Some bacteria are extremely adept at evading a host immune response by secreting a protective “biofilm” that plays a role in delayed healing in chronic wounds. As described by Lynn Pezzanite, DVM, MS, PhD, DACVAS-LA from Colorado State University, a biofilm is an organized community of bacteria that mostly attaches to surface tissue. A slimy matrix that is formed enables the survival of bacteria even if tissue is depleted of nutrients and oxygen.
Biofilms form within 24 hours and mature rapidly within 3–4 days, at which time they are more difficult to resolve. As they detach and disperse from their original location, not only does the host’s immune response provide nutrients that promote biofilm survival, but bacteria also develop increased resistance to antimicrobial drugs (AMDs), antiseptics, and environmental challenges. Communities of more than one species of biofilm-producing bacteria act synergistically to amplify virulence. The primary bacteria responsible for biofilms are: Pseudomonas spp., Enterococcus spp., and Staphylococcus spp.
Biofilm-producing bacteria can occur in any wound type although predisposing factors are important and are based on immunocompetency of the patient, age, nutritional deficiencies, sepsis, corticosteroid administration, antibody deficiency, chronic stress, or PPID (Cushing’s disease). Reduced blood perfusion to the region and extensive wound contamination, foreign bodies, surgical implants, or a bone sequestrum are other predisposing factors.
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A greater frequency of biofilms is found in chronic wounds (60–100%) as compared to acute wounds (6%). An affected wound may not look particularly infected but if it is non-healing and chronic, not progressing, and lacks epithelialization (migration of skin cells), or the wound worsens when AMDs are discontinued, then biofilms should be suspected. A chronic inflammatory state that is poorly responsive to treatment is typical of a biofilm-infected wound. Certain indirect clinical indicators may tip you off to the presence of a biofilm: a) excess moisture; b) poor quality granulation tissue; c) heat, swelling, sensitivity/pain, and redness; d) history of persistent or recurrent infection; and e) negative culture results despite overt signs or suspicion of infection. Diagnosis is best accomplished with electron microscopy of deep tissue biopsies.
Successful management relies on early recognition and targeted treatment, stresses Pezzanite. To overcome the persistence of biofilms in a wound, it is necessary to have the wound repeatedly lavaged and aggressively debrided with a sharp scalpel to remove unhealthy tissue. This better exposes bacteria to antimicrobial drugs (AMDs). The other ingredient for success relies on the delivery of appropriately-identified AMDs at the appropriate concentration for a sufficient period of time.
Topical treatment uses surfactants and/or topical dressings impregnated with AMDs with daily bandage changes. If improvement is not seen in 3-4 days, have your vet repeat a bacterial culture and sensitivity, and be sure to discuss with your veterinarian details of treatment costs and the on-going medical plan.