Pandemic fluoroquinolone resistant Escherichia coli clone ST1193 emerged via simultaneous homologous recombinations in 11 gene loci

Abstract

Significance Global growth in antibiotic resistance is a major social and health problem. The most common mechanism of high resistance to fluoroquinolones is the sequential acquisition of 3 mutations in 2 DNA topoisomerases, GyrA and ParC. We show that Escherichia coli ST1193 acquired the mutant variants of gyrA and parC not by a conventional stepwise evolution but rather all at once. This was likely a result of a single transfer of about 1 Mb of chromosomal DNA from a phylogenetically distant donor E. coli strain, followed by 11 homologous recombination events involving the transferred DNA. Thus we describe a highly effective mechanism of acquisition of antimicrobial resistance by pathogenic bacteria, which led to the emergence of pandemic E. coli clone ST1193. Global growth in antibiotic resistance is a major social problem. A high level of resistance to fluoroquinolones requires the concurrent presence of at least 3 mutations in the target proteins—2 in DNA gyrase (GyrA) and 1 in topoisomerase IV (ParC), which occur in a stepwise manner. In the Escherichia coli chromosome, the gyrA and parC loci are positioned about 1 Mb away from each other. Here we show that the 3 fluoroquinolone resistance mutations are tightly associated genetically in naturally occurring strains. In the latest pandemic uropathogenic and multidrug-resistant E. coli clonal group ST1193, the mutant variants of gyrA and parC were acquired not by a typical gradual, stepwise evolution but all at once. This happened as part of 11 simultaneous homologous recombination events involving 2 phylogenetically distant strains of E. coli, from an uropathogenic clonal complex ST14 and fluoroquinolone-resistant ST10. The gene exchanges swapped regions between 0.5 and 139 Kb in length (183 Kb total) spread along 976 Kb of chromosomal DNA around and between gyrA and parC loci. As a result, all 3 fluoroquinolone resistance mutations in GyrA and ParC have simultaneously appeared in ST1193. Based on molecular clock estimates, this potentially happened as recently as <12 y ago. Thus, naturally occurring homologous recombination events between 2 strains can involve numerous chromosomal gene locations simultaneously, resulting in the transfer of distant but tightly associated genetic mutations and emergence of a both highly pathogenic and antibiotic-resistant strain with a rapid global spread capability.