Optimizing Antimicrobial Susceptibility Testing: Cost and Environmental Benefits of MIC Volume Reduction

The determination of Minimum Inhibitory Concentrations (MICs) is essential for evaluating antimicrobial efficacy, guiding both clinical treatment decisions and drug development. The standard broth microdilution method is widely used but requires significant reagent volumes, which can be limiting when working with novel or expensive antimicrobials. This study assesses the feasibility of reducing assay volumes without compromising MIC accuracy. We compared the MIC values obtained in standard 96-well plates (200 µL) to those in 384-well plates with reduced volumes (30–50 µL) for a range of Gram-negative and Gram-positive bacteria, as well as yeast species. Our results demonstrate that except for micafungin, MIC values obtained with reduced volumes remained within the acceptable variability ranges defined by EUCAST and CLSI. Evaporation, a potential source of bias in smaller volumes, was mitigated by conducting experiments in a water-saturated atmosphere. Furthermore, reduced assay volumes significantly lowered material costs and antimicrobial consumption, particularly for expensive drugs such as cefiderocol. This miniaturization approach offers a cost-effective, high-throughput alternative for antimicrobial susceptibility testing while maintaining accuracy and reproducibility.