Q: A soil sample is serially diluted and plated. If you plate $$0.1\ \mathrm{mL} of a$$ $$10^{-4}$$ dilution and count 50 colonies, what is the CFU/g in the original sample?

$$5 \times 10^6$\ \mathrm{CFU/g}$$

Question 1

Which staining procedure does NOT require heat-fixing the smear, and what is the primary reason for this?

Accepted Answer

Capsule Stain; because heat-fixing can shrink or destroy the capsule.

Question 2

During the capsule stain, why is the slide rinsed with $$\mathrm{CuSO}_4$$ instead of water?

Accepted Answer

Water would dissolve the capsule, while $$\mathrm{CuSO}_4$$ preserves it and acts as a counterstain.

Question 3

Which stain employs steam in conjunction with the primary stain, and what is the purpose of using steam?

Accepted Answer

Endospore Stain; steam allows malachite green to penetrate the tough endospore coat.

Question 4

A student observes a slide under the microscope and sees round cells arranged in clusters. Which term best describes this morphology and arrangement?

Accepted Answer

Cocci in clusters

Question 5

Which of the following microorganisms is commonly used as an example for the acid-fast stain in laboratory exercises?

Accepted Answer

Mycobacterium smegmatis

Question 6

When performing an isolation streak, what is the primary goal?

Accepted Answer

To obtain pure colonies by separating individual cells.

Question 7

Which media would you use to select for Gram-negative bacteria and differentiate between lactose fermenters and non-fermenters?

Accepted Answer

MacConkey Agar

Question 8

On a Mannitol Salt Agar (MSA) plate, which ingredient is responsible for selecting for Staphylococcus species?

Accepted Answer

Sodium chloride ($$\mathrm{NaCl})$$

Question 9

A soil sample is serially diluted and plated. If you plate $$0.1\ \mathrm{mL} of a$$ $$10^{-4}$$ dilution and count 50 colonies, what is the CFU/g in the original sample?

Accepted Answer

$$5 	imes 10^6$\ \mathrm{CFU/g}$$

Question 10

Which of the following is a strong lactose fermenter on EMB agar, and what is the typical colony appearance?

Accepted Answer

Escherichia coli; metallic green sheen

Question 11

What is the function of eosin and methylene blue in EMB agar?

Accepted Answer

They inhibit Gram-positive bacteria and differentiate lactose fermenters.

Question 12

Which of the following best describes the procedure for inoculating an agar deep?

Accepted Answer

Use a needle to stab straight down into the agar.

Question 13

Why aren't capsule stains blotted after staining?

Accepted Answer

Blotting would cause the capsule to shrink or be destroyed.

Question 14

Which ingredient in MacConkey agar differentiates lactose fermenters from non-fermenters?

Accepted Answer

Lactose

Question 15

A student streaks a plate and observes isolated colonies. What does this indicate about the culture?

Accepted Answer

The culture contains pure, genetically identical cells.

Question 16

Which of the following is a non-lactose fermenter on MacConkey agar, and what is the colony appearance?

Accepted Answer

Salmonella enterica; colorless colonies

Question 17

What is the purpose of making serial dilutions before performing viable plate counts?

Accepted Answer

To ensure colonies are countable and not confluent.

Question 18

Which microorganism is commonly used as an example for the endospore stain in laboratory exercises?

Accepted Answer

Bacillus subtilis

Question 19

On MSA plates, which organism is a mannitol fermenter and what color change occurs?

Accepted Answer

Staphylococcus aureus; yellow

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