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Colistin resistance due to the  mcr -type genes in  Escherichia coli  is well characterized. In order to study the resistance mechanism in  mcr -negative colistin-resistant  E. coli , strains were selected from a nationwide antimicrobial resistance surveillance program in Taiwan for further investigation. A total of 11  mcr -negative colistin-resistant isolates among 7,942 (0.1%) clinical  E. coli  isolates were identified between 2008 and 2018. Their prevalence was low and remained stable during the study period. Since 2012, ST131 and ST1193 clones with multiple drug-resistant phenotypes have emerged. All resistant strains displayed higher expression levels of the operons  pmrHFIJKLM  and  pmrCAB  than the control MG1655 strain. Although several amino acid substitutions were identified in PmrA or PmrB, only R81H in PmrA was associated with overexpression of  pmrHFIJKLM  and colistin resistance. The effect of substitution R81H in PmrA in colistin resistance was confirmed by complementation experiments. Although some strains harbored substitutions in PmrB, the identified mutations in  pmrB  did not contribute to colistin resistance. In conclusion, the amino acid substitution R81H in PmrA is an independent factor contributing to colistin resistance in non- mcr   E. coli .   IMPORTANCE  The molecular epidemiology and resistance mechanisms of  mcr -negative colistin-resistant  E. coli  are not well described. In this study, a total of 11  mcr -negative colistin-resistant  E. coli  isolates were selected from a nationwide antimicrobial resistance surveillance program in Taiwan for further investigation. We determined the resistance mechanism of non- mcr  colistin-resistant strains using gene knockout and complementation experiments. We observed the occurrence of the global multiple-drug-resistant  E. coli  clones ST131 and ST1193 starting in 2012. Moreover, for the first time, we proved that the amino acid substitution R81H in PmrA is an independent factor contributing to colistin resistance in non- mcr   E. coli . The study results helped to gain an insight into the diversity and complexity of chromosome-encoded colistin resistance in  E. coli .

microbiology spectrum

A Resistance Mechanism in Non- <i>mcr</i> Colistin-Resistant Escherichia coli in Taiwan: R81H Substitution in PmrA Is an Independent Factor Contributing to Colistin Resistance

A Resistance Mechanism in Non- mcr Colistin-Resistant Escherichia coli in Taiwan: R81H Substitution in PmrA Is an Independent Factor Contributing to Colistin Resistance