Influence of IS on Genome Variability and Formation of Small-Colony Variants in Staphylococcus aureus.

has acquired resistance to nearly all antibiotics used in clinical practice. Whereas some resistance mechanisms are conferred by uptake of resistance genes, others evolve by mutation. In this study, IS has been shown to play a role, e.g., in strains displaying intermediate resistance to vancomycin (VISA). To characterize the IS insertion sites in the genomes of two closely related sequence type 247 (ST247) VISA strains, all insertions were mapped in both VISA and a susceptible control strain. The results showed that the three ST247 strains contained the highest number so far of IS insertions for all sequenced strains. Furthermore, in contrast to the case with the other IS elements in these genomes, the IS insertion sites were not identical in the closely related strains, indicating a high transposition frequency of IS When IS was introduced into a laboratory strain which was then cultured in the presence of antibiotics, it was possible to isolate small-colony variants (SCVs) that possessed IS insertions in and that displayed increased resistance to vancomycin and aminoglycosides, respectively. For these clones, a very rapid reversion to the wild type that resembled the fast reversion of clinical SCVs was observed. The reversion was caused by excision of IS in a small number of fast-growing clones that quickly outcompeted the SCVs in broth cultures. In conclusion, the presence of IS confers a strong genomic plasticity that is useful for adaptation to antibiotic stress.