Resistance of selected saprobic and zoopathogenic fungi to cycloheximide

Spore germination was used as an assay to measure the sensitivities of selected fungi (Achlya bisexualis, Cladosporium sp., Trichophyton mentagrophytes and Microsporum gypseum) to cyclohexamide and to determine their abilities to adapt to the drug. Two patterns of response were noted. The saprobes, A. bisexualis and Cladosporium sp., demonstrated acquired resistance. Spores from hyphae previously exposed to cycloheximide either germinated in the presence of concentrations of the drug that completely inhibited spores from unexposed hyphae (Achlya), or germinates with a shorter lag and to a greater extent in the presence of the antibiotic than did spores from unexposed hyphae (Cladosporium). Hyphae of Achlya adapted at concentrations of cycloheximide in which spores did not germinate and hyphae of Cladosporium adapted more rapidly than spores. Achlya adapted to only 12 muM-cycloheximide whereas Cladosporium acquired resistance to 18 mM-cycloheximide. These fungi lost this acquired resistance after a single transfer to media lacking cycloheximide. The zoopathogens, T. mentagrophytes and M. gypseum, had a contrasting response, indicating constitutive resistance. Conidia from unexposed hyphae showed 90 to 100% germination on media containing up to 18 mM-cycloheximide; prior exposure to the drug did not affect their response.