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Microbiology: Bacteriology: Gram-Positive Identification Flashcards

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  1. Which enzyme produced by Staphylococcus aureus converts fibrinogen to fibrin, forming a clot around the bacteria?

    Answer: Coagulase

    Coagulase is a key virulence factor of S. aureus that clots plasma, distinguishing it from coagulase-negative staphylococci.

  2. A gram-positive rod that forms a central spore and appears 'drumstick'-shaped on smear is most consistent with:

    Answer: Clostridium tetani

    C. tetani produces a terminal (subterminal) spore that swells the cell end, creating the classic drumstick morphology.

  3. Which characteristic distinguishes Enterococcus faecalis from Streptococcus pyogenes on laboratory media?

    Answer: Growth in 6.5% NaCl broth

    Enterococci are salt-tolerant and grow in 6.5% NaCl, while Streptococcus pyogenes does not.

  4. The CAMP test is used to presumptively identify which gram-positive organism?

    Answer: Streptococcus agalactiae

    S. agalactiae (Group B Strep) produces CAMP factor that enhances the hemolysis of S. aureus, creating an arrowhead pattern.

  5. A gram-positive coccus that is alpha-hemolytic, optochin-resistant, and bile-insoluble is most likely:

    Answer: Viridans streptococci

    Viridans streptococci are alpha-hemolytic like S. pneumoniae but are resistant to optochin and not lysed by bile salts.

  6. Which gram-positive bacillus causes disease by producing a toxin that ADP-ribosylates EF-2, halting protein synthesis?

    Answer: Corynebacterium diphtheriae

    C. diphtheriae exotoxin ADP-ribosylates elongation factor 2 (EF-2), blocking protein synthesis in host cells.

  7. On Mannitol Salt Agar (MSA), a yellow halo around colonies indicates:

    Answer: Mannitol fermentation by S. aureus

    S. aureus ferments mannitol, producing acid that turns the phenol red indicator yellow on MSA.