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[False sensitivity of Enterococcus faecium to amoxicillin-clavulanic acid with the MicroScan system]
P Joyanes1, A Pascual, A I Suárez
1Departamento de Microbiología, Hospital Universitario Virgen Macarena, Sevilla.
Background:
We have observed by using MicroScan automatic system discrepancies in susceptibility results of Enterococcus faecium strains to ampicillin and amoxicillin/clavulanic acid.
Methods And Results:
Seventy-six strains of E. faecium were studied with MicroScan; 98.7% of them were inhibited by 16 mg/l of amoxycillin-clavulanic acid versus 60.5% inhibited by the same concentration of ampicillin. We have evaluated the susceptibility to ampicillin, amoxicillin and amoxicillin-clavulanic acid of 7 strains of E. faecium by both MicroScan and standard microdilution assay. MIC values of ampicillin were similar by both methods but MIC values of amoxicillin/clavulanic acid obtained by MicroScan (< or = 4/2 mg/l in 6 strains) were considerably lower than those obtained by standard microdilution (> or = 16/8 in 6 strains). This phenomenon was not dependent of betalactamase production or bacterial inocula.
Conclusions:
When using MicroScan, E. faecium strains resistant to ampicillin (betalactamase non producers) must be also considered resistant to amoxicillin/clavulanic acid without considering the values obtained by this system.
Insights
MicroScan system shows discrepancies in Enterococcus faecium susceptibility testing. For ampicillin-resistant strains, consider them resistant to amoxicillin-clavulanic acid, disregarding MicroScan results.
Area of Science:
- Clinical microbiology
- Antimicrobial susceptibility testing
Background:
- Discrepancies observed with the MicroScan system for Enterococcus faecium susceptibility to ampicillin and amoxicillin/clavulanic acid.
- Enterococcus faecium is an important human pathogen.
Purpose of the Study:
- To evaluate discrepancies in amoxicillin/clavulanic acid susceptibility testing of Enterococcus faecium using the MicroScan system compared to standard methods.
- To determine the reliability of MicroScan for predicting amoxicillin/clavulanic acid resistance in E. faecium.
Main Methods:
- Susceptibility testing of 76 Enterococcus faecium strains using the MicroScan automated system.
- Comparison of MicroScan results with standard microdilution assays for ampicillin, amoxicillin, and amoxicillin/clavulanic acid.
- Evaluation of the impact of bacterial inoculum and betalactamase production on MIC values.
Main Results:
- MicroScan identified 98.7% of E. faecium strains as inhibited by amoxicillin-clavulanic acid (16 mg/l) versus 60.5% for ampicillin.
- MIC values for amoxicillin/clavulanic acid were significantly lower with MicroScan (< or = 4/2 mg/l) compared to standard microdilution (> or = 16/8 mg/l) for most strains.
- The observed discrepancies were independent of betalactamase production or bacterial inoculum size.
Conclusions:
- The MicroScan system may underestimate amoxicillin-clavulanic acid resistance in Enterococcus faecium.
- Enterococcus faecium strains found resistant to ampicillin (non-betalactamase producers) should be considered resistant to amoxicillin-clavulanic acid, irrespective of MicroScan-generated values.
- Clinical laboratories should exercise caution when interpreting MicroScan results for amoxicillin-clavulanic acid susceptibility in E. faecium.