Comparative evaluation of colistin broth disc elution (CBDE) and broth microdilution (BMD) in clinical isolates of Pseudomonas aeruginosa with special reference to heteroresistance

Purpose: Antimicrobial resistance (AMR) in Pseudomonas aeruginosa has been ever-increasing. Among other reasons, colistin resistance might be attributed to limited routine testing by approved methods. Both broth micro dilution (BMD) and colistin broth disc elution (CBDE) methods have been advocated, with limited data on the performance of these methods in the Indian settings. This prospective study was conducted to determine colistin resistance in P. aeruginosa, compare the BMD and CBDE methods with special reference to heteroresistance.Materials and methods: A total of 100 isolates of P. aeruginosa from admitted patients were included. Antimicrobial susceptibility testing was done against standard antibiotics by disc diffusion test. Minimum inhibitory concentration (MIC) against polymyxins was studied by BMD and CBDE (for colistin only). Heteroresistance to colistin was studied by population analysis profile (PAP). CBDE and BMD were compared by performance calculations. Discrepancy in results were analyzed based on heteroresistance.Results: Majority of the P. aeruginosa isolates were from pus samples (62, 62 %). Disc diffusion method revealed maximum susceptibility towards aztreonam (74, 74 %) followed by meropenem (68, 68 %) and piperacillintazobactam (65, 65 %). Polymyxin B resistance was seen in 6 % (6) while colistin resistance was seen in 9 % (9) isolates by BMD. CBDE revealed 8 % (8) resistance to colistin, having 97 % essential agreement and 95 % categorical agreement with BMD. Further, by PAP analysis, 9 isolates were resistant to colistin which included 9 resistant isolates by BMD. On discrepancy analysis, 1 isolate was found to be heteroresistant to colistin. No heteroresistance was seen in the isolates that were susceptible by all the methods.Conclusions: Heteroresistance to colistin in P. aeruginosa accounted for the discrepancy in results where CBDE method failed to detect heteroresistant isolate. As heteroresistance is a least studied phenotype, it's exact prevalence should be studied so that challenges in susceptibility testing could be addressed.