Streptococcus pneumoniae


  • Aerobic, Gram-positive diplococcus [Fig 1] with a capsule.
  • Grows on blood agar.
  • Capsular swelling w/ application of Quelling antisera [Fig 2].
  • Serotypes: immunity is serotype-specific.
    • Abx resistance is often serotype-specific.
    • Serotypes 19A and 19F had emerged as predominant pathogens following increased immunization.
      • Serotype 24F, with the advent of PCV-13, is now problematic and often with multi-drug resistance[6].
  • Minimum inhibitory concentration (MIC) to penicillin (mcg/mL) breakpoints for non-CNS specimens.
    • Susceptible: 2 mcg/mL
    • Intermediate: 4 mcg/mL
    • Resistant: 8 mcg/mL
    • Previously, MICs were determined solely based on possible meningeal disease.
      • The breakpoints above are based on anticipated respiratory or non-CNS isolates.
      • The susceptible breakpoint for meningitis caused by S. pneumoniae remains unchanged.
        • S= 0.06 mcg/mL
        • I = 0.12 - 1.0 mcg/mL
        • R ≥ 2 mcg/mL
      • This means that >95% of strains are sensitive to penicillin, the preferred drug.
  • Penicillin resistance is increasing due to "replacement strains" associated with the extensive use of conjugated pneumococcal vaccines, although PCV-15 and PCV-20 are now on the scene. Replacement serotypes tend to be relatively nonsusceptible to penicillin.

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Last updated: February 10, 2024