Genetic changes associated with tigecycline resistance in Staphylococcus aureus in vitro-selected mutants belonging to different lineages.

Tigecycline (TGC) resistance remains rare in Staphylococcus aureus worldwide. In this study, 12 TGC-resistant S. aureus mutants (TRSAm) were obtained displaying an increase in efflux activity. The isolates belonged to seven different genetic lineages, with a predominance of clonal complex 5 (CC5). Diverse genetic changes in mepA and mepR genes were found producing alterations in the amino acid sequences of the corresponding proteins (MepA and MepR, respectively). The most frequent amino acid change in MepA was Glu287Gly. All of the TRSAm exhibited different single nucleotide polymorphisms (SNPs) or insertions/deletions (InDels) in mepR causing premature stop codons or amino acid changes in MepR. Expression of mepA was significantly increased in TRSAm with different mutations in mepA and mepR. Of the 12 TRSAm, 6 also harboured mutations in rpsJ that resulted in amino acid changes in the S10 ribosomal protein, with Lys57 being the most frequently mutated site. Our findings demonstrate that these acquired mechanisms of TGC resistance are not restricted to a single type of genotypic background and that different lineages might have the same plasticity to develop TGC resistance. The impact of TGC selective pressure assessed by whole-genome sequencing in four selected strain pairs revealed mutations in other singular genes and IS256 mobilisation.