Free AP-MICRO Bacteriology: Gram-Negative Identification Questions and Answers 1 — 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?
- Escherichia coli
- Klebsiella pneumoniae
- Salmonella enterica (Correct answer)
- Pseudomonas aeruginosa
Correct answer: Salmonella enterica
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?
- X factor only
- V factor only
- Both X and V factors (Correct answer)
- Neither X nor V factor
Correct answer: Both X and V factors
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?
- Glucose positive, Maltose negative, Lactose negative, Sucrose negative (Correct answer)
- Glucose positive, Maltose positive, Lactose negative, Sucrose negative
- Glucose negative, Maltose positive, Lactose positive, Sucrose positive
- Glucose positive, Maltose positive, Lactose positive, Sucrose positive
Correct answer: Glucose positive, Maltose negative, Lactose negative, Sucrose negative
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?
- Proteus vulgaris
- Proteus mirabilis (Correct answer)
- Escherichia coli
- Providencia stuartii
Correct answer: Proteus mirabilis
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?
- Acinetobacter baumannii
- Stenotrophomonas maltophilia
- Escherichia coli
- Pseudomonas aeruginosa (Correct answer)
Correct answer: Pseudomonas aeruginosa
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.
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?
- Klebsiella pneumoniae
- Enterobacter cloacae
- Escherichia coli (Correct answer)
- Serratia marcescens
Correct answer: Escherichia coli
This classic IMViC pattern (++--) is characteristic of Escherichia coli. The organism is a lactose fermenter (pink colonies on MacConkey). It produces indole from tryptophan and performs mixed-acid fermentation (Methyl Red positive). In contrast, Klebsiella and Enterobacter typically have a --++ IMViC pattern. Serratia is variable but often VP positive.
Free AP-MICRO Bacteriology: Gram-Negative Identification Questions and Answers 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?
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