Characterization of polyvalent and safe Bacillus thuringiensis strains with potential use for biocontrol

Sixteen Bacillus thuringiensis (Bt) strains were screened for their anti-insect, antibacterial and antifungal determinants by phenotypic tests and PCR targeting major insecticidal proteins and complements, chitinases, lactonases, β-1,3-glucanases and zwittermicinA. Six strains had genes of at least two major insecticidal toxins and of insecticidal complements. With regard to fungal biocontrol, all the strains inhibited Fusarium oxysporum and Aspergillus flavus growth and four strains had all or most of the antifungal determinants examined, with strain Bt HD932 showing the widest antifungal activity spectrum. Autolysins, bacteriocin and AHL-lactonases were produced by all or most of the tested strains with different activity spectra including pathogens like Listeria monocytogenes. Safety evaluation was carried out via PCR by screening the B. cereus psychrotolerance-related genes, toxin genes and the virulence pleiotropic regulator plcR. Diarrheal enterotoxins and other toxin genes were widespread among the collection with strains Bt HD9 and H45 lacking psychrotolerance-related genes, while five strains were positive. Only three strains (BMG1.7, H172, H156) resulted positive with primer sets targeting partial or complete plcR gene. By Vero Cell Assays, Bt HD868 followed by Bt HD9 were shown to be the safest strains. These polyvalent and safe Bt strains could be very promising in field application.