Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2009
Publication Date: 12/7/2009
Citation: Poole, T.L., Edrington, T.S., Harhay, D.M., Carattoli, A., Ramlachan, N., Anderson, R.C., Nisbet, D.J. 2009. Conjugative transferability of the A/C plasmids from Salmonella enterica isolates that possess or lack blaCMY in the A/C plasmid backbone. Foodborne Pathogens and Disease. 6:1185-1194.
Interpretive Summary: Antibiotics have been in use for fifty years to combat bacterial infections. When antibiotics were first discovered, many thought bacterial diseases would be eradicated; however, this has not happened. In fact, some disease-causing bacteria are reemerging due to the development of resistance to antibiotics. The use of antibiotics among food-producing animals is believed to have created a reservoir of antimicrobial resistant microflora. One of the problems is that bacteria can share or transfer resistance genes by a process called conjugation. Conjugation occurs when a bridge is formed between two bacterial cells and a circular piece of DNA called a plasmid migrates across the bridge. After this process is completed, the recipient cell that acquired the plasmid will be resistant to the same antibiotics as the original bacterial cell. In this study, epidemic strains of Salmonella Newport prevalent in dairy cattle were found to primarily carry a plasmid called A/C. Some plasmids transfer more easily than others. A/C does not transfer very easily; however, it was not known why this was true. As part of this study, we determined that a second plasmid could help A/C transfer. This process is called mobilization, and we found several plasmids that were capable of mobilizing A/C. Additionally, we had S. Newport isolates in our collection that carried an ancestor to the epidemic form of A/C. These ancestor A/C plasmids were capable of conjugation. Subsequence analysis of a recent publication of the gene sequence of A/C showed that integration of an antibiotic resistance gene interrupted the genes on the plasmid for transfer. The results of this work give a great deal of insight into the mechanism of how plasmids transfer between bacteria and may help food animal producers with strategies to combat the spread of antibiotic resistance.
Technical Abstract: The objective of this study was to gain a better understanding of the conjugative transfer of antimicrobial resistance plasmids from 205 Salmonella enterica strains, isolated from cattle to E. coli or Salmonella recipients. PCR-based replicon typing (PBRT) was used to type incompatibility plasmid replicons. Conjugation experiments were done at 30 deg C and 37 deg C to determine transferability of the resistance plasmids. Approximately 86% (n=177) of the Salmonella isolates were resistant to multiple antimicrobials (3 to 12). Of these, approximately 83% (n=146) were resistant to expanded-spectrum cephalosporins and possessed the AmpC blaCMY-2 gene. A/C was the predominant replicon detected, present in 90% of the multidrug-resistant (MDR) isolates. Twenty-three percent of the A/C positive strains were positive for a second plasmid replicon. Replicons found to be co-resident with A/C included: I, N, B/O, HI1, and HI2. Twenty-eight percent of the MDR isolates produced transconjugants, and most of these isolates carried multiple replicons. A/C co-transferred with B/O, N, and I1 at both 30 deg C and 37 deg C and with HI2 at 30 deg C. Eight isolates that were positive for only A/C produced transconjugants. Four of these possessed A/C but not blaCMY-2. These results suggest that A/C may have been a conjugative plasmid prior to the integration of blaCMY-2 into the type IV conjugative transfer region of the plasmid. Additionally, A/C replicons with disrupted transfer functions may still be mobilized in the presence of certain compatible conjugative plasmids.