SCCmectransformation requires living donor cells in mixed biofilms

Abstract

AbstractMethicillin-resistantStaphylococcus aureus(MRSA) is an important human pathogen that has emerged through the horizontal acquisition of the staphylococcus cassette chromosomemec(SCCmec). Previously, we showed that SCCmecfrom 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 SCCmecwith frequencies up to 10-2using co-cultured biofilms with living donor cells. The Ccr-attB system played an important role in SCCmectransfer, and the deletion ofccrABrecombinase genes reduced the frequency ∼30-fold. SCCmeccould be transferred from either MRSA or methicillin-resistant coagulase-negative staphylococci to some of methicillin-sensitiveS. aureusrecipients. 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 ofS. aureusand in MRSA emergence.ImportanceMethicillin-resistantStaphylococcus 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 genemecA, which is harboured within the staphylococcal cassette chromosome (SCCmec). Our study sheds light on a noteworthy discovery: when methicillin-sensitiveS. aureusrecipient cells are grown alongside viable methicillin-resistant donor cells in co-cultured biofilms, a highly efficient transfer of SCCmecoccurs, 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.