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 (see gram 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.