Abstract
BACKGROUND: Biofilm production by canine otitis externa (COE) pathogens and resistance development to multiple antimicrobials are commonly reported problems in veterinary practice. The use of adjuvants to disrupt biofilms may be a viable adjunctive therapy. HYPOTHESIS/OBJECTIVES: To compare the in vitro antibiofilm activity against COE pathogens of three otic cleansers: PHMB-EDTA (poly [hexamethylene] biguanide hydrochloride and disodium edetate), N-acetylcysteine (NAC) and Triz-EDTA. ANIMALS/ISOLATES: Thirty isolates of each species, including Staphylococcus pseudintermedius, Pseudomonas aeruginosa, Streptococcus canis, Proteus mirabilis, Escherichia coli, and Malassezia pachydermatis, were collected from COE cases and stored at -80°C until tested. METHODS AND MATERIALS: Biofilm production was determined by Congo-red agar and microtitre plate-assay methods. Ten of the best biofilm-producing isolates per species were selected to determine minimum biofilm eradication concentration (MBEC) values. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values were determined to compare MBEC/MIC and MBEC/MBC. RESULTS: PHMB-EDTA possessed antibiofilm activity at low concentrations (MBEC range 3.9/2.3-500/300 μg/mL) against all tested COE pathogens. NAC demonstrated antibiofilm activity for all tested bacterial COE pathogens (MBEC range 4,925-19,700 μg/mL); however, most M. pachydermatis isolates exhibited MBEC values >20,000 μg/mL. Triz/EDTA at the highest concentration tested (3,025/19,520 μg/mL) did not demonstrate antibiofilm activity against most COE pathogens except for S. canis (94.5/610 μg/mL). CONCLUSIONS AND CLINICAL RELEVANCE: PHMB-EDTA had intrinsic antibiofilm activity at low concentrations against all COE pathogens. Therefore, it is likely to be a very effective adjuvant when used in conjunction with other antimicrobials for the treatment of COE caused by biofilm-producing pathogens.