Q: Which of the following equations correctly represents the scoring system for the API 20E triplet tests?

$$\text{Score} = 1 + 2 + 4 = 7$$

Question 1

A patient’s stool sample is streaked onto Hektoen Enteric (HE) agar. After incubation, colonies appear blue-green with black centers. Which organism is most likely present, and what metabolic process is responsible for the black coloration?

Accepted Answer

Salmonella; sulfur reduction

Question 2

Which of the following best explains why MacConkey agar (MAC) is both selective and differential?

Accepted Answer

It contains bile salts and crystal violet to inhibit Gram-positive bacteria and neutral red to indicate lactose fermentation.

Question 3

A microbiologist performs an oxidase test on a bacterial isolate. The test area turns purple within 30 seconds. Which of the following is the most likely identity of the organism?

Accepted Answer

Pseudomonas aeruginosa

Question 4

Which of the following equations correctly represents the scoring system for the API 20E triplet tests?

Accepted Answer

$$	ext{Score} = 1 + 2 + 4 = 7$$

Question 5

A bacterial isolate grows on Mannitol Salt Agar (MSA), turning the medium yellow. What can you conclude about this organism?

Accepted Answer

It is salt tolerant and ferments mannitol.

Question 6

Which of the following best distinguishes fermentation from respiration in bacteria?

Accepted Answer

Fermentation is anaerobic and does not use cytochrome c oxidase.

Question 7

A Gram-negative rod is isolated from a urine sample. Which of the following is the most appropriate initial step before using the API 20E system for identification?

Accepted Answer

Perform a Gram stain to confirm Gram-negative morphology.

Question 8

Which of the following media would you use to differentiate between Staphylococcus aureus and Staphylococcus epidermidis, and what is the key indicator?

Accepted Answer

Mannitol Salt Agar; phenol red

Question 9

A clinical specimen is streaked onto MacConkey agar. After incubation, colonies are colorless. Which of the following is the most likely explanation?

Accepted Answer

The organism is a Gram-negative, non-lactose fermenter.

Question 10

Which of the following statements about selective and differential media is correct?

Accepted Answer

Differential media distinguish organisms based on visible reactions.

Question 11

A laboratory technician is testing a bacterial isolate for cytochrome c oxidase using a chromogenic reducing agent. The test is negative. Which of the following is most likely true about the organism?

Accepted Answer

It is likely a member of the Enterobacteriaceae family.

Question 12

Which of the following best describes the function of neutral red in MacConkey agar?

Accepted Answer

It acts as a pH indicator for lactose fermentation.

Question 13

A stool sample is streaked onto Hektoen Enteric agar. Colonies appear blue-green without black centers. Which organism is most likely present?

Accepted Answer

Shigella

Question 14

Which of the following is NOT a correct pairing of medium and its selective agent?

Accepted Answer

SBA – bile salts

Question 15

A microbiologist observes green discoloration around colonies on sheep blood agar. What type of hemolysis is this, and what is the likely cause?

Accepted Answer

Alpha hemolysis; partial breakdown of hemoglobin

Question 16

Which of the following organisms would you expect to produce pink colonies on MacConkey agar and orange/salmon colonies on Hektoen Enteric agar?

Accepted Answer

E. coli

Question 17

A bacterial isolate is tested on MSA and shows growth with a red color. What is the most likely identity and metabolic property of this organism?

Accepted Answer

Staphylococcus epidermidis; does not ferment mannitol

Question 18

Which of the following best explains why Enterobacteriaceae are oxidase negative?

Accepted Answer

They lack cytochrome c oxidase and do not use oxygen as a terminal electron acceptor.

Question 19

Which of the following is a correct sequence for using the API 20E system to identify a Gram-negative rod?

Accepted Answer

Perform Gram stain → Inoculate API strip → Add day 2 reagents → Interpret results

Question 20

A clinical isolate is oxidase positive, grows on MacConkey agar as colorless colonies, and produces a blue-green pigment on nutrient agar. Which organism is most likely present?

Accepted Answer

Pseudomonas aeruginosa

Microbial Identification and Differential Media: Key Concepts and Applications

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