“Investigating mcr Genes in CRAB Isolates during a NICU outbreak and Advancements in Colistin Sensitivity Testing"

Purpose : Colistin-resistant Acinetobacter baumannii (CRAB) poses a growing challenge in neonatal intensive care units (NICUs) due to the emergence of plasmid-mediated Mobile Colistin Resistance (mcr) genes that can rapidly spread among neonates. This pioneering study conducted a comprehensive analysis of mcr-1, mcr-2, mcr-3, mcr-4, and mcr-5 genes in A. baumannii strains isolated from a Pakistani NICU. Notably, this investigation marks the first of its kind in this setting. In addition to genotypic analyses, the study incorporated phenotypic assays to identify the most effective method for detecting colistin resistance in these A. baumannii strains. The research also encompassed the examination of A. baumannii blood isolates exhibiting low to mid-level colistin resistance during an outbreak in a tertiary care hospital. Methods: In the genotypic investigation, DNA was extracted from strains of colistin-resistant A. baumannii, collected from the NICU of the local hospital of Islamabad, Pakistan. The specific set of primers for each mcr gene was used to detect their presence. Various phenotypic methods such as; disc diffusion, broth macro dilution, Minimum Inhibition Concentration (MIC), disc elusion and colistin agar methods were performed using predetermined calculations for phenotypic studies. Results: The study confirms the distribution of the mcr-1 gene among the eight strains, which contributes 66 % of the total number, whereas mcr-2, 3, 4 and 5 genes were not detected in all strains. Based on phenotypic analysis, A. baumannii had shown resistance against colistin at < 2µg/mL. In contrast, MIC varies from strain to strain. Conclusions: The horizontal transfer of the mcr-1 gene is responsible for developing resistance against colistin among A. baumannii in neonates. All phenotypic methods adopted in this study generated the same results, so the method selection depends upon individual comfort. However, we propose that the colistin agar method offers multiple advantages over other methods as it is more economical, easy to perform, multiple samples can be assessed simultaneously and fewer calculations are involved for colistin resistance determination and finding out clinical breakpoints.