Mixed-biofilm natural transformation assay reveals the presence of staphylococci in human environments that can transfer SCCmec to Staphylococcus aureus.

Methicillin-resistant Staphylococcus aureus (MRSA) is an important pathogen that causes healthcare-, community-, and livestock-associated infections. The methicillin resistance gene mecA is embedded in the mobile genetic element termed Staphylococcal Cassette Chromosome (SCCmec). SCCmec is shared among staphylococci inhabiting human and animal hosts, which are recognized epidemiologically as the genetic reservoir of SCCmec. However, the ability of diverse methicillin-resistant staphylococci (MRS) to serve as SCCmec donors for S. aureus has not been tested experimentally. Here, we investigated the ability of 157 MRS isolates from pets, meat, livestock, and humans to transfer SCCmec to methicillin-sensitive S. aureus strains using a recently developed natural transformation protocol in mixed biofilms. We found that 25 out of 157 isolates were able to transfer SCCmec to S. aureus. The most effective donor species were S. epidermidis (~33% of the tested isolates), S. felis (40%), and S. capitis (30%). Isolates from meat and livestock (collected in Vietnam and Thailand) had lower transfer rates of SCCmec (5% and 3%, respectively), compared to human and pet isolates from Japan (35% and 25%, respectively). The SCCmec transfer depended on site-specific integration/excision mediated by an intact attB site, which is recognized by the SCC recombinase Ccr. Our study experimentally demonstrates the presence of SCCmec donors in our living environments, highlighting the importance of specific staphylococcal species.IMPORTANCEHow MRSA emerges has long been the pivotal question regarding the ever-increasing burden of antimicrobial resistance (AMR) issues for over half a century. Extensive research efforts in bacteriology, epidemiology, genome biology, and healthcare fields have led to the common understanding that SCCmec is transmitted among distinct staphylococcal species. However, global efforts to provide empirical evidence for intercellular SCCmec transmission have yielded limited results. We recently established the mixed-biofilm transformation assay to evaluate intercellular and interspecies SCCmec transmission. This novel assay system allows us to gain insight into the question "How MRSA emerges," and here, we provide the first experimental results about the potential donor species and habitats. This is the first report to show the ability of staphylococci from distinct sources to transfer SCC to S. aureus. Moreover, the new finding of S. felis as an effective donor that is not commensal to humans reinforces the importance of the One Health concept.