Escherichia coli belonging to ST131 rarely transfers blactx-m-15 to fecal Escherichia coli isolates

ESBL producing Escherichia coli causing urinary tract infections often belong to sequence type 131 (ST131), serotype O25, carrying blaCTX-M-15. The main aim of this study was to examine the conjugational frequencies of E. coli with plasmids carrying blaCTX-M-15 to E. coli isolates from the fecal flora of healthy humans to determine whether ST131 is more likely to uptake or donate extended spectrum beta-lactamase (ESBL) resistance compared to other E. coli clones. Donors and recipients were all clinical isolates and did not harbour plasmids with identical incompatibility groups (Inc-groups) based on in silico analyses of Inc-groups and restriction/modification systems (R/M-systems). The in vitro conjugation experiments were performed as filter conjugation with verification of transconjugants by random amplified polymorphic DNA (RAPD) PCR and blaCTX-M-15 PCR. The frequencies of conjugation with blaCTX-M-15-carrying plasmids were found to be very rare with detectable conjugation frequencies in the range of 4x10-9-7x10-7 transconjugants/recipient. Recipients of O25/ST131 type yielded significantly lower conjugation frequencies compared to recipients of other O-types (P=0.004). The applied ST131/O25 donors did not yield detectable levels of transconjugants regardless of the applied recipient. Presence of sub-MIC levels of ampicillin increased plasmid transfer frequencies x100 fold (P=0.07). The results indicate that blaCTX-M-15 is rarely transferred by conjugation to E. coli isolates of the intestinal flora, even though the gene is plasmid-borne.