Epidemiology and risk factors.

The skin and mucous membranes are colonized by microorganisms which are ref erred to as the normal or indigenous flora. The inhabited surfaces of the h uman body include skin, mouth, throat, lower intestinal tract and vagina. M icroorganisms colonizing these region are present in appreciable numbers an d they are largely anaerobes. Under usual circumstances, these organisms ar e not pathogenic. Anaerobic infections arise after disruption of an anatomi c barrier; with the invasion of local flora into a previously sterile site; in cases where host defenses are impaired, flora components are capable of producing serious disease. The infection is generally a mixed infection of aerobic and anaerobic bacteria, but the number of anaerobes is greater tha n the number of aerobic or facultative organims. While the normal flora is composed of several hundred anaerobic bacteria species, a few of these spec ies are routinely found in clinical specimens. The anaerobic species which predominate in clinical infections are Bacteroides fragilis, pigmented Prev otella, Porphyromonas, Fusobacterium or Peptostreptococcus species. These b acteria have one or more virulence factor properties which contribute to vi rulence. (C) 2000 Editions scientifiques et medicales Elsevier SAS.