Q: Which of the following equations best represents the acid production from lactose fermentation by coliforms on EMB agar?

$$\text{C}_{12}\text{H}_{22}\text{O}_{11} + 2\text{H}_2\text{O} \rightarrow 4\text{CH}_3\text{COOH} + 4\text{CO}_2 + 4\text{H}_2$$

Question 1

Which of the following best describes the main function of selective media in microbiology?

Accepted Answer

To encourage the growth of specific microorganisms while inhibiting others.

Question 2

A microbiologist wants to isolate and differentiate members of the Enterobacteriaceae family based on their ability to ferment lactose. Which medium should they use?

Accepted Answer

MacConkey Agar

Question 3

On MacConkey Agar, what color will colonies of lactose-fermenting bacteria appear, and why?

Accepted Answer

Pink to red, due to acid production lowering the pH and activating the neutral red indicator.

Question 4

Which component in MacConkey Agar is responsible for inhibiting the growth of gram-positive bacteria?

Accepted Answer

Bile salts and crystal violet

Question 5

A sample is streaked on Eosin Methylene Blue (EMB) Agar. After incubation, colonies appear dark purple with a green metallic sheen. What does this indicate?

Accepted Answer

The organism is a vigorous lactose fermenter, likely Escherichia coli.

Question 6

Which of the following best explains why Mannitol Salt Agar is selective for Staphylococcus species?

Accepted Answer

It has a high salt concentration that inhibits most bacteria except Staphylococci.

Question 7

A microbiologist observes yellow colonies on Mannitol Salt Agar. What does this result indicate?

Accepted Answer

The bacteria are fermenting mannitol, producing acid and lowering the pH.

Question 8

Which medium would be most appropriate to isolate Staphylococcus species from a mixed culture containing both gram-positive and gram-negative bacteria?

Accepted Answer

Mannitol Salt Agar

Question 9

Phenylethyl Alcohol Agar (PEA) is used primarily for which purpose?

Accepted Answer

To select for gram-positive bacteria by inhibiting gram-negative bacteria.

Question 10

A clinical sample is streaked on MacConkey Agar and produces colorless colonies. What is the most likely explanation?

Accepted Answer

The organism is a non-lactose fermenter.

Question 11

Which of the following equations best represents the acid production from lactose fermentation by coliforms on EMB agar?

Accepted Answer

$$	ext{C}_{12}	ext{H}_{22}	ext{O}_{11} + 2	ext{H}_2	ext{O} ightarrow 4	ext{CH}_3	ext{COOH} + 4	ext{CO}_2 + 4	ext{H}_2$$

Question 12

Which of the following statements about complex media is correct?

Accepted Answer

Complex media supports the growth of microorganisms but contains at least one ingredient that is not chemically definable.

Question 13

A patient sample is streaked on EMB agar. After incubation, the colonies are pink but not dark purple or metallic. What does this indicate?

Accepted Answer

The organism is a less-aggressive lactose fermenter, such as Enterobacter or Klebsiella.

Question 14

Which of the following best explains why crystal violet is sometimes omitted from MacConkey Agar formulations?

Accepted Answer

To allow for the growth of Enterococcus and Staphylococcus species.

Question 15

A microbiologist is testing an unknown bacterium. It grows well on PEA agar but not on MacConkey or EMB agar. What is the most likely identity of the bacterium?

Accepted Answer

Gram-positive cocci

Question 16

Which of the following best describes the role of phenylethyl alcohol in PEA agar?

Accepted Answer

It disrupts the cell membrane of gram-negative bacteria, inhibiting their growth.

Question 17

A clinical laboratory receives a stool sample suspected of containing Salmonella. Which medium would be most useful for initial isolation and why?

Accepted Answer

MacConkey Agar, because it inhibits gram-positive bacteria and differentiates lactose fermenters from non-fermenters.

Question 18

Which of the following best describes a differential medium?

Accepted Answer

A medium that contains indicators to distinguish between different types of microorganisms based on their biochemical properties.

Question 19

A microbiologist observes that a bacterial colony on MacConkey Agar is colorless. Which of the following best explains this observation?

Accepted Answer

The bacterium is unable to ferment lactose and does not lower the pH.

Selective and Differential Media in Microbiology

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