Biofilms and Wound Infection

Wounds provide an ideal environment for bacterial growth and are readily colonised by microorganisms of indigenous, human origin and in many cases by environmental contaminants. Bacteria attach to a coating known as the extracellular matrix, a mesh of proteins that encompasses the cells of the wound bed. Once attached, bacteria proliferate to form micro-colonies, secreting a thick polysaccharide layer that protects against environmental challenges such as the host immune system or antimicrobial treatments. Micro-colonies eventually develop into a biofilm, which is a dynamic microbial community comprised of diverse bacterial species.
Biofilm is common within infected wounds being found in 4 percent of acute and 40 percent of chronic wounds. It is difficult to see by eye and presents challenges to the clinical microbiologist; bacteria can be exceptionally difficult to culture from wound swabs with a proportion of the population failing to be recovered. Therefore, treatment of biofilm-infected wounds can be problematic resulting in persistent or recurrent infection. Furthermore it is estimated that bacteria residing within a biofilm are up to 1000 times more resistant to antimicrobial treatment, a characteristic conferred in part by the polysaccharide layer surrounding the biofilm.
Topical antimicrobials in the form of impregnated dressings are often applied to infected wounds. The efficacy of such dressings is dependent on continued release of the antimicrobial component, sufficient to provide a sustained inhibitory, or lethal dose. The polysaccharide layer of the biofilm impairs diffusion of antimicrobials through the biofilm, meaning that not all bacteria are exposed to an appropriate treatment dose. Consequently upon cessation of antimicrobial treatment, infection can recur because bacteria still remain within the wound.
Despite this, research continues to aid scientists’ understanding of the biofilm lifestyle and how best to tackle these resilient microbial communities. New strategies to prevent and treat biofilms in wounds include the use of anti-adhesive (or anti-biofilm) compounds to prevent attachment of bacteria to the wound, or anti-virulence compounds which diminish the damage that infecting bacteria cause and give the immune system a chance to clear the infection. Other alternatives include the use of bacteria-specific viruses which target and kill biofilm bacteria without infecting or damaging the host. At present many of these are still in early investigative or developmental stages and so traditional antimicrobial treatments are still widely utilised.
Biofilm is not just problematic in wound infection. Bacteria can grow as a biofilm on almost any surface meaning that in humans, biofilm can be found in the lungs of patients with chronic pulmonary infection, on the heart valves of patients with infective endocarditis, within the urethra of patients with persistent or recurrent urinary tract infection, and - the most well known biofilm - on the teeth as dental plaque. Therefore understanding the biofilm lifestyle in infection and the challenges posed to effective eradication is imperative, and as more is understood the better equipped clinical practitioners will be to overcome the biofilm burden.

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Sarah Maddocks
Dr Sarah Maddocks works as a lecturer in Microbiology at Cardiff Metropolitan University (UK), where her work focuses on the host-pathogen interaction and biofilm development during infection.