Beta-hemolytic streptococci infection was implicated in most cases of nontraumatic cellulitis.
Cellulitis is a diffuse infection that causes redness, heat, and swelling of the skin and underlying soft tissue, particularly on the legs. It should be distinguished from cutaneous inflammation associated with a suppurative focus, such as an abscess, furuncle, or underlying osteomyelitis. The microbial cause of cellulitis has been difficult to define, because cultures of blood, skin aspirates, or cutaneous biopsies are generally positive in less than 20% of patients. Findings from previous studies that made use of cultures, serology, immunofluorescent staining of skin biopsies for streptococcal antigens, and experimental models in animals have suggested that the vast majority of cases are caused by beta-hemolytic streptococci, not only Group A (Streptococcus pyogenes), but other groups as well. Some cases may be caused by Staphylococcus aureus, but the role of methicillin-resistant S. aureus (MRSA) strains has been unclear.
Investigators evaluated 179 patients with cellulitis, excluding those with animal or human bites, foreign bodies, or neutropenia. The patients were tested for acute and convalescent titers of anti-streptolysin O and anti-DNaseB; the former helps to detect infection with streptococcal Groups A, C, and G; the latter detects Group A infections alone. Along with results from blood cultures, these tests implicated beta-hemolytic streptococci in 73% of cases. In a medical center where MRSA was common in cutaneous abscesses and other skin and soft-tissue infections, 96% of patients receiving beta-lactam antibiotics ineffective against MRSA had a successful outcome.
Comment: This study may underestimate the frequency of a streptococcal etiology for cellulitis, because the serologic tests employed may miss some infections caused by Groups A, C, and G, and because they do not detect infections with other beta-hemolytic streptococci, such as Groups B and F, that can also cause cellulitis. In any event, the excellent response to beta-lactam antibiotics indicates that MRSA is a very uncommon cause of cellulitis and that the increasing use of antimicrobial therapy directed against that organism, such as trimethoprim-sulfamethoxazole or doxycycline, is rarely necessary in cellulitis. Instead, to treat patients with typical nonculturable cellulitis, clinicians can prescribe beta-lactam penicillins, such as parenteral oxacillin or oral dicloxacillin, or first-generation cephalosporins, such as parenteral cefazolin or oral cephalexin.
