Highly conjugative IncX4 plasmids carrying blaCTX-M in Escherichia coli from humans and food animals

This study investigated the prevalence of IncX plasmid subtypes in commensal and pathogenic Escherichia coli isolates and the biological features of the IncX4 subtype. Two hundred and twenty-five E. coli isolates from multiple sources (47 chickens, 41 pigs, 30 cattle and 107 humans) obtained during the period 2006-2012 were tested for the presence of IncX1 to IncX5. Overall, the prevalence of IncX plasmids in chicken, pig, cattle and human isolates were 21.2% (10/47), 19.5% (8/41), 3.3% (1/30) and 4.8% (5/107), respectively. IncX4 was the most common subtype, followed by IncX1 and IncX3, while no IncX2 or IncX5 were found. Seven out of 16 (43.8%) IncX4 plasmids were found to carry bla(CTX-M) genes and six of them originating from different host sources (four chickens, one pig and one human) had identical or highly similar RFLP patterns. Three IncX4 plasmids carrying bla(CTX-M) from different host sources were investigated further. It was found that the IncX4 plasmids had little effect on bacterial host growth parameters after their introduction to J53 recipients. Conjugation experiments demonstrated that the IncX4 plasmids could be efficiently transferred at 30-42 degrees C at rates which were generally 10(2)-10(5)-fold higher than those for the epidemic IncFII plasmid carrying bla(CTX-M) (pHK01). In conclusion, the IncX plasmids are more common than previously recognized. The efficient transfer of IncX4 plasmid at different temperatures and the lack of fitness burden on bacterial hosts highlight the ability of this plasmid replicon to be an important vehicle for dissemination of antimicrobial resistance.