An experimental study to evaluate the effect of polymixin E (Colistin) alone or in combination with gentamicin in McCarey-Kaufman corneal preservation medium on various drug resistant bacterial and fungal isolates

Abstract

Purpose: To assess the efficacy of the addition of polymyxin E (colistin) in the McCarey-Kaufman (MK) corneal storage solution against multi-drug resistant strains of Enterobacteriaceae, Staphylococcus aureus, and Candida spp.

Methods: A standard micro broth dilution test and a checkerboard assay were performed for five multi-drug resistant (MDR) clinical strains of P. aeruginosa and five clinical strains of methicillin-resistant S. aureus (MRSA) and C. albicans against colistin and gentamicin alone and in combination. The minimum inhibitory concentration (MIC) and the fractional inhibitory concentration index (FICI) were calculated to assess the efficacy of each combination.

Results: The MIC of colistin was in the range of 1-2 μg/mL for P. aeruginosa, whereas it was 256-1024 μg/mL against S. aureus. In comparison, the MIC of gentamicin was found to be 0.5-512 μg/mL and 0.5-8 μg/mL against P. aeruginosa and S. aureus, respectively. All five isolates of C. albicans did not exhibit any susceptibility to either colistin or gentamicin even at a concentration of ≥ 512 μg/mL each. The checkerboard assay was performed to evaluate the nature of the interaction of the combination of colistin and gentamicin. Based on the FICI, it was observed that the colistin and gentamicin combination has a maximum synergistic effect (FIC <0.5) in 80% (4/5) for S. aureus isolates, whereas the maximum additive effect (FIC >0.5-4) was 100% (5/5) for P. aeruginosa and the minimum additive effect was 20% (1/5) for S. aureus isolates. Antagonism (FIC ≥ 4) was not observed in any combination between the strains used in the study. Both colistin and gentamicin alone or in combination were, however, ineffective against Candida spp.

Conclusion: The addition of colistin has an inhibitory effect on bacterial contamination that could be possibly caused by MDR strains and could potentially be considered as an additional additive in corneal storage media.

Keywords: Colistin; MK medium; Pseudomonas aeruginosa; endophthalmitis; post keratoplasty infections.

Figure 1

Depicts the way gentamicin and colistin were added to the 96-well plates for the checkerboard assay

Figure 2

A representative image of 96-well plates containing varying concentrations of colistin ranging from 16 µg/mL to 0.25 µg/mL against five isolates of MDR P. aeruginosa. The MIC is indicated by the lowest concentration of colistin required to completely inhibit the growth of MDR P. aeruginosa

Figure 3

Graph depicting the percentage of synergism and additive effects of a combination of colistin and gentamicin on P. aeruginosa and S. aureus isolates

Figure 4

Graph depicting the percentage of viable corneal endothelial cells in Opti-MEM (control 1), MK media (control 2), and MK media with different concentrations of colistin