Q: A bacterial culture has a generation time of 30 minutes. If you start with 1 cell, how many cells will there be after 3 hours, assuming exponential growth? Use the formula $$N = N_0$ \times 2^n$$, where $$n is $$the number of generations.

$$N = 1 \times 2$$^{12}$$ = 4096$$

Question 1

A microbiologist is culturing bacteria that thrive at temperatures around $$70^\circC. $$Which classification best describes these bacteria, and what is a likely natural environment for them?

Accepted Answer

Thermophiles; hot springs

Question 2

A bacterium is found growing in a salt lake with a NaCl concentration of 20%. Which term best describes this organism's adaptation to osmotic pressure?

Accepted Answer

Obligate halophile

Question 3

Which of the following best explains why oxygen is potentially dangerous to microbial cells?

Accepted Answer

Oxygen can form reactive oxygen species (ROS) such as $$\mathrm{O_2$^-}$$ and $$\mathrm{H_2O_2}$$, which damage cellular components.

Question 4

A culture tube is inoculated with a bacterium and incubated in an upright position. After 24 hours, growth is observed only at the bottom of the tube. Which oxygen requirement classification fits this organism?

Accepted Answer

Obligate anaerobe

Question 5

Which enzyme is responsible for converting superoxide radicals ($$\mathrm{O_2$^-}) $$into hydrogen peroxide ($$\mathrm{H_2O_2}) in $$aerobic bacteria?

Accepted Answer

Superoxide dismutase

Question 6

A microbiologist wants to isolate a single species from a mixed culture. Which technique should they use, and why is it important?

Accepted Answer

Streak for isolation; it separates individual cells to form pure colonies

Question 7

Which of the following best describes a chemically defined medium?

Accepted Answer

A medium in which the exact chemical composition is known

Question 8

During which phase of the bacterial growth curve does the population size remain constant because the rate of cell division equals the rate of cell death?

Accepted Answer

Stationary phase

Question 9

A researcher is using a spectrophotometer to measure the cloudiness of a bacterial culture. Which method of enumeration is being used, and what is its main limitation?

Accepted Answer

Indirect method: turbidity; it cannot distinguish between live and dead cells

Question 10

Which of the following best describes the function of an anaerobic jar in a microbiology laboratory?

Accepted Answer

It removes oxygen to create an environment suitable for obligate anaerobes

Question 11

A microbiologist is working with a medium that contains bile salts and crystal violet to inhibit Gram-positive bacteria, but also contains lactose and a pH indicator to distinguish lactose fermenters. What type of medium is this?

Accepted Answer

Selective and differential medium

Question 12

Which of the following best explains why biofilms are advantageous for microbes?

Accepted Answer

Biofilms provide protection from environmental stresses and antibiotics

Question 13

A bacterial culture has a generation time of 30 minutes. If you start with 1 cell, how many cells will there be after 3 hours, assuming exponential growth? Use the formula $$N = N_0$ 	imes 2^n$$, where $$n is $$the number of generations.

Accepted Answer

$$N = 1 	imes 2$$^{12}$$ = 4096$$

Question 14

Which of the following is NOT a direct method for enumerating microbes?

Accepted Answer

Turbidity measurement

Question 15

Which process is used to preserve microbial cultures by rapidly freezing them and then removing water by sublimation under vacuum?

Accepted Answer

Lyphilization

Question 16

A culture medium contains peptone, beef extract, and agar. Which type of medium is this?

Accepted Answer

Complex medium

Question 17

Which of the following best describes the role of agar in culture media?

Accepted Answer

It acts as a solidifying agent that is not metabolized by most microbes

Question 18

Which of the following statements about the log phase of bacterial growth is correct?

Accepted Answer

Cells are dividing at a constant and maximum rate

Question 19

Which of the following is an organic growth factor required by some bacteria for growth?

Accepted Answer

Amino acids

Question 20

A scientist is using the most probable number (MPN) method to estimate bacterial numbers in a water sample. Which principle does this method rely on?

Accepted Answer

Statistical probability based on dilution and positive growth in tubes

Dynamics of Microbial Growth: Environmental and Chemical Requirements, Culture Methods, and Enumeration

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