Genotypic and Phenotypic Characterization of Colistin-Resistant Gram-Negative Bacteria from Clinical Isolates in Ibadan, Oyo State

Colistin is a last-resort antibiotic reserved for carbapenem-resistant Gram-negative infections, and the emergence of plasmid-mediated colistin resistance genes (mcr genes) poses a catastrophic threat to the global antimicrobial arsenal. This study characterized colistin resistance mechanisms, both phenotypic and genotypic, in Gram-negative bacteria isolated from clinical specimens at the University College Hospital (UCH), Ibadan, Oyo State, South West Nigeria. Two hundred and fifteen Gram-negative clinical isolates, predominantly E. coli, K. pneumoniae, and Acinetobacter baumannii, were screened for colistin resistance by broth microdilution (MIC above 2 mg/L). Phenotypically resistant isolates were subjected to PCR for mcr-1 through mcr-5 genes. Chromosomal resistance mechanisms including mutations in mgrB and pmrA/pmrB regulatory genes were investigated by PCR and Sanger sequencing in mcr-negative isolates. Colistin resistance phenotype was detected in 11.6 percent of isolates. mcr-1 was identified in 52 percent of phenotypically resistant E. coli isolates. mcr-3 was additionally detected in one K. pneumoniae isolate. mgrB gene disruption was confirmed in 68 percent of colistin-resistant K. pneumoniae that were mcr-negative. Colistin-resistant isolates carried concurrent carbapenemase genes (blaNDM or blaOXA-48) in 60 percent of cases, defining extensively drug-resistant (XDR) phenotypes. These findings document the emergence of plasmid-mediated colistin resistance alongside carbapenem resistance in Ibadan and highlight the urgent need for colistin resistance surveillance in Nigeria's AMR monitoring framework. Keywords: colistin resistance, mcr genes, mgrB, extensively drug-resistant, Ibadan.