Streptococcus pneumoniae is an obligate, gram-positive, non-motile, encapsulated cocci. The lancet shaped bacteria is the most common cause of pneumonia and otitis media, and an important cause of menigitis and bacteremia/sepsis. The bacteria are common in healthy adults, but disease is generally confined to the old and the young and people with chronic illnesses.

Approximately 85 different serotypes have been identified based on their polysaccharide capsule. The capsule is both antiphagocytic and antigenic and is a major virulence factor. Autolysin and pneumolysin, both enzymes associated with S.pneumoniae are also contributers to pathogenicity.

Treatment. The appearance of resistance in the late 1980's has reduced the usage of penicillin G in S.pneumoniae treatment. Currently resistant strains are still susceptible to cephalosporins (eg cefotaxine and cetriaxone) and vanomycin.

Vaccines. High risk individuals over two years old can be protected using a polyvalent capsular pollysaccharide vaccine. The vaccine immunizes against 23 serotypes of S.pneumoniae, which make up 85-90% of infections.