Dual Effects of Feed-Additive-Derived Chelerythrine in Combating Mobile Colistin Resistance

The emergence and spread of the mobile colistin-resistance gene, mcr-1 , and its variants pose a challenge to the use of colistin, a last-resort antibiotic used to treat severe infections caused by extensively drug-resistant (XDR) Gram-negative pathogens. Antibiotic adjuvants are a promising strategy to enhance the efficacy of colistin against colistin-resistant pathogens; however, few studies have considered the effects of adjuvants on limiting resistance-gene transmission. We found that chelerythrine (4 mg∙L −1 ) derived from Macleaya cordata extract, which is used as an animal feed additive, reduced the minimal inhibitory concentration (MIC) of colistin against an mcr-1 positive E. coli strain by 16-fold (from 2 to 0.125 mg∙L −1 ), eliminated approximately 10 4 colony-forming units (CFUs) of an mcr-1 -carrying strain in a murine intestinal infection model, and inhibited the conjugation of an mcr-1 -bearing plasmid in vitro (by >100-fold) and in a mouse model (by up to 5-fold). A detailed analysis revealed that chelerythrine binds to phospholipids on bacterial membranes and increases cytoplasmic membrane fluidity, thereby impairing respiration, disrupting proton motive force (PMF), generating reactive oxygen species (ROS), and decreasing intracellular adenosine triphosphate (ATP) levels, which subsequently downregulates mcr-1 and conjugation-associated genes. These dual effects of chelerythrine can expand the use of antibiotic adjuvants and may provide a new strategy for circumventing mobile colistin resistance.