An Overview of Mobile Colistin Resistance (mcr) Genes in Gram-Negative Bacilli

The increasing spread of mobile colistin resistance (mcr) genes is becoming a major concern in the treatment of infections caused by multidrug-resistant Gram-negative bacilli. Colistin is often used as a last treatment option, but the emergence of mcr genes is reducing its effectiveness. These genes are most commonly found in bacteria, such as Escherichia coli, Klebsiella pneumoniae, and Salmonella, and have also been reported, though less frequently, in organisms like Pseudomonas aeruginosa. This review provides an overview of the occurrence, diversity, and mechanisms of mcr genes in Gram-negative bacilli. These genes are usually carried on plasmids, which allows them to spread easily between different bacteria. They produce enzymes that modify lipid A in the bacterial outer membrane, reducing the ability of colistin to bind and act effectively. In addition, changes in chromosomal regulatory systems such as polymyxin resistance A and B (pmrAB), phosphate regulon P and Q (phoPQ), and polymyxin adaptive resistance R and S (parRS) can further increase resistance. The spread of mcr genes is mainly driven by horizontal gene transfer, making it easier for resistance to move across different bacterial species and environments. From a clinical point of view, infections caused by mcr-positive bacteria can make treatment more difficult, increase the risk of complications, and put more pressure on healthcare systems. Therefore, early detection, regular monitoring, and careful use of antibiotics are important to control the spread of resistance. Understanding how these genes spread and persist in different environments will be important for developing better strategies to manage this growing problem.