The species B. thuringiensis is a typical member of the B. cereus-Group of the genus Bacillus. Like B. cereus, B. thuringiensis is motile, otherwise they are facultatively anaerobic Gram-positive spore-bearing bacilli (1.0 µm wide or wider). The central to sub-terminal spores do not distend the cells. They are haemolytic and not sensitive to penicillin. They form white to gray colonies on bacteriological media. The main distinguishing feature is that they form a parasporal crystal inclusion. The chromosomal DNA of B. thuringiensis has a GC content of 33.8 – 34.3 mol%.

They are lecithinase positive. They are motile, ferment a few carbohydrates producing acid from glucose but not gas. They do not grow above 50°C but grow in up to 7% NaCl. They produce catalase, give a positive Voges-Proskauer reaction and reduce nitrate to nitrite. They hydrolyse casein and starch.

Most of the genetic studies on B. thurigiensis , have focussed on their production of the insect toxins. Typically strains of B. thurigiensis contain a large number of plasmids ranging in size from 2 - >200 kb. Up to 17 plasmids comprising up to 20% of the total DNA have been noted in some strains. The ability of a strain to maintain so many plasmids is being intensively investigated. The plasmids can be transferred with relatively high frequency between strains including other species of Bacillus.

There are repeated elements around the protein crystal genes, which are believed to aid their dispersal. Transfer systems, including transduction have been reported in B. thurigiensis.

As many as 20 different flagellar (H) serotypes of B. thurigiensis have been recognized. Similarly, the crystal toxin antigens have been used to subdivide strains of B. thurigiensis.

Insect Toxins:

The crystal toxin, which affects exclusively insect larvae has a mode of action similar to that of cholera toxin in humans. The toxin affects the adenyl cyclase system of the insect cells. The toxin binds to a specific receptor on the brush border membrane of the insect’s midgut epithelium. Its mode of action is then to induce pore formation in the cells resulting in leakage of cell contents followed by cell death.

B. thurigiensis strains can be grown very easily on most microbiological media. Apart from their production of the parasporal crystal there are few distinguishing features between B. thurigiensis and B. cereus.

B. thurigiensis survives well in soil. However, epizootics caused by B. thurigiensis are rare.

The main industrial uses of B. thurigiensis are in the production of insecticides. Such insecticides have been formulated to be lethal to only a limited range of insects and not to affect other insects or birds and mammals. There are stability problems for the toxin in soil, however its biodegradable nature gives it a great advantage over chemical insecticides.

There are no B. thurigiensis vaccines.