Objectives: The aims of this study were to identify small staphylococcal plasmids that carry either the trimethoprim resistance gene dfrK or the apramycin resistance gene apmA and analyse them for their structure and organization with regard to their potential role as precursors of large multiresistance plasmids that carry these genes.
Methods: Trimethoprim- or apramycin-resistant staphylococci from the strain collections of the two participating institutions were investigated for the presence of plasmid-borne dfrK or apmA genes. The dfrK- or apmA-carrying plasmids were sequenced completely and compared with sequences deposited in the databases.
Results: Two small plasmids, the 4957 bp dfrK-carrying plasmid pKKS966 from porcine Staphylococcus hyicus subsp. hyicus and the 4809 bp apmA-carrying plasmid pKKS49 from porcine methicillin-resistant Staphylococcus aureus were identified. Structural analysis revealed that both plasmids had a similar organization, comprising a single resistance gene (dfrK or apmA), a plasmid replication gene (rep) and three partly overlapping genes for mobilization proteins (mobA, mobB and mobC). Comparisons showed 71%-82% amino acid identity between the Rep and Mob proteins of these two plasmids; however, distinctly lesser percentages of identity to Rep and Mob proteins of staphylococci and other bacteria deposited in the databases were detected.
Conclusions: Both plasmids, pKKS966 and pKKS49, appeared not to be typical staphylococcal plasmids. The homology to larger plasmids that harbour the genes apmA and/or dfrK was limited to these resistance genes and their immediate upstream and downstream regions and thus suggested that these small plasmids were not integrated into larger plasmids.