Streptococcus pneumoniae

MICROBIOLOGY

• Aerobic, gram-positive diplococcus with capsule.
• Grows on blood agar.
• Capsular swelling w/ application of Quelling antisera.
• Serotypes: immunity is serotype-specific.
  • Abx resistance is often serotype-specific.
  • Serotype 19A has emerged as predominant pathogen.
• Minimum Inhibitory Concentration (MIC) to penicillin (mcg/mL) breakpoints for non-CNS specimens.
  
  • Susceptible: (2)
  • Intermediate: (4)
  • Resistant: (8)
  • Previously S = 0.06, I = 0.12-1.0 and R ≥ 2 breakpoints were determined solely on the basis of possible meningeal disease. New breakpoints above are based on anticipated respiratory or non-CNS isolates. Susceptible breakpoint for meningitis caused by S. pneumoniae remains unchanged (S= 0.06 mcg/mL). This now means that >95% of strains are sensitive to penicillin, and penicillin is the preferred drug.
• Penicillin resistance is increasing due to "replacement strains" associated with extensive use of Prevnar 7 (13) vaccine with emergence of 19A strain and more recently serotypes 6C and 35. These serotypes are relatively non-susceptible to penicillin.