Background: Antimicrobial resistance in Salmonella enterica is often plasmid encoded. A key resistance plasmid group is the incompatibility group (Inc) A/C plasmids that often carry multiple resistance determinants. Previous studies showed that IncA/C plasmids were often co-located with other plasmids. The current study was undertaken to evaluate the impact of plasmid co-carriage on antimicrobial resistance and plasmid transfer. Methods: A total of 1267 Salmonella isolates, representing multiple serotypes and sources were previously subjected to susceptibility testing and 251 isolates with resistance to at least 5 antimicrobial agents were identified for further study. Each isolate was subjected to PCR-based replicon typing, and those with IncA/C plasmids were selected for plasmid isolation, PCR-based mapping of IncA/C plasmid backbone genes, and conjugation assays to evaluate resistance plasmid transferability. Results: Of the 87 identified IncA/C positive isolates, approximately 75% carried a plasmid with another identified replicon type, with the most common being I1 (39%), FIA, FIIA, FIB and HI2 (each 15%). PCR-based mapping indicated significant diversity in IncA/C backbone content, especially in regions encoding transfer-associated and hypothetical proteins. Conjugation experiments showed that nearly 68% of the isolates transferred resistance plasmids, with 90% containing additional identified plasmids or larger (>50 kb) non-typeable plasmids. Conclusions: The majority of IncA/C-positive strains were able to conjugally transfer antimicrobial resistance to the recipient, encoded by IncA/C and/or co-carried plasmids. These findings highlight the importance of co-located plasmids for resistance dissemination either by directly transferring resistance genes or by potentially providing the needed conjugation machinery for IncA/C plasmid transfer.
Bibliographical noteFunding Information:
The authors would like to thank Drs. Carl Cerniglia, Ashraf Khan and Mohamed Nawaz for their insightful review and critique of the manuscript and acknowledge Dr. Bashar Shaheen for his assistance with the project. Mr. Sean Pendelton was supported through the Oak Ridge Institute for Science and Engineering for administering the fellowship programs. Also, the authors would like to thank Drs. Alessandra Carattoli for providing positive control isolates for replicon typing and George Jacoby for isolate E . coli J53. Disclaimer: the information in this manuscript is not a formal dissemination of information by FDA and does not represent agency position or policy.
- Antimicrobial resistance
- Incompatibility groups
- Salmonella enterica