Colistin exerts potent activity against mcr+ Enterobacteriaceae via synergistic interactions with the host defense.

Colistin (COL) is a cationic cyclic peptide that disrupts the membranes of Gram-negative bacteria and is often used as a last resort antibiotic against multidrug-resistant strains. The emergence of plasmid-borne mcr genes, which confer transferable COL resistance, has raised serious concerns, particularly in strains also carrying extended-spectrum β-lactamase and carbapenemase genes. Standard antimicrobial susceptibility testing (AST), performed in enriched bacteriological media, indicates no activity of COL against mcr+ strains, leading to its exclusion from treatment regimens. However, these media poorly reflect in vivo physiology and lack host immune components. Here we show that COL retained bactericidal activity against mcr-1+ Escherichia coli, Klebsiella pneumoniae, and Salmonella enterica when tested in tissue culture medium containing physiological bicarbonate. COL enhanced serum complement deposition on bacterial surfaces and synergized with human serum to kill pathogens. At clinically achievable concentrations, COL killed mcr-1+ strains in freshly isolated human blood and was effective as monotherapy in a murine E. coli bacteremia model. These findings suggest that COL, currently dismissed based on conventional AST, may offer clinical benefit against mcr-1+ infections when evaluated under more physiological conditions - warranting reconsideration in clinical microbiology practices and future trials for high-risk patients.