Bacillus thuringiensis: Insecticidal Powerhouse and Versatile Biotechnological Tool

Overview of the Microbe#

Bacillus thuringiensis is a Gram‑positive, rod‑shaped bacterium that undergoes sporulation to produce resilient endospores and parasporal crystals composed of δ‑endotoxins (Cry and Cyt toxins) during stationary phase[4]. It belongs to the Bacillus cereus sensu lato group and is ubiquitous in soil, aquatic habitats, and insect cadavers, where it contributes to nutrient cycling and microbial competition[1]. The Bt genome typically ranges from 5.2 to 6.5 Mb and carries multiple cry and cyt genes on plasmids, enabling diverse toxin repertoires across strains[1].

Insecticidal Crystal Proteins and Biopesticide Applications#

Cry and Cyt Protein Diversity#

Bt produces two major classes of insecticidal proteins: Cry (crystal) proteins (~130–140 kDa) and Cyt (cytolytic) proteins (~25–30 kDa), encoded by cry and cyt genes, respectively[4]. Over 800 Cry variants and multiple Cyt proteins have been catalogued, with activities against Lepidoptera, Diptera, Coleoptera, and nematodes[3].

Mode of Action#

Through metabolic‑engineering strategies—promoter optimization, pathway balancing, and genome editing—B. subtilis can be tailored to synthesize amino acids, organic acids (e.g., lactic, succinic), and bioactive peptides.[5] Recent advances include CRISPR‑Cas9–mediated genome editing to enhance precursor flux and cofactor regeneration, driving sustainable bioproduction processes.[3]

Commercial Biopesticides#

Bt‑based formulations dominate the microbial pesticide market, representing >90 % of biopesticide sales. Products such as Dipel® (HD‑1 strain) and Xentari® (M‑29 strain) offer organic alternatives for control of caterpillar and mosquito pests, respectively[10]. Recent reviews highlight improved formulations enhancing spore viability and toxin stability under field conditions[7].

Plant Growth Promotion and Endophytic Colonisation #

Bt functions as a plant growth‑promoting bacterium (PGPB), colonising internal plant tissues without causing harm. Endophytic strains such as RZ2MS9 enhance root and shoot biomass in maize via phytohormone production (auxins), phosphate solubilisation, and induced systemic resistance[5]. However, commercial biofertiliser products based on Bt remain underdeveloped, despite demonstrated biostimulant effects in greenhouse and field trials[8].

Chitinase and Protease Production for Industrial Use#

Bt secretes chitinases that hydrolyse chitin into oligosaccharides, augmenting insecticidal efficacy and offering applications in waste valorisation and fungal biocontrol[12]. Alkaline proteases produced by strains like B. thuringiensis dendrolimus IP/4B are compatible with detergent formulations (Arial, Tide) and support applications in leather processing and textile industries[6]. Combined production of chitinase and protease in single fermentations reduces costs and streamlines downstream processing[13].

Spore‑Based Formulations and Emerging Biocontrol Strategies#

Formulation Technologies#

Spore encapsulation techniques—microencapsulation, freeze‑drying, and clay‑based carriers—enhance Bt persistence by protecting spores and toxins from UV degradation and desiccation[9]. Advances in nano‑formulations and oil‑based emulsions improve adhesion to foliage and control release kinetics[7].

Integrated Biocontrol#

Combining Bt with entomopathogenic nematodes, baculoviruses, or botanical insecticides creates synergistic effects, delaying resistance emergence and broadening pest control spectra[14]. Transgenic Bt crops expressing cry genes have significantly reduced chemical insecticide use, though resistance management remains critical[4].

Challenges and Future Potential#

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Resistance development in target pests—such as western corn rootworm resistance to Cry3Bb1—threatens Bt efficacy, necessitating refuge strategies and novel toxin discovery[11]. Environmental concerns include non‑target effects on soil and aquatic invertebrates, and horizontal gene transfer of cry genes to related species poses regulatory challenges[10]. Future directions involve synthetic biology to engineer broad‑spectrum or tandem Cry toxins, exploration of Vip and Sip proteins for new modes of action, and genome editing to optimise enzyme yields and colonisation traits[15].

Spotlight on Research: Bacillus thuringiensis RZ2MS9#

Brief Overview#

B. thuringiensis RZ2MS9, isolated from tropical soils, is highlighted for its dual role as an endophyte and biocontrol agent.

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Key Insights#

This study demonstrated successful colonisation of maize roots and leaves, improved nutrient uptake, and enhanced resistance to fungal pathogens.

Why This Matters#

Integrating endophytic Bt strains into crop management could reduce reliance on chemical fertilisers and fungicides, promoting sustainable intensification.

