Colistin resistance in Gram-negative ocular infections: prevalence, clinical outcome and antibiotic susceptibility patterns.

To study the prevalence, antibiotic susceptibility profile, clinical outcomes and plasmid-mediated transfer of colistin resistance (CLR) among Gram-negative bacilli (GNB) isolates from different ocular infections. Prospective case-control study in eastern India. Consecutive ocular samples with GNB isolates from clinically diagnosed cases of microbial keratitis, infectious endophthalmitis and orbital infections were included. Inclusion criteria were significant GNB growth from ocular samples and > 6 weeks follow-up. Clinical outcomes were determined by disease-specific criteria for each clinical group. Antibiotic susceptibility was tested by broth microdilution for colistin and Kirby-Bauer disc diffusion method for others. Plasmid detection for CLR genes mcr-1 and mcr-2 genes was done by standard protocols. Sixty GNB isolates were studied. Overall prevalence of CLR (intrinsic plus acquired) was 40% (n = 24), acquired being 37.5% of CLR isolates (n = 9). The prevalence varied from 45.5% (10/22) and 45% (9/20) in microbial keratitis and infectious endophthalmitis, respectively, to 26.3% (5/19) in orbital infections. Clinical outcomes in CLR patients were significantly worse in microbial keratitis (p = 0.018) and orbital infections (p = 0.018), and comparable to colistin-susceptible ones (p = 0.77) in infectious endophthalmitis. CLR isolates had significantly higher resistance to Amikacin, Gentamicin and Ceftazidime but were susceptible to Piperacillin, Carbapenems and fluoroquinolones. Plasmids mcr-1 and mcr-2 were detected in 6.25% (n = 1) and 25%(n = 4), respectively, of the 16 tested isolates. CLR is highly prevalent in ocular isolates and affects clinical outcomes. CLR isolates may still remain susceptible to Carbapenems, Piperacillin and fluoroquinolones. Plasmid mcr-1- and mcr-2-mediated CLR remains low in ocular infections.