A SIMPLIFIED NEW ASSAY FOR ASSESSMENT OF FUNGAL CELL-DAMAGE WITH THE TETRAZOLIUM DYE, NITRO-5-SULFENYL)-(2H)-TETRAZOLIUM-5-CARBOXANILIDE (XTT)

Studies of antimycotic host defenses have been limited by the paucity of rapid, reproducible quantitative assays for fungal cell damage. Pri or studies defined a colorimetric method that uses MTT, a tetrazolium dye, to quantify polymorphonuclear leukocyte (PMNL)-mediated damage to fungi. These relatively simple, rapid, and reproducible assays requir e cumbersome extraction of precipitated MTT-formazan and high cell den sities to overcome relatively low sensitivity. In experiments that com pared assays with MTT and another tetrazolium dye, itro-5-sulphenyl)-( 2H)-tetrazolium-5-carboxanilide (XTT), estimates of damage to Aspergil lus fumigatus hyphae by human PMNL were similar with both dyes. Howeve r, XTT reduction was more rapid and sensitive, yielding accurate resul ts with fewer organisms and PMNL. The water-soluble XTT-formazan produ ct also simplified measurements by eliminating the need for solvent-ex traction steps that are obligatory in MTT assays. Thus, XTT is advanta geous for quantitative assessment of fungal cell damage, although MTT remains useful for assessing individual fungal cell viability by direc t microscopic visualization of precipitated formazan.