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

Escherichia coli

Answer

Klebsiella pneumoniae

Answer

Pseudomonas aeruginosa

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

X factor only

Answer

V factor only

Answer

Neither X nor V factor

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

Glucose positive, Maltose positive, Lactose negative, Sucrose negative

Answer

Glucose negative, Maltose positive, Lactose positive, Sucrose positive

Answer

Glucose positive, Maltose positive, Lactose positive, Sucrose positive

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

Proteus vulgaris

Answer

Escherichia coli

Answer

Providencia stuartii

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

Acinetobacter baumannii

Answer

Stenotrophomonas maltophilia

Answer

Escherichia coli

Question 6

A Gram-negative rod is isolated from a wound culture. It grows on MacConkey agar, producing pink to red colonies. The IMViC reaction is ++-- (Indole positive, Methyl Red positive, Voges-Proskauer negative, Citrate negative). Which of the following organisms best fits this profile?

Accepted Answer

Escherichia coli

Answer

Klebsiella pneumoniae

Answer

Enterobacter cloacae

Answer

Serratia marcescens