Cytoplasmic Membrane,

or Inhibit DNA Synthesis

A few antimicrobial chemotherapeutic agents inhibit normal synthesis of the acid-fast cell

wall

A few antimicrobial chemotherapeutic agents inhibit normal synthesis of the acid-fast cell wall

of the genus Mycobacterium (see Common Antibiotics).. INH(isoniazid) appears to block the

synthesis of mycolic acid, a key component of the acid-fast cell wall of mycobacteria (see

Figure 4.2.94.2.9). Ethambutol interferes with the synthesis of the outer membrane of acid-fast

cell walls (see Figure 4.2.94.2.9).

Concept map for How Antibiotics and

Chemical Agents Affect Bacterial Structures

and Function:

Agents that Inhibit Cell Wall Synthesis,

Alter the Cytoplasmic Membrane,

or Inhibit DNA Synthesis

A very few antibiotics alter the bacterial cytoplasmic membrane causing leakage of

molecules and enzymes needed for normal bacterial metabolism.

A very few antibiotics, such as polymyxins, colistins, and daptomycin (Common Antibiotics),

as well as many disinfectants and antiseptics, such as orthophenylphenol, chlorhexidine,

hexachlorophene, zephiran, alcohol, and triclosans, alter the bacterial cytoplasmic membrane

causing leakage of molecules and enzymes needed for normal bacterial metabolism.

a. Polymyxins and colistins act as detergents and alter membrane permeability in Gram-

negative bacteria. They cannot effectively diffuse through the thick peptidoglycan layer

in gram-positives.

b. Daptomycin disrupts the bacteria cytoplasmic membrane function by apparently

binding to the membrane and causing rapid depolarization. This results on a loss of

membrane potential and leads to inhibition of protein, DNA and RNA synthesis,

resulting in bacterial cell death.

c. Pyrazinamide inhibits fatty acid synthesis in the membranes of Mycobacterium

tuberculosis.

Concept map for How Antibiotics and

Chemical Agents Affect Bacterial Structures

and Function:

Agents that Inhibit Cell Wall Synthesis,

Alter the Cytoplasmic Membrane,

or Inhibit DNA Synthesis

Some antimicrobial chemotherapeutic agents inhibit normal nucleic acid replication in

bacteria (see Common Antibiotics).

a. Fluoroquinolones

Fluoroquinolones (norfloxacin, lomefloxacin, fleroxacin, ciprofloxacin, enoxacin,

trovafloxacin, gatifloxacin, etc., (Common Antibiotics))) work by inhibiting one or more of a

group of enzymes called topoisomerase, enzymes needed for supercoiling, replication, and

separation of circular bacterial DNA (see Figure 4.2.104.2.10). For example, DNA gyrase

(topoisomerase II) catalyzes the negative supercoiling of the circular DNA found in bacteria.

It is critical in bacterial DNA replication, DNA repair, transcription of DNA into RNA, and

genetic recombination. Topoisomerase IV, on the other hand, is involved in the relaxation of

the supercoiled circular DNA, enabling the separation of the interlinked daughter chromosomes

at the end of bacterial DNA replication.

In Gram-negative bacteria, the main target for fluoroquinolones is DNA gyrase (topoisomerase

II), an enzyme responsible for supercoiling of bacterial DNA during DNA replication; in Gram-

positive bacteria, the primary target is topoisomerase IV, an enzyme responsible for relaxation

of supercoiled circular DNA and separation of the inter-linked daughter chromosomes.

Flash animation illustrating a normal bacterial enzyme reaction.

html5 version of animation for iPad illustrating a normal bacterial enzyme

reaction.

Flash animation illustrating antimicrobial agents may inactivate a bacterial

enzyme.

html5 version of animation for iPad illustrating antimicrobial agents may

inactivate a bacterial enzyme.

For More Information: The Nucleoid from Unit 1.

b. Sulfonamides

Sulfonamides (sulfamethoxazole, sulfanilamide) and diaminopyrimidines (trimethoprim) (see

Common Antibiotics) block enzymes in the bacteria pathway required for the synthesis of

tetrahydrofolic acid, a cofactor needed for bacteria to make the nucleotide bases thymine,

guanine, uracil, and adenine (see Figure 4.2.114.2.11).

