A Type 2 A/C2 plasmid carrying the aacC4 apramycin resistance gene and the erm(42) erythromycin resistance gene recovered from two Salmonella enterica serovars

11 March 2016

Objective: To determine the relationships between RepA/C2 plasmids carrying several antibiotic resistance genes found in isolates of Salmonella enterica serovars Ohio and Senftenberg from pigs. Methods: Illumina HiSeq was used to sequence seven S. enterica isolates. BLAST searches identified relevant A/C2 plasmid contigs, and contigs were assembled using PCR. Results: Two serovar Ohio isolates were ST329 and the five Senftenberg isolates were ST210. The A/C2 plasmids recovered from the seven isolates belong to Type 2 and contain two resistance islands. Their backbones were closely related, differing by five or fewer single nucleotide polymorphisms. The sul2-containing resistance island ARI-B is 19.9 kb and also contains the kanamycin and neomycin resistance gene aphA1, the tetracycline resistance gene tetA(D), and an erythromycin resistance gene, erm(42), not previously seen in A/C2 plasmids. A second 30.3 kb resistance island, RI-119, is in a unique location in the A/C2 backbone 8.2 kb downstream of rhs. RI-119 contained genes conferring resistance to apramycin, netilmicin, tobramycin (aacC4), hygromycin (hph), sulphonamides (sul1) and spectinomycin and streptomycin (aadA2). In one of the seven plasmids, this resistance region contained two IS26-mediated deletions. A discrete 5.7 kb segment containing the aacC4 and hph genes and bounded by IS26 on one side and the IR of Tn5393 on the other was identified. Conclusions: The presence of almost identical A/C2 plasmids in two serovars indicates a common origin. Type 2 A/C2 plasmids continue to evolve via addition of new resistance regions such as RI-119 and evolution of existing ones.