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High-level resistance to gentamicin in Enterococcus faecium
N Woodford1, E McNamara, E Smyth
1Antibiotic Reference Laboratory, Central Public Health Laboratory, London, UK.
Abstract:
During a six-month period in a hospital in Ireland, four patients were infected (isolation from blood cultures) and two were colonized (isolation from rectal swabs) with strains of Enterococcus faecium highly resistant to gentamicin. MICs of gentamicin were greater than 1000 mg/L for all six strains, and each possessed a plasmid of approximately 50 MDa. Resistance to gentamicin was transferable by conjugation from two of the six strains, and was associated with transfer of the 50 MDa plasmid. This plasmid hybridized with a DNA probe specific for the bifunctional AAC(6')-APH(2") aminoglycoside-modifying enzyme. The mechanism of high-level gentamicin resistance in these strains appeared therefore, to be identical to that encountered in Enterococcus faecalis strains worldwide and reported in E. faecium strains in the USA.
Insights
High-level gentamicin resistance in Enterococcus faecium strains was identified in an Irish hospital. This resistance is linked to a transferable plasmid encoding a specific aminoglycoside-modifying enzyme.
Area of Science:
- Microbiology
- Infectious Diseases
- Molecular Biology
Background:
- Enterococcus faecium is an opportunistic pathogen.
- High-level gentamicin resistance (HLGR) is a growing concern in Gram-positive bacteria.
- Previous reports of HLGR in Enterococcus faecium were primarily from the USA.
Purpose of the Study:
- To investigate the prevalence and mechanism of high-level gentamicin resistance in Enterococcus faecium strains isolated from a hospital in Ireland.
- To characterize the genetic basis of gentamicin resistance in these strains.
Main Methods:
- Six strains of Enterococcus faecium were isolated from patients over a six-month period.
- Minimum Inhibitory Concentrations (MICs) for gentamicin were determined.
- Plasmid DNA was analyzed, and conjugation experiments were performed.
- Plasmid DNA was hybridized with a DNA probe specific for the AAC(6 extprime)-APH(2 extprime extprime) enzyme.
Main Results:
- Four patients were infected and two were colonized with Enterococcus faecium strains exhibiting high-level gentamicin resistance (MICs > 1000 mg/L).
- All six strains possessed a 50 MDa plasmid.
- Gentamicin resistance was transferable via conjugation, associated with the 50 MDa plasmid.
- The plasmid hybridized with a probe for the bifunctional AAC(6 extprime)-APH(2 extprime extprime) aminoglycoside-modifying enzyme.
Conclusions:
- The mechanism of high-level gentamicin resistance in these Irish Enterococcus faecium strains is mediated by the bifunctional AAC(6 extprime)-APH(2 extprime extprime) enzyme.
- This mechanism is identical to that observed in Enterococcus faecalis and previously reported in Enterococcus faecium strains in the USA.
- The findings highlight the spread of gentamicin resistance in Enterococcus faecium.