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Microbiolgy Math Problems

Show your work for all problems.

1. Human red blood cells are about 8 µm in diameter. A single Staphylococcus bacterium is about 1µm in diameter. You are looking at a blood smear containing both of these using the 100x objective, which has a field of view 0.2mm in diameter.

a. How many red blood cells would fit end to end across the field of view?

b. How many Staphylococcus bacteria would fit end to end across the field of view?

2. When performing a use-dilution test, you meant to add 1 ml of your sample to 4 ml of water. However, you lost count and ended up adding 1ml of your sample to 5 ml of water.

a. What was the intended dilution factor for your chemical?

b. What was the actual dilution factor for your chemical?

c. In this instance, would you be testing the chemical at a higher (stronger) or a lower (weaker) concentration than what you originally intended to use?

3. Wade brewed a batch of India pale ale and added active yeast to his homebrew. After a day, he saw evidence of fermentation in his brew. Inspired to be a budding micro-brewer, he was very excited and wanted to know how many yeast cells he had in his homebrew at various steps. He remembered that he learned some very useful math from his wife, a microbiology teacher. He set up a serial dilution experiment and got the results described below.

He took one milliliter of brew and performed a serial dilution in a series of blanks. (Adding 1ml into Bottle 1, then taking 1 ml from Bottle 1 and transferring it into Bottle 2, etc.)

Bottle 1 Bottle 2 Bottle 3 Bottle 4

99 ml of water 99 ml of water 9 ml of water 9 ml of water

He then took 100 (l from each bottle, and spread it on a yeast growth plate.

(Sample from Bottle 1 onto Plate 1, sample from Bottle 2 onto Plate 2, etc.)

Plate 1 Plate 2 Plate 3 Plate 4

TMTC TMTC 500 CFU 50 CFU

a. What is the total dilution factor in each bottle?

b. Which plate is the best to use for your estimate?

c. What was the total number of CFU in the 100µl sample for the plate you chose to estimate from?

d. For the sample you used in C, what was the number of CFU per milliliter in the bottle that sample was taken from?

e. What was the number of CFU in the original milliliter of brew added to Bottle1?

f. If Wade had 10 liters of brew, about how many yeast organisms did he have in total in his brew?