Involvement of a transmissible plasmid in heat-stable exotoxin and delta-endotoxin production inBacillus thuringiensis subspeciesdarmstadiensis

A strain ofBacillus thuringiensis subsp.darmstadiensis (serotype 10), which produces heat-stable exotoxin and delta-endotoxin (Exo⁺Cry⁺), was used for curing and conjugation-like transformation experiments. After treatment with sodium dodecyl sulfate, nine independent mutants that lacked exotoxin productivity (Exo^–) were obtained. Agarose gel electrophoresis showed that all Exo^– strains had lost a plasmid, whose size was 62 megadaltons (Mdal). WhenB. thuringiensis was mated with a streptomycin-resistant (Str^r)B. cereus strain, five Exo⁺Str^r transformants that had acquired the 62-Mdal plasmid were isolated. Furthermore, the Cry⁺ phenotype was consistently associated with the Exo⁺ phenotype. These results indicate that a transmissible plasmid is involved in production of both heat-stable exotoxin and delta-endotoxin.