Q: Which of the following equations correctly calculates the number of bacteria per milliliter using a direct microscopic count?

$$\text{Number of bacteria/ml} = \frac{\text{Number of cells counted}}{\text{Volume of area counted}}$$

Question 1

Which of the following best describes the role of phosphate salts in bacterial growth media?

Accepted Answer

They serve as a buffer to maintain pH and provide an essential nutrient.

Question 2

A microbiologist wants to isolate a bacterium that can grow in the absence of oxygen but is not harmed by its presence. Which type of organism is she most likely working with?

Accepted Answer

Aerotolerant anaerobe

Question 3

Which equation correctly represents the action of catalase in detoxifying hydrogen peroxide?

Accepted Answer

$$2\ H_2O_2 \rightarrow 2\ H_2O + O_2$

Question 4

A food safety inspector finds that a large pot of rice left at room temperature overnight has a high risk of Bacillus cereus growth. Which factor most contributes to this risk?

Accepted Answer

The large volume of rice cooled slowly, remaining in the danger zone for hours.

Question 5

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 6

A bacterium is found to grow only at the bottom of a test tube of solid medium. Which of the following best describes this organism?

Accepted Answer

Obligate anaerobe

Question 7

Which of the following statements about biofilms is FALSE?

Accepted Answer

Biofilms are rarely involved in human infections.

Question 8

A microbiologist is using a chemically defined medium to grow Escherichia coli. Which of the following is a key characteristic of this medium?

Accepted Answer

Its exact chemical composition is known.

Question 9

Which of the following best describes the process of binary fission in bacteria?

Accepted Answer

A cell elongates, replicates its DNA, forms a cross-wall, and separates into two cells.

Question 10

A microbiologist wants to count the number of viable bacteria in a sample. Which method should she use?

Accepted Answer

Plate count method

Question 11

Which of the following equations correctly calculates the number of bacteria per milliliter using a direct microscopic count?

Accepted Answer

$$	ext{Number of bacteria/ml} = \frac{	ext{Number of cells counted}}{	ext{Volume of area counted}}$$

Question 12

Which type of culture medium would be most appropriate for isolating Salmonella Typhi from a fecal sample?

Accepted Answer

Bismuth sulfite agar

Question 13

A bacterium is classified as a thermophile. At which temperature range would you expect its optimum growth?

Accepted Answer

50–60°C

Question 14

Which of the following best explains why hypertonic environments inhibit bacterial growth?

Accepted Answer

They cause plasmolysis by drawing water out of the cell.

Question 15

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

Accepted Answer

Iron

Question 16

A microbiologist is using the streak plate method. What is the primary purpose of this technique?

Accepted Answer

To isolate pure colonies from a mixed sample

Question 17

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

Accepted Answer

Stationary phase

Question 18

A bacterium that can use $$N_2$$ directly from the atmosphere for its nitrogen needs is performing which process?

Accepted Answer

Nitrogen fixation

Question 19

Which of the following best describes a selective medium?

Accepted Answer

A medium that suppresses unwanted microbes and encourages desired ones

Question 20

A microbiologist is using the most probable number (MPN) method. Which scenario best justifies this choice?

Accepted Answer

The bacteria cannot grow on solid media.

Microbial Growth: Requirements, Culture, and Measurement

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