An increase in antimicrobial resistance in Salmonella enterica is a serious concern to both animal and public health. In this study, S. Dublin and S. Heidelberg isolates (2011-2018) in the Penn State Animal Diagnostic Laboratory Culture Repository were selected and subjected to whole-genome sequence analysis. Genomic and bioinformatic tools were used to assess the unique and shared antimicrobial resistance genes and mobile genetic elements in S. Dublin and S. Heidelberg isolates from veal calf operations in Pennsylvania. S. Dublin and S. Heidelberg shared antimicrobial genes including floR, tet(A), sul2, blaCMY-2, aac(6’), aph(6), aph(3’’), and aph(3’). blaTEM-214, blaTEM-206, and blaTEM-1B were only found in S. Dublin isolates while qnrB19, tet(B), tet(M), tet(O), aac(3), aadA1, aadA5, aadA5, fosA7, mph(A), dfrA17, dfrA12, dfrA34, and qacE were only found in S. Heidelberg isolates. It was also observed that both serotypes, S. Dublin and S. Heidelberg, harbored identical Col(pHAD28), IncC, and IncFII(S) plasmids that likely encoded for antimicrobial genes including qnrB19, sul2, tet(A), blaCMY-2, floR, and aphA. S. Dublin also contained IncX1 plasmid and S. Heidelberg contained IncFIB(S), IncHI2, IncHI2A, and IncI1-I(Alpha) plasmids. All three of the S. Heidelberg plasmids occur widely in gram-negative bacteria and encode for multiple antimicrobial resistance genes, indicating a broad host range. Based on these observations it can be speculated that co-habitation or sharing the same environment could facilitate sharing of plasmids that encode for antimicrobial-resistant determinants, and this could further promote the dissemination of antimicrobial-resistant genes in food animal agricultural operations.
