FREE AP-MICRO Bacteriology: Gram-Negative Identification Questions and Answers

Question 1

A Gram-negative bacillus isolated from a stool culture produces colorless colonies on MacConkey agar. On Triple Sugar Iron (TSI) agar, it produces an alkaline (red) slant and an acid (yellow) butt, with the production of H₂S. Which of the following is the most likely organism?

Accepted Answer

Salmonella enterica

Answer

The organism is a non-lactose fermenter, indicated by the colorless colonies on MacConkey agar. The TSI reaction of alkaline slant/acid butt (K/A) indicates fermentation of only glucose. The production of H₂S (blackening of the butt) is a key characteristic of Salmonella species. E. coli and Klebsiella are lactose fermenters (pink on MAC), and Pseudomonas is a non-fermenter (K/K on TSI).

Question 2

A microbiologist is attempting to identify a fastidious Gram-negative coccobacillus isolated from a respiratory specimen. The organism fails to grow on sheep blood agar but grows on chocolate agar. Satellite growth is observed around Staphylococcus aureus colonies on a blood agar plate. This organism requires which of the following for growth?

Accepted Answer

Both X and V factors

Answer

The organism described is characteristic of Haemophilus influenzae. It is fastidious and requires both X factor (hemin) and V factor (NAD) for growth. Chocolate agar contains both factors because the red blood cells are lysed, releasing them. Sheep blood agar contains X factor, but the V factor is trapped within the intact red blood cells. Staphylococcus aureus produces V factor as a metabolic byproduct, allowing Haemophilus to grow as satellite colonies around it on blood agar.

Question 3

A Gram-negative diplococcus is isolated from a urogenital specimen on modified Thayer-Martin agar. The organism is oxidase-positive. To confirm the identification as Neisseria gonorrhoeae, which carbohydrate utilization pattern is expected?

Accepted Answer

Glucose positive, Maltose negative, Lactose negative, Sucrose negative

Answer

Neisseria gonorrhoeae is identified by its characteristic growth on selective media, Gram stain morphology (Gram-negative diplococci), positive oxidase test, and a specific carbohydrate utilization profile. It characteristically utilizes glucose only. Neisseria meningitidis, a closely related species, utilizes both glucose and maltose.

Question 4

A urine culture grows >100,000 CFU/mL of a Gram-negative rod that exhibits swarming motility on blood agar. The organism is a non-lactose fermenter on MacConkey agar and is rapidly urease-positive. The indole test is negative. Which organism is most likely identified?

Accepted Answer

Proteus mirabilis

Answer

The key features described—swarming motility, non-lactose fermentation, and rapid urease production—are characteristic of the genus Proteus. The indole test is used to differentiate the two most common species. Proteus mirabilis is indole-negative, whereas Proteus vulgaris is indole-positive.

Question 5

Which of the following Gram-negative organisms is a non-fermenter, oxidase-positive, and often produces a characteristic grape-like odor and a blue-green pigment?

Accepted Answer

Pseudomonas aeruginosa

Answer

Pseudomonas aeruginosa is a classic example of a non-fermentative Gram-negative rod. It is distinguished by its positive oxidase test, production of the blue-green pigment pyocyanin, and a characteristic fruity or grape-like odor. Acinetobacter is oxidase-negative, Stenotrophomonas is also typically oxidase-negative, and E. coli is a fermenter and member of the Enterobacteriaceae family.

AP Practice Test

AP Practice Test

FREE AP-MICRO Bacteriology: Gram-Negative Identification Questions and Answers