Streptococcus pneumoniae

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 have emerged as predominant pathogens following increased immunization.
• 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 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 now PCV-15 and PCV-20 are on the scene. These replacement serotypes are relatively nonsusceptible to penicillin.