The Importance of the Group A Streptococcus Capsule in the Pathogenesis of Human Infections: A Historical Perspective
Departments of 1Medicine and 2Public Health, Boston University School of Medicine, Boston, Massachusetts; and Departments of 3Medicine and 4Molecular Sciences, University of Tennessee Health Science Center, and 5Veterans Affairs Medical Center, Memphis, Tennessee
The virulence of group A streptococci (GAS) correlates closely with expression of its surface antigen, M protein, and its hyaluronic acid capsule. In studies of human GAS infection, the former has received considerable attention. For several decades, however, systematic identification of encapsulated virulent strains by the mucoid colonies they produce has been neglected in clinical studies. In part, this may be due to the capsule’s evanescent expression on artificial media, its repression during convalescent carriage, lack of expertise in recognizing its colonial morphology, and the growing tendency for clinical laboratories to eschew throat cultures in favor of rapid laboratory tests for group A polysaccharide. Older and more recent studies are reviewed here that emphasize the capsule’s basic role in infection. We believe that it is time to refocus newer clinical studies and techniques on achieving early recognition of potentially dangerous, heavily encapsulated strains of GAS for which spread may be prevented.
Received 17 September 2007; accepted 15 November 2007; electronically published 27 February 2008.