This is done through a process called competitive antagonism whereby a drug chemically

resembles a substrate in a metabolic pathway. Because of their similarity, either the drug or the

substrate can bind to the substrate's enzyme. While the enzyme is bound to the drug, it is unable

to bind to its natural substrate and that blocks that step in the metabolic pathway (see

Figure 4.2.124.2.12). Typically, a sulfonamide and a diaminopyrimidine are combined. Co-

trimoxazole, for example, is a combination of sulfamethoxazole and trimethoprim.

Flash animation showing competitive antagonism.

html5 version of animation for iPad showing competitive

antagonism.

Sulfonamides such as sulfamethoxazole tie up the first enzyme in the pathway, the conversion

of para-aminobenzoic acid to dihydropteroic acid (see Figure 4.2.114.2.11). Trimethoprim

binds to the third enzyme in the pathway, an enzyme that is responsible for converting

dihydrofolic acid to tetrahydrofolic acid (see Figure 4.2.114.2.11). Without the tetrahydrofolic

acid, the bacteria cannot synthesize DNA or RNA.

c. Metronidazole

Metronidazole (see Common Antibiotics) is a drug that is activated by the microbial proteins

flavodoxin and feredoxin found in microaerophilc and anaerobic bacteria and certain

protozoans. Once activated, the metronidazole puts nicks in the microbial DNA strands.

Concept map for How Antibiotics and

Chemical Agents Affect Bacterial Structures

and Function:

Agents that Inhibit Cell Wall Synthesis,

Alter the Cytoplasmic Membrane,

or Inhibit DNA Synthesis

d. Rifampin

Rifampin (rifamycin) (see Common Antibiotics) blocks transcription by inhibiting bacterial

RNA polymerase, the enzyme responsible for transcription of DNA to mRNA.

For More Information: Transcription from Unit 7

Concept map for How Antibiotics and

Chemical Agents Affect Bacterial Structures

and Function:

Agents that Alter Prokaryotic Ribosomal

Subunits,

Inhibit RNA Polymerase, and Denature

Enzymes

Many antibiotics alter bacterial ribosomes, interfering with translation of mRNA into

proteins and thereby causing faulty protein synthesis (see Common Antibiotics).

To learn more detail about the specific steps involved in translation during bacterial protein

synthesis, see the animation that follows. Protein synthesis is discussed in greater detail in Unit

For More Information: Ribosomes from Unit 1

For More Information: Translation from Unit 7

Flash animation illustrating the early stages of translation during

bacterial protein synthesis.

html5 version of animation for iPad illustrating the early stages of

translation during bacterial protein synthesis.

a. Aminoglycosides

The aminoglycosides (streptomycin, neomycin, netilmicin, tobramycin, gentamicin, amikacin,

etc. (see Common Antibiotics)) bind irreversibly to the 16S rRNA in the 30S subunit of

bacterial ribosomes. Although the exact mechanism of action is still uncertain, there is evidence

that some prevent the transfer of the peptidyl tRNA from the A-site to the P-site, thus preventing

the elongation of the polypeptide chain. Some aminoglycosides also appear to interfere with

the proofreading process that helps assure the accuracy of translation (see Figure 4.2.134.2.13).

Possibly the antibiotics reduce the rejection rate for tRNAs that are near matches for the codon.

This leads to misreading of the codons or premature termination of protein synthesis (see

Figure 4.2.144.2.14). Aminoglycosides may also interfere directly or indirectly with the

function of the bacterial cytoplasmic membrane. Because of their toxicity, aminoglycosides are

generally used only when other first line antibiotics are not effective.

Flash animation illustrating aminoglycosides preventing the

translocation of tRNA from the A-site to the P-site of bacterial

ribosomes.

html5 version of animation for iPad illustrating aminoglycosides

preventing the translocation of tRNA from the A-site to the P-site of

bacterial ribosomes.

