In Vitro efficacy and antiprotease activity of an antimicrobial ophthalmic drug combination against corneal pathogens of horses

Abstract

Background: Despite limited information, topical drug combinations are being routinely used by equine veterinarians for the treatment of equine keratitis in order to minimize the intensive labor and costs associated with such treatment with individual drugs. Typical drug combination includes antimicrobials, atropine and antiproteases for the treatment of infection, uveitis and keratomalacia respectively. The goal of this study was to evaluate the in vitro efficacy of an ophthalmic drug combination against protease activity and common equine corneal pathogens. Methods: Multiple representative isolates of three bacterial and two fungal corneal pathogens of horses were subjected to minimum inhibitory concentration testing of a drug combination that consisted of equal volumes of natamycin, tobramycin, and cefazolin. Proteinase inhibitory activity of the drug combination was assessed by use of a fluorescence microplate assay with gelatin and collagen I as substrates. The Minimal inhibitory concentrations were compared between the drug combination and each of its components; antiproteinase activity of the drug combination was compared with that of serum by use of paired t-tests and a one-way analysis of variance, respectively. Results: The drug combination did not inhibit the collagenase and protease activity compared to control (P>0.49). The three bacterial pathogens were susceptible to both the drug mix and cefazolin alone. Although one of four isolates of S. aureus was resistant to tobramycin alone, the rest of the isolates showed susceptibility to tobramycin. None of the bacterial isolates was sensitive to natamycin alone. All fungal isolates were susceptible to the drug combination and natamycin alone, with the exception of one Fusarium isolate which was susceptible to the drug combination but not to natamycin alone. None of the fungal isolates was susceptible to cefazolin or tobramycin alone. Conclusions: This drug combination effectively inhibited common corneal pathogens of horses in vitro, but was not effective as an antiprotease treatment. Combining anitimicrobial drugs with serum, does not increase the antimicrobials potency, but reduces the efficacy of the serum as an antiprotease agent.