The plcR regulon is involved in the opportunistic properties of Bacillus thuringiensis and Bacillus cereus in mice and insects

Bacillus thuringiensis has been widely used for 40 years as ii safe biopest icide for controlling agricultural pests and mosquitoes because it produces insecticidal crystal proteins. However, spores have also been shown to con tribute to overall entomopathogenicity. Here. the opportunistic properties of acrystalliferous B. thuringiensis Cry(-) and Bacillus cereus strains wer e investigated in an insect species, Galleria mellonella, and in a mammal, BALB/c mice. In both animal models, the pathogenicity of the two bacterial species was similar. Mutant strains were constructed in which the plcR gene , encoding a pleiotropic regulator of extracellular factors, was disrupted. In larvae, co-ingestion of 10(6) spores of the parental strain with a subl ethal concentration of Cry1C toxin caused 70% mortality whereas only 7% mor tality was recorded if spores of the Delta plcR mutant strain were used. In mice, nasal instillation of 10(8) spores of the parental strain caused '11 00% mortality whereas instillation with the same number of Delta plcR strai n spores caused much lower or no mortality. Similar effects were obtained i f vegetative cells were used instead of spores. The cause of death is unkno wn and is unlikely to be due to actual growth of the bacteria in mice. The lesions caused by B. thuringiensis supernatant in infected mice suggested t hat haemolytic toxins were involved. The cytolytic properties of strains of a. thuringiensis and B. cereus, using sheep, horse and human erythrocytes and C. mellonella haemocytes, were therefore investigated. The level of cyt olytic activity is highly reduced in Delta plcR strains. Together, the resu lts indicate that the pathogenicity of B. thuringiensis strain 407 and B. c ereus strain ATCC 14579 is controlled by PlcR.