AP-Micro Microbiology: Bacteriology: Gram-Negative Identification 3 — Questions and Answers
Question 1: Which gram-negative organism is the causative agent of cat-scratch disease?
- Bartonella henselae (Correct answer)
- Pasteurella multocida
- Capnocytophaga canimorsus
- Francisella tularensis
Correct answer: Bartonella henselae
Bartonella henselae, transmitted via cat scratches or flea feces, causes cat-scratch disease presenting with regional lymphadenopathy.
Question 2: On MacConkey agar, Salmonella colonies appear:
- Pink/red (lactose fermenter)
- Colorless (non-lactose fermenter) (Correct answer)
- Mucoid and pink
- Yellow (acid production)
Correct answer: Colorless (non-lactose fermenter)
Salmonella is a non-lactose fermenter and appears colorless/transparent on MacConkey agar because it does not lower the pH to trigger the neutral red indicator.
Question 3: Which feature of Klebsiella pneumoniae is responsible for its mucoid colony appearance?
- Peptidoglycan layer
- Polysaccharide capsule (Correct answer)
- Flagella
- Lipopolysaccharide O-antigen
Correct answer: Polysaccharide capsule
Klebsiella pneumoniae produces a thick polysaccharide capsule that gives colonies a mucoid, string-test-positive appearance.
Question 4: The CAMP test is used to identify Listeria monocytogenes, but which gram-negative organism uses a similar beta-hemolysin enhancement mechanism in its own identification?
- Haemophilus influenzae in satellite phenomenon (Correct answer)
- Pseudomonas aeruginosa in oxidase testing
- Bordetella pertussis on Bordet-Gengou agar
- Acinetobacter on blood agar
Correct answer: Haemophilus influenzae in satellite phenomenon
The satellite phenomenon uses beta-hemolysin from S. aureus to provide factor V (NAD) that supports H. influenzae growth around the streak, analogous to CAMP enhancement.
Question 5: Which gram-negative pathogen is most associated with waterborne outbreaks and produces a cholera toxin that activates adenylyl cyclase?
- Campylobacter jejuni
- Vibrio cholerae (Correct answer)
- Enterotoxigenic E. coli
- Yersinia enterocolitica
Correct answer: Vibrio cholerae
Vibrio cholerae produces cholera toxin (CT), which ADP-ribosylates Gs protein, permanently activating adenylyl cyclase and causing massive secretory diarrhea.
Question 6: A curved, gram-negative rod with a single polar flagellum isolated from a patient with bloody diarrhea after eating undercooked chicken is most likely:
- Helicobacter pylori
- Campylobacter jejuni (Correct answer)
- Vibrio parahaemolyticus
- Arcobacter butzleri
Correct answer: Campylobacter jejuni
Campylobacter jejuni is the most common bacterial cause of foodborne diarrhea in the US, transmitted via undercooked poultry, and is curved/comma-shaped.
Question 7: Which identification test differentiates Neisseria meningitidis from Neisseria gonorrhoeae?
- Oxidase test
- Gram stain morphology
- Maltose fermentation (Correct answer)
- Coagulase test
Correct answer: Maltose fermentation
N. meningitidis ferments both glucose and maltose, while N. gonorrhoeae ferments only glucose; this maltose fermentation test is the key differentiator.
AP-Micro Microbiology: Bacteriology: Gram-Negative Identification 3
Which gram-negative organism is the causative agent of cat-scratch disease?