Virulence of Bacillus thuringiensis

Bacillus thuringiensis and B. cereus are genetically and phenotypically indistinguishable, except for the plasmid encoded ability of B. thuringiensis to produce insecticidal parasporal inclusion bodies (Cry toxins). Some B. cereus are known to cause diarrhoeal and emetic symptoms in humans, but also somatic infections have been reported. The pathogenesis of B. cereus is based on a number of virulence factors produced during vegetative- and stationary phases. An increasing number of reports demonstrate the similarity of virulence factors of B. thuringiensis and B. cereus, but there are only few reports on B. thuringiensis involved in human pathogenesis. This lack of reports might either be caused by an real lack of cases, or might be due to diagnostic procedures which do not distinguish between B. cereus and B. thuringiensis. Despite the overall homogeneity of strains within B. thuringiensis and B. cereus, high variations in pathogenic potential are found. Clinical B. cereus isolates tend to show higher toxicity than environmental strains generally. Besides being active in human pathogenesis, these non specific virulence factors also seem to be of important for effective insect pathogenesis. To avoid undesired effects from use of B. thuringiensis, conditions promoting vegetative growth in food products should be avoided, e.g. by introduction of a pre-harvest spray free period- Further, strains used for commercial purposes should be tested for vegetative toxicity in comparison with pathogenic and apathogenic B. cereus. And asprorogenic commercialised Bt could be promoted. However, evaluation of overall risks from using B. thuringiensis should be a holistic process done by comparison of risks and inconvenience from other pest control methods, related to environmental, social and economic parameters.