Seasonal distribution and characterization of Bacillus thuringiensis isolated from sericultural environments in Korea.

To isolate naturally occurring novel Bacillus thuringiensis strains, we investigated the distribution and characteristics of B. thuringiensis from samples of sericultural farms in various regions of Korea in the spring and fall. Fifty-four B. thuringiensis strains out of 164 samples and 34 B. thuringiensis strains out of 135 samples were isolated in the spring and fall, respectively. Seventy percent of the isolates in the spring and 15% in the fall were toxic to lepidopteran larvae. Dipteran-active isolates were rare (7% in spring and 3% in fall isolation). Particularly, B. thuringiensis isolates, which are toxic to both Lepidoptera and Diptera, were widely distributed (19% in spring and 62% in fall isolation). Non-toxic isolates were also found (4% in spring and 20% in fall isolation). B. thuringiensis isolates in the sericultural farms represented 11 H serotypes; they were principally B. thuringiensis subsp. aizawai in the spring and kurstaki in the fall. B. thuringiensis isolates of serotypes 1, 3a, 3a3b, 4a4c, 6, 7 and 12 were toxic to Lepidoptera. Seventy isolates produced typical rhomboidal inclusions, and the remainder produced parasporal inclusions with various morphologies. PCR analysis using cryI gene type-specific primers showed that cryIAa and cryIC genes are frequently found in the spring and cryIAa gene is a predominant type in the fall. Toxicity, H serotype and the cryI gene contents of B. thuringiensis isolated from sericultural farms showed that distribution varied depending on the season.