Chromosomal origin of acetyltransferase AAC(6') specifying amikacin resistance in Serratia marcescens.

Two clinical isolates of Serratia marcescens resistant to aminoglycoside-aminocyclitols and other antibiotics have been examined for aminoglycoside-modifying enzymes. Both strains were amikacin-resistant, and this resistance was mediated by an acetyltransferase AAC(6'). S. marcescens 737 contains a single conjugative plasmid, pUZ 737, of 135 kilobases, which confers resistance to gentamicin and tobramycin by a nucleotidyltransferase, ANT(2"), and to kanamycin, neomycin, butirosin and lividomycin by a phosphotransferase, APH(3'). S. marcescens 1830 does not contain extrachromosomal DNA, and it produced only the above mentioned AAC(6'). The presence of AAC(6') and associated aminoglycoside resistance are not dependent on the presence of a detectable plasmid, not transferred by conjugation, and not cured. Therefore, this enzyme is probably encoded by a chromosomal gene.