By screening cultures of soil bacteria, we re-discovered an old antibiotic (nybomycin) as an antibiotic
with a novel feature. Nybomycin is active against quinolone-resistant Staphylococcus aureus strains
with mutated gyrA genes but not against those with intact gyrA genes against which quinolone antibiotics
are effective. Nybomycin-resistant mutant strains were generated from a quinolone-resistant,
nybomycin-susceptible, vancomycin-intermediate S. aureus (VISA) strain Mu50. The mutants, occurring
at an extremely low rate (