AP-Micro Microbiology: Bacteriology: Gram-Positive Identification 3 — Questions and Answers
Question 1: Which virulence factor of Streptococcus pyogenes mimics host proteins to evade immune recognition?
- M protein (Correct answer)
- Streptolysin O
- Hyaluronidase
- Streptokinase
Correct answer: M protein
M protein on the surface of S. pyogenes has regions that resemble human cardiac proteins, contributing to molecular mimicry and rheumatic fever.
Question 2: Bacillus anthracis is differentiated from other Bacillus species by its:
- Non-hemolytic, non-motile, capsule-forming colonies (Correct answer)
- Beta-hemolytic, motile, spore-forming growth
- Alpha-hemolytic, urease-positive growth
- Catalase-negative, anaerobic growth
Correct answer: Non-hemolytic, non-motile, capsule-forming colonies
B. anthracis is uniquely non-hemolytic, non-motile, and produces a poly-D-glutamate capsule that inhibits phagocytosis.
Question 3: A newborn presents with meningitis; CSF Gram stain shows gram-positive rods with tumbling motility. The most likely pathogen is:
- Listeria monocytogenes (Correct answer)
- Clostridium tetani
- Bacillus cereus
- Corynebacterium diphtheriae
Correct answer: Listeria monocytogenes
L. monocytogenes is a key cause of neonatal meningitis and is characterized by end-over-end tumbling motility at room temperature.
Question 4: The PYR test (pyrrolidonyl arylamidase) is positive for which two organisms?
- Streptococcus pyogenes and Enterococcus spp. (Correct answer)
- Streptococcus agalactiae and S. pneumoniae
- Staphylococcus aureus and S. epidermidis
- Listeria and Bacillus spp.
Correct answer: Streptococcus pyogenes and Enterococcus spp.
Both S. pyogenes (Group A Strep) and Enterococcus spp. are PYR-positive, helping distinguish them from other streptococci.
Question 5: Clostridium difficile produces two main toxins that cause pseudomembranous colitis. What do Toxin A and Toxin B primarily do?
- Glucosylate Rho GTPases, disrupting the cytoskeleton (Correct answer)
- ADP-ribosylate EF-2, halting protein synthesis
- Cleave SNARE proteins, blocking neurotransmitter release
- Activate adenylyl cyclase, increasing cAMP
Correct answer: Glucosylate Rho GTPases, disrupting the cytoskeleton
Both C. difficile toxins A and B glucosylate Rho family GTPases, destroying the actin cytoskeleton and causing cell death.
Question 6: Which medium is selective and differential for isolating Streptococcus pneumoniae from sputum?
- Blood agar with optochin disk (Correct answer)
- Mannitol Salt Agar
- MacConkey agar
- Chocolate agar with bacitracin
Correct answer: Blood agar with optochin disk
Blood agar with an optochin (ethylhydrocupreine) disk identifies S. pneumoniae by its zone of inhibition and alpha-hemolysis.
Question 7: A gram-positive organism showing 'Chinese letter' or 'palisade' arrangement on Gram stain with metachromatic granules is characteristic of:
- Corynebacterium diphtheriae (Correct answer)
- Bacillus anthracis
- Streptococcus mutans
- Clostridium perfringens
Correct answer: Corynebacterium diphtheriae
C. diphtheriae's irregular pleomorphic arrangement results from snapping division, and metachromatic (Babes-Ernst) granules are visible with special stains.
AP-Micro Microbiology: Bacteriology: Gram-Positive Identification 3
Which virulence factor of Streptococcus pyogenes mimics host proteins to evade immune recognition?