Flash animation illustrating aminoglycosides causing a misreading

of codons.

html5 version of animation for iPad illustrating aminoglycosides

causing a misreading of codons.

b. Tetracyclines

The tetracyclines (tetracycline, doxycycline, demeclocycline, minocycline, etc. (see Common

Antibiotics)) bind reversibly to the 16S rRNA in the 30S ribosomal subunit, distorting it in such

a way that the anticodons of charged tRNAs cannot align properly with the codons of the

mRNA (see Figure 4.2.154.2.15).

Flash animation illustrating how tetracyclines bind to the 30S

ribosomal subunit and block translation.

html5 version of animation for iPad illustrating how tetracyclines

bind to the 30S ribosomal subunit and block translation.

c. Macrolides

The macrolides (erythromycin, azithromycin, clarithromycin, dirithromycin, troleandomycin,

etc. (see Common Antibiotics)) bind reversibly to the 23S rRNA in the 50S subunit of bacterial

ribosomes. They appear to inhibit elongation of the protein by preventing the enzyme

peptidyltransferase from forming peptide bonds between the amino acids (see

Figure 4.2.164.2.16). They may also prevent the transfer of the peptidyl tRNA from the A-site

to the P-site (see Figure 4.2.174.2.17) as the beginning peptide chain on the peptidyl tRNA

adheres to the ribosome, creates friction, and blocks the exit tunnel of the 50S ribosomal

subunit.

Flash animation illustrating how macrolides bind to the 50S

ribosomal subunit and block translation by blocking

peptidyltransferase.

Flash animation illustrating how macrolides bind to the 50S

ribosomal subunit and block translation by preventing the transfer

of the peptidyl tRNA from the A-site to the P-site.

html5 version of animation for iPad illustrating how macrolides bind

to the 50S ribosomal subunit and block translation by blocking

peptidyltransferase.

html5 version of animation for iPad illustrating how macrolides bind

to the 50S ribosomal subunit and block translation by preventing the

transfer of the peptidyl tRNA from the A-site to the P-site.

d. Oxazolidinones

The oxazolidinones (linezolid, sivextro) (see Common Antibiotics), following the first cycle of

protein synthesis, interfere with translation sometime before the initiation phases. They appear

to bind to the 50S ribosomal subunit and interfere with its binding to the initiation complex (see

Figure 4.2.184.2.18).

Flash animation illustrating how oxazolidinones block the binding

of the 50S ribosomal subunit to the initiation complex.

html5 version of animation for iPad illustrating how oxazolidinones

block the binding of the 50S ribosomal subunit to the initiation

complex.

e. Streptogramins

The streptogramins (synercid, a combination of quinupristin and dalfopristin (see Common

Antibiotics)) bind to two different locations on the 23S rRNA in the 50S ribosomal subunit and

work synergistically to block translation. There are reports that the streptogramins may inhibit

the attachment of the charged tRNA to the A-site or may block the peptide exit tunnel of the

50S ribosomal subunit.

Concept map for How Antibiotics and

Chemical Agents Affect Bacterial Structures

and Function:

Agents that Alter Prokaryotic Ribosomal

Subunits,

Inhibit RNA Polymerase, and Denature

Enzymes

For a more detailed description of any specific antimicrobial agent, see the website of RxList

- The Internet Drug Index.

Modes of action for disinfectants, antiseptics, and sanitizers

Disinfection is the elimination of microorganisms, but not necessarily endospores, from

inanimate objects or surfaces, whereas decontamination is the treatment of an object or

inanimate surface to make it safe to handle. Sterilization is the process of destroying all living

organisms and viruses. A sterile object is one free of all life forms, including bacterial

endospores, as well as viruses.

The term disinfectant is used for an agent used to disinfect inanimate objects or surfaces but is

generally too toxic to use on human tissues. An antiseptic refers to an agent that kills or inhibits

growth of microbes but is safe to use on human tissue. A sanitizer describes an agent that

reduces microbial numbers to a safe level. Because disinfectants and antiseptics often work

slowly on some viruses - such as the hepatitis viruses, bacteria with an acid-fast cell wall such

as Mycobacterium tuberculosis, and especially bacterial endospores, produced by the

genus Bacillus and the genus Clostridium, they are usually unreliable for sterilization - the

destruction of all life forms.

