Abstract
The objective of this study was to understand the conjugative transmissibility of resistance plasmids present in 205 Salmonella enterica isolates from bovine sources. Polymerase chain reaction (PCR)–based replicon typing was used to type plasmid replicons. Conjugation experiments were preformed in triplicate at 30°C and 37°C on solid medium. PCR mapping of the A/C transfer gene operon was done on 17 Salmonella Newport isolates that were only positive for A/C. Eighty-six percent (n = 177) of the Salmonella isolates were multidrug resistant (MDR) with resistance to 3–12 antimicrobial agents. Of these, 82% (n = 146) were resistant to extended-spectrum cephalosporins and possessed a bla CMY gene. A/C was the predominant replicon detected, present in 90% (n = 160) of the MDR isolates. Twenty-three percent (n = 37) of the A/C-positive strains were positive for a second replicon. Replicons coresident with A/C included I1, N, B/O, HI1, and HI2. Only 31% (n = 54) of the MDR isolates produced transconjugants, and most of these donors carried multiple replicons. A/C cotransferred with B/O, N, and I1 at both 30°C and 37°C and with HI2 at 30°C. Seven Salmonella Newport isolates that produced transconjugants possessed only the single A/C replicon and lacked bla CMY. PCR mapping of the A/C transfer gene operon in ten Salmonella Newport isolates that carried bla CMY revealed a bla CMY inverted repeat element integrated between the traA and traC genes. These results suggest that A/C may have been a conjugative plasmid before the integration of bla CMY into the transfer gene operon. Additionally, transfer deficient A/C replicons may be mobilized in the presence of certain compatible conjugative plasmids.