Streptococcus pneumoniae strains, commonly called pneumococci, differ from other streptococci in that they contain complex polysaccharide capsules that consitute the major virulence determinant in the species. Pneumococci are respiratory pathogens that colonize upper airways and cause bronchitis or pneumonia after aspiration of respiratory secretions.
The capsule inhibits alveolar macrophage phagocytosis and allows the pneumococcus to multiply in the lung. Patients with abnormal mucocillary reflexes or decreased alveolar macrophage function are more susceptible to pulmonary infection. Patients with decreased systemic clearance of bacteria are susceptible to disseminated disease.
Type-specific antibody is elicited and is protective, but there are more than 80 serotypes of pneumococci although relatively few of these serotypes represent a large majority of infectious strains. Thus, reinfection with a different serotype is common in susceptible persons. The polysaccharide capsule is sometimes cross-reactive with the capsular polysaccharides of different genera, including Haemophilus and Klebsiella.
A vaccine containing capsular polysaccharide from the 23 most prevalent or virulent serotypes is available for adult patients at high risk for pneumococcal disease. Conjugated protein polysaccharide vaccines containing the most common infecting serotypes have been developed and have been shown to prevent disease in young children.
