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Back to Bacteria Index

Streptococcus pneumoniae

Streptococcus pneumoniae is a species of Streptococcus that is a major human pathogen.

It was recognized as a major cause of pneumonia in the 1880s, and is the subject of many humoral immunity studies.


The organism was originally named Diplococcus pneumonia in 1926 because of its characteristic appearance in Gram-stained sputum. It was renamed Streptococcus pneumoniae in 1974 because of its growth in chains in liquid media. Because of its role as the etiologic agent of pneumonia, it has long been known informally as the pneumococcus.

Cultural characteristics

They grow on blood agar to produce a zone of alpha-hemolysis


Pneumococcus does not produce toxins but owes its virulance to its capsule which allows it to invade tissue and resist phagocytosis.

Diseases caused by pneumococcus

Despite the name, the organism causes many types of infection other than pneumonitis, including acute sinusitis, otitis media, meningitis, osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis, cellulitis, and brain abscess.

S. pneumonia is the most common cause of bacterial meningitis in adults, and is one of the top two isolates found in otitis media. Pneumococcal pneumonia is more common in the very young and the very old.

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