Various factors can be used to classify NSTIs, including the depth of tissue involvement, severity of infection, and microbiology. According to the FDA, NSTIs are classified as: 1) uncomplicated infections that respond to a simple course of antibiotics or incision and drainage, or 2) complicated infections that involve deeper tissues and may require surgery. Based on the organisms isolated, there are three types of NSTIs: type I, type II, and type III.[1,2]
Type I Infections
These are the most common type and usually involve four or more causative organisms, which are frequently a mix of aerobic and anaerobic bacteria. The most prominent aerobic organisms are streptococci; one may also find staphylococci, enterococci, and gram-negative rods. Bacteroides species are the most common anaerobes, followed by Peptostreptococcus. The presence of multiple pathogenic organisms is usually a sign of a compromised immune system, and patients with type I infections commonly have diabetes mellitus, obesity, peripheral vascular disease, chronic kidney disease, or alcohol abuse.
Fournier's gangrene is a type I NSTI involving the perineum or genital areas. It can arise from genitourinary, colorectal, or dermatologic sources. Ludwig's angina (cervical necrotizing fasciitis) is another type I infection that involves the submandibular space. Common complications associated with it include stridor and airway obstruction.
Type II Infections
These are much less common and account for 10% to 15% of NSTIs. They are increasingly caused by Staphylococcus aureus, specially methicillin-resistant strains. Additionally, streptococci may be isolated from such infections. Type II NSTIs are typically located on the extremities and normally originate from minor injuries that allow the entry for bacteria or create an environment that is conducive to the survival of hematogenously transported bacteria. Type II NSTIs may also be associated with the use of nonsteroidal anti-inflammatory drugs (NSAIDs).
Type III Infections
These are the least common type of NSTIs, but the cause of these infections has not been universally agreed upon. While some sources consider these infections to be caused by V vulnif-icus, others attribute them to clostridial myonecrosis.[3,4] Type II infections typically occur as a result of a deeply penetrating wound or a crush injury accompanied by local devascularization, intestinal surgery, or "black tar" heroin injection.
US Pharmacist. 2013;38(4):HS-8-HS-12. © 2013 Jobson Publishing