Question 1

A microbiologist uses a Petroff-Hausser counting chamber to enumerate cells in a sample. If 12 cells are counted in one large square (0.04 mm²), and the chamber depth is 0.02 mm, what is the calculated number of cells per milliliter in the original sample?

Accepted Answer

$$1.5 	imes 10^7$

Question 2

Which of the following best describes the main limitation of using direct microscopic counts for environmental water samples?

Accepted Answer

It cannot distinguish between live and dead cells without special stains.

Question 3

A researcher wants to isolate and count only living, reproducing bacteria from a soil sample. Which method should they use?

Accepted Answer

Viable plate count using serial dilution and spread-plate method

Question 4

Why is it important to dilute a sample before performing a viable plate count?

Accepted Answer

To obtain a countable number of colonies (30–300) on the plate

Question 5

Which type of culture medium contains ingredients of unknown chemical composition, such as yeast extract or peptone?

Accepted Answer

Complex medium

Question 6

A microbiologist is using MacConkey agar to isolate bacteria from a fecal sample. What is the primary function of this medium?

Accepted Answer

It is both selective for Gram-negative enteric bacteria and differential for lactose fermentation.

Question 7

Which of the following best explains why the 'Great Plate Count Anomaly' occurs when comparing direct microscopic counts to viable plate counts in natural samples?

Accepted Answer

Many microbes in natural samples cannot grow under laboratory conditions.

Question 8

A culture is grown in a chemostat. What happens if the dilution rate is set too high?

Accepted Answer

Cells are washed out of the vessel faster than they can grow.

Question 9

A bacterial colony starts from a single cell and grows exponentially with a doubling time of 30 minutes. How many cells will be present after 24 hours?

Accepted Answer

$$2.8 	imes 10$$^{14}$$

Question 10

Which of the following is a key adaptation of psychrophilic bacteria that allows them to thrive at low temperatures?

Accepted Answer

High levels of unsaturated fatty acids in their membranes

Question 11

Which equation correctly relates optical density (OD) to the amount of unscattered light in a turbidimetric measurement?

Accepted Answer

$$\mathrm{OD} = \log \left( \frac{I_0$}{I} \right)$$

Question 12

Which macronutrient is typically the most abundant element in a bacterial cell by dry weight?

Accepted Answer

Carbon

Question 13

A scientist is studying a microorganism that grows optimally at 90°C. How would this organism be classified?

Accepted Answer

Hyperthermophile

Question 14

Which of the following best describes a differential medium?

Accepted Answer

A medium that contains an indicator to distinguish between different metabolic activities

Question 15

Which of the following is a micronutrient required by some bacteria for enzyme activity?

Accepted Answer

Iron (Fe)

Question 16

A sample is diluted 1:10,000 and 0.1 mL is plated, resulting in 159 colonies. What is the concentration of viable cells in the original sample?

Accepted Answer

$$1.59 	imes 10^6$ CFU/mL

Question 17

Which of the following best explains why turbidimetric measurements may not always accurately reflect cell number?

Accepted Answer

Cell clumping can cause deviations from the standard curve.

Question 18

Which of the following is a real-world consequence of nutrient runoff (nitrogen and phosphorus) into aquatic environments?

Accepted Answer

Cyanobacterial (blue-green algae) blooms that produce toxins

Question 19

Which of the following best describes a pure (axenic) culture?

Accepted Answer

A culture containing only a single kind of microorganism

Question 20

Which of the following is a critical reason for using aseptic techniques in microbiology laboratories?

Accepted Answer

To prevent contamination of cultures with unwanted organisms

Microbial Growth: Culturing, Counting, and Environmental Effects

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