In Vitro Activity of Manuka Honey, Either Alone or in Combination With Topical Antibiotics, Against Bacteria Commonly Found in Equine Ulcerative Keratitis

Objective To assess the antibacterial activity of manuka honey against bacterial isolates commonly associated with infected corneal ulcerations in horses, and to investigate possible combined effects of manuka honey and commonly prescribed topical antibiotics. Procedures Four Staphylococcus aureus , including three methicillin‐resistant (MRSA), two methicillin‐resistant coagulase‐negative staphylococci ( S. sciuri and S. haemolyticus ), and two Streptococcus equi subspecies zooepidemicus isolates from horses with stromal ulcerative keratitis were selected. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of manuka honey were determined with microdilution assays and spectrophotometric analyses. Potential combined antibacterial effects were explored by assessing bacterial growth inhibition using disk diffusion and E‐tests, both with and without a sub‐inhibitory concentration of manuka honey. Tested antibiotics included chloramphenicol, tetracycline, gentamicin, ofloxacin, tobramycin, cloxacillin, and fusidic acid. Results The spectrophotometric MIC of manuka honey was 12% (w/v) for all tested isolates. MBC values ranged between 20% (w/v) and 28% (w/v) for all isolates, except S. sciuri . Combining 5% (w/v) manuka honey with tetracycline, chloramphenicol, or fusidic acid enhanced the antibacterial effect against Staphylococcus spp. (including methicillin‐resistant isolates). The antibacterial effect of ofloxacin against S. aureus and streptococci and of cloxacillin against S. aureus appeared slightly reduced when combined with 5% manuka honey. No consistent difference was observed when manuka honey was combined with gentamicin or tobramycin. Conclusions Manuka honey has in vitro inhibitory and bactericidal activity against equine corneal surface pathogens including multi‐resistant isolates. Further studies are required to assess potential synergistic and antagonistic effects of manuka honey in combination with antibiotics.