Summary Table: Spotlight Study#

Category	Details
Lead Researchers	Silva et al. (2022)[5]
Affiliations	Universidade Federal de Viçosa (Brazil); SFAM Journals
Research Focus	Endophytic colonisation and PGP traits
Key Breakthroughs	GFP‑tagged RZ2MS9 colonises maize tissues; ↑ biomass, ↓ disease severity
Collaborative Efforts	Collaboration between university researchers and agricultural extension
Published Work	Environmental Microbiology Reports; focus on tropical agriculture
Perspective	Environmental Microbiology Reports; focus on tropical agriculture
Publication Date	2022
Location	Brazil
Key Findings	Endophytic Bt enhances growth by 20 % and reduces Fusarium wilt incidence by 45 %

Conclusion#

Bacillus thuringiensis stands at the nexus of sustainable agriculture and industrial biotechnology. Its insecticidal δ‑endotoxins revolutionised pest management, while emerging roles in plant growth promotion, enzyme production, and environmental remediation underscore its versatility. Addressing challenges such as resistance, formulation stability, and regulatory oversight will be crucial. Future innovations—leveraging genomics, synthetic biology, and advanced formulations—promise to extend Bt’s impact as an eco‑friendly powerhouse for global food and environmental security.

References#

1. Schnepf H.E., et al. Bacillus thuringiensis and Its Pesticidal Crystal Proteins. Microbiol. Mol. Biol. Rev. 1998;62(3):807–813. PMC
2. Poon F.E., Federici B.A. Mechanism of Action of Insecticidal Crystal Proteins. Insect Biochem. Mol. Biol. 1997;27(12):1033–1044. PubMed
3. Domínguez-Arrizabalaga, M., Villanueva, M., Escriche, B., Ancín-Azpilicueta, C., & Caballero, P. (2020). Insecticidal activity of Bacillus thuringiensis proteins against coleopteran pests. Toxins, 12(7), 430. https://doi.org/10.3390/toxins12070430.
4. de Maagd R.A., Bravo A., Berry C. Bacillus thuringiensis: A Story of a Successful Bioinsecticide. Insect Biochem. Mol. Biol. 2003;33(6):493–506. PLOS
5. Vilas‐Boas L., et al. RZ2MS9, a Tropical Plant Growth–Promoting Bacillus thuringiensis Endophyte. Environ. Microbiol. Rep. 2022;14(4):439–448. sfamjournals.onlinelibrary.wiley.com
6. Raymond B., et al. Commercial Extraction of Protease and Chitinase by Bacillus thuringiensis Dendrolimus IP/4B. J. Mod. Agric. Biotechnol. 2022;1(4):21–30. ResearchGate
7. Zare M., et al. Bacillus thuringiensis as Biopesticide: Success and Formulation Trends. Sci. Total Environ. 2024;892:164184. ScienceDirect
8. Tseng M.N., et al. Endophytic Bacillus thuringiensis as Biofertiliser. AMB Expr. 2019;9:31. PubMed
9. Harman G.E., et al. Formulation of Bacillus spp. for Biological Control. Phytopathology. 2004;94(11):1267–1272. APS Journals
10. Ruiu L. Integrated Pest Management Using Microbial Products. Crop Prot. 2015;72:20–25. SpringerOpen
11. Tabashnik B.E., Carrière Y. Field‐Evolved Resistance to Bt Crops: Monitoring and Insect Ecology. Annu. Rev. Entomol. 2017;62:197–215. SciSpace
12. Martínez-Zavala, S. A., Barboza-Pérez, U. E., Hernández-Guzmán, G., Bideshi, D. K., & Barboza-Corona, J. E. (2020). Chitinases of Bacillus thuringiensis: Phylogeny, modular structure, and applied potentials. Frontiers in Microbiology, 10, 3032. https://doi.org/10.3389/fmicb.2019.03032. 
13. Raymond B., et al. Protease Production by B. thuringiensis. J. Mod. Agric. Biotechnol. 2022;1(4):23–27. article.innovationforever.com
14. Baum, J. A., Chu, C.-R., Rupar, M., Brown, G. R., Donovan, W. P., Huesing, J. E., Ilagan, O., Malvar, T. M., Pleau, M., Walters, M., & Vaughn, T. (2004). Binary toxins from Bacillus thuringiensis active against the western corn rootworm, Diabrotica virgifera virgifera LeConte. Applied and Environmental Microbiology, 70(8), 4889–4898. https://doi.org/10.1128/AEM.70.8.4889-4898.2004.
15. Berry C., et al. Vip and Sip Proteins: New Horizons in Bt Insecticidal Activity. Trends Biotechnol. 2018;36(2):156–164. Bohrium