Effects of Cryptococcus humicola killer toxin upon Cryptococcus terreus envelope: combined fluorometric and microscopic studies.

Killer toxin (microcin) produced by Cryptococcus humicola 9-6 induced interaction of the fluorogenic dyes, ethidium bromide, propidium iodide, and hemimagnesium 8-anilino-1-naphtalenesulfonate, with the sensitive strain of Cryptococcus terreus VKM Y-2253. The toxin also made the cells susceptible to cetyltrimethylammonium bromide and leaky for K+. When excited at 360 nm, cell-bound ethidium (propidium) fluorescence was enhanced by 8-anilino-1-naphtalensulfonate, and cell-bound 8-anilino-1-naphtalensulfonate fluorescence was quenched by ethidium (propidium), indicating energy transfer from 8-anilino-1-naphtalensulfonate to ethidium (propidium). These results suggest that at least a portion of the probe molecules had the same binding site, possibly the cytoplasmic membrane. The parameters of kinetics of microcin action were evaluated fluorometrically. They were found to be identical for all probes and depended on microcin concentration. The fluorescence increment of ethidium and 8-anilino-1-naphtalensulfonate upon binding to microcin-treated cells correlated with the fraction of stainable cells and viability.