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 (100 and 200 µL) to those in 384-well plates with reduced volumes (30 and 50 µL) for a range of ATCC gram-negative and gram-positive bacteria, as well as yeast species. Our results demonstrate that, except for micafungin against yeast, MIC values obtained with reduced volumes remained within the acceptable variability ranges defined by EUCAST and CLSI. The consistency of the MIC evaluation in 30 µL final volume was then confirmed against a selection of gram-negative 30 clinical isolates. 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. This miniaturization approach provides a cost-effective and high-throughput alternative for antimicrobial susceptibility testing, ensuring accuracy and reproducibility and is particularly advantageous in research settings where compound availability is limited or associated with high costs.