Azithromycin restores colistin activity against multidrug-resistant bacteria.

The spread of multidrug-resistant bacteria through agricultural and food systems poses a growing threat to public health. The extensive use of colistin as an antimicrobial agent in livestock farming has served as a key driver behind the emergence and worldwide spread of plasmid-mediated colistin resistance genes (mcr), establishing a persistent reservoir of resistant bacteria within the food chain. This study investigates azithromycin (AZI) as an adjuvant to restore colistin (COL) activity against resistant bacteria relevant to foodborne and environmental exposure. We demonstrate that a low concentration of AZI effectively reverses COL resistance in MDR isolates, including carbapenem-resistant Enterobacteriaceae (CRE). Mechanistic studies reveal that AZI competitively inhibited ArnT, an enzyme responsible for lipopolysaccharide modification, thereby preventing the transfer of 4-amino-4-deoxy-L-arabinose to lipid A and restoring bacterial surface electronegativity essential for COL binding. In a murine peritonitis model, the AZI-COL combination significantly enhanced survival and reduced systemic bacterial load. These findings propose a viable combination strategy for existing antibiotics, offering potential applications in controlling resistant bacterial infections associated with environmental safety.