Multidrug resistance and lipopolysaccharide remodeling associated with mcr-1 acquisition in Shiga toxin-producing Escherichia coli

The emergence of plasmid-mediated colistin resistance in Shiga toxin-producing Escherichia coli (STEC) is an important public health concern because these strains can combine antimicrobial resistance with intestinal pathogenic potential. However, although mcr -harboring STEC has been reported from food-related sources, the phenotypic consequences of mcr-1 acquisition in STEC remain poorly understood. In this study, we investigated the prevalence of mcr variants and associated antimicrobial resistance determinants among 144 mcr -harboring STEC isolates and examined the phenotypic impact of mcr-1 in E. coli O157:H7 ATCC 43894. Among the mcr -harboring STEC isolates, mcr-1 was the predominant mcr variant (63.9%), and 88.9% of mcr-1 -positive isolates were multidrug-resistant. Notably, genomically predicted fosfomycin resistance determinants were detected in 84.8% of mcr-1 -positive isolates. Acquisition of the mcr-1 -harboring plasmid in E. coli O157:H7 ATCC 43894 increased adhesion to HEp-2 epithelial cells but reduced swimming motility, biofilm formation, and Galleria mellonella mortality. The reduced G. mellonella mortality may reflect altered innate immune stimulation. These changes were accompanied by remodeling of the lipopolysaccharide profile, with reduced banding in both the O-antigen and core-lipid A regions, reduced endotoxin levels, increased cell surface hydrophobicity, a less negative zeta potential, and reduced autoagglutination, all of which were mcr-1 -dependent in knockout and complementation experiments. Collectively, these findings indicate that mcr-1 contributes beyond colistin resistance, converging on a coordinated reshaping of cell surface architecture and host interaction together with co-selection of resistance to other clinically relevant antibiotics, including fosfomycin.