Among the bioinsecticides used to control pests, the bacteriumBacillus thuringiensis(Bt) and its Cry toxins represent 90 % of the market. Bt bioinsecticides are widely used in organic farming (Btkurstakiand Btaizawai) and in mosquito control (Btisraelensis). Each toxin is highly specific, meaning that it targets a limited number of insect species. Therefore, regarding acute toxicity, Bt bioinsecticides should have no detrimental effect on the other species of the ecosystem. However, the long-term effects of a chronic exposure, on both the environment and human health are still largely unexplored.
Using both fundamental and applied approaches, our team aims to answer three main questions:
- What are the physiological, cellular and molecular mechanisms putatively affected by Bt bioinsecticides?
- What are the consequences of the ingestion of Bt bioinsecticides on individuals predisposed to develop physiopathologies?
- What are the environmental and public health risks associated with an increasing use of Bt bioinsecticides?
A-Sporange of Bacillus thuringiensis ; B-3D modelling of Cry toxins ; C-Apico-basal view of the Drosophila midgut ; D-Drosophila adult midgut ; E-Coomassie blue staining of Cry toxin ; F-Transversal section of the Drosophila posterior midgut ; G-Longevity curves of adult Drosophila ; H-Cell counting of the Drosophila midgut.
We use one of the best genetic models: Drosophila melanogaster. Drosophila presents the advantage to be insensitive to Bt bioinsecticides relative to the acute toxicity, and therefore allows to study the "unintended " or "adverse" effects on a model organism. In addition, many genetic, cellular and molecular tools have been already developed to study this organism (http://flybase.org/). Moreover, the high conservation of the main physiological mechanisms and signaling pathways between Drosophila and vertebrate allows to rapidly transpose the results to other animals.
To this end, we have established collaborations within our institute with the TEAPEA team that studies the behavior of Trichogramma (a parasitoids used for biological control), and with the ESIM team studying relationship host/parasitoid with the couple Drosophila/Leptopilina. We have also established collaborations with the IMT team (located in Toulouse, France) and working on the impact of mycotoxins on human and animal health (using cell culture and pigs). Finally, we collaborate with the IMI team that studies the development of allergic diseases using the mouse model.
Scientific originality of the team
Our team is the first one to develop systematic researches on the unintended effects of Bt bioinsecticides by combining ecotoxicological and functional studies. This is achieved in particular by using one of the best genetic models i.e.Drosophila melanogaster. We can carried out simultaneously:
- a finalized approach by studying the pathophysiological consequences;
- a fundamental approach by identifying the underlying cellular and molecular mechanisms.
Research topics and objectives
The gut is the first organ and barrier in contact with ingested toxins and microbes. Because Bt bioinsecticides are made up of a mixture of bacteria and toxins that can, on one hand, affect the integrity of the intestinal tract and on the other hand, stimulate and/or modify its local immune response. Although the deleterious impact of an acute intoxication can be overcome by the regenerative capacity of the gut epithelium, a prolonged or chronic aggression may compromise the physiological equilibrium of the gut. Such an impact can have dramatic consequences for both intestinal and systemic health of an organism, affecting both its overall fitness and longevity.
1. Bt and the cellular homeostasis of the Gut
The gastrointestinal tract is permanently attacked either through food pathogens (viruses, bacteria), toxins or chemicals (such as drugs or pesticides). The cellular and physiological balance (the homeostasis) of the intestinal epithelium ensures the efficient replacement of damaged cells while limiting the risk of developing pathologies. The rate of stem cell proliferation is closely linked to the requirement for tissue renewal at a given time and is regulated by several signalling pathways such as JNK, Hippo, Wg/Wnt, JAK/STAT, BMP/TGFβ, Hh and EGFR pathways. The activation/repression of these pathways allows the adaptation of the cellular composition of the gut depending on the environmental cues. In addition, upon damaging bacterial intoxication of the intestinal epithelium, these same cell signaling pathways are also required to rapidly restore the integrity of the digestive tract.
Therefore, we are studying the impact of Bt bioinsecticides on the structure and the cellular homeostasis of the gut in order to assess the potential risks associated with their ingestion.
2.Btand the local immune response
The immune response is essential to fight against pathogens (bacteria, viruses, fungi, etc ...). Any impact on its functioning alters its effectiveness and thus the ability of an individual to defend itself in a hostile environment.
The midgut of Drosophila (and insects in general), like the intestines of mammals, is endowed of a local immune response capable of fighting against pathogens present in the food bowl. This response is essential for the survival of an organism