Streptococcus pneumoniae

Paul G. Auwaerter, M.D.

MICROBIOLOGY

  • 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[8].
  • 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 PCV15 and PCV20 are now on the scene. Replacement serotypes tend to be relatively nonsusceptible to penicillin.

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Last updated: December 15, 2024