Q: Which of the following equations best represents the relationship between colony-forming units (CFU), dilution factor (DF), and plated volume (V) when calculating the original cell concentration?

$$\text{original cell concentration} = \frac{CFU}{DF \times V}$$

Question 1

Which of the following best describes the purpose of a streak plate in microbiology?

Accepted Answer

To isolate single colonies from a mixed culture

Question 2

Mannitol Salt Agar (MSA) is both selective and differential. Which component makes it selective, and for which group of bacteria?

Accepted Answer

7.5% NaCl; selects for salt-tolerant Staphylococcus species

Question 3

On EMB agar, a metallic green sheen is observed. Which organism is most likely responsible for this appearance, and what does it indicate?

Accepted Answer

E. coli; vigorous lactose fermentation

Question 4

A student streaks an unknown mixed culture on MSA. After incubation, one colony turns the medium yellow, while another leaves it red. What can be inferred about these two colonies?

Accepted Answer

The yellow colony ferments mannitol; the red colony does not

Question 5

Which of the following best explains why EMB agar is considered both selective and differential?

Accepted Answer

It contains eosin and methylene blue to inhibit Gram-positive bacteria and differentiates based on lactose fermentation

Question 6

A microbiologist observes no growth of a bacterial sample on EMB agar but robust growth on nutrient agar. What is the most likely explanation?

Accepted Answer

The organism is Gram-positive and inhibited by EMB dyes

Question 7

Which of the following is the correct sequence of steps for streaking an unknown mixed culture for isolation?

Accepted Answer

Streak sector 1, flame loop, streak sector 2, flame loop, streak sector 3, flame loop, streak sector 4

Question 8

Which of the following best describes the role of phenol red in MSA?

Accepted Answer

It is a pH indicator that turns yellow in acidic conditions

Question 9

A student observes colorless colonies on EMB agar. Which of the following is the most likely explanation?

Accepted Answer

The organism does not ferment lactose

Question 10

Which of the following equations best represents the relationship between colony-forming units (CFU), dilution factor (DF), and plated volume (V) when calculating the original cell concentration?

Accepted Answer

$$	ext{original cell concentration} = \frac{CFU}{DF 	imes V}$$

Question 11

Which of the following best describes an enrichment medium?

Accepted Answer

A medium that enhances the growth of a particular organism without necessarily inhibiting others

Question 12

A student forgets to flame the loop between sectors while streaking for isolation. What is the most likely result?

Accepted Answer

The plate will have confluent growth with few or no isolated colonies

Question 13

Which of the following statements about nutrient agar is correct?

Accepted Answer

It is neither selective nor differential

Question 14

A microbiologist observes that S. aureus turns MSA yellow, while S. epidermidis does not. What is the most likely explanation?

Accepted Answer

S. aureus ferments mannitol, producing acid

Question 15

Which of the following best explains why E. coli does not grow on MSA?

Accepted Answer

E. coli is not salt-tolerant and is inhibited by the high NaCl concentration

Question 16

A student streaks a plate and observes growth on EMB with a metallic green sheen, but no color change on MSA. Which organism is most likely present?

Accepted Answer

E. coli

Question 17

Which of the following is the correct interpretation if a bacterium grows on MSA but the medium remains red?

Accepted Answer

The bacterium is salt-tolerant but does not ferment mannitol

Question 18

Why is it important to label the bottom (agar side) of a petri dish rather than the lid?

Accepted Answer

The lid can be easily switched between plates, leading to misidentification

Question 19

Which of the following best describes the function of a differential medium?

Accepted Answer

It allows all bacteria to grow but distinguishes them based on a biochemical property

Question 20

A student observes that a bacterial sample grows on nutrient agar, does not grow on EMB, and grows on MSA with no color change. Which organism is most likely present?

Accepted Answer

S. epidermidis

Streak Plate Technique and Special Media in Microbiology

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