Isolation and molecular characterization of Bacillus thuringiensis subsp. kurstaki toxic to lepidopteran pests Spodoptera spp. and Plutella xylostella

Abstract

Background: Bacillus thuringiensis (Bt) has been widely used as a biological control agent for lepidopteran pests. However, resistance to Bt is a major concern associated with Spodoptera spp. (Noctuidae) and Plutella xylostella (Plutellidae). For efficient control of Noctuidae and Plutellidae, novel Bt strains which have high toxicity and a broad host range are needed.Results: To develop novel Bt strains as used for bio-insecticides, the Bt IMBL-B9 with high toxicity against Spodoptera exigua, Spodoptera frugiperda and P. xylostella was isolated and characterized. The Bt kurstaki IMBL-B9 strain produced bipyramidal and cuboidal crystals consisting of cry toxins with molecular weights of 130 and 65 kDa, respectively. This strain harbors eight crystal protein genes in total, including cry1Ea and one vegetative insecticidal protein gene. The median lethal concentration (LC 50 ) values of IMBL-B9 against S. exigua and S. frugiperda were 21.8-and 19.3-fold lower than those of the Bt kusrstaki strain, and 5.6-and 4.9-fold lower than those of Bt aizawai strain, respectively. To evaluate the insecticidal activity of Cry proteins from IMBL-B9, cry gene-sourced recombinant Bt strains were constructed. These strains have insecticidal activity and synergic action against lepidopteran pests. Conclusion:In this study, a novel Bt kurstaki IMBL-B9 strain was isolated and this could be useful for the development of new bio-insecticide or cry gene-based recombinant products as an alternative solution against lepidopterans, including Noctuidae and Plutellidae.

Relative activity of 15 bacterial strains against the larvae of Helicoverpa armigera, Spodoptera exigua, and Spodoptera litura (Lepidoptera: Noctuidae)

“…The toxicities of B6 and P6 strains reported in this study were higher than those of the 2 reported Btk strains. Thus, our study supports previous research and demonstrates that Btk has a high insecticidal effect against S. exigua (Moar et al 1990, Hosseinzadeh et al 2016, Park et al 2022). Furthermore, previous studies have shown the toxicity of commercial Btk (53,000 SU/mg, Delfin, WG) to S. litura , with an LC 50 of 633 g/ml (Devi et al 2021).…”

“…In this study, we evaluated the insecticidal activity of 15 culturable bacterial strains isolated from naturally infected insect cadavers against second-instar larvae of laboratory-reared lepidopteran pests. Currently, Bt strains can be tentatively identified by esterase type, and by physiological and biochemical reactions (Ramya et al 2016, Li et al 2022, Park et al 2022). Most of the isolated strains showed catalase amylohydrolysis and indole properties (Table 2), which are consistent with the physiological and biochemical characteristics of Bt subsp.…”

“…Most of the isolated strains showed catalase amylohydrolysis and indole properties (Table 2), which are consistent with the physiological and biochemical characteristics of Bt subsp. kurstaki strain HD-1 (Ref) (Porcar et al 2000, Porcar et al 2003, Ardahan Eski et al 2018, Gaona Mendoza et al 2021, Park et al 2022). In the present study, 11 general primers previously used for cry gene identification in lepidopteran pests were used.…”

“…Both LB and Nutrient-Broth (NB) are commonly used to culture Bt strains (Jisha et al 2015, Park et al 2022). Before the determination of Cry protein concentration, the LB and NB medium-cultivated Bt strains were compared, and the LB medium was determined as better for the Bt strains to produce δ-endotoxin than the NB medium during our assays (data not shown).…”