Question 1

Which of the following best describes the primary difference between EMB agar and MacConkey agar in the isolation of enteric bacteria?

Accepted Answer

EMB agar uses eosin and methylene blue to inhibit Gram positive bacteria and differentiate lactose fermenters, while MacConkey agar uses bile salts and crystal violet for inhibition and neutral red for lactose fermentation.

Question 2

A patient presents with severe diarrhea described as 'rice water stools.' Which organism is most likely responsible, and what is a key laboratory feature for its identification?

Accepted Answer

Vibrio cholerae; Gram negative pleomorphic rods and does not ferment lactose.

Question 3

Which of the following results would you expect from a Triple Sugar Iron (TSI) slant inoculated with a bacterium that ferments only glucose and produces hydrogen sulfide?

Accepted Answer

Red slant/yellow butt with black precipitate

Question 4

A Gram negative rod isolated from a burn wound grows at 42°C, produces a green pigment on Mueller Hinton agar, and has a fruity odor. Which test would best confirm its identity?

Accepted Answer

Oxidase test

Question 5

Which of the following statements about the IMViC tests is correct?

Accepted Answer

The Indole test detects the ability to produce indole from tryptophan.

Question 6

A bacterium produces red colonies on MacConkey agar and a metallic green sheen on EMB agar. Which of the following is most likely true about this organism?

Accepted Answer

It is a lactose fermenter, likely Escherichia coli.

Question 7

Which of the following best explains why Pseudomonas aeruginosa is a significant concern in hospital settings?

Accepted Answer

It is resistant to many antibiotics and can survive in harsh environments.

Question 8

A TSI slant shows a yellow butt and red slant after incubation. What does this indicate about the organism's metabolic capabilities?

Accepted Answer

It ferments only glucose.

Question 9

Which of the following is a distinguishing feature of Pseudomonas aeruginosa compared to Enterobacteriaceae?

Accepted Answer

It does not ferment glucose.

Question 10

A clinical isolate is Gram negative, forms colorless colonies on MacConkey agar, and produces black precipitate in TSI agar. Which organism is most likely?

Accepted Answer

Salmonella species

Question 11

Which of the following best describes the principle behind the Citrate utilization test?

Accepted Answer

Detection of an organism's ability to use citrate as the sole carbon source, resulting in a color change to blue

Question 12

A patient develops gastroenteritis after eating raw oysters. Which organism is most likely responsible, and what is a key feature of its identification?

Accepted Answer

Vibrio parahemolyticus; salt water organism causing diarrhea and burning sensation in the stomach

Question 13

Which of the following is a correct interpretation of a positive Methyl Red test?

Accepted Answer

The organism is a mixed acid fermenter, lowering the pH below 4 and turning the indicator red.

Question 14

Which of the following best explains why EMB and MacConkey agars are considered both selective and differential?

Accepted Answer

They inhibit Gram positive bacteria and differentiate Gram negative rods based on lactose fermentation.

Question 15

A laboratory technician observes a red color after adding Kovac’s reagent to a SIM tube. What does this indicate?

Accepted Answer

The organism produces indole from tryptophan.

Question 16

Which of the following is a common source of Salmonella infection in humans?

Accepted Answer

Consumption of raw or undercooked eggs

Question 17

A bacterium is isolated from a patient with dysentery. It is Gram negative, does not ferment lactose, and is positive for the methyl red test but negative for the Voges-Proskauer test. Which organism is most likely?

Accepted Answer

Shigella species

Question 18

Which of the following best describes the result and interpretation of a TSI slant with a yellow slant, yellow butt, and gas bubbles?

Accepted Answer

The organism ferments lactose and/or sucrose with gas production.

Question 19

Which of the following is a unique feature of the Enterotube system for bacterial identification?

Accepted Answer

It allows for the performance of 15 standard biochemical tests from a single colony.

Question 20

A bacterium is oxidase positive, grows at 42°C, and produces blue-green colonies on Pseudosel agar. Which pigment is specifically produced by this organism?

Accepted Answer

Pyocyanin

Enterobacteriaceae and Pseudomonas: Identification and Differentiation in the Clinical Laboratory

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