Improvement the media culture for spore/crystal production of Bacillus thuringiensis subsp. kurstaki and evaluate their virulence against second larval instar of Ephestia kuehniella L.

In the present study, the pathogenicity of spores/crystals produced byBacillus thuringiensis subsp. kurstaki (HD-1) in different culture media within fermented and non fermented conditions were determined against the second larval instar of Ephestia kuehniella L. The colony forming unit (CFU), spore count, optical density (OD) and cell mass indices in enriched and non-enriched liquid media of fermented condition after 96 h cultivation were (28.2×‌1014 CFU.mL-1, 11.8×‌1010 spore.mL-1, 28.0×‌108 mg.mL-1 and 11.0 g.L-1) and (26.6×‌1014 CFU.mL-1, 10.0×‌1010 spore.mL-1, 24.4×‌108 mg.mL-1 and 9.9 g.L-1), respectively while those were found to be (18.1×‌1014 CFU.mL-1, 8.1×‌1010 spore.mL-1, 16.4×‌108 mg.mL-1 and 8.9 g.L-1) and (16.8×‌1014 CFU.mL-1, 7.4 ×‌1010 spore.mL -1, 15.6×‌108 mg.mL-1 and 7.9 g.L-1) in non-fermented condition, respectively. The LC50 value of Btk spore/crystal cultured on enriched and non-enriched liquid media for 96h cultivation in fermented condition after 2 days treatment against larval instar were obtained (0.9 and 3.0) and in non-fermented condition (1.5 and 4.2) µg.mL-1, respectively.The LT50 value in fermented condition (6.2 and 10.7) and in non-fermented condition (7.2 and 12.6) h were, respectively. Bioassay studies indicated that the Btk spore/crystal suspention cultured on enriched and non-enriched media in fermented condition for 96h cultivation after 2 days treatment had (61.0 and 46.6-%) and in non-fermented condition (53.3 and 38.6%) mortality, respectively. The results confirm that enriched culture medium in fermented conditions produced the highest amount of spores and crystals and thus increased bacterial toxicity on the second larval instar of E. kuehniella after treatment.