AP-Micro Microbiology: Bacteriology: Gram-Positive Identification 2 — Questions and Answers
Question 1: Which enzyme produced by Staphylococcus aureus converts fibrinogen to fibrin, forming a clot around the bacteria?
- Coagulase (Correct answer)
- Catalase
- Urease
- Hyaluronidase
Correct answer: Coagulase
Coagulase is a key virulence factor of S. aureus that clots plasma, distinguishing it from coagulase-negative staphylococci.
Question 2: A gram-positive rod that forms a central spore and appears 'drumstick'-shaped on smear is most consistent with:
- Clostridium tetani (Correct answer)
- Bacillus anthracis
- Listeria monocytogenes
- Clostridium perfringens
Correct answer: Clostridium tetani
C. tetani produces a terminal (subterminal) spore that swells the cell end, creating the classic drumstick morphology.
Question 3: Which characteristic distinguishes Enterococcus faecalis from Streptococcus pyogenes on laboratory media?
- Growth in 6.5% NaCl broth (Correct answer)
- Beta-hemolysis on blood agar
- Gram-positive cocci in chains
- Catalase-negative reaction
Correct answer: Growth in 6.5% NaCl broth
Enterococci are salt-tolerant and grow in 6.5% NaCl, while Streptococcus pyogenes does not.
Question 4: The CAMP test is used to presumptively identify which gram-positive organism?
- Streptococcus agalactiae (Correct answer)
- Streptococcus pneumoniae
- Listeria monocytogenes
- Staphylococcus epidermidis
Correct answer: Streptococcus agalactiae
S. agalactiae (Group B Strep) produces CAMP factor that enhances the hemolysis of S. aureus, creating an arrowhead pattern.
Question 5: A gram-positive coccus that is alpha-hemolytic, optochin-resistant, and bile-insoluble is most likely:
- Viridans streptococci (Correct answer)
- Streptococcus pneumoniae
- Enterococcus faecalis
- Staphylococcus aureus
Correct answer: Viridans streptococci
Viridans streptococci are alpha-hemolytic like S. pneumoniae but are resistant to optochin and not lysed by bile salts.
Question 6: Which gram-positive bacillus causes disease by producing a toxin that ADP-ribosylates EF-2, halting protein synthesis?
- Corynebacterium diphtheriae (Correct answer)
- Bacillus cereus
- Clostridium botulinum
- Listeria monocytogenes
Correct answer: Corynebacterium diphtheriae
C. diphtheriae exotoxin ADP-ribosylates elongation factor 2 (EF-2), blocking protein synthesis in host cells.
Question 7: On Mannitol Salt Agar (MSA), a yellow halo around colonies indicates:
- Mannitol fermentation by S. aureus (Correct answer)
- Catalase production by S. epidermidis
- Coagulase activity by S. saprophyticus
- Beta-hemolysis by streptococci
Correct answer: Mannitol fermentation by S. aureus
S. aureus ferments mannitol, producing acid that turns the phenol red indicator yellow on MSA.
AP-Micro Microbiology: Bacteriology: Gram-Positive Identification 2
Which enzyme produced by Staphylococcus aureus converts fibrinogen to fibrin, forming a clot around the bacteria?