Isolation and characterization of two highly insecticidal, endophytic strains of Bacillus thuringiensis

Abstract

Recent discovery of endophytic strains of Bacillus thuringiensis significantly improves the knowledge on its ecology. It also may be a new source for the isolation of insecticidal strains. This report shows the characterization of two endophytic, highly insecticidal strains of B. thuringiensis. Strains LBIT-1250L and LBIT-1251P were isolated from lavender and Poinsettia sap, respectively. Their parasporal crystals were very similar in morphology to those shown by serotypes israelensis and kurstaki, respectively. Bioassays on Aedes aegypti fourth instar larvae and on Manduca sexta first instar larvae, respectively, showed significantly higher levels of toxicity than those of their standard counterparts, IPS-82 (israelensis) and HD-1 (kurstaki) strains, respectively. Characterization of both strains included the sequencing of flagellin (hag) gene, plasmid and Bc Rep-PCR patterns, and crystal protein content. All four characterization features indicated that LBIT1250L is highly related to the IPS-82 standard (serotype H-14: israelensis); while the LBIT-1251P was highly related to the HD-1 standard (serotype H-3a3b3c kurstaki). These results indicate that endophytic strains of B. thuringiensis may be a new source of potential insecticidal strains and opens more in-depth studies about the role of this bacterium in such a specialized habitat.

Bacillus thuringiensis as a biofertilizer in crops and their implications in the control of phytopathogens and insect pests

“…However, the toxicity of Bt acting as an endophyte/rhizospheric bacterium is not well characterized. The toxicity results reported to date of Bt associated with plants, corroborate that Bt can be toxic to different kinds of phytopathogens (fungi, bacteria, viruses and oomycetes) and predators (insects, nematodes) 15,16,18,31–38,40,41,43–48,65 (Fig. 1).…”

“…Few reports are published that provide toxicity of isolated Bt strains from the plants naturally colonized 39,41 or artificially inoculated 5,40 with Bt isolates. To date, three studies 5,39,41 have reported toxicity of Bt isolated from colonized (natural 39 and artificial 5,41 bacterial colonization) plants of cotton, lavender, poinsettia and Arabidopsis thaliana . Monnerat et al 39 .…”

“…Monnerat et al 39 . and García‐Suárez et al 41 . performed toxicity assays after the Bt strains were isolated from plants.…”

“…Monnerat et al 39 . and García‐Suárez et al 41 . do not indicate the mortality of the respective pests in the Bt‐inoculated plants (and comparison of dose rates may be difficult when plant tissue containing endophytes is used).…”

“…Endophytic bacteria exist inside the plant tissues and this gives them an ability to contact with the plant's cells continually and to influence directly the plant host's metabolism 10–12 . Several studies have reported that rhizospheric/endophytic Bt isolates can stimulate both plant growth 13–30 and resistance against pathogens and pests 16,31–48 . Endophytic locations may also be advantageous since the toxicity of the Bt strains is affected by ultraviolet (UV) light (toxin inactivation) and flushing away of spores by precipitation (toxin washing) 49–51 .…”