Antimicrobial resistance in enteric or urinary Escherichia coli is a risk factor for invasive E. coli infections. Due to widespread trimethoprim resistance amongst urinary <E. coli and increased bacteraemia incidence, a national recommendation to prescribe nitrofurantoin for uncomplicated urinary tract infection was made in 2018. Nitrofurantoin resistance is reported in < 6% urinary E. coli isolates in the UK. However, mechanisms underpinning nitrofurantoin resistance in these isolates remain unknown. This study aimed to identify genetic determinants of nitrofurantoin resistance in a local E. coli collection and assess their prevalence in a larger dataset of E. coli genomes. Deleterious point mutations and gene-inactivating insertion sequences in both chromosomal nitroreductase genes nfsA and nfsB were identified in genomes of nine nitrofurantoin-resistant urinary E. coli isolates collected from north west London. Eight types of genetic alterations were identified when comparing sequences of nfsA, nfsB, and the associated gene ribE in 12,412 E. coli genomes collected from across the UK. Evolutionary analysis revealed homoplasic mutations and explained the order of stepwise mutations. An algorithm was developed to predict nitrofurantoin susceptibility and predictions for 20 accessible isolates were experimentally validated. Only one genome carrying oqxAB, a mobile gene complex associated with reduced nitrofurantoin susceptibility, was identified. In conclusion, mutations and insertion sequences in nfsA and nfsB are leading causes of nitrofurantoin resistance in UK E. coli. As nitrofurantoin exposure increases in human populations, the prevalence of nitrofurantoin resistance in carriage E. coli isolates and those from urinary and bloodstream infections should be monitored.

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