Determination of MICING: a new assay for assessing minimal inhibitory concentration for invasive growth

Our work was focused on a new assay for characterising clinically important yeast. This assay was developed due to the need for new diagnostic methods for recognising potentially virulent strains of increasingly important non-albicans yeast pathogens, such as Saccharomyces cerevisiae and Candida glabrata. With the great diversity among strains for virulence and virulence factors, identification to the species level is not sufficient; therefore, testing for specific virulent traits remains the best option. We show here that the proposed assay uncovers the relationships between the three most important yeast virulence traits in a single test: the ability of a strain to invade solid medium, while resisting the presence of an antimycotic and high temperature (37 °C). We combined the quantitative agar invasion assay with classical antimycotic susceptibility testing into a single assay. Similarly to the minimal inhibitory concentration (MIC) value, we defined the MICING (minimal inhibitory concentration of antimycotic for invasive growth) as the concentration of an antimycotic above which the yeast invasive growth is significantly repressed. In this study, we tested three of the most common antimycotics: fluconazole, itraconazole and amphotericin B. The response of yeast strains invasion was characteristic of each antimycotic, indicating their mechanisms of action. In addition to MICING, the assay provides quantitative information about the superficial and invasive growth, and also about the relative invasion, which helps in identifying clinically important yeast, such as azole-resistant and/or invasive strains of S. cerevisiae and C. glabrata.