a large pool of antibiotic-resistant and auxotrophic mutants from strains ut0002-19 and ut0017 were isolated by using ( i ) antibiotic gradient plates , ( ii ) trimethoprim selection , and ( iii ) nitrosoguanidine mutagenesis , which sometimes was coupled by enrichment with penicillin or methicillin .
C0030842
penicillin
C0025643
methicillin