Determination of Minimum Inhibitory Concentration of Liposomes: A Novel Method

Abstract

For determination of minimum inhibitory concentration (MIC) of antimicrobial agents the Clinical and Laboratory Standards Institute (CLSI) guidelines are considered as the golden standards. Unfortunately, there is no standard guideline for MIC determination of liposomal and nano-formulations loaded with antimicrobial agents in fluid medium. In this study, a new method has been developed and standardized for determining MIC of suspensions, which is the major limitation of MIC determination in fluid media. The new method makes use of the fluorescent dye resazurin for testing MIC. The fluorescence assay was standardized and was found equally valid as that of the conventional method. The CLSI broth microdilution method and the microtiter plate based resazurin assay were evaluated for the liposomal MIC determination. The fluorescence assay was found to be highly feasible, repeatable and cost effective. The assay was found to have high sensitivity, specificity, positive and negative predictive value. Even though the sensitivity and negative predictive value of broth microdilution assay were found good, specificity and positive predictive value were low especially in case of the tested Pseudomonas aeruginosa strain. Hence the new fluorescence assay is a valid test for liposomal MIC determination and can replace the broth microdilution assay.