In VitroStudies Indicate a High Resistance Potential for the Lantibiotic Nisin in Staphylococcus aureus and Define a Genetic Basis for Nisin Resistance

Abstract

ABSTRACTLantibiotics such as nisin (NIS) are peptide antibiotics that may have a role in the chemotherapy of bacterial infections. A perceived benefit of lantibiotics for clinical use is their low propensity to select resistance, although detailed resistance studies with relevant bacterial pathogens are lacking. Here we examined the development of resistance to NIS inStaphylococcus aureus, establishing that mutants, including small-colony variants, exhibiting substantial (4- to 32-fold) reductions in NIS susceptibility could be selected readily. Comparative genome sequencing of a single NISrmutant exhibiting a 32-fold increase in NIS MIC revealed the presence of only two mutations, leading to the substitutions V229G in the purine operon repressor, PurR, and A208E in an uncharacterized protein encoded by SAOUHSC_02955. Independently selected NISrmutants also harbored mutations in the genes encoding these products. Reintroduction of these mutations into theS. aureuschromosome alone and in combination revealed that SAOUHSC_02955(A208E) made the primary contribution to the resistance phenotype, conferring up to a 16-fold decrease in NIS susceptibility. Bioinformatic analyses suggested that this gene encodes a sensor histidine kinase, leading us to designate it “nisin susceptibility-associated sensor (nsaS).” Doubling-time determinations and mixed-culture competition assays between NISrand NISsstrains indicated that NIS resistance had little impact on bacterial fitness, and resistance was stable in the absence of selection. The apparent ease with whichS. aureuscan develop and maintain NIS resistancein vitrosuggests that resistance to NIS and other lantibiotics with similar modes of action would arise in the clinic if these agents are employed as chemotherapeutic drugs.