Q: Which equation correctly represents the calculation for 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 group of microorganisms is most likely to cause spoilage in refrigerated foods, and why?

Accepted Answer

Psychrotrophs, because they can grow at low temperatures found in refrigerators.

Question 2

A bacterial culture is growing in a medium with a pH of 4.5. Which type of microorganism is most likely to thrive in this environment?

Accepted Answer

Acidophiles

Question 3

Why does a cell placed in a hypertonic solution undergo plasmolysis?

Accepted Answer

Because water leaves the cell, causing the cytoplasm to shrink away from the cell wall.

Question 4

Which of the following best describes the role of carbon in microbial growth?

Accepted Answer

It is the structural backbone of organic molecules.

Question 5

A scientist wants to grow obligate anaerobes in the laboratory. Which method would be most appropriate?

Accepted Answer

Using reducing media containing sodium thioglycollate.

Question 6

Which biosafety level (BSL) requires the use of a positive pressure suit and sealed, negative pressure laboratory?

Accepted Answer

BSL-4

Question 7

What is the main advantage of using phosphate salts as buffers in microbial growth media, besides controlling acidity?

Accepted Answer

They provide a source of phosphorus for nucleic acids and ATP.

Question 8

Which of the following is NOT a direct method for measuring microbial growth?

Accepted Answer

Turbidity measurement with a spectrophotometer

Question 9

A microbe is found growing only at the bottom of a test tube of solid growth medium. What is the most likely classification of this microbe?

Accepted Answer

Obligate anaerobe

Question 10

Which equation correctly represents the calculation for 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 11

Which type of media would you use to distinguish between lactose-fermenting and non-lactose-fermenting bacteria?

Accepted Answer

Differential media

Question 12

Why are biofilms particularly problematic in medical settings?

Accepted Answer

They are 1000x more resistant to microbicides and can form on medical devices.

Question 13

Which phase of bacterial growth is characterized by an equilibrium between cell division and cell death?

Accepted Answer

Stationary phase

Question 14

A microbe requires oxygen but only grows where oxygen concentration is lower than atmospheric levels. What is its classification?

Accepted Answer

Microaerophile

Question 15

Which of the following elements is NOT required in large amounts for microbial growth?

Accepted Answer

Iron

Question 16

What is the main reason primitive civilizations used food preservation techniques that relied on osmotic pressure?

Accepted Answer

To inhibit microbial growth by causing plasmolysis in bacteria.

Question 17

Which of the following best describes the streak plate method?

Accepted Answer

It is used to isolate pure cultures from a mixed population.

Question 18

Which of the following statements about agar is TRUE?

Accepted Answer

Agar solidifies at around 40°C and is generally not metabolized by microbes.

Question 19

Which equation best describes the exponential increase in bacterial population during the log phase, assuming binary fission and a generation time of $$t$$ minutes?

Accepted Answer

$$N = N_0$ 	imes 2$$^{\frac{t}$${g}}$$

Question 20

Which method would be most appropriate for estimating the vitamin content in a product when no direct chemical assay is available?

Accepted Answer

Metabolic activity measurement

6 Microbial Growth: Physical and Chemical Requirements, Biofilms, and Culture Methods

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