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

## TL;DR

Manuka honey shows antibacterial effects against bacteria causing horse eye ulcers, and combining it with some antibiotics may enhance or reduce their effectiveness.

## Contribution

Demonstrates manuka honey's in vitro antibacterial activity and its combined effects with antibiotics against equine corneal pathogens, including MRSA.

## Key findings

- Manuka honey inhibited bacterial growth at 12% concentration and killed bacteria at 20-28%.
- Combining manuka honey with tetracycline, chloramphenicol, or fusidic acid enhanced antibacterial effects against Staphylococcus species.
- Manuka honey reduced the effectiveness of ofloxacin and cloxacillin against some bacteria.

## Abstract

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.

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.

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.

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.