Streptococcus pneumoniae, or pneumococcus, is a Gram-positive, alpha-hemolytic
diplococcus bacterium and a members of the genus Streptococcus. A significant human
pathogen, S. pneumoniae was recognized as a major cause of pneumonia in the late 19th
century and is the subject of many humoral immunity studies.
Despite the name, the organism causes many types of infection other than pneumonia,
including acute sinusitis, otitis media, meningitis, osteomyelitis, septic arthritis,
endocarditis, peritonitis, pericarditis, cellulitis, and brain abscess.
S. pneumoniae is the most common cause of bacterial meningitis in adults and children,
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.
S. pneumoniae can be differentiated from Streptococcus viridans, which is also alpha
hemolytic, using an optochin test, as S. pneumoniae is optochin sensitive. The
encapsulated, gram-positive coccoid bacteria have a distinctive morphology on gram stain,
the so-called, "lancet shape." It has a polysaccharide capsule that acts as a virulence
factor for the organism; 91 different capsular types are known, and these types differ in
virulence, prevalence, and extent of drug resistance.
S. pneumoniae has several virulence factors, including the polysaccharide capsule
mentioned earlier, that help it evade a host's immune system. It has pneumococcal surface
proteins that inhibit complement-mediated opsonization, and it secretes IgA1 protease that
will destroy secretory IgA produced by the body.
The risk of pneumococcal infection is much increased in persons with impaired IgG
synthesis, impaired phagocytosis, or defective clearance of pneumococci. In particular, the
absence of a functional spleen, through congenital asplenia, splenectomy, or sickle-cell
disease predisposes one to a more severe course of infection (Overwhelming postsplenectomy infection) and prevention measures are indicated (see asplenia).