Characterization of two highly divergent lineages ofexfoliative toxin B-encoding plasmids revealed in impetigostrains of Staphylococcus aureus

Staphylococcal epidermolytic diseases are caused by S. aureus strains producing exfoliative toxins A and B encoded by mobile genetic elements: plasmids and prophages. We analyzed various etb gene-positive plasmids isolated from the impetigo strains implicated in outbreaks of pemphigus neonatorum in Czech maternity hospitals. Most of the plasmids were related to archetypal plasmid pETBTY4. They shared a same core sequence, but they differed from each other in the content of additional DNA regions such as antibiotic resistance-encoding transposons. The ETB plasmids of this lineage were carried by strains of clonal complex CC121. Some of these strains also harbored exfoliative toxin A-encoding prophages. A novel distinct type of ETB plasmid originated from a prophageless impetigo strain was isolated and characterized in detail. Carrying conjugative cluster genes, as well as new variants of ETB and EDIN genes, it could be regarded as a source of a new lineage of ETB plasmids. Based on our findings, we have designed a helpful detection assay, which facilitates the precise identification of the all described types of ETB plasmids.