Acquired nisin resistance in Staphylococcus aureus involves constitutive activation of an intrinsic peptide antibiotic detoxification module

Resistance to the lantibiotic nisin (NIS) arises readily in Staphylococcus aureus as a consequence of mutations in the nsaS gene, which encodes the sensor kinase of the NsaRS two-component regulatory system. Here we present a series of studies to establish how these mutational changes result in reduced NIS susceptibility. Comparative transcriptomic analysis revealed upregulation of the NsaRS regulon in a NIS-resistant mutant of S. aureus versus its otherwise-isogenic progenitor, indicating that NIS resistance mutations prompt gain-of-function in NsaS. Two putative ABC transporters (BraDE and VraDE) encoded within the NsaRS regulon that have been reported to provide a degree of intrinsic protection against NIS were shown to be responsible for acquired NIS resistance; as is the case for intrinsic NIS resistance, NIS detoxification was ultimately mediated by VraDE, with BraDE participating in the signaling cascade underlying VraDE expression. Our study revealed new features of this signal transduction pathway, including that BraDE (but not VraDE) physically interacts with NsaRS. Furthermore, while BraDE has been shown to sense stimuli and signal to NsaS in a process that is contingent upon ATP hydrolysis, we established that this protein complex is also essential for onward transduction of the signal from NsaS through energy-independent means. NIS resistance in S. aureus therefore joins the small number of documented examples in which acquired antimicrobial resistance results from the unmasking of an intrinsic detoxification mechanism through gain-of-function mutation in a regulatory circuit.