Pneumococcal serotypes and their clinical relevance

Pneumococcal disease is endemic all over the world. Epidemics may occur, most often in closed institutional settings such as schools and military camps,1-3 but account for only a minority of all the cases. The total incidence of pneumococcal pneumonia in Western populations is 1–5 per 1000 person-years, and several times higher in the very young and the elderly. In developing countries it is estimated that at least one million children below the age of five die each year from pneumococcal pneumonia.4 Streptococcus pneumoniae is also the most important pathogen in bacterial meningitis, otitis media, and sinusitis. The annual incidence of pneumococcal bacteraemia in North America and Europe is at least 10–20 per 100 000 individuals, the majority of cases being due to pneumonia. However, the most common host-parasite interrelationship is the asymptomatic carriage of one (or occasionally two or more) pneumococcal strains in the nasopharynx.5 6 As in other Gram positive bacteria, the pneumococcal cell wall is composed of peptidoglycan and teichoic acid (fig 1). A predominant component of teichoic acid is the C-polysaccharide which is covalently bound to the peptidoglycan layer and distributed both on its inner and outer side.7 The exact chemical structure of the C-polysaccharide has been established as a ribitol teichoic acid containing glucose, 2-acetamido-2,4,6-trideoxygalactose, galactosamine, ribitol phosphate, and phosphorylcholine.8 Interestingly, each of these components has been identified as constituents of different capsular polysaccharides, possibly pointing to an evolutionary origin of the capsular substances from the C-polysaccharide. Figure 1 Schematic illustration of the arrangement of the pneumococcal cell wall. All wild strains of Streptococcus pneumoniae are provided with a polysaccharide capsule. To date, 90 distinct capsular types have been described, six of them very recently.9Some of the types are antigenically related to each other and such related types are included together in …