MICROBIOLOGY
- Streptococcus pneumoniae (pneumococcus): Gram-positive diplococcus with polysaccharide capsule, demonstrated by Quellung reaction or capsular swelling when antibodies bind to capsular polysaccharide antigen.
- Grows well on standard media, but prior antibiotics may preclude growth.
- Ecologic niche is nasopharynx, colonizes 5-10% of adults, 20-40% of children. Definite pathogen when isolated from sterile site (blood, CSF, joint fluid, etc). Probable pathogen in respiratory specimens (Gram stain or Cx) or when urinary antigen positive in pt w/pneumonia.
- Dx: sputum Gram stain and Cx, blood Cx, and urine antigen. Blood Cx more sensitive in HIV+ pts due to greater likelihood of bacteremia. Cx data important due to increased risk of TMP/SMX and macrolide resistance in pts on ABx for OI prophylaxis.
- Rapid urinary antigen test (Binax): 70-90% sensitive, 90% specific for bacteremic pneumonia; lower yield in non-bacteremic pneumonia; data show test results are independent of HIV status.
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