A convenient microdilution method for screening natural products against bacteria and fungi

Candida albicans, Clyptococcus neoformans, Mycobacterium tuberculosis, Pseudomonas aeruginosa, and Staphylococcus aureus (methicillin-susceptible and -resistant strains) are some of the most common pathogens of immunocompromised individuals. Since multi-drug resistance of these microorganisms is a major medical problem, we propose a convenient microdilution method for screening of natural products in a search for new antimicrobial agents that would be active against these organisms. To enable the screening process under standard laboratory conditions, Cr. albidus and M. smegmatis were used as model-organisms in place of their pathogenic counterparts. Antibiotics were used as positive controls, and their MIC values were in agreement with the MIC ranges recommended by the National Committee for Clinical Laboratory Standards. Organic plant extracts from Lemna minor and Ilex cornuta were used for developing the microdilution assay. The method can be used as a reliable tool for discovering antimicrobial agents with novel chemistry. It is adopted for organic extracts at a microgram scale and is currently being used for screens of more than one thousand extracts from the National Cancer Institute Open Repository.