Efficacy of tetracycline, chloramphenicol and colistimethate sodium fixed combination against Candida species

Purpose. To evaluate the potential antifungal activity of a commercially available ophthalmic product as fixed antibiotic combination (AC) containing tetracycline, chloramphenicol and colistimethate sodium, using in vitro and ex vivo models. Methods. In vitro: Candida albicans ATCC 2091 and 10231 and 14 clinical isolates of C. albicans (7), C. glabrata (5), C. utilis (1) and C. tropicalis (1) were used. The MICs of each antibiotic included in the AC were determined either individually or in combination for all strains. Fluconazole was tested for comparison. The Fractional Inhibitory Concentration (FIC) index was also calculated. Time-killing curves for C. albicans ATCC 10231 and C. albicans n. 4 clinical isolate were performed. Ex vivo: cultured rabbit corneal buttons were divided in two groups and each group was injected intrastromally with 50 µl of suspension of C. albicans ATCC 10231 and C. albicans n. 4 containing 5 x 104 colony forming units/ml, respectively. Two hours after the injection, each group was divided into 3 sub-groups. The sub-groups were treated with six instillations of AC, fluconazole or saline solution (control) up to 24 h after injection. Then the tissues were homogenized and plated to determine the number of recovered CFU/g. Results. In vitro: MIC values for the individual antibiotics were higher than that observed for fluconazole. When the AC was used a reduction of the concentration of each antibiotic able to inhibit the yeast growth was observed. In 88% of strains tested the FIC index was between 0.5 and 2.0, indicative of an additive effect. The Time-killing curves showed that the AC and fluconazole were able to maintain under control the charge for both C. albicans strains up to 10 h. After 24 h, the AC reduced of 1 Log the charge compared to the inoculum at the 0 time point. Ex vivo: the AC was effective in decreasing the load of C. albicans ATCC 10231 and C. albicans n. 4. AC reduced the both loads of strains by 4 log10 CFU/g respect to control after six doses up to 24 h after infection. Moreover, AC treatment kept under control the corneal opacity better than fluconazole. Conclusions. The ophthalmic AC, already widely used in eye drop/ointment to treat bacterial eye infections, has a potent antifungal activity and therefore can be effectively used in the treatment of ocular mycotic infections due to Candida species. The authors wish to thank the “University of Messina” within the “Research and Mobility” Project.