Bt GS57 Interaction With Gut Microbiota Accelerates Spodoptera exigua Mortality

Abstract

The Beet armyworm Spodoptera exigua (Lepidoptera: Noctuidae, Spodoptera) is an important global polyphagous pest. Pathogen infection could destroy the intestinal microbial homeostasis of insects, leading to the death of the host. However, the effect of the host intestinal microbial community on the insecticidal effect of Bacillus thuringiensis is rarely studied. In this study, the genome characteristics of Bt GS57 and the diversity and functions of the gut bacteria in S. exigua are investigated using crystal morphology, biological activity, and Illumina HiSeq high-throughput sequencing. The total size of the Bt GS57 genome is 6.17 Mbp with an average G + C content of 35.66%. Furthermore, the Bt GS57 genome contains six cry genes: cry1Ca, cry1Da, cry2Ab, cry9Ea, cry1Ia, and cry1Aa, and a vegetative insecticidal protein gene vip3Aa. The Bt GS57 strain can produce biconical crystals, mainly expressing 70 kDa and 130 kDa crystal proteins. The LC50 value of the Bt GS57 strain against the S. exigua larvae was 0.339 mg mL–1. Physiological and biochemical reactions showed that Bt GS57 belongs to B.t. var. thuringiensis. In addition, we found that B. thuringiensis can cause a dynamic change in the gut microbiota of S. exigua, with a significant reduction in bacterial diversity and a substantial increase in bacterial load. In turn, loss of gut microbiota significantly decreased the B. thuringiensis susceptibility of S. exigua larvae. Our findings reveal the vital contribution of the gut microbiota in B. thuringiensis-killing activity, providing new insights into the mechanisms of B. thuringiensis pathogenesis in insects.

Group V Chitin Deacetylases Influence the Structure and Composition of the Midgut of Beet Armyworm, Spodoptera exigua

“…The maintenance of PM mechanical strength in H. armigera is probably correlated with CDA [ 17 ]. A change in the peritrophic membrane might make insects more prone to infection, which may eventually be lethal [ 46 ]. It was shown via scanning electron microscopy (SEM) analysis of the surface structure of the S. exigua midgut after ds SeCDAV treatment that the microvilli on the epithelia of the larval midgut did not alter substantially.…”

“…Three biological repeats, each with three technical replicates, were set for each cDNA, and β -actin was used as the internal reference gene to calibrate the total amount of RNA. β -actin proved to be a suitable housekeeping gene applied in the qRT-PCR of S. exigua in our previous study [ 46 ]. The amplification efficiencies of the target and reference were approximately equal ( Supplementary Figure S5 ).…”

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

“…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.…”

Research Progresses on the Function and Detection Methods of Insect Gut Microbes

“…After S. exigua fed on Bt GS57, gut microbial diversity was significantly reduced. Bt GS57 accelerates insect death when complexed with the gut commensal B. cereus [ 81 ]. To sum up, the interaction between conditioned pathogenic bacteria and insect intestinal microorganisms will affect their insecticidal activity in insects, which provides a new research direction into the pathogenic mechanisms of fungi/bacteria in insects.…”

“…These results suggest that intestinal microbial homeostasis is crucial to host health. Some gut commensal bacteria may be transformed into pathogenic bacteria in the presence of dysregulated insect gut microbial homeostasis, such as S. marcescens, Pseudomonas protegens Ramette, and B. cereus, which can elicit host immune responses and disrupt gut microbial homeostasis, and thus pose a serious threat to insect health [80][81][82]. S. marcescens, a symbiotic bacteria in the mosquito gut, secretes a secreted protein called SmEnhancin to facilitate arbovirus infection [75,83].…”