Q: A bacterial culture yields 250 colonies from a 0.1 mL plated sample. What is the CFU/mL?

$$\text{CFU/mL} = \frac{250}{0.1} = 2500$$

Question 1

Which of the following statements best describes metachromatic granules in bacteria?

Accepted Answer

They are large inclusions that act as an inorganic phosphate reserve and stain red with certain blue dyes.

Question 2

A microbiologist is trying to identify Corynebacterium diphtheriae. Which cellular inclusion would help in identification?

Accepted Answer

Metachromatic granules

Question 3

Which of the following best distinguishes true bacterial motility from Brownian movement?

Accepted Answer

True motility is caused by flagella, while Brownian movement is random motion due to water molecules.

Question 4

A student observes bacteria moving in one direction under a microscope, but the movement is due to currents within the slide. What is this phenomenon called?

Accepted Answer

Streaming

Question 5

Which method is best for isolating pure bacterial colonies from a mixed sample?

Accepted Answer

Streak plate method

Question 6

In the spread plate method, why are colonies only found on the surface of the agar?

Accepted Answer

Because the sample is spread after the agar has solidified, preventing growth within the medium.

Question 7

A pour plate is used to count bacteria. Which of the following is a disadvantage of this method?

Accepted Answer

Heat from melted agar can kill heat-sensitive bacteria.

Question 8

During heat fixation, what is the primary purpose of heating the slide?

Accepted Answer

To attach microorganisms to the slide and kill them, preserving their structure.

Question 9

Calculate the grams of soy broth needed to prepare 500 mL, given the ratio $$30\,	ext{g}/1000\,	ext{mL}.$$

Accepted Answer

$$15\,	ext{g}$$

Question 10

Which staining method is best for observing bacterial cell shape without distortion?

Accepted Answer

Negative staining

Question 11

In Gram staining, what is the role of iodine?

Accepted Answer

It acts as a mordant, forming a crystal violet-iodine complex that is larger and less easily washed out.

Question 12

Which two elements are required within an endospore for germination?

Accepted Answer

Calcium and dipicolinic acid

Question 13

A bacterial culture yields 250 colonies from a 0.1 mL plated sample. What is the CFU/mL?

Accepted Answer

$$	ext{CFU/mL} = \frac{250}{0.1} = 2500$$

Question 14

Which dye is classified as acidic, and why?

Accepted Answer

Nigrosine, because its chromophore ion is negative and does not color the inside of the cell.

Question 15

What is the main advantage of using the hanging drop method over a wet mount for observing motility?

Accepted Answer

Bacteria are not squished and have more area to move, with less heat from the lamp.

Question 16

Which phase of bacterial growth is characterized by preparation for cell division but no increase in cell number?

Accepted Answer

Lag phase

Question 17

A mannitol salt agar plate is used in a lab. Why is it considered both selective and differential?

Accepted Answer

It selects for salt-tolerant organisms and differentiates based on mannitol fermentation, indicated by phenol red pH changes.

Question 18

Which enzyme allows bacteria to survive in the presence of toxic oxygen by converting it to water and oxygen?

Accepted Answer

Catalase

Question 19

During the log phase of bacterial growth, what is the main activity occurring?

Accepted Answer

Cells are actively dividing, typically every 10-20 minutes.

Question 20

Which formula is used to calculate absorbance from percent transmittance in a spectrophotometry experiment?

Accepted Answer

$$A = 2 - \log(\%T)$$

Microbiology Lab Techniques and Bacterial Cell Structures: Study Notes

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