Question 1

You isolate several colonies from a selective and differential agar plate. What can you infer about these organisms?

Accepted Answer

They possess characteristics that allow them to grow in the presence of the selective agent and can be distinguished based on their metabolic properties.

Question 2

Which of the following is NOT a commonly used method to count the number of microorganisms in a sample?

Accepted Answer

Gram staining

Question 3

A direct microscopic count is performed using which of the following tools?

Accepted Answer

Hemocytometer or Petroff-Hausser counting chamber

Question 4

Which of the following is an advantage of the direct microscopic count method?

Accepted Answer

It is rapid and does not require incubation.

Question 5

Does measuring turbidity provide an absolute number of bacteria in a sample? Why or why not?

Accepted Answer

No, because turbidity only provides a relative estimate based on light scattering, not an exact cell count.

Question 6

What instrument is used to measure the turbidity of a bacterial culture?

Accepted Answer

Spectrophotometer

Question 7

How does a spectrophotometer work in measuring bacterial growth?

Accepted Answer

It measures the amount of light absorbed or transmitted by a bacterial suspension.

Question 8

Which of the following is a disadvantage of using turbidity to estimate bacterial numbers?

Accepted Answer

It cannot distinguish between live and dead cells.

Question 9

What is the difference between absorbance and transmittance in spectrophotometry?

Accepted Answer

Absorbance is the amount of light absorbed by the sample, while transmittance is the fraction of light that passes through the sample.

Question 10

A highly turbid bacterial culture indicates:

Accepted Answer

A high concentration of bacterial cells.

Question 11

Which of the following best describes a plate count (viable count) method?

Accepted Answer

It estimates the number of viable (living) cells capable of forming colonies.

Question 12

When setting up serial dilutions, what do you expect to happen to the number of bacterial cells as the dilution increases?

Accepted Answer

The number of bacterial cells decreases.

Question 13

A countable plate in a viable plate count should have how many colonies?

Accepted Answer

30-300 colonies

Question 14

What does CFU stand for in microbiology?

Accepted Answer

Colony Forming Unit

Question 15

You plate 0.1 mL of a $$10^{-4}$$ dilution and count 50 colonies. What is the concentration of CFUs per mL in the original sample?

Accepted Answer

$$5 	imes 10$$^{6}$$ CFU/mL

Question 16

Which of the following steps is essential when setting up a serial dilution for a plate count?

Accepted Answer

Mixing each dilution thoroughly before transferring to the next tube.

Question 17

A student prepares a series of dilutions: $$10^{-1}$$, $$10^{-2}$$, $$10^{-3}$$, and $$10^{-4}. $$Which plate is most likely to yield a countable number of colonies if the original sample is highly concentrated?

Accepted Answer

$$10^{-4}$$

Question 18

If a culture has an absorbance (A) of 0.8 at 600 nm, and another has an absorbance of 0.2, what can you infer about the relative cell concentrations?

Accepted Answer

The culture with A = 0.8 has a higher cell concentration.

Question 19

A microbiologist wants to determine the number of viable bacteria in a water sample. Which method should they use and why?

Accepted Answer

Viable plate count, because it only counts living cells capable of forming colonies.

Question 20

Which equation correctly relates absorbance (A) and transmittance (T) in spectrophotometry?

Accepted Answer

$$A = -\log_{10}(T)$$

Microbiology Lab Techniques: Counting and Characterizing Microorganisms

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