Phylogenetic and molecular insights into the evolution of multidrug-resistant porcine enterotoxigenic Escherichia coli in Australia

Abstract This study investigated the phylogeny and molecular epidemiology of Australian porcine enterotoxigenic Escherichia coli (ETEC) isolates ( n = 70) by performing multilocus sequence typing (MLST), random amplified polymorphic DNA (RAPD) analysis, virulence gene analysis, plasmid, bacteriocin, integron and antimicrobial resistance gene typing, and antimicrobial susceptibility phenotyping. Isolates of the most commonly observed O serogroup (O149) were highly clonal with a lower frequency of antimicrobial resistance compared with the less common O141 serogroup isolates, which were more genetically diverse and resistant to a greater array of antimicrobials. The O149 and O141 isolates belonged to sequence types (STs) ST100 and ST1260, respectively. A small number of new STs were identified for the least common serogroups, including O157 (ST4245), O138 (ST4244), O139 (ST4246) and O8 (ST4247). A high frequency of plasmid replicons was observed among all ETEC isolates. However, O149 isolates predominantly carried IncFIB, I1, HI1 and FIC, whereas O141 isolates carried a more varied array, including IncI1, FIB, FIC, HI1, I1, Y and, most significantly, A/C. O141 isolates also possessed a greater diversity of bacteriocins, with almost one-half of the isolates carrying colicin E3 (44.4%; 12/27) and E7 (48.1%; 13/27). This study shows that Australian porcine ETEC are distinct from isolates obtained in other parts of the world with respect to the MLST profile and the absence of resistance to critically important antimicrobials, including third-generation cephalosporins and fluoroquinolones.