What is the role of the bacterial capsule in the pathophysiology of pneumococcal infection?

Updated: Jun 08, 2020
  • Author: Eduardo Sanchez, MD; Chief Editor: John L Brusch, MD, FACP  more...
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The capsule is composed of polysaccharides that cover the cell wall, which is made up of peptidoglycan and teichoic acid, characterizing the classic gram positive structure; It acts as the principal antiphagocytic and protective element that prevents access of the leukocytes to the underlying cell wall elements. The capsular polysaccharides have served as means of serotyping and identifying these organisms. The Quellung reaction is the criterion standard method for pneumococcal capsular serotyping. More than 9 serotypes of S pneumoniae have been identified; currently, serotypes 6, 14, 18, 19, and 23 are the most prevalent agents that cause infections. Serotyping provides important epidemiological information, especially with the widespread use of vaccination, but rarely provides timely clinical information.

The virulence of each organism is determined in part by two distinct states: opaque and transparent colony types that influence the capacity to evade host defenses. The nasopharynx is predominantly colonized by the transparent phenotype. Conversely, the opaque type predominates in lung, CNS, and bloodstream infections; it has increased capsular polysaccharide and produces more biofilm. [26] In addition, in vitro and in vivo studies of clinical isolates have shown that pneumococci have the ability to obtain DNA from other pneumococci (or other bacteria) via transformation, allowing them to switch to serotypically distinct capsular type.

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