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Spheroidal bacterium (Streptococcus pneumoniae) that causes human diseases including pneumonia, sinusitis, ear infection, and meningitis. Usually occurring in the upper respiratory tract, this gram-positive (seegram stain) coccus is often found in a chain configuration and surrounded by a polysaccharide capsule. Pneumococci are separated into types depending on the specific capsular polysaccharide formed. Their disease-causing ability resides in the capsule, which delays or prevents their destruction by cells in the bloodstream that normally engulf foreign material.
This entry comes from Encyclopædia Britannica Concise. For the full entry on pneumococcus, visit Britannica.com.