Fluorometholone-antibiotic interactions in canine ocular bacteria: in vitro susceptibility changes in common corneal infection pathogens.

INTRODUCTION

Corticosteroid-antibiotic combinations are commonly used in veterinary ophthalmic practice, yet the potential for corticosteroids to modulate antimicrobial susceptibility remains incompletely characterized. This study evaluated the in vitro effects of fluorometholone (FML) on the antimicrobial susceptibility of three major canine ocular pathogens-Staphylococcus pseudintermedius, Streptococcus canis, and Pseudomonas aeruginosa-and compared its interaction profile with that of dexamethasone.

METHODS

Thirty clinical isolates from dogs with bacterial keratitis were tested using standardized broth microdilution assays with 17 ophthalmic antibiotics at increasing FML concentrations (0, 0.25, 0.5, and 1 mg/mL). Changes in median minimum inhibitory concentrations (MICs) and susceptibility distributions were analyzed using the Friedman test and Cochran's Q test.

RESULTS

FML exposure produced selective, species- and antibiotic-dependent effects. S. pseudintermedius showed significant MIC increases for amikacin, ticarcillin, and tobramycin, with susceptibility reductions identified for amikacin and ticarcillin. S. canis demonstrated the most pronounced alterations, with significant MIC increases for seven antibiotics-neomycin, oxytetracycline, amikacin, gentamicin, chloramphenicol, tobramycin, and ofloxacin-and corresponding susceptibility reductions for several agents. P. aeruginosa exhibited minimal changes, with no statistically significant alterations in either MIC or susceptibility analysis. Ciprofloxacin consistently maintained stable MIC values and susceptibility classifications across all three species.

DISCUSSION: FML selectively modulated antimicrobial susceptibility in a species- and antibiotic-dependent manner, with a more restricted interaction profile than dexamethasone. Aminoglycosides demonstrated the greatest susceptibility variability, whereas ciprofloxacin maintained stable activity across all species, suggesting that it may represent a compatible option when concurrent FML use is clinically considered.