The strains are differentiated by the capsular polysaccharide coat, which stimulates the production of serotype-specific, protective antibody. In 1977 a strain highly resistant to penicillin emerged in South Africa. S. Pneumoniae under the microscope appears as small, dark dots. In hematogenous spread the bacteria usually initially infect the lungs, causing pneumonia.
Infection spreads to the blood resulting in bacteremia.
The inflammatory response to pneumococcal cell walls. In this case the lung is the site of infection. See text for discussion. The cell wall is acted upon by components of the acute phase response and has the ability to fix complement. The trouble is that many people, especially the elderly and very young, are unable to successfully withstand the response.