Objectives: The recently documented presence of almost identical, small, non-self-transmissible, erm(T)-carrying plasmids in clonally unrelated erythromycin-resistant isolates of Streptococcus pyogenes and Streptococcus agalactiae suggests that these plasmids somehow circulate in the streptococcal population. The objective of this study was to characterize the erm(T)-carrying genetic element in a clinical isolate of Streptococcus dysgalactiae subsp. equisimilis (Sde5580) and to provide a possible explanation for the spread of erm(T)-carrying plasmids in streptococci.
Methods: The erm(T)-carrying element of Sde5580 was investigated by plasmid analysis, PCR experiments and sequencing. Transfer and retransfer experiments were performed using S. pyogenes, S. agalactiae and Streptococcus suis strains as recipients and by selection in the presence of suitable drug concentrations. Transconjugants were analysed by SmaI-macrorestriction analysis. Genetic studies also included PCR-restriction fragment length polymorphism analysis using HindIII endonuclease.
Results: Sde5580 contained two mobile genetic elements: a 4950 bp erm(T)-carrying plasmid (p5580) almost identical to the non-self-transmissible erm(T)-carrying plasmids of S. pyogenes and S. agalactiae mentioned above, and an ~63 kb cadC/cadA-carrying integrative and conjugative element (ICESde3396-like) of the ICESa2603 family. p5580 was transferable at high frequency to the recipients of all three species through in trans mobilization by the coresident ICESde3396-like element. p5580 and ICESde3396-like were able to be transferred either separately or together.
Conclusions: This is the first evidence of horizontal transfer of an erm(T)-carrying plasmid between streptococci. In trans mobilization by coresident ICEs may be one mechanism for the spread of erm(T)-carrying plasmids in the streptococcal population.