Variation in plasmid conjugation among nontyphoidal Salmonella enterica serovars

Conjugation is a complex phenomenon involving multiple plasmid, bacterial, and environmental factors. Here we describe an IncI1 plasmid encoding multidrug antibiotic resistance to aminoglycosides, sulfonamides, and third-generation cephalosporins. This plasmid is widespread geographically among animal, human, and environmental sectors. We present data on the transmissibility of this plasmid from Salmonella enterica ser. Kentucky into 40 strains of S. enterica (10 strains each from serovars Enteritidis, Heidelberg, Infantis, and Typhimurium). Thirty seven out of 40 strains were able to take up the plasmid. Rates of conjugation were variable between strains ranging from 10^-8 to 10^-4. Overall, serovars Enteritidis and Typhimurium demonstrated the highest rates of conjugation, followed by Heidelberg, and then Infantis. No relationships were observed between the recipient cell surface and rate of conjugation. Recipient cell numbers correlated positively with conjugation rate and strains with high conjugation rates had marginally but significantly higher growth parameters compared to strains that took up the plasmid at lower frequencies. Environmental conditions known to impact cell growth, such as temperature, nutrient availability, and the presence of antibiotics, had a modulating effect on conjugation. Collectively, these results will further understanding of plasmid transmission dynamics in Salmonella, which is a critical first step towards the development of mitigation strategies.