Question 1

Which phase of the bacterial growth curve is characterized by cells repairing damage and synthesizing essential components before active division begins?

Accepted Answer

Lag phase

Question 2

A culture of E. coli is grown in a batch system. If the generation time is 20 minutes, how many cells will be present after 2 hours if the initial population is 1 cell? Use the formula $$N = N_0$ 	imes 2$$^{n}$$, where n$ is the number of generations.

Accepted Answer

Question 3

Which laboratory method allows for the direct counting of both live and dead microbial cells, but cannot distinguish between them without special stains?

Accepted Answer

Microscopic counts

Question 4

During which phase of the microbial growth curve does the population size remain constant, but cells continue to carry out metabolism?

Accepted Answer

Stationary phase

Question 5

Which of the following best describes the relationship between optical density (OD) and colony forming units (CFU) in a bacterial culture, as measured by a spectrophotometer?

Accepted Answer

OD increases linearly with CFU up to a certain point

Question 6

A researcher uses the spread-plate method to determine viable cell counts. Why must the sample be diluted before plating?

Accepted Answer

To obtain a countable number of colonies

Question 7

Which category of microbes grows best at pH values below 6, and may have membranes destroyed at neutral pH?

Accepted Answer

Acidophiles

Question 8

What is the primary reason that microbial culture media typically contain buffers?

Accepted Answer

To maintain a constant pH during growth

Question 9

Which group of microorganisms would be most likely to thrive in hot springs with temperatures above 80°C?

Accepted Answer

Hyperthermophiles

Question 10

A batch culture is monitored using a spectrophotometer. If the optical density (OD) at 600 nm is 0.8, and the standard curve equation is $$Y = 2 	imes 10^8$ X + 4 	imes 10^6$, what is the estimated CFU/mL?

Accepted Answer

$$1.64 	imes 10^8$

Question 11

Which of the following is NOT a limitation of microscopic cell counting?

Accepted Answer

Selective media targets only particular species

Question 12

Which temperature category includes most human pathogens, and what is their optimal growth temperature?

Accepted Answer

Mesophiles; 37°C

Question 13

What happens to microbial enzymes and cell membranes when exposed to temperatures above their maximum growth temperature?

Accepted Answer

Enzymes denature and membranes collapse

Question 14

Which method is most appropriate for measuring the total cell count in a sample with low cell density and motile cells?

Accepted Answer

Microscopic counts with immobilization

Question 15

A bacterial culture is in the stationary phase. Which of the following best explains why growth has ceased?

Accepted Answer

An essential nutrient is depleted or waste products accumulate

Question 16

Which pH category would you expect to find in soda lakes, and what adaptation might these microbes have?

Accepted Answer

Alkaliphiles; sodium motive force

Question 17

If a bacterial culture is incubated past the stationary phase, what is the expected trend in cell number?

Accepted Answer

Cell number decreases exponentially

Question 18

Which of the following statements about binary fission is correct?

Accepted Answer

The septum forms by inward growth of the cell membrane and cell wall from opposite directions

Question 19

A sample is diluted 1:10 five times and 17 colonies are counted on the final plate. What is the original cell concentration per mL? Use the formula $$	ext{CFU/mL} = 	ext{number of colonies} 	imes 	ext{dilution factor}.$$

Accepted Answer

$$1.7 	imes 10^5$

Question 20

Which microbial group would be most likely to grow in refrigerated foods, and what is their optimal temperature range?

Accepted Answer

Psychrotrophs; 0–20°C

The Effects of Temperature & pH on Microbial Growth

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