Flow cytometric quantitation of yeast a novel technique for use in animal model work and in vitro immunologic assays

Animal models of fungal and other infectious diseases often require that the number of organisms in tissue be quantified, traditionally by grinding organs, plating them on agar and counting colony forming units (CFU). This method is labor intensive, slow as some fungi require two weeks of culture and limited in reliability by poor plating efficiency. To circumvent these problems, we developed a flow cytometric method to quantify yeast. In vitro cultured Blastomyces dermatitidis, Cryptococcus neoformans, Candida albicans and Histoplasma capsulatum yeast were labelled with specific monoclonal or polyclonal antibodies to stain surface determinants or with Calcofluor to stain cell-wall chitin. A defined number of fluorescently labelled beads were added prior to acquisition by flow cytometry as a reference standard for quantitation. Beads were readily distinguished from yeast by forward scatter, side scatter and intensity of fluorescence. Cultured yeast were enumerated by both standard CFU determination and flow cytometry in a range of 10(2) to 10(7) cells. Only flow cytometry enabled discrimination of live and dead yeast by using appropriate fluorescent dyes. The flow cytometric method was applied to murine models of histoplasmosis and blastomycosis to quantify the burden of fungi in the lungs of infected mice. Labelling yeast with Calcofluor alone resulted in unacceptably high levels of nonspecific binding to mouse cell debris. In contrast, labelling H. capsulatum with a rabbit polyclonal antiserum and B. dermatitidis with a monoclonal antibody to the surface protein WI-1 permitted accurate quantitation. We conclude that this flow cytometry technique is rapid, efficient and reliable for quantifying the burden of infection in animal models of fungal disease. The technique also should lend itself to performing cytotoxicity assays that require discrimination of live and dead fungi, or phagocytosis assays that require discrimination of intracellular and extracellular organisms.