Natural occurrence of Bacillus thuringiensis and Bacillus cereus in eukaryotic organisms: a case for symbiosis

Bacillus thuringiensis and Bacillus cereus, two members of the Bacillus cereus sensu lato group, are most noted for their virulence in, respectively, arthropods and mammals including humans. Because of their pathogenicity to insects in particular, and their narrow host range, strains of B. thuringiensis have been utilised successfully as biocontrol agents of insect pests. Whereas B. cereus is not an established entomopathogen, certain strains of this species are well known to be etiological agents of gastrointestinal and emetic syndromes in humans. While much is known about the taxonomic properties and molecular basis for virulence of B. thuringiensis and B. cereus, comparatively less is known about their ecology in natural environments. Thus, there are limited data regarding their resilience, i.e. recycling of vegetative and sporulated phases of growth in soil, ecolgical niches including symbiotic interactions with other organisms, and the impact on ecosystems in which they proliferate. Nevertheless, based on recent data, a picture is beginning to emerge that B. thuringiensis and B. cereus are capable of establishing mutual and commensal relationships with both animals and plants. In this regard, these bacilli can proliferate in the digestive tracts of animals, where upon defecation they form dormant spores in the soil, and to a lesser extent on the phylloplane and rhizospheres of plants. Altogether, current evidence strongly suggests that B. thuringiensis and B. cereus are capable of completing their life cycles and recycling through various reservoirs, including animals, plants, and soil. This review focuses on the current knowledge pertaining to the ecologic interactions between B. thuringiensis and B. cereus and eukaryotic hosts, with special emphasis on symbiosis.