There are a number of factors which influence the antimicrobial action of disinfectants and

antiseptics, including:

1. The concentration of the chemical agent.

2. The temperature at which the agent is being used. Generally, the lower the temperature,

the longer it takes to disinfect or decontaminate.

3. The kinds of microorganisms present. Endospore producers such

as Bacillus species, Clostridium species, and acid-fast bacteria like Mycobacterium

tuberculosis are harder to eliminate.

4. The number of microorganisms present. The more microorganisms present, the harder

it is to disinfect or decontaminate.

5. The nature of the material bearing the microorganisms. Organic material such as dirt

and excreta interferes with some agents.

The best results are generally obtained when the initial microbial numbers are low and when

the surface to be disinfected is clean and free of possible interfering substances.

Concept map for Lab 19 - Using

disinfectants, antisepticics, and sanitizers to

control microorganisms

There are 2 common antimicrobial modes of action for disinfectants, antiseptics, and sanitizers:

1. They may damage the lipids and/or proteins of the semipermeable cytoplasmic membrane

of microorganisms resulting in leakage of cellular materials needed to sustain life.

2. They may denature microbial enzymes and other proteins, usually by disrupting the

hydrogen and disulfide bonds that give the protein its three-dimensional functional shape. This

blocks metabolism.

A large number of such chemical agents are in common use. Some of the more common groups

are listed below:

1. Phenol and phenol derivatives: Phenol (5-10%) was the first disinfectant commonly used.

However, because of its toxicity and odor, phenol derivatives (phenolics) are now generally

used. The most common phenolic is orthophenylphenol, the agent found in O-syl®,

Staphene®, and Amphyl®. Bisphenols contain two phenolic groups and typically have chlorine

as a part of their structure. They include hexachlorophene and triclosan. Hexachlorophene in a

3% solution is combined with detergent and is found in PhisoHex®. Triclosan is an antiseptic

very common in antimicrobial soaps and other products. Biguanides include chlorhexadine and

alexidine. A 4% solution of chlorhexidine in isopropyl alcohol and combined with detergent

(Hibiclens® and Hibitane®) is a common hand washing agent and surgical handscrub. These

agents kill most bacteria, most fungi, and some viruses, but are usually ineffective against

endospores. Chloroxylenol (4-chloro-3,5-dimethylphenol) is a broad spectrum antimicrobial

chemical compound used to control bacteria, algae, fungi and virus and is often used in

antimicrobial soaps and antiseptics. Phenol and phenolics alter membrane permeability and

denature proteins. Bisphenols, biguanides, and chloroxylenol alter membrane permeability.

2. Soaps and detergents: Soaps are only mildly microbicidal. Their use aids in the mechanical

removal of microorganisms by breaking up the oily film on the skin (emulsification) and

reducing the surface tension of water so it spreads and penetrates more readily. Some cosmetic

soaps contain added antiseptics to increase antimicrobial activity.

Detergents may be anionic or cationic. Anionic (negatively charged) detergents, such as

laundry powders, mechanically remove microorganisms and other materials but are not very

microbicidal. Cationic (positively charged) detergents alter membrane permeability and

denature proteins. They are effective against many vegetative bacteria, some fungi, and some

viruses. However, bacterial endospores and certain bacteria such as Mycobacterium

tuberculosis and Pseudomonas species are usually resistant. Soaps and organic materials like

excreta also inactivate them. Cationic detergents include the quaternary ammonium compounds

such as benzalkonium chloride, zephiran®, diaprene, roccal, ceepryn, and phemerol.

Household Lysol® contains alkyl dimethyl benzyl ammonium chloride and alcohols.

3. Alcohols

70% solutions of ethyl or isopropyl alcohol are effective in killing vegetative bacteria,

enveloped viruses, and fungi. However, they are usually ineffective against endospores and

non-enveloped viruses. Once they evaporate, their cidal activity will cease. Alcohols denature

membranes and proteins and are often combined with other disinfectants, such as iodine,

mercurials, and cationic detergents for increased effectiveness.

4. Acids and alkalies

Acids and alkalies alter membrane permeability and denature proteins and other molecules.

Salts of organic acids, such as calcium propionate, potassium sorbate, and methylparaben, are

commonly used as food preservatives. Undecylenic acid (Desenex®) is used for dermatophyte

infections of the skin. An example of an alkali is lye (sodium hydroxide).

5. Heavy metals

Heavy metals, such as mercury, silver, and copper, denature proteins. Mercury compounds

(mercurochrome, metaphen, merthiolate) are only bacteriostatic and are not effective against

endospores. Silver nitrate (1%) is sometimes put in the eyes of newborns to prevent gonococcal

ophthalmia. Copper sulfate is used to combat fungal diseases of plants and is also a common

algicide. Selinium sulfide kills fungi and their spores.

6. Chlorine

Chlorine gas reacts with water to form hypochlorite ions, which in turn denature microbial

enzymes. Chlorine is used in the chlorination of drinking water, swimming pools, and sewage.

Sodium hypochlorite is the active agent in household bleach. Calcium hypochlorite, sodium

hypochlorite, and chloramines (chlorine plus ammonia) are used to sanitize glassware, eating

utensils, dairy and food processing equipment, hemodialysis systems, and treating water

supplies.

7. Iodine and iodophores

Iodine also denatures microbial proteins. Iodine tincture contains a 2% solution of iodine and

sodium iodide in 70% alcohol. Aqueous iodine solutions containing 2% iodine and 2.4%

sodium iodide are commonly used as a topical antiseptic. Iodophores are a combination of

iodine and an inert polymer such as polyvinylpyrrolidone that reduces surface tension and

slowly releases the iodine. Iodophores are less irritating than iodine and do not stain. They are

generally effective against vegetative bacteria, Mycobacterium tuberculosis, fungi, some

viruses, and some endospores. Examples include Wescodyne®, Ioprep®, Ioclide®, Betadine®,

and Isodine®.

8. Aldehydes

Aldehydes, such as formaldehyde and glutaraldehyde, denature microbial proteins. Formalin

(37% aqueous solution of formaldehyde gas) is extremely active and kills most forms of

microbial life. It is used in embalming, preserving biological specimens, and in preparing

vaccines. Alkaline glutaraldehyde (Cidex®), acid glutaraldehyde (Sonacide®), and

glutaraldehyde phenate solutions (Sporocidin®) kill vegetative bacteria in 10-30 minutes and

endospores in about 4 hours. A 10 hour exposure to a 2% glutaraldehyde solution can be used

for cold sterilization of materials. Ortho-phthalaldehyde (OPA) is dialdehyde used as a high-

level disinfectant for medical instruments.

9. Peroxygens

Peroxygens are oxidizing agents that include hydrogen peroxide and peracetic acid. Hydrogen

peroxide is broken down into water and oxygen by the enzyme catalase in human cells and is

not that good of an antiseptic for open wounds but is useful for disinfecting inanimate objects.

The high concentrations of hydrogen peroxide overwhelm the catalase found in microbes.

Peracetic acid is a disinfectant that kills microorganisms by oxidation and subsequent

disruption of their cytoplasmic membrane. It is widely used in health care, food processing,

and water treatment.

10. Ethylene oxide gas

Ethylene oxide is one of the very few chemicals that can be relied upon for sterilization (after

4-12 hours exposure). Since it is explosive, it is usually mixed with inert gases such as freon

or carbon dioxide. Gaseous chemosterilizers, using ethylene oxide, are commonly used to

sterilize heat-sensitive items such as plastic syringes, petri plates, textiles, sutures, artificial

heart valves, heart-lung machines, and mattresses. Ethylene oxide has very high penetrating

power and denatures microbial proteins. Vapors are toxic to the skin, eyes, and mucous

membranes and are also carcinogenic. Another gas that is used as a sterilant is chlorine dioxide

which denatures proteins in vegetative bacteria, bacterial endospores, viruses, and fungi.