RNase-mediated defence and its association with SCCmec in Staphylococcus aureus.

The rise of antibiotic resistance in Staphylococcus aureus is a major threat to global health. Resistance often emerges when bacteria acquire foreign DNA elements, such as the SCCmec cassette that makes strains resistant to nearly all β-lactam antibiotics. Yet, not all S. aureus lineages take up these elements equally, raising the question of what prevents some strains from becoming drug-resistant. Here we identify a previously unknown defence factor that explains this mystery in a common skin lineage (ST188), frequently isolated from patients with atopic dermatitis. Such defence factor is encoded by a gene, which we name sadR, that is present in methicillin-susceptible ST188 strains but absent from resistant counterparts. SadR exhibits RNase activity that provides anti-bacteriophage and anti-plasmid defence, thus acting as a barrier to the uptake of mobile genetic elements. Gene sadR is found in methicillin-susceptible strains in S. aureus lineages other than ST188, and is consistently located adjacent to rlmH, the canonical SCCmec integration site. Moreover, SadR homologues are widespread across staphylococci and other bacteria suggesting a previously unrecognised family of RNase-based defence systems.