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Microbiology: Bacteriology: Gram-Positive Identification Flashcards

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  1. Which gram-positive organism is a leading cause of nosocomial infections and is associated with biofilm formation on prosthetic devices?

    Answer: Staphylococcus epidermidis

    S. epidermidis, a coagulase-negative staphylococcus, forms robust biofilms on catheters and implants, making it a significant hospital pathogen.

  2. Streptolysin O from S. pyogenes is detectable by the ASO titer test because it is:

    Answer: Oxygen-labile and highly immunogenic

    Streptolysin O is inactivated by oxygen and is highly immunogenic, making anti-streptolysin O (ASO) titers useful for detecting prior strep infections.

  3. A patient with a wound infection caused by gas gangrene has a Gram stain showing large gram-positive rods with no spores visible. The most likely causative agent is:

    Answer: Clostridium perfringens

    C. perfringens rarely shows spores in clinical specimens and is the primary cause of gas gangrene, producing alpha-toxin (lecithinase) that destroys tissue.

  4. The Nagler reaction is used to identify which organism by detecting its lecithinase activity on egg yolk agar?

    Answer: Clostridium perfringens

    C. perfringens produces alpha-toxin (phospholipase C/lecithinase) that creates an opaque precipitate on egg yolk agar, inhibited on one half by antitoxin.

  5. Which gram-positive coccus is the most common cause of lobar pneumonia in otherwise healthy adults?

    Answer: Streptococcus pneumoniae

    S. pneumoniae (pneumococcus) is encapsulated and the leading bacterial cause of community-acquired pneumonia in adults.

  6. In the Lancefield classification system, Group B streptococci (GBS) carry which carbohydrate antigen?

    Answer: N-acetylglucosamine and galactose (Group B antigen)

    GBS (S. agalactiae) is defined by the Group B carbohydrate antigen, a polysaccharide containing N-acetylglucosamine and galactose.

  7. A foodborne illness characterized by a preformed heat-stable toxin causing vomiting within 1-6 hours of eating is caused by:

    Answer: Staphylococcus aureus

    S. aureus produces heat-stable enterotoxins that cannot be destroyed by cooking, leading to rapid-onset vomiting without fever.