Q: A sample contains 1 mL of a bacterial suspension. After serial dilution and plating, 50 colonies are counted on a plate that received a 1:1000 dilution. What is the original concentration of bacteria per mL?

$$50 \times 1000 = 50,000$$ bacteria/mL

Question 1

Which of the following best explains why salt is used to preserve foods such as meats and pickles?

Accepted Answer

Salt increases the osmotic pressure, causing plasmolysis in bacteria, which inhibits their growth.

Question 2

A microbiologist is culturing a bacterium that requires high salt concentrations to grow. Which term best describes this organism?

Accepted Answer

Obligate halophile

Question 3

Which of the following statements about the pH requirements of microorganisms is correct?

Accepted Answer

Most bacteria grow best at pH 6.5–7.5, while molds and yeasts prefer pH 5–6.

Question 4

A patient is infected with a bacterium that cannot survive in the presence of oxygen. Which of the following best describes this organism?

Accepted Answer

Obligate anaerobe

Question 5

Which of the following is a correct description of the log phase in a bacterial growth curve?

Accepted Answer

Cells are dividing at a constant, exponential rate and are most sensitive to adverse conditions.

Question 6

A water sample is suspected of containing very low numbers of coliform bacteria. Which direct method would be most appropriate for quantifying the bacteria?

Accepted Answer

Filtration followed by culturing on a nutrient pad

Question 7

Which of the following best describes the function of phosphate salts in bacterial culture media?

Accepted Answer

They act as buffers to maintain a stable pH.

Question 8

A microbiologist wants to isolate a pure culture from a mixed sample. Which method is most commonly used for this purpose?

Accepted Answer

Streak plate method

Question 9

Which of the following equations correctly represents the calculation of bacterial population after $$n$$ generations, starting from a single cell, assuming binary fission?

Accepted Answer

$$N = 2^n$$

Question 10

A bacterial culture has a generation time of 30 minutes. If you start with 100 cells, how many cells will there be after 3 hours of exponential growth? (Assume no cell death.)

Accepted Answer

6400

Question 11

Which type of culture medium would you use to specifically encourage the growth of Salmonella while inhibiting other bacteria, and also allow you to distinguish it from other organisms?

Accepted Answer

MacConkey agar

Question 12

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

Accepted Answer

The rate of cell division equals the rate of cell death.

Question 13

Which of the following is NOT a chemical requirement for microbial growth?

Accepted Answer

Sodium chloride as a universal energy source

Question 14

A researcher is working with a bacterium that grows best at 37°C and is pathogenic to humans. To which temperature group does this bacterium most likely belong?

Accepted Answer

Mesophiles

Question 15

Which of the following best explains why obligate anaerobes cannot survive in the presence of oxygen?

Accepted Answer

They lack enzymes to neutralize harmful forms of oxygen.

Question 16

Which of the following methods would be most appropriate for long-term preservation of a bacterial culture for several years?

Accepted Answer

Lyophilization (freeze-drying)

Question 17

A microbiologist is measuring the cloudiness of a bacterial culture using a spectrophotometer. Which method is being used?

Accepted Answer

Turbidity measurement

Question 18

Which of the following is a limitation of the direct microscopic count method?

Accepted Answer

It counts both live and dead cells.

Question 19

A sample contains 1 mL of a bacterial suspension. After serial dilution and plating, 50 colonies are counted on a plate that received a 1:1000 dilution. What is the original concentration of bacteria per mL?

Accepted Answer

$$50 	imes 1000 = 50,000$$ bacteria/mL

Question 20

Which of the following best describes an enrichment culture?

Accepted Answer

A selective culture used to increase very small numbers of a desired organism to detectable levels.

Microbial Growth: Physical and Chemical Requirements, Culture Methods, and Measurement

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