- 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.
- 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 now PCV-15 and PCV-20 are on the scene. These replacement serotypes are relatively nonsusceptible to penicillin.
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