Effects of environment on compensatory mutations to ameliorate costs of antibiotic resistance

Most types of antibiotic resistance impose a biological cost on bacterial f itness. These costs can be compensated, usually without Loss of resistance, by second-site mutations during the evolution of the resistant bacteria in an experimental host or in a Laboratory medium. Different fitness-compensa ting mutations were selected depending on whether the bacteria evolved thro ugh serial passage in mice or in a Laboratory medium. This difference in mu tation spectra was caused by either a growth condition-specific formation o r selection of the compensated mutants. These results suggest that bacteria l evolution to reduce the costs of antibiotic resistance can take different trajectories within and outside a host.