Selection of netilmicin resistance, associated with increased 6' aminoglycoside acetyltransferase activity, in Serratia marcescens.

Eight aminoglycoside-sensitive clinical strains of Serratia marcescens were serially passaged through broth containing increasing sub-inhibitory concentrations of netilmicin. Before each subculture the aminoglycoside acetyltransferase activity of the bacteria was determined. After up to eight transfers all the strains showed an increase in resistance to netilmicin (MIC greater than 32 mg/l) and MICs for two of the strains exceeded 1000 mg/l. Six of the eight strains showed significant increases in aminoglycoside acetylating activity in accompaniment with the development of resistance. The greatest rises in enzyme activity (eight- and four-fold) were in the strains where the netilmicin MICs for the resistant derivatives exceeded 1000 mg/l. The aminoglycoside acetylating enzyme present was identified as AAC 6'-I by two methods. After five subcultures in antibiotic-free medium, the two derivatives with the highest resistance to netilmicin remained highly resistant and continued to produce AAC 6'-I copiously. These results suggest that exposure of some aminoglycoside sensitive strains of Ser. marcescens to netilmicin leads to the development of high level netilmicin resistance associated with the production of AAC 6'-I. The implication of these findings for the study of aminoglycoside resistance in Ser. marcescens are discussed.