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Microbiology: Bacteriology: Gram-Positive Identification Flashcards

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  1. Which test definitively distinguishes Streptococcus from Staphylococcus at the genus level?

    Answer: Catalase test

    Staphylococci are catalase-positive (produce bubbles with H₂O₂) while Streptococci are catalase-negative, making this the first-line genus differentiation test.

  2. Botulinum toxin from Clostridium botulinum causes flaccid paralysis by:

    Answer: Cleaving SNARE proteins at the neuromuscular junction

    Botulinum toxin is a zinc-dependent protease that cleaves SNARE complex proteins, preventing acetylcholine vesicle fusion and release.

  3. Bacitracin sensitivity is used as a presumptive identification test for which organism?

    Answer: Streptococcus pyogenes (Group A Strep)

    Group A Strep (S. pyogenes) is sensitive to bacitracin (0.04 U disk), while Group B and other streptococci are resistant.

  4. Methicillin-resistant Staphylococcus aureus (MRSA) acquires resistance through which primary mechanism?

    Answer: Acquisition of mecA gene encoding altered penicillin-binding protein PBP2a

    The mecA gene encodes PBP2a, which has low affinity for all beta-lactam antibiotics, conferring broad resistance regardless of beta-lactamase activity.

  5. Which gram-positive organism can cause both toxic shock syndrome and necrotizing fasciitis through its superantigen toxins?

    Answer: Streptococcus pyogenes

    S. pyogenes produces streptococcal pyrogenic exotoxins (superantigens) that massively activate T-cells, and its invasive strains cause rapidly progressing necrotizing fasciitis.

  6. A selective medium used to isolate Listeria monocytogenes from food samples that uses lithium chloride and phenylethanol to suppress other organisms is:

    Answer: Oxford agar

    Oxford agar contains lithium chloride, acriflavine, and other inhibitors that suppress background flora while allowing Listeria to grow with characteristic black halos from esculin hydrolysis.

  7. The quellung (capsular swelling) reaction is used as a rapid diagnostic test primarily for which organism?

    Answer: Streptococcus pneumoniae

    In the quellung reaction, type-specific antibodies bind to the pneumococcal capsule, causing it to appear swollen and refractile under the microscope.