Conjugative transferability of the A/C plasmids from salmonella enterica isolates that possess or lack blaCMY in the A/C plasmid backbone

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 blaCMY 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 blaCMY. PCR mapping of the A/C transfer gene operon in ten Salmonella Newport isolates that carried blaCMY revealed a blaCMY 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 blaCMY into the transfer gene operon. Additionally, transfer deficient A/C replicons may be mobilized in the presence of certain compatible conjugative plasmids. © 2009, Mary Ann Liebert, Inc.