Use of genetically modified bakers yeast in high sensitivity screening of antifungal and multidrug resistance modulatory activities in extracts of plants used in traditional medicine

The majority of human fungal infections are caused by two yeast species Candida albicans and Candida glabrata, with the prevalence rate of about 70% and 15% respectively. Although less frequent, Candida glabrata infections are difficult to treat and are associated with high mortality rates due to innate increased tolerance towards azole antifungal drugs. Azoles constitute the most important of the limited number of antifungals classes suitable for treatment of systemic infections. The incidence of Candida albicans infections resistant to azoles treatment also increases. The underlying mechanism frequently found in azole resistant strains is the overproduction of efflux transporters Cdr1p and Cdr2p from the ATP binding cassette superfamily that actively extrude azoles out of cells. In face of the raising resistance there is an urgent need for the identification of novel antifungal compounds not affected by efflux transporters as well as inhibitors of these proteins able to sensitize resistant strains towards currently available antifungals. Bakers yeast Saccharomyces cerevisiae due to its well characterized network of genes involved in multidrug resistance, close similarity to pathogenic yeast and ease of genetic manipulation is well suited for antifungal screening purposes. In this study we have screened a panel of 63 crude extracts of plants used in traditional medicine, mainly from South and Southeast African regions, for their ability to inhibit growth of strains showing different levels of expression of endogenous multidrug efflux transporters. We have also used a strain heterologously overproducing Cdr1p to screen for modulators of its efflux activity. Extracts were also assayed against Candida species. Our results indicate the advantage of genetic modulation of activity of drug efflux transporters in the process of antifungal activity screening in natural products.