$$1.6 \times 10^7$ (correct calculation: 10^6$ \times 2^4$ = 1.6 \times 10^7$)

$$\text{Number of cells counted} \times \frac{1}{\text{Volume of area counted}}$$

Question 1

Which of the following best describes a psychrotroph, and why are they significant in food microbiology?

Accepted Answer

They grow between 0°C and 30°C and are significant because they cause food spoilage at refrigeration temperatures.

Question 2

A bacterium is found growing only at the bottom of a test tube of thioglycolate broth. Which of the following best describes its oxygen requirements?

Accepted Answer

Obligate anaerobe

Question 3

Which of the following statements about biofilms is FALSE?

Accepted Answer

Biofilms are rarely involved in human infections.

Question 4

Which of the following best explains why high salt concentrations inhibit the growth of most bacteria?

Accepted Answer

High salt creates a hypertonic environment, causing plasmolysis due to water leaving the cell.

Question 5

A microbiologist wants to culture a fastidious organism that requires many specific nutrients. Which type of medium should she use?

Accepted Answer

Chemically defined medium

Question 6

Which phase of the bacterial growth curve is characterized by a balance between cell division and cell death?

Accepted Answer

Stationary phase

Question 7

A sample contains $$10^6$$ bacteria. If the generation time is 30 minutes, how many bacteria will be present after 2 hours of exponential growth?

Accepted Answer

$$1.6 	imes 10^7$ (correct calculation: 10^6$ 	imes 2^4$ = 1.6 	imes 10^7$)

Question 8

Which of the following is a trace element required by bacteria as an enzyme cofactor?

Accepted Answer

Iron

Question 9

Which method would be most appropriate for counting bacteria in a water sample with very low bacterial concentration?

Accepted Answer

Filtration followed by plate count

Question 10

Which of the following best describes the purpose of selective media?

Accepted Answer

To suppress unwanted microbes and encourage the growth of desired microbes

Question 11

A microbiologist is working with a pathogen that requires biosafety level 3 precautions. Which of the following is a required safety feature?

Accepted Answer

Biosafety cabinets to prevent airborne transmission

Question 12

Which of the following equations correctly represents the calculation for the number of bacteria per milliliter using a Petroff-Hausser cell counter?

Accepted Answer

$$	ext{Number of cells counted} 	imes \frac{1}{	ext{Volume of area counted}}$$

Question 13

Which of the following best explains why agar is used as a solidifying agent in culture media?

Accepted Answer

It remains solid at incubation temperatures and is not generally metabolized by microbes.

Question 14

A bacterial culture is in the log phase of growth. Which of the following best describes what is happening?

Accepted Answer

Cells are dividing at the fastest possible rate for the conditions.

Question 15

Which of the following is a correct statement about the streak plate method?

Accepted Answer

It is used to isolate pure cultures by spreading bacteria over the surface of an agar plate.

Question 16

Which of the following best describes the function of reducing media?

Accepted Answer

To remove oxygen and support the growth of obligate anaerobes

Question 17

A microbiologist wants to preserve a bacterial culture for several years. Which method should she use?

Accepted Answer

Deep-freezing at –50°C to –95°C

Question 18

Which of the following is a chemical requirement for microbial growth that is used in DNA, RNA, ATP, and found in membranes?

Accepted Answer

Phosphorus

Question 19

A sample is diluted 1:10,000 and 54 colonies are counted on the plate. What is the number of bacteria per milliliter in the original sample?

Accepted Answer

$$54 	imes 10,000 = 540,000$$

Question 20

Which of the following best describes the role of quorum sensing in biofilm formation?

Accepted Answer

It allows bacteria to communicate and coordinate gene expression for biofilm development.

Chapter 6 - Microbial Growth: Requirements, Culture, and Measurement

Study Guide - Practice Questions