Forty amikacin-resistant strains of Serratia marcescens isolated from four different hospitals (A, B, C, and D) were examined for modifying enzymes and plasmids. Twenty-one of the isolates produced acetyltransferase that modified amikacin. Eighteen of the 21 acetyltransferase-bearing isolates were from different inpatients in hospital A and the other three were from hospital C. Amikacin resistance was mediated by conjugative plasmid of 24 megadaltons in 15 of the 18 acetyltransferase-bearing isolates of hospital A and by nonconjugative plasmids, derivatives of the 24-megadalton plasmids, in the remaining three isolates of the same hospital. The 24-megadalton plasmid determined aminoglycoside acetyltransferase (6') IV. This plasmid-borne enzyme conferred amikacin resistance on S. marcescens but not on Escherichia coli K12. The frequency of transfer of the 24-megadalton plasmid from the S. marcescens isolate to E. coli K12 by conjugation was approximately 10-7 (transconjugants/donors) and was 0.1 % of that between E. coli strains. In acetyltransferase-bearing isolates from hospital C, the enzyme was mediated by a nonconjugative plasmid in one case and could not be associated with a plasmid in the remaining two cases. Neither enzymes nor plasmids could be associated with amikacin resistance of the isolates of the other two hospitals.Serratia marcescens is primarily a hospital-acquired pathogen that exhibits resistance to various antibiotics including aminoglycoside antibiotics (AGs) (8, 12,18,29). Resistance to gentamicin, tobramycin, dibekacin, and kanamycin is known to be mediated by modifying enzymes encoded by a conjugative R plasmid (7,8,13,14).During the last 7 years, we continuously surveyed the conjugal transfer of AGs resistance from hospital isolates of S. marcescens to E. coli K 12 and obtained the preliminary information that transferable amikacin (AMK) resistance, though rare, was present (15,17). To confirm the presence of transferable AMK resistance in our isolates, we examined the strains for the modifying enzyme and plasmids by a molecular epidemiological procedure. As a result, it was determined that a conjugative R plasmid mediating AAC(6')IV and some nonconjugative plasmids were associated with AMK resistance.

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