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Discrepancy in antimicrobial susceptibility test results obtained for oral streptococci with the Etest and agar
Eiman M Mokaddas1, Nathaneal O Salako, Leeba Philip
1Department of Microbiology, Faculty of Medicine, Kuwait University, P.O. Box 24923, Safat 13110, Kuwait. e.mokaddas@hsc.edu.kw
Abstract:
A total of 270 viridans group streptococci (VS) isolated from healthy children, identified to the species level, were tested for their susceptibilities to penicillin, imipenem, erythromycin, and vancomycin. A total of 270 isolates and 1,080 organism-antibiotic combinations were evaluated. The overall susceptibility rates of all isolates obtained by the Etest (ET) versus agar dilution (AD) were 60.4% versus 61.8% for penicillin, 63.8% versus 63.9% for erythromycin, 90.6% versus 96% for vancomycin, and 99.1% versus 96.0% for imipenem, respectively. Major discrepancies occurred in the testing of the susceptibility of Streptococcus mutans to vancomycin, with 59.5% (ET) versus 100% (AD), followed by S. salivarius, with 84.1% versus 100%; S. oralis, with 82.1% versus 96.4%; and S. mitis, with 90% versus 100%, respectively. There were also differences in the rates of susceptibility of S. mutans, 66.5% (ET) versus 85.1% (AD), and S. intermedius, 82.9% versus 72.1%, respectively, to penicillin. General agreement between the results of ET and AD was obtained for 973 organism-antibiotic combinations out of 1,080 antibiotic combinations, i.e., 90.1%. Very major errors were found for 6.8% of isolates, and major errors were found for 3.2% of isolates; the minor errors were negligible. Agreement between the results of the two methods was 98.7% for penicillin, 94.6% for vancomycin, 96.9% for imipenem, and 99.9% for erythromycin. The highest rate of very major errors was for vancomycin, at 5.4%. The ET appears to be as efficient as AD for susceptibility testing of VS, except for vancomycin, where very major errors in the results were relatively high.
Insights
This study compared Etest (ET) and agar dilution (AD) for antibiotic susceptibility testing of viridans group streptococci (VS) in children. While both methods showed high agreement overall, Etest had significant errors for vancomycin susceptibility testing.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Antimicrobial Resistance
Background:
- Viridans group streptococci (VS) are common commensals in the human oropharynx.
- Accurate antibiotic susceptibility testing (AST) is crucial for guiding treatment, especially with rising antimicrobial resistance.
- Comparing different AST methods ensures reliable diagnostic results.
Purpose of the Study:
- To evaluate and compare the performance of Etest (ET) and agar dilution (AD) for antimicrobial susceptibility testing of VS.
- To identify discrepancies and potential errors between the two AST methods for specific antibiotics and bacterial species.
Main Methods:
- 270 viridans group streptococci isolates from healthy children were tested.
- Susceptibility to penicillin, imipenem, erythromycin, and vancomycin was determined using both Etest and agar dilution methods.
- Agreement and error rates (very major, major, minor) between the two methods were analyzed.
Main Results:
- Overall agreement between ET and AD was 90.1% across 1080 organism-antibiotic combinations.
- High agreement was observed for penicillin (98.7%), vancomycin (94.6%), imipenem (96.9%), and erythromycin (99.9%).
- Significant discrepancies, particularly very major errors, were noted for vancomycin susceptibility testing of several VS species (e.g., S. mutans, S. salivarius, S. oralis, S. mitis).
Conclusions:
- Etest is generally comparable to agar dilution for AST of VS.
- However, Etest demonstrates a higher rate of very major errors for vancomycin susceptibility testing, necessitating caution.
- Further investigation into vancomycin testing discrepancies with Etest is warranted for reliable clinical interpretation.
