Pathogenesis of Streptococcus pneumoniae
المؤلف:
Cornelissen, C. N., Harvey, R. A., & Fisher, B. D
المصدر:
Lippincott Illustrated Reviews Microbiology
الجزء والصفحة:
3rd edition , p85
2025-07-01
593
The bacterial capsule of S. pneumoniae is the most important virulence factor and is the basis for the classification of serotypes of this organism. The cell-associated enzymes pneumolysin and autolysin contribute to its pathogenicity (Figure 1).
Fig1. Cytolytic toxins produced by Streptococcus pneumoniae.
1. Capsule: The S. pneumoniae polysaccharide capsule is both antiphagocytic and antigenic. Antiphagocytic properties of the capsule protect the bacteria from polymorphonuclear leukocyte attack, facilitating growth of the bacteria prior to the appearance of anti-capsular antibodies. There are approximately 85 distinct capsular serotypes, some of which endow strains with greater virulence than others, as reflected by the fact that about 20 serotypes account for the vast majority of pneumococcal infections.
2. Pili: Pili enable the attachment of encapsulated pneumococci to the epithelial cells of the upper respiratory tract. Not all pneumococci are piliated, but those clinical isolates that express pili are more virulent. The genes required for regulation and assembly of the pilus are not present in all pneumococcal strains, but they can be horizontally transferred between strains on a pathogenicity “islet”, which is small pathogenicity island. The chromosomal region responsible for production of the pneumococcal pilus is called the rlrA islet, named for the regulatory gene (rlrA) that is required for expression.
3. Choline-binding protein A: Choline binding protein A is a major adhesin allowing the pneumococcus to attach to carbohydrates on epithelial cells of the human nasopharynx.
4. Autolysins: Autolysins are enzymes that hydrolyze the components of a biological cell in which it is produced. LytA, B and C are peptidoglycan-hydrolyzing enzymes that are present in the bacterial cell wall and are normally inactive. However, these enzymes are readily activated (for example, by surface-active agents, β-lactam antibiotics, or stationary phase), resulting in cell lysis. Autolysin is thus responsible for the release of intracellular virulence factors (notably, pneumolysin).
5. Pneumolysin: Although retained within the cytosol of intact pneumococci, pneumolysin is thought to be an important virulence factor by virtue of its ability to attack mammalian cell membranes, causing lysis once it is released by autolysin from the interior of the bacterium. Pneumolysin binds to cholesterol and therefore interacts indiscriminately with all cell types. This toxin stimulates production of proinflammatory cytokines, inhibits the activity of polymorphonuclear leukocytes and activates complement.
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