Escherichia coli B-strains are intrinsically resistant to colistin and not suitable for characterization and identification of mcr genes

Antimicrobial resistance is an increasing threat to human and animal health. Due to the rise of multi- and extreme drug resistance, last resort antibiotics, such as colistin, are extremely important in human medicine. While the distribution of colistin resistance genes can be tracked through sequencing methods, phenotypic characterization of putative antimicrobial resistance (AMR) genes is still important to confirm the phenotype conferred by different genes. While heterologous expression of AMR genes (e.g., in E. coli) is a common approach, so far, no standard method for heterologous expression and characterization of mcr genes exists. E. coli B-strains, designed for optimum protein expression, are frequently utilized. Here we report that four E. coli B-strains are intrinsically resistant to colistin (MIC 8 - 16 g/mL). Additionally, the three tested B-strains that encode T7 RNA show growth defects when transformed with empty or gene-expressing pET17b plasmids and grown in the presence of IPTG; K-12 or B-strains without T7 RNA polymerase do not show these growth defects. The E. coli Shuffle T7 express strain carrying empty pET17b also showed a heteroresistance phenotype when exposed to colistin in the presence of IPTG. These phenotypes could explain why B-strains were erroneously reported as colistin susceptible. Analysis of existing genome data identified one nonsynonymous change in each pmrA and pmrB in all four E. coli B-strains; the E121K change in PmrB has previously been linked to intrinsic colistin resistance. We conclude that E. coli B-strains are not appropriate heterologous expression hosts for identification and characterization of mcr genes.