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A Reference Broth Microdilution Method for Dalbavancin In Vitro Susceptibility Testing of Bacteria that Grow Aerobically
Published on: September 9, 2015
Manual Reading of Sensititre Broth Microdilution System Panels Improves Accuracy of Susceptibility Reporting for
Michelle M Bellerose1, Andrew E Clark1, Jung-Ho Youn1
1Department of Laboratory Medicine, National Institutes of Health Clinical Centergrid.410305.3, Bethesda, Maryland, USA.
Abstract:
Accurate and reproducible antimicrobial susceptibility testing (AST) of polymyxin antibiotics is critical, as these drugs are last-line therapeutic options for the treatment of multidrug-resistant Gram-negative bacterial infections. However, polymyxin AST in the routine laboratory remains challenging. In this study, we evaluated the performance of an automated broth microdilution (BMD) system (Sensititre, ThermoFisher) compared to that of agar dilution (AD) for colistin and polymyxin B AST of 129 Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii complex clinical isolates. MICs derived from the Sensititre instrument based on two operator comparisons demonstrated overall categorical agreement (CA) of 86% and 89% compared to AD for colistin and 89% and 92% compared to AD for polymyxin B. However, error rates were higher than recommended by CLSI. Manual inspection of microdilution wells revealed microbial growth and skip wells which were erroneously interpreted by the Aris 2X instrument. Using manually interpreted BMD MICs read by two operators increased the overall categorical agreements to 88% and 95% compared to AD for colistin and 92% and 96% compared to AD for polymyxin B. Laboratories choosing to use the Sensititre platform for polymyxin AST should consider manual evaluation of wells as part of their algorithm.
Insights
Accurate polymyxin antibiotic susceptibility testing is crucial for treating resistant infections. Manual review of automated broth microdilution results improves accuracy for colistin and polymyxin B testing.
Area of Science:
- Clinical microbiology
- Antimicrobial resistance
- Pharmacology
Background:
- Polymyxin antibiotics are vital for treating multidrug-resistant Gram-negative bacterial infections.
- Accurate antimicrobial susceptibility testing (AST) for polymyxins is challenging in routine laboratories.
Purpose of the Study:
- To evaluate the performance of an automated broth microdilution (BMD) system (Sensititre) against agar dilution (AD) for colistin and polymyxin B AST.
- To assess the impact of manual interpretation on BMD accuracy.
Main Methods:
- Compared Sensititre BMD with AD for colistin and polymyxin B MIC determination.
- Tested 129 clinical isolates of Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii complex.
- Evaluated automated and manual interpretations of BMD results.
Main Results:
- Automated BMD showed 86-92% categorical agreement with AD for colistin and polymyxin B.
- Error rates exceeded CLSI recommendations.
- Manual interpretation of BMD improved categorical agreement to 88-96%.
Conclusions:
- The Sensititre automated BMD system requires manual well evaluation for reliable polymyxin AST.
- Manual review is recommended to minimize errors in polymyxin susceptibility testing.
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