Q: A sample is diluted and plated, resulting in 50 colonies from a 1:10,000 dilution of 1 mL. What is the original concentration of bacteria per mL?

$50 \times 10,000 = 500,000$ cells/mL

Question 1

Which of the following best describes a chemoheterotroph?

Accepted Answer

An organism that uses organic compounds as both an energy and carbon source.

Question 2

A bacterium is found growing at the bottom of a test tube of thioglycollate broth, away from oxygen. Which term best describes its oxygen requirement?

Accepted Answer

Obligate anaerobe

Question 3

Which enzyme catalyzes the following reaction, allowing some bacteria to survive in the presence of hydrogen peroxide?
$
2H_2O_2 \rightarrow 2H_2O + O_2
$

Accepted Answer

Catalase

Question 4

Nitrogen fixation is essential for life on Earth because:

Accepted Answer

It converts N$_2$ into forms usable for amino acid and nucleotide synthesis.

Question 5

A bacterium that grows optimally at pH 10 is classified as a(n):

Accepted Answer

Alkalinophile

Question 6

Which of the following best explains why obligate anaerobes are killed by oxygen?

Accepted Answer

They lack enzymes to detoxify reactive oxygen species.

Question 7

A microbiologist wants to isolate a single species from a mixed sample. Which technique should they use?

Accepted Answer

Streak-plate method

Question 8

Which of the following media would you use to distinguish between bacteria that ferment lactose and those that do not?

Accepted Answer

Differential medium

Question 9

A culture is maintained at a constant population size and nutrient level by continuously adding fresh medium and removing spent medium. This system is called a:

Accepted Answer

Chemostat

Question 10

A bacterial population doubles every 30 minutes. If you start with 1 cell, how many cells will there be after 3 hours? Use the formula $N = N_0 	imes 2^n$, where $n$ is the number of generations.

Accepted Answer

$N = 1 	imes 2^6 = 64$

Question 11

Which of the following is NOT a benefit of biofilm formation for bacteria?

Accepted Answer

Increased susceptibility to UV radiation

Question 12

A sample is diluted and plated, resulting in 50 colonies from a 1:10,000 dilution of 1 mL. What is the original concentration of bacteria per mL?

Accepted Answer

$50 	imes 10,000 = 500,000$ cells/mL

Question 13

Which of the following best describes a psychrophile?

Accepted Answer

An organism that grows best at temperatures below 15$^\circ$C

Question 14

Which of the following is a trace element required in small amounts by microorganisms?

Accepted Answer

Zinc

Question 15

Why are endospores able to survive in dry environments?

Accepted Answer

They cease most metabolic activity, requiring little water.

Question 16

Which method would be most appropriate for counting bacteria in a sample that cannot be cultured in the lab?

Accepted Answer

Genetic methods (e.g., DNA sequence detection)

Question 17

A bacterium that uses H$_2$S as an electron source and CO$_2$ as a carbon source is classified as a:

Accepted Answer

Chemoautotroph

Question 18

Which of the following best describes the stationary phase of a bacterial growth curve?

Accepted Answer

Cells are dividing at the same rate as they are dying.

Question 19

Which of the following is a reason why defined (synthetic) media are used in microbiology?

Accepted Answer

To precisely control the nutrients available to the microorganism.

Question 20

Which of the following is a direct method of measuring microbial growth that does NOT require incubation?

Accepted Answer

Petroff-Hausser counting chamber

Microbial Nutrition and Growth: Study Notes for Microbiology Majors

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