Toxin and virulence gene expression in Bacillus thuringiensis

Abstract

At the end of vegetative growth and in response to various nutrient stresses, Bacillus species produce specific components (degradative enzymes, antibiotics, toxins) allowing the bacteria to rapidly adapt to the environment, to eliminate competitors and to gain access to novel sources of nutrients. The entomopathogenic properties of Bacillus thuringiensis are partly due to the production of larvicidal toxins, called δ-endotoxins (the Cry and Cyt proteins). These proteins are synthesised during the stationary phase and/or during sporulation. They accumulate in the mother cell to form a crystal inclusion which can account for up to 25% of the dry weight of the cells. This massive production of proteins and its coordination with the stationary phase result from a variety of mechanisms occurring at the transcriptional, post-transcriptional and post-translational levels. Some B. thuringiensis strains produce another set of insecticidal toxins, called the Vip proteins. These toxins are synthesised from the vegetative phase to the early stages of the stationary phase. In addition to these insecticidal proteins, B. thuringiensis produces a variety of extracellular proteins which might be involved in the virulence of the bacterium and specifically in its ability to provoke a septicaemia in susceptible insects. These potential virulence factors are degradative enzymes (proteinases, phospholipases...), secondary metabolites (antibiotics, β-exotoxin...), cell surface proteins (flagellin, S-layer proteins...) and cytotoxic components (haemolysins, enterotoxins). The expression of the genes encoding these compounds is generally activated at the onset of the stationary phase.