Frozen storage stability of pre-diluted colistin microdilution plates for routine susceptibility testing
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Abstract
Broth microdilution (BMD) is the reference method recommended by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) for determining colistin susceptibility testing; however, its routine implementation in clinical laboratories remains challenging due to factors such as storage conditions and technical variability. To provide a more practical and standardized approach, this study evaluated the stability of pre-diluted colistin stored at -20°C in a 96-well microdilution plate format. Serial twofold dilutions of colistin in Mueller-Hinton broth were prepared, frozen, and tested at 15-day intervals over 90 days using Klebsiella pneumoniae (colistin-resistant clinical isolate), Acinetobacter baumannii (colistinintermediate isolate), and Pseudomonas aeruginosa ATCC27853 (quality control strain). Minimum inhibitory concentrations (MICs) were determined according to CLSI guidelines to monitor changes in antimicrobial activity during storage. The MIC values of all strains remained consistent throughout the 90-day period, indicating that colistin retained its biological activity under frozen storage. These findings demonstrate that colistin microdilution plates stored at -20 °C remain stable and reliable for at least three months, offering laboratories a convenient and reproducible method for preparing ready-to-use colistin BMD plates to support standardized susceptibility testing.
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