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

Purpose: 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.
Design: Prospective case-control study in eastern India.
Methods: 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.
Results: 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.
Conclusions: 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.