Q: Which formula is used to calculate the concentration of bacteria in a sample using the standard plate count method?

$$ \text{CFU/mL} = \frac{\text{Number of colonies}}{\text{Volume plated (mL)} \times \text{Dilution factor}} $$

Question 1

Which of the following best describes a chemically defined medium used in microbiology?

Accepted Answer

A medium in which the exact chemical composition is known, including all individual components and their concentrations.

Question 2

A microbiologist wants to isolate bacteria that can grow in high salt concentrations. Which type of medium should she use, and why?

Accepted Answer

Selective medium, because it inhibits the growth of organisms that cannot tolerate high salt.

Question 3

Which of the following is an example of a medium that is both selective and differential?

Accepted Answer

MacConkey agar

Question 4

A patient develops a recurrent infection after a catheter is inserted. Which microbiological concept best explains this phenomenon?

Accepted Answer

Biofilm formation on the medical device protects bacteria from antibiotics and immune responses.

Question 5

Which group of microbes would be most likely to grow in a refrigerator at 4°C?

Accepted Answer

Psychrotrophs

Question 6

Why are salts and sugars effective in preserving foods?

Accepted Answer

They create a hypertonic environment, causing microbial cells to lose water and inhibiting growth.

Question 7

Which biosafety level is appropriate for handling microbes that pose a high risk of aerosol-transmitted infections and require special containment procedures?

Accepted Answer

Biosafety Level 3

Question 8

A culture tube shows bacterial growth only at the bottom. Which oxygen requirement group does this organism belong to?

Accepted Answer

Obligate anaerobe

Question 9

During which phase of bacterial growth are cells most susceptible to penicillin?

Accepted Answer

Log (exponential) phase

Question 10

Which formula is used to calculate the concentration of bacteria in a sample using the standard plate count method?

Accepted Answer

$$ 	ext{CFU/mL} = \frac{	ext{Number of colonies}}{	ext{Volume plated (mL)} 	imes 	ext{Dilution factor}} $$

Question 11

A microbiologist needs to culture bacteria from a hot spring with temperatures above 80°C. Which group of microbes should she expect to find?

Accepted Answer

Hyperthermophiles

Question 12

Which method is considered a direct measurement of microbial growth?

Accepted Answer

Direct microscopic count

Question 13

What is the main advantage of the standard plate count method?

Accepted Answer

It counts only viable (living) cells.

Question 14

Which phase of bacterial growth is characterized by the rate of cell division equaling the rate of cell death?

Accepted Answer

Stationary phase

Question 15

A microbiologist wants to culture bacteria that require high levels of carbon dioxide. Which technique should she use?

Accepted Answer

Candle jar

Question 16

Which of the following best describes a biofilm?

Accepted Answer

A community of microorganisms attached to a surface and encased in a self-produced matrix.

Question 17

Which of the following is NOT a major element required in large amounts for microbial growth?

Accepted Answer

Calcium

Question 18

A food manufacturer wants to preserve food without electricity. Which method would be most effective?

Accepted Answer

Adding salt or sugar to create a hypertonic environment

Question 19

Which group of microbes grows best at a pH below 5.5?

Accepted Answer

Acidophiles

Question 20

A laboratory technician is culturing bacteria in a medium with a pH of 9. Which group of microbes is most likely to thrive?

Accepted Answer

Alkaliphiles

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