Biofilm and Equine Limb Wounds

Abstract

In chronic wounds in humans, biofilm formation and wound chronicity are linked, as biofilms contribute to chronic inflammation and delayed healing. Biofilms are aggregates of bacteria, and living as biofilms is the default mode of bacterial life; within these aggregates, the bacteria are protected from both antimicrobial substances and the immune response of the host. In horses, delayed healing is more commonly seen in limb wounds than body wounds. Chronic inflammation and hypoxia are the main characteristics of delayed wound healing in equine limbs, and biofilms might also contribute to this healing pattern in horses. However, biofilm formation in equine wounds has been studied to a very limited degree. Biofilms have been detected in equine traumatic wounds, and recent experimental models have shown that biofilms protract the healing of equine limb wounds. Detection of biofilms within wounds necessitates advanced techniques that are not available in routine diagnostic yet. However, infections with biofilm should be suspected in equine limb wounds not healing as expected, as they are in human wounds. Treatment should be based on repeated debridement and application of topical antimicrobial therapy.

Keywords: bacterial aggregates; biofilm; delayed wound healing; horse; infection; wound; wound chronicity.

Figure 1

(A) Normally bacteria growing in shaken cultures in the lab are planktonic (single cells); (B) Bacteria in most situations in nature, including wounds, are present as biofilms. Biofilms are aggregates of bacteria, and within these biofilms, the bacteria are protected from immune defences and antimicrobial products. The microimage shows bacterial biofilm in an equine limb wound biopsy detected with peptide nucleic acid (PNA) fluorescence in situ hybridization (FISH) and subsequent confocal laser scanning microscopy (CLSM). Bacterial ribosomal RNA is displayed with a red fluorophore and eukaryotic cell nuclei (DNA) are blue due to DAPI stain (4′,6′-diamidino-2-phenylindole). Many eukaryotic cell nuclei (seen as blue spots) surround the biofilms and leaked DNA (blue stain not shaped as nuclei) from the cells can be seen, this leaked DNA is most likely from neutrophils dying in the combat against the biofilms. Figure created with BioRender.com (accessed on 7 July 2021).

Figure 2

(A) Wound with exudate and presumed infection with biofilm. This wound will need reduction of bioburden to heal. This may be achieved by surgical debridement of the wound surface followed by topical application of antimicrobials (e.g., dressings containing cadexomer iodine, nanocrystalline silver, silver sulfadiazine, polyhexamethylene biguanide, or acetic acid). The treatment/dressing is reapplied at each bandage change, normally performed every 2 to 5 days. Reduction of bioburden and removal of exudates may also occur through application of dressings containing hypertonic saline; these are applied daily for one or a few days, where after the wound is re-evaluated. When treating wounds with crevices in the wound surface, it is important to ensure debridement of the crevice and to pack dressings into the crevice to ensure that the entire wound surface is in contact with the antimicrobial substance; (B) Application of acetic acid to a limb wound. Acetic acid soaked gaze is secured over the wound (cling wrap is applied on top to further secure), the gaze can be kept soaked by multiple applications of acetic acid (1–2%) over a 20–30-min treatment period.