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- Streptococcus pneumoniae (pneumococcus): Gram-positive diplococcus with polysaccharide capsule, demonstrated by Quellung reaction or capsular swelling when antibodies bind to capsular polysaccharide antigen.
- Ecologic niche is nasopharynx, colonizes 5-10% of adults, 20-40% of children.
- Definite pathogen when isolated from sterile site (blood, CSF, pleural or joint fluid).
- Probable pathogen in respiratory specimens (Gram stain or culture) or when urine Ag+ in setting of pneumonia.