The involvement of microbial interactions in natural transformation of bacteria was evaluated using an aquatic model system. For this purpose, the naturally transformable Bacillus subtilis was used as the model bacterium which was co-cultivated with the protist Tetrahymena thermophila (a consumer) and/or the hotosynthetic alga Euglena gracilis (a producer). Co-cultivation with as few as 10E+2 individuals ml-1 of T. thermophila lowered the number of transformants to less than the detectable level (<1x100ml-1), while co-cultivaton with E. gracilis did not.Metabolites from co-cultures of T. thermophila and B. subtilis also decreased the number of transformants to less than the detectable level, while metabolites from co-culture of T. thermophila and B. subtilis with E. gracilis did not. Thus, the introduction of transformation inhibitory factor(s) by the grazing of T. thermophila and the attenuation of this inhibitory factor(s) by E. gracilis is indicated. These observations suggest that biological components do affect the natural transformation of B. subtilis . The study described is the first to suggest that ecological interactions are responsible not only for the carbon and energy cycles, but also for the processes governing horizontal transfer of genes, in microbial ecosystems
