Here is an excellent review of the topic.
A few key points---
Definitions and classifications:
The FDA classification is broken down into complicated and uncomplicated skin and soft tissue infections. Complicated soft tissue infection basically means any infection at deep enough levels to require surgical intervention. However, the FDA designations exclude necrotizing infections.
Necrotizing infections are designated types I, II, III and miscellaneous.
Type I infections are mixed infections, with both aerobic and anerobic organisms. Their pathogenicity is driven largely by host factors especially diabetes. Fournier's gangrene represents but one example and is defined by location in the perineal or genital area.
Type II infections are monomicrobial, involving certain necrotizing (popularly known as “flesh eating”) strains of either beta strep or Ca-MRSA. Host factors play a lesser role.
Type III infections are rare, very severe infections, with gas gangrene as the prototype.
Miscelaneous infections include water borne infections with Vibrio vulnificus as a prototype.
Ancillary diagnostic tools, including imaging and clinical scoring, as well as a table of antibiotic recommendations, are included in the article.