Bacillus thuringiensis

Bacillus thuringiensis (Bt) is a Gram-positive, soil-dwelling bacterium widely recognised for its role as the most commonly used biological pesticide in global agriculture. It occurs naturally in diverse environments such as the gut of caterpillars, leaf surfaces, aquatic habitats, insect-rich soils, animal faeces, flour mills, and grain-storage facilities. Its ability to form insecticidal crystal proteins during sporulation has driven extensive use in pest management and in the development of genetically modified crops.

Taxonomy and Discovery

The organism was first identified in 1902 by a Japanese sericultural engineer investigating disease in silkworms, who initially named it Bacillus sotto due to its association with bacillary paralysis. A later discovery in 1911 by the German microbiologist Ernst Berliner isolated the bacterium from diseased caterpillars in Thuringia, leading to the species designation thuringiensis. Subsequent taxonomic reassessment merged B. sotto under the thuringiensis classification.
In the mid-1970s, the discovery of plasmids in Bt strains helped clarify its genetic behaviour, particularly the plasmid-encoded nature of its crystal proteins. Bt is part of the Bacillus cereus group, a cluster of species that includes B. cereus, B. anthracis, B. mycoides, B. pseudomycoides, B. cytotoxicus, and B. weihenstephanensis. Members of this group are closely related and differentiated largely on the basis of plasmid content. Bt shares the capacity to form endospores with its relatives but is distinguished primarily by its insecticidal plasmids.
Bt subpopulations extend across several dozen identified subspecies, many of which are named after the locations where they were isolated. Examples include subsp. aizawai, berliner, darmstadiensis, fukuokaensis, kurstaki, israelensis, tenebrionis, shandongiensis, tolworthi, and many others. Subspecies such as kurstaki (Btk) and israelensis (Bti) are among the most widely applied in commercial agriculture and public health programmes.

Habitat and Natural Occurrence

Bt is frequently found in environments associated with insects, reflecting its ecological relationships. It can act as a natural pathogen of caterpillars, as observed in laboratory studies on Cadra calidella, where infected individuals displayed disease symptoms attributable to Bt. Its distribution on plant surfaces and in soils further facilitates interactions with herbivorous insect populations.

Genetics and Plasmid Diversity

Bt’s characteristic insecticidal traits are primarily encoded by cry genes, which are typically located on large plasmids rather than the bacterial chromosome. Loss of these plasmids renders Bt nearly indistinguishable from B. cereus, underscoring the centrality of plasmid-borne genes to its phenotype. Plasmid exchange has been reported both naturally and in laboratory settings, occurring not only among Bt strains but also between Bt and close relatives such as B. cereus and B. mycoides.
Key genetic regulators include plcR, an essential transcription factor governing multiple virulence factors, and its partner peptide papR, which functions in quorum sensing. Some Bt strains possess pXO1-like or pXO2-like plasmids related to those found in B. anthracis, although their pathogenicity islands vary considerably and typically lack mammalian virulence factors.
Genomic studies have revealed the presence of group I and group II introns within the B. cereus group, along with several conserved elements such as efflux pumps and chemotaxis proteins that appear unique to this species cluster.

Subspecies Diversity

Bt’s extensive subspecies diversity reflects its global distribution and varied ecological niches. Subspecies include, among many others:

• subsp. aizawai
• subsp. berliner
• subsp. coreanensis
• subsp. fukuokaensis
• subsp. galleriae
• subsp. kurstaki
• subsp. israelensis
• subsp. tenebrionis
• subsp. tolworthi
• subsp. wuhanensis

These subspecies may differ in their host range, crystal morphology, plasmid composition, and environmental adaptations.

Crystal Protein Formation and Insecticidal Mechanism

Bt’s global importance stems from its production of delta endotoxins, crystalline proteins formed during sporulation. These are mainly of two types:

• Cry proteins (crystal toxins)
• Cyt proteins (cytolytic toxins)

Cry toxins target specific insect orders, including Lepidoptera, Diptera, Coleoptera, and Hymenoptera, and can also affect certain nematodes. When susceptible insects ingest Bt spores and crystals:

1. Alkaline gut conditions solubilise the toxin crystals.
2. Proteases in the gut cleave the soluble proteins into active toxins.
3. Cry toxins bind to specific midgut epithelial receptors, forming pores in the membrane.
4. The insect’s digestive system becomes paralysed, leading to starvation.
5. Bt bacteria may proliferate within the insect, contributing to mortality.

Organisms lacking the specific receptor—such as humans, vertebrates, and most non-target insects—are unaffected, making Bt a safe option for targeted pest control.

Bt in Agriculture and Biological Control

Bt is widely used in biological pest management due to its selective toxicity. Subspecies kurstaki is commonly applied against lepidopteran pests in agriculture, while israelensis is used in public health settings for mosquito control due to its effectiveness against larvae of Anopheles, Aedes, and Culex species.
Genetic engineering has incorporated Bt cry genes into various crop species, producing genetically modified varieties such as Bt maize and Bt cotton. These crops express endogenous Cry proteins, providing resistance against key insect pests and reducing reliance on chemical insecticides.

Species Group Relationships and Proteomic Features

Within the B. cereus group, Bt exhibits proteomic diversity comparable to that of B. cereus. Shared characteristics include the potential for enterotoxin production, although the typical insecticidal function of Bt is plasmid-dependent. Common genetic markers within the group include unique efflux systems and chemotactic proteins.

Mechanisms of Environmental Adaptation

Although evidence remains incomplete, ongoing research explores whether coevolution between plasmids and chromosomes has enabled specific Bt strains to adapt to specialised environments. Genomic analyses continue to shed light on the bacterium’s ecological flexibility, pathogenicity mechanisms, and evolutionary relationships.