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Microbiology: Bacteriology: Gram-Positive Identification Flashcards

7 cards from real AP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. Which virulence factor of Streptococcus pyogenes mimics host proteins to evade immune recognition?

    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.

  2. Bacillus anthracis is differentiated from other Bacillus species by its:

    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.

  3. A newborn presents with meningitis; CSF Gram stain shows gram-positive rods with tumbling motility. The most likely pathogen is:

    Answer: Listeria monocytogenes

    L. monocytogenes is a key cause of neonatal meningitis and is characterized by end-over-end tumbling motility at room temperature.

  4. The PYR test (pyrrolidonyl arylamidase) is positive for which two organisms?

    Answer: Streptococcus pyogenes and Enterococcus spp.

    Both S. pyogenes (Group A Strep) and Enterococcus spp. are PYR-positive, helping distinguish them from other streptococci.

  5. Clostridium difficile produces two main toxins that cause pseudomembranous colitis. What do Toxin A and Toxin B primarily do?

    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.

  6. Which medium is selective and differential for isolating Streptococcus pneumoniae from sputum?

    Answer: Blood agar with optochin disk

    Blood agar with an optochin (ethylhydrocupreine) disk identifies S. pneumoniae by its zone of inhibition and alpha-hemolysis.

  7. A gram-positive organism showing 'Chinese letter' or 'palisade' arrangement on Gram stain with metachromatic granules is characteristic of:

    Answer: Corynebacterium diphtheriae

    C. diphtheriae's irregular pleomorphic arrangement results from snapping division, and metachromatic (Babes-Ernst) granules are visible with special stains.