Effect of plasmids harbouring blaCTX-M on the virulence and fitness of Escherichia coli ST131 isolates.

The effect of plasmids harbouring blaCTX-M on the virulence and fitness on Escherichia coli sequence type 131 (ST131) isolates was investigated. Plasmids harbouring blaCTX-M-14 or blaCTX-M-15 were transferred by transconjugation into five non-extended-spectrum β-lactamase (ESBL)-producing ST131 isolates. Clinical non-ESBL-producing ST131 isolates demonstrated a higher degree of biofilm formation and serum resistance compared with CTX-M-producing ST131 isolates. In addition, non-ESBL-producing isolates were more competitive than CTX-M-producing isolates. Transconjugants showed no significant defect in growth rate and competitiveness compared with their hosts. However, serum resistance and biofilm formation were diminished in the transconjugants. In conclusion, non-ESBL-producing E. coli ST131 isolates were more competitive and virulent than CTX-M-producing E. coli ST131 isolates. However, transconjugants harbouring blaCTX-M were no less competitive than their susceptible hosts, which may partially explain the global dissemination of CTX-M-14- and CTX-M-15-producing E. coli ST131 isolates, in addition to their increased antimicrobial resistance.