Cellulitis and necrotizing fasciitis: microbiology and pathogenesis.

Streptococcus pyogenes is a common cause of necrotizing cutaneous infection s in otherwise healthy children and adults. Several surface components are involved in the processes of adherence and invasiveness, such as protein M and capsulae. Streptolysin O and other bacterial products, such as pyrogeni c exotoxins, are involved in tissue injury and necrosis. Toxins A and C act as superantigens and are expressed by strains associated with the toxic sh ock syndrome. Staphylococcus aureus, alone or in association with streptoco cci, is also commonly isolated form all body sites, but bacteremia is incon stant. Capsule, protein A, and the staphylococcal toxic shock syndrome toxi n are the major pathogenicity factors. in infections of the face and the ne ck, the predominant anaerobes recovered in association with group A strepto cocci are Peptostreptococcus magnus, oral Prevotella, Porphyromonas spp., a nd Fusobacterium spp. Bacteroides fragilis, Clostridium, enterobacteria, an d enterococci are recovered in infections located next to the perineal area . Penicillin is the drug of choice for the treatment of streptococcal infec tions. However benzylpenicillin may be not sufficient for severe infections and large inoculum, therefore the administration of clindamycin or another inhibitor of protein synthesis is recommended. Since the infection may be polymicrobial, the initial therapy should include treatment for staphylococ ci and anaerobes. In some cases broad- spectrum antibiotics also, effective on enterobacteria, are needed. The efficacy of appropriate parenteral anti biotics, however, depends on the prompt and aggressive exploration and debr idement of suspected deep seated infection, and supportive care of shock an d multiple organ failure.