Q: Which of the following is a correct equation for calculating the original cell density (OCD) in a viable plate count experiment?

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

Question 1

Which of the following best describes the process of sterilization?

Accepted Answer

A process that kills or destroys all forms of life, including bacterial endospores.

Question 2

A bacteriostatic agent is best described as:

Accepted Answer

An agent that inhibits the growth of bacteria without killing them.

Question 3

Which physical method of microbial control is most appropriate for sterilizing heat-sensitive liquids?

Accepted Answer

Filtration using a 0.22-μm pore size filter

Question 4

Which of the following statements about non-ionizing radiation is correct?

Accepted Answer

It is most effective at a wavelength of 260 nm because DNA absorbs maximally at this wavelength.

Question 5

In the Kirby-Bauer method, what is the purpose of using a 0.5 McFarland standard?

Accepted Answer

To standardize the turbidity of the bacterial inoculum for reproducible results.

Question 6

Refer to the image showing a ruler measuring a zone of inhibition. If the diameter is 20 mm, what does this indicate?

Accepted Answer

The antimicrobial agent is highly effective against the test organism.

Question 7

Which of the following is a correct equation for calculating the original cell density (OCD) in a viable plate count experiment?

Accepted Answer

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

Question 8

Which of the following best explains why ionizing radiation is more effective against Gram-negative bacteria than viruses?

Accepted Answer

Gram-negative bacteria have more DNA, making them more susceptible to DNA damage.

Question 9

A student performs a viable plate count and observes 150 colonies on a plate inoculated with 0.1 mL of a $$10^{-3}$$ dilution. What is the original cell density (OCD)?

Accepted Answer

$$1.5 	imes 10^6$ CFU/mL

Question 10

Which of the following is a key difference between antisepsis and disinfection?

Accepted Answer

Antisepsis is used on living tissue, while disinfection is used on inanimate surfaces.

Question 11

Which of the following best describes the role of lactic acid in yogurt production?

Accepted Answer

It imparts a sour taste and causes milk proteins to curdle.

Question 12

Which of the following statements about the Kirby-Bauer method is correct?

Accepted Answer

A thicker agar layer will make the antimicrobial agent appear less effective.

Question 13

Which of the following is the correct sequence of steps for preparing a 1:1000 dilution from an original milk sample?

Accepted Answer

1 mL milk to 9 mL water, then 1 mL of that to 99 mL water

Question 14

Which of the following best describes the function of the enzyme lactase in yogurt production?

Accepted Answer

It hydrolyzes lactose into glucose and galactose.

Question 15

Which of the following is a plausible consequence of using too little bacterial inoculum in the Kirby-Bauer method?

Accepted Answer

The antimicrobial agent will appear more effective than it actually is.

Question 16

Which of the following statements about chemotherapeutic agents is correct?

Accepted Answer

They are chemicals that interfere with microbial proliferation at concentrations tolerated by the host.

Question 17

Which of the following is the correct chemical equation for alcoholic fermentation?

Accepted Answer

$$	ext{Glucose} ightarrow 2\ 	ext{Ethanol} + 2\ 	ext{CO}_2$$

Question 18

Which of the following is a reason why pasteurization is used instead of sterilization for milk?

Accepted Answer

Sterilization would alter the taste and nutritional value of milk.

Control of Microbial Growth and Food Microbiology: Study Notes

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