Factors affecting the magnitude of toxicant interactions in microbial bioassays

A toxicant interaction method was used to study the effects of various bioassay parameters on interaction responses obtained in microbial bioassays. The fungus Pythium ultimum was employed as the test organism, and was exposed to various combinations of the fungicide captan and several organic solvents, using a poisoned agar technique. In all cases the fungicide and solvents interacted synergistically toward culture growth. For most experiments acetone was used as the test solvent. Where pH and temperature were altered, the magnitude of the interaction response between captan and acetone increased dramatically as the pH or temperature was raised from 4.5 to 7.5, or 15 to 30°C, respectively. This corresponded to similar increases in the culture growth rate and decreases in the toxicity of captan. When the medium composition was changed, interaction magnitudes were again greatest in media eliciting the fastest growth rate. These media also yielded the lowest captan toxicity. The largest interaction magnitudes occurred with V8 juice agar, followed by corn meal agar, potato dextrose agar, and malt extract agar. When the solvent used in the interaction experiments was changed, a similar response was obtained, in that the greatest interaction magnitudes occurred in systems eliciting the lowest captan toxicity. The largest magnitudes were measured with acetone, followed by hexane, ethanol, dimethylformamide, methanol, and dimethyl sulfoxide. The significance of these data in toxicant interaction bioassays is discussed.