SCCmec transformation requires living donor cells in mixed biofilms

Methicillin-resistant Staphylococcus aureus (MRSA) is an important human pathogen that has emerged through the horizontal acquisition of the staphylococcus cassette chromosome mec (SCCmec). Previously, we showed that SCCmec from heat-killed donors can be transferred via natural transformation in biofilms at frequencies of 10-8-10-7. Here, we show an improved transformation assay of SCCmec with frequencies up to 10-2 using co-cultured biofilms with living donor cells. The Ccr-attB system played an important role in SCCmec transfer, and the deletion of ccrAB recombinase genes reduced the frequency [~]30-fold. SCCmec could be transferred from either MRSA or methicillin-resistant coagulase-negative staphylococci to some of methicillin-sensitive S. aureus recipients. In addition, the transformation of other plasmid or chromosomal genes is enhanced by using living donor cells. This study emphasizes the role of natural transformation as an evolutionary ability of S. aureus and in MRSA emergence. ImportanceMethicillin-resistant Staphylococcus aureus (MRSA) stands out as the leading contributor to fatalities attributed to antibiotic-resistant infections. To comprehend its emergence and dissemination, it is crucial to understand the mechanisms behind it. MRSA has arisen through the horizontal acquisition of the methicillin resistance gene mecA, which is harboured within the staphylococcal cassette chromosome (SCCmec). Our study sheds light on a noteworthy discovery: when methicillin-sensitive S. aureus recipient cells are grown alongside viable methicillin-resistant donor cells in co-cultured biofilms, a highly efficient transfer of SCCmec occurs, both within the same species and between different species. This remarkable transfer process is facilitated by natural transformation, underscoring its pivotal role in the evolution of staphylococci and the emergence of MRSA.