Purpose: To evaluate the antifungal activity of a fixed antibiotic combination (AC) containing tetracycline (TET), chloramphenicol (CAF), and colistimethate sodium (CS).
Methods: In vitro: Candida ATCC and clinical strains were used. The minimum inhibitory concentrations (MICs) of AC and of each antibiotic were determined. Fluconazole (FLC) was tested for comparison. Time-killing curves of selected strains were performed. Ex vivo keratitis: corneas were injected intrastromally with the selected strains. After the injection, corneas were divided into groups of treatments: AC, FLC, or saline. Then, the tissues were analyzed for colony-forming units per gram (CFU/g). Propidium iodide (PI) and MitoTracker (MTR) staining were used to investigate the mode of action.
Results: Values of MIC required to inhibit the growth of 90% of organisms for the antibiotics alone were higher than FLC. However, their activity was enhanced when used in combination against Candida yeasts. Time-killing curves showed that at 24 hours, AC reduced the load of both strains of approximately 1 Log10 CFU/g compared with the initial inoculum (P < 0.0001). This effect was also significant versus FLC. In ex vivo, AC was effective in decreasing the loads of both strains by 4 Log10 CFU/g with respect to the control. Moreover, it showed higher activity than FLC against Candida albicans ATCC 10231 (1 Log10 CFU/g, P < 0.01 versus control). PI staining demonstrated that CS changed the membrane's permeability, whereas MTR staining demonstrated that TET or CAF altered mitochondrial function. The cells treated with AC and stained showed both effects.
Conclusions: In this study, AC showed antifungal efficacy versus Candida spp.; this activity can be due to the synergistic effects of antibiotics in it.