tailieunhanh - Chapter 130. Streptococcal and Enterococcal Infections (Part 2)

Group A Streptococci Lancefield's group A consists of a single species, S. pyogenes. As its species name implies, this organism is associated with a variety of suppurative infections. In addition, GAS can trigger the postinfectious syndromes of ARF (which is uniquely associated with S. pyogenes infection; Chap. 315) and PSGN (Chap. 277). Worldwide, GAS infections and their postinfectious sequelae (primarily ARF and rheumatic heart disease) account for an estimated 500,000 deaths per year. Although data are incomplete, the incidence of all forms of GAS infection and that of rheumatic heart disease are thought to be tenfold higher in resourcelimited countries than. | Chapter 130. Streptococcal and Enterococcal Infections Part 2 Group A Streptococci Lancefield s group A consists of a single species S. pyogenes. As its species name implies this organism is associated with a variety of suppurative infections. In addition GAS can trigger the postinfectious syndromes of ARF which is uniquely associated with S. pyogenes infection Chap. 315 and PSGN Chap. 277 . Worldwide GAS infections and their postinfectious sequelae primarily ARF and rheumatic heart disease account for an estimated 500 000 deaths per year. Although data are incomplete the incidence of all forms of GAS infection and that of rheumatic heart disease are thought to be tenfold higher in resourcelimited countries than in developed countries Fig. 130-1 . Figure 130-1 Prevalence of rheumatic heart disease in children 5-14 years old. The circles within Australia and New Zealand represent indigenous populations and also Pacific Islanders in New Zealand . From Carapetis et al 2005 with permission. Pathogenesis GAS elaborates a number of cell-surface components and extracellular products important in both the pathogenesis of infection and the human immune response. The cell wall contains a carbohydrate antigen that may be released by acid treatment. The reaction of such acid extracts with group A-specific antiserum is the basis for definitive identification of a streptococcal strain as S. pyogenes. The major surface protein of GAS is M protein which occurs in more than 100 antigenically distinct types and is the basis for the serotyping of strains with specific antisera. The M protein molecules are fibrillar structures anchored in the cell wall of the organism that extend as hairlike projections away from the cell surface. The amino acid sequence of the distal or amino-terminal portion of the M protein molecule is quite variable accounting for the antigenic variation of the different M types while more proximal regions of the protein are relatively conserved. A newer technique