Toxicity of Xanthene Dyes to Entomopathogenic Fungi

Effects of xanthene dyes on mycelial growth and conidial germination in three species of entomopathogenic fungi, Beauveria bassiana, Metarhizium anispoliae and Paecilomyces fumosoroseus, were evaluated in a variety of assay systems. In a disk-diffusion assay, erythrosin B and phloxine B (but not eosin B) produced zones of inhibition in colonies of all three species under continuous exposure to light at disk-loadings of 100mug. None of the dyes produced zones of inhibition in the absence of light at disk loadings of 100mug. Both erythrosin B and phloxine B inhibited mycelial growth of all three species in the light in a dose-dependent manner. Weaker dose-responses for inhibition of growth in the dark were observed for some fungus/dye combinations. Erythrosin B, tested singly, completely inhibited conidial germination in the light in all eight fungal strains tested at 100mug ml-1 medium, but failed to inhibit conidial germination in any of these strains in the dark at the same concentration of dye. For single strains of each of the three fungi, erythrosin B and phloxine B inhibited conidial germination in a dose-dependent manner in the light with IC50s < 6.2mug dye ml-1 medium for all fungus/dye combinations. Phloxine B was a more potent inhibitor of germination than erythrosin B for all three fungal species. At fixed dosages of erythrosin B and phloxine B, inhibition of conidial germination in all three species increased with time of exposure to light. These results constitute the first quantitative demonstration of photodynamic inhibition of conidial germination in fungi by xanthene dyes.