Efficacy of a fixed combination of three antibiotics 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 (Colbiocin, SIFI, Italy), using in vitro and ex vivo models. Methods: 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 for 1 clinical isolate were performed at 10 x MIC values of AC and Fluconazole. Cultured corneal buttons with sclera rims were injected intrastromally with 50 µl of suspension of C. albicans ATCC 10231 containing 5 x 104 colony forming units/ml. Two hours after the injection, corneas were divided into 3 groups of 4 corneas each. The groups were treated with six instillations of AC (first-group), Fluconazole (second) or balanced salt solution (third-control) up to 24 h after injection. Then the tissues were homogenized and plated to determine the number of recovered CFU/g. Results: 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. Only in the strains of C. glabrata and C. tropicalis the FIC index was < 0.5 or > 2.0 respectively, indicating a synergic or antagonist effect. The Time-killing curves showed that the AC and Fluconazole, at a concentration 10 x MIC were able to maintain under control the charge for both C. albicans strains up to 10 h (p<0.01 vs. control). After 24 h, the AC reduced of 1 Log the charge compared to the inoculum at the 0 time point. This effect was statistically superior to that of Fluconazole (p<0.001). The AC and Fluconazole were also effective (p<0.01 vs control) in the ex vivo keratitis experiments in decreasing the charge of C. albicans ATCC 10231. Conclusions: The ophthalmic AC has a potent antifungal activity and therefore can be effectively used in the treatment of ocular mycotic infections due to Candida species. Layman Abstract This study was performed to demonstrate the antifungal activity of fixed antibiotic combination containing tetracycline, chloramphenicol and colistimethate sodium (Colbiocin, SIFI, Italy), using in vitro and ex vivo models, to support the observations of the clinical experience, that highlighted a good activity of antibiotic-combination eye drops versus keratitis induced by Candida species. The results showed that the ophthalmic fixed antibiotic combination has a potent antifungal activity and therefore can be effectively used in the treatment of ocular mycotic infections due to Candida species.