In the late nineteenth century, pairs of micrococci were first recognized in the blood of rabbits injected with human saliva by both Louis Pasteur working in France and George Sternberg, an American army physician. The important role of these micrococci in human disease was not appreciated at that time. By 1886, when the organism was designated “pneumokokkus” and Diplococcus pneumoniae, the pneumococcus had been isolated by many independent investigators, and its role in the etiology of pneumonia was well known. In the 1930s, pneumonia was the third leading cause of death in the United States (after heart disease and cancer) and was responsible for ∼7% of all deaths both in the United States and in Europe. While pneumonia was caused by a host of pathogens, lobar pneumonia—a pattern more likely to be caused by the pneumococcus—accounted for approximately one-half of all pneumonia deaths in the United States in 1929. In 1974, the organism was reclassified as Streptococcus pneumoniae.
Pneumococci are spherical gram-positive bacteria of the genus Streptococcus. Within this genus, cell division occurs along a single axis, and bacteria grow in chains or pairs—hence the name Streptococcus, from the Greek streptos, meaning “twisted,” and kokkos, meaning “berry.” At least 22 streptococcal species are recognized and are divided further into groups based on their hemolytic properties. S. pneumoniae belongs to the α-hemolytic group that characteristically produces a greenish color on blood agar because of the reduction of iron in hemoglobin (Fig. 134-1). The bacteria are fastidious and grow best in 5% CO2 but require a source of catalase (e.g., blood) for growth on agar plates, where they develop mucoid (smooth/shiny) colonies. Pneumococci without a capsule produce colonies with a rough surface. Unlike that of other α-hemolytic streptococci, their growth is inhibited in the presence of optochin (ethyl hydrocuprein hydrochloride), and they are bile soluble.
Pneumococci growing on blood agar, illustrating α hemolysis and optochin sensitivity (zone around optochin disk). Inset: Gram's stain, illustrating gram-positive diplococci. (Photographs courtesy of Paul Turner, Shoklo Malaria Research Unit, Thailand.)
In common with other gram-positive bacteria, pneumococci have a cell membrane beneath a cell wall, which in turn is covered by a polysaccharide capsule. Pneumococci are divided into serogroups or serotypes based on capsular polysaccharide structure, as distinguished with rabbit polyclonal antisera; capsules swell in the presence of specific antiserum (the Quellung reaction). The 91st and 92nd serotypes, 6C and 6D, have most recently been identified with monoclonal antibodies and by serologic, genetic, and biochemical means, respectively. Within the 92 serotypes there are 21 serogroups, each containing two to five serotypes with closely related capsules. The capsule protects the bacteria from phagocytosis by host cells in the absence of type-specific antibody and is arguably the most important determinant ...