Bacterial endometritis (infection of the uterine mucosa) that is refractory to traditional antimicrobial treatment is a significant challenge to the equine breeding industry. A common survival strategy employed by bacterial pathogens is the formation of a biofilm—a complex and dynamic structure composed of aggregates of bacteria surrounded by a thick protective layer of exopolysaccharide (a polymer composed of sugar residues).
Biofilms confer resistance to immune-mediated clearance by reducing the host’s ability to recognize infection. Additionally, biofilms protect bacteria from antibiotics by providing a diffusion barrier and creating a microenvironment that slows down bacterial metabolism and replication, which makes them more tolerant to antimicrobial agents. Using a model of equine infectious endometritis, we have clearly identified the ability of the bacterium Pseudomonas aeruginosa to form a biofilm within the uterus of the mare.
The biofilm forms in multiple locations with the greatest number of adherent bacteria occurring between the tissue folds and in the uterine horns. This suggests that a traditional guarded culture swab might not be ideal for detecting biofilm-associated infections; a low-volume lavage might be a better diagnostic tool. The bacteria are in greater numbers deep within the endometrial glands as compared to the luminal surface. To be successful in clearing these infections, treatment options need to be capable of penetrating deeper into the glands and tissue. For microscopic visualization of biofilms within endometrial biopsies, Bouin’s solution provides significantly better preservation of the biofilm matrix on the surface of the endometrium as compared to traditional formalin fixation.
Bacteria residing in a biofilm can be up to 1,000 times more refractive to treatment with antibiotics as compared to free-living (planktonic) bacteria. The simple administration of more or a higher concentration of antibiotics has failed to eliminate chronic biofilm infections in both human and veterinary medicine. The goal in treating a biofilm-associated infection is to disrupt the biofilm material and kill the bacteria residing within the biofilm.
A series of in vitro (within a laboratory setting, such as in a test tube) studies were conducted to assess biofilm dispersal and/or bacterial killing by antibiotics and non-antibiotic agents alone or in combination against Gram-negative bacteria (P. aeruginosa, Escherichia coli, and Klebsiella pneumoniae). Data would indicate that antibiotics and nonantibiotic agents are more effective against biofilms if administered concurrently.
When dealing with bacterial infections protected in biofilms, the treatment period should be at least 72 hours in duration, with repeated treatments every 24 hours (i.e., a uterine infusion of the selected combination once every 24 hours for three consecutive days). Following this treatment protocol, the biofilm was completed disrupted and bacterial killing ensued. Assessment of antibiotic sensitivity of the offending pathogen(s) is still important as inherent genetic resistance of the bacteria involved will not be overcome solely by the addition of the nonantibiotic compounds.
A recent in vivo (in the live horse) study evaluated the intrauterine treatment of a preformed P. aeruginosa biofilm with a combination of ceftiofur and tris-ethylenediaminetetraacetic acid (EDTA) or ceftiofur or tris-EDTA alone. Of five mares treated with the combination, all effectively cleared the infection. This contrasted with only two of five mares treated with ceftiofur alone and one of five mares with tris-EDTA alone. The findings confirmed greater efficiency in killing preformed biofilm within the uterus by using a combination of antibiotic (ceftiofur) and nonantibiotic (tris-EDTA) agents.
Advances in our understanding of the significance of biofilms in human and veterinary medicine will, in time, lead to improved diagnostics and more effective treatment modalities. Fortunately, several therapeutic options are currently available to clinicians for the treatment of biofilm-associated endometritis in horses.