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Selection of netilmicin resistance, associated with increased 6' aminoglycoside acetyltransferase activity, in
1Department of Microbiology, University of Leeds, UK.
Abstract:
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.
Insights
Exposure to netilmicin increased resistance in Serratia marcescens strains. This antibiotic resistance was linked to higher production of the enzyme aminoglycoside acetyltransferase (AAC 6´-I).
Area of Science:
- Microbiology
- Molecular Biology
- Antimicrobial Resistance
Background:
- Serratia marcescens is an opportunistic pathogen.
- Aminoglycosides are crucial antibiotics for treating Gram-negative infections.
- Understanding resistance mechanisms is vital for effective treatment.
Purpose of the Study:
- To investigate the development of netilmicin resistance in Serratia marcescens.
- To determine the role of aminoglycoside acetyltransferase activity in resistance.
- To identify the specific enzyme involved in netilmicin resistance.
Main Methods:
- Serial passage of Serratia marcescens strains in increasing sub-inhibitory concentrations of netilmicin.
- Determination of minimum inhibitory concentrations (MICs) for netilmicin.
- Assay of bacterial aminoglycoside acetyltransferase activity.
- Enzyme identification using two distinct methods.
Main Results:
- All eight clinical strains developed increased resistance to netilmicin after serial passage.
- Six strains exhibited significant increases in aminoglycoside acetylating activity correlating with resistance.
- The enzyme aminoglycoside acetyltransferase (AAC 6´-I) was identified as responsible for the resistance.
- Highly resistant derivatives maintained resistance and enzyme production after antibiotic withdrawal.
Conclusions:
- Exposure to netilmicin can induce high-level resistance in Serratia marcescens.
- The production of aminoglycoside acetyltransferase (AAC 6´-I) is a key mechanism for this resistance.
- Findings have implications for understanding and combating aminoglycoside resistance in clinical settings.