Question 1

Which of the following is NOT one of the four fundamental components of selective media?

Accepted Answer

A fermentation by-product

Question 2

What is the main difference between defined and undefined media?

Accepted Answer

Defined media has a known chemical composition, while undefined media contains complex ingredients of unknown composition.

Question 3

Which of the following is a major by-product of fermentation that affects the pH of the media?

Accepted Answer

Acid

Question 4

During fermentation, which by-product is responsible for lowering the pH of the medium?

Accepted Answer

Acid

Question 5

What color does phenol red turn in a medium with a basic (alkaline) pH?

Accepted Answer

Pink/fuchsia

Question 6

Which property makes Mannitol Salt Agar (MSA) selective?

Accepted Answer

High salt concentration inhibits growth of most bacteria except Staphylococcus species.

Question 7

On EMB agar, what does a metallic green sheen indicate?

Accepted Answer

The organism ferments lactose vigorously, producing strong acid.

Question 8

What is the purpose of a Durham tube in a phenol red broth?

Accepted Answer

To detect gas production during fermentation

Question 9

Which enzyme is detected in the catalase test, and what is the reaction it catalyzes?

Accepted Answer

Catalase; $$2H_2O_2 \rightarrow 2H_2O + O_2$

Question 10

A positive oxidase test is indicated by which result?

Accepted Answer

A blue or purple color change within 20 seconds

Question 11

In the nitrate reduction test, what does the appearance of gas in the Durham tube indicate?

Accepted Answer

Denitrification has occurred, producing nitrogen gas ($$N_2).$$

Question 12

Why is zinc added in the nitrate reduction test?

Accepted Answer

To confirm the presence of nitrite by reducing nitrate to nitrite

Question 13

What is the carbon source in Simmons' Citrate Agar?

Accepted Answer

Citrate

Question 14

A positive result on Simmons' Citrate Agar is indicated by which color change?

Accepted Answer

Green to blue

Question 15

Why is the Simmons' Citrate slant inoculated with a needle instead of a loop?

Accepted Answer

To prevent over-inoculation and ensure only citrate-utilizing bacteria grow

Question 16

In the TSI (Triple Sugar Iron) test, what does a black precipitate indicate?

Accepted Answer

Production of hydrogen sulfide ($$H_2S)$$

Question 17

Before sulfur can be reduced in the TSI test, what must occur?

Accepted Answer

Fermentation of sugars to produce acid

Question 18

Which amino acid is broken down into indole in the indole test, and what is the enzyme responsible?

Accepted Answer

Tryptophan; tryptophanase

Question 19

In a motility test, what does diffuse growth away from the stab line indicate?

Accepted Answer

Motile organism

Question 20

Which of the following best describes the difference between aerobic and anaerobic respiration?

Accepted Answer

Aerobic respiration uses oxygen as the final electron acceptor, while anaerobic respiration uses other molecules such as nitrate or sulfate.

Microbiology Lab Media and Biochemical Tests Study Guide

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