An INRA team of Jouy-en-Josas discovered that the insect pathogen Bacillus thuringiensis switches from a virulence state in the living host, to a survival state in the host cadaver by a cell-cell communication system called quorum sensing. This system allows the bacteria to adapt and coordinate their behaviour according to their population density. By ensuring survival of these bacteria after the death of their host, this system contributes to their spread into the environment. These findings are published in the journal PLoS Pathogens 12 April 2012. | ||
The virulence mechanisms that allow pathogenic bacteria to infect and kill their hosts have been extensively studied. In sharp contrast, the fate of bacteria after death of the host has been generally overlooked. Didier Lereclus' team developed an original infection model based on bacterium-insect interactions. This model system was used to characterize the Bacillus thuringiensis infectious cycle. The work of this team shows that, during the infectious cycle, the pathogenic and necrotrophic(1) lifestyles of B. thuringiensis occur successively. They are tightly controlled by two cell-cell communication systems, called quorum sensing(2), which act sequentially. The first regulator of quorum sensing, PlcR, induces the production of virulence factors, allowing the bacteria to kill the insect. The second, NprR, activates transcription of target genes during sporulation.
After the death of the insect, the regulator NprR activates the expression of about 40 genes specifying the synthesis of degrading enzymes and of a biosurfactant named kurstakin. These enzymes enable the bacteria to grow on necrosed tissues – necrotrophism – and the kurstakin allows B. thuringiensis to survive and eventually sporulate. Thus, the kurstakin increases the ability of these bacteria to disseminate in the environment as resistant spores. 1. Ability to grow on necrosed tissues, and thus in a cadaver.Entomopathogenic bacteria belonging to the B. thuringiensis species have been used for over fifty years to control insect pests. They produce insecticidal toxins forming parasporal crystal inclusions. The present work clarifies the mechanisms used by these bacteria to survive and disseminate after killing their insect host. The new results reported by Lereclus and coworkers are pertinent to agriculture, but also to public health. Indeed, B. thuringiensis belongs to the Bacillus cereus group including bacteria found in various environments and contaminating all stages of the food chain. B. cereus is a recurring problem in the dairy industry and is a source of two types of food poisoning: one causing diarrhoeal diseases and the other causing emetic syndromes. As the regulatory systems elucidated here for B. thuringiensis are also present in B. cereus, the findings will give insight into the mechanisms involved in B. cereus infection. 2. These systems, which involve signalling molecules, allow bacteria to adapt and coordinate their behavior in response to the density of bacterial populations belonging to the same family. http://www.international.inra.fr/ |
