Interaction between lpxABCD and pmrABC genes and Biofilm Formation in Colistin-resistant Acinetobacter baumannii and Klebsiella pneumoniae Strains.

Antibiotic resistance represents a major global health threat, particularly in infections caused by multidrug-resistant (MDR) Gram-negative bacteria. Understanding the molecular mechanisms underlying colistin resistance and their association with biofilm formation is critical for improving treatment strategies. In this study, a total of 46 colistin-resistant clinical isolates (Acinetobacter baumannii, n = 12; Klebsiella pneumoniae, n = 34) obtained from intensive care unit patients were analyzed. Antimicrobial susceptibility was determined using Kirby–Bauer disk diffusion and broth microdilution methods, and biofilm formation capacity was evaluated using the crystal violet microdilution plate assay. The presence of plasmid-mediated (mcr-1 to mcr-4) and chromosomal (pmrA, pmrB, pmrC, lpxA, lpxB, lpxC, lpxD) colistin resistance genes was investigated by PCR, while expression levels of pmrA, pmrB, and pmrC genes were quantified using RT-qPCR. All isolates were classified as MDR. Carbapenem resistance was detected in 91.67% of A. baumannii and 82.35% of K. pneumoniae isolates. None of the isolates carried mcr genes, indicating that colistin resistance was primarily mediated by chromosomal mechanisms. The pmrB gene was detected in all isolates, whereas lpxB and lpxD genes were detected in 19.57% of cases. Biofilm formation was observed in 95.65% of the isolates, with varying degrees of biofilm strength. A statistically significant positive correlation (p < 0.05) was identified between pmrCAB gene expression levels and biofilm formation capacity, particularly for pmrC expression. These findings suggest that colistin resistance in A. baumannii and K. pneumoniae is mainly driven by chromosomal mechanisms involving pmrCAB and lpxABCD pathways and that pmrCAB expression is associated with enhanced biofilm formation. Targeting biofilm associated resistance mechanisms may provide new therapeutic opportunities for managing colistin-resistant infections in intensive care settings.