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Background  Our observation that in the Mexican  Salmonella  Typhimurium population none of the ST19 and ST213 strains harbored both the  Salmonella  virulence plasmid (pSTV) and the prevalent IncA/C plasmid (pA/C) led us to hypothesize that restriction to horizontal transfer of these plasmids existed. We designed a conjugation scheme using ST213 strain YU39 as donor of the  bla  CMY-2  gene (conferring resistance to ceftriaxone; CRO) carried by pA/C, and two  E. coli  lab strains (DH5α and HB101) and two Typhimurium ST19 strains (SO1 and LT2) carrying pSTV as recipients. The aim of this study was to determine if the genetic background of the different recipient strains affected the transfer frequencies of pA/C.    Results  YU39 was able to transfer CRO resistance, via a novel conjugative mechanism, to all the recipient strains although at low frequencies (10 -7  to 10 -10 ). The presence of pSTV in the recipients had little effect on the conjugation frequency. The analysis of the transconjugants showed that three different phenomena were occurring associated to the transfer of  bla  CMY-2 : 1) the co-integration of pA/C and pX1; 2) the transposition of the CMY region from pA/C to pX1; or 3) the rearrangement of pA/C. In addition, the co-lateral mobilization of a small (5 kb) ColE1-like plasmid was observed. The transconjugant plasmids involving pX1 re-arrangements (either via co-integration or IS Ecp1 -mediated transposition) obtained the capacity to conjugate at very high levels, similar to those found for pX1 (10 -1 ). Two versions of the region containing  bla  CMY-2  were found to transpose to pX1: the large version was inserted into an intergenic region located where the “genetic load” operons are frequently inserted into pX1, while the short version was inserted into the  stbDE  operon involved in plasmid addiction system. This is the first study to report the acquisition of an extended spectrum cephalosporin (ESC)-resistance gene by an IncX1 plasmid.    Conclusions  We showed that the transfer of the YU39  bla  CMY-2  gene harbored on a non- conjugative pA/C requires the machinery of a highly conjugative pX1 plasmid. Our experiments demonstrate the complex interactions a single strain can exploit to contend with the challenge of horizontal transfer and antibiotic selective pressure.

Conjugative transfer of an IncA/C plasmid-borne blaCMY-2gene through genetic re-arrangements with an IncX1 plasmid