Bacterial antibiotic resistance is often associated with a fitness cost in the absence of the antibiotic  and . We have examined a resistance mechanism in Staphylococcus aureus that negates these costs. Exposure to gentamicin both in vitro and in vivo has been reported to result in the emergence of a gentamicin-resistant small colony variant (SCV) , , , ,  and . We show that the emergence of SCVs following exposure to gentamicin results from a rapid switch and that bacteria exposed to cycles of gentamicin followed by antibiotic-free medium repeatedly switched between a resistant SCV and a sensitive parental phenotype (revertants). The fitness of revertants relative to S. aureus with stable gentamicin resistance was greater in drug-free media, which suggests that S. aureus has evolved an inducible and reversible resistance mechanism that circumvents a permanent cost to fitness.
Massey, R. C., Buckling, A., & Peacock, S. J. (2001). Phenotypic switching of antibiotic resistance circumvents permanent costs in Staphylococcus aureus. Current Biology, 11(22), 1810-1814. https://doi.org/10.1016/S0960-9822(01)00507-3