Survey of modifying enzymes and plasmids in amikacin-resistant Serratia marcescens.

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.