Micro strep and gram pos rods

Review of Beta and Gamma Streps

  • Key Organisms & Tests Reviewed:
    • Beta Strep: Group A Strep (Streptococcus pyogenes), Group B Strep (Streptococcus agalactiae), Group C, F, and G Streps.
      • A Disc (Bacitracin Disc): Susceptible for Group A Strep (A disc for Group A). Resistant for Group B Strep.
      • CAMP Test: Positive for Group B Strep, indicated by arrowhead hemolysis. Requires Staph. aureus as a complementary organism for setup.
      • Hippurate Test: Positive for Group B Strep, indicated by a purple precipitate (hippurate hydrolysis).
      • PYR Test: Rapid test, results in 5 minutes.
        • Positive for Group A Strep and Enterococcus.
        • Can largely replace A disc or CAMP test for presumptive identification of Group A Strep.
      • SXT Disc: Both Group A and Group B Streps are resistant to it.
    • Gamma/Alpha Strep: Enterococcus and Group D Strep.
      • Bile Esculin Test: Universally positive for both Enterococcus and Group D Strep, indicated by a black color change.
      • 6.5\% Salt Tolerance Test: Differentiates Enterococcus from Group D Strep.
        • Enterococcus is positive (turbidity or yellow color change with pH indicator).
        • Group D Strep is negative (no turbidity, stays purple with pH indicator).
      • Enterococcus is also PYR positive.

Alpha Streps

  • General Importance: Differentiating hemolytic patterns (alpha, beta, gamma) is crucial for strep organisms. For Gram-negative rods, beta-hemolysis is more significant than alpha or gamma.
  • Overall Strategy: A flowchart available in the strep lecture is highly recommended for memorization, especially for students during exams (e.g., regurgitating it on a scratch sheet).
    • The chart generally lists: A, B (beta), C, F, G (beta), D (alpha or gamma), Strep. pneumo (alpha), Viridans (alpha).
  • Differentiation: Knowing both common and scientific names (e.g., Streptococcus pyogenes = Group A Strep; Streptococcus agalactiae = Group B Strep) is required.

Streptococcus pneumoniae (Strep pneumo / Pneumococcus)

  • Morphology:
    • Gram Stain: Gram-positive diplococci, typically lancet-shaped or bullet-shaped (tapered ends). Always found in pairs. Can be encapsulated, appearing as a clearing around the organism in stained smears.
    • Culture: Catenophilic (loves CO2) – requires CO2 incubation.
      • Hemolysis: Alpha hemolytic (green coloration around colonies).
      • Colony Morphology: Two distinct forms, making presumptive identification easier:
        • Mucoid: Looks like