What is the evidence that biofilm forms in chronic wounds?
Evidence from literature review
Multiple species of bacteria have been isolated from chronic and acute wounds, including wounds without any signs of infection. A literature review by Bowler examined culture data from 62 published studies dating between 1969 and 1997 (Bowler, 1998). The most predominant isolate was Staphylococcus aureus (reported in 63% of the studies), followed by coliforms (45%), Bacteroides spp. (39%), Peptostreptococcus spp. (36%), Pseudomonas aeruginosa (29%), Enterococcus spp. (26%), and Streptococcus pyogenes (13%).
This literature review also indicated that anaerobic bacteria were more commonly reported isolates from infected wounds. A later culture-based study compared the microflora of chronic and acute wounds (Bowler and Davies, 1999). In this study, Staphylococcus aureus predominated in infected acute wounds, while infected chronic wounds were primarily colonized by anaerobic bacteria (Peptostreptococcus spp., Bacteroides spp., Prevotella/ Porphyromonas spp., Clostridium spp.), coliforms, and fecal streptococci. More recent studies have used molecular techniques to assess bacterial populations colonizing wounds (Davies et. al., 2004). These studies have shown that microbial communities in chronic wounds are more diverse than indicated by culture-based techniques. Furthermore, the molecular analysis indicated culture techniques underestimated bacterial populations for bacteria generally considered culturable (Pseudomonas aeruginosa).
Overall, both culture- and molecular-based microbiological analyses have shown that wounds harbor diverse populations of bacteria. However, no single bacterial species has been identified that correlates with chronic wounds, and many of the same bacterial species are found in both chronic and acute wounds. These observations are contrary to Koch’s postulates for determining the causative agent of a disease. However, the possibility of a complex polymicrobial cause for a disease is becoming more accepted. For example, periodontal disease is believed to involve multiple species of bacteria working in consort over an extended period of time.
It has been speculated for several years that bacteria colonizing chronic wounds exist as biofilm communities (Serralta et. al., 2001; Percival and Bowler, 2004). Wounds are an ideal environment for the formation of biofilms due to their susceptibility to contamination and the availability of substrate and surface for biofilm attachment. Chronic wound infections share two important attributes with other biofilm diseases: persistent infection that is not cleared by the host immune system, and resistance to systemic and topical antimicrobial agents. Frequent debridement is one of the most clinically effective treatments to help heal chronic wounds. This may be an effective treatment because it removes the biofilm from the wound. This is similar to resolving infections from biofilm-colonized catheters: where antibiotic therapy is ineffective the most effective approach is to remove the colonized catheter. However, direct evidence of biofilm involvement in chronic wound infections is scarce. Using a porcine model, Serralta et. al. demonstrated that Pseudomonas aeruginosa inoculated onto wounds do indeed form biofilm. These researchers also demonstrated that a P. aeruginosa strain isolated from a burn wound rapidly formed biofilms in-vitro. The CBE recently conducted a microscopic examination of clinical specimens of chronic and acute wounds for the presence of biofilm. Overall, preliminary evidence indicate polymicrobial biofilm forms on chronic wounds and clinical aspects of chronic wound infections resemble those of other biofilm infections. However, the role of biofilms in preventing wound healing and mechanisms involved have yet to be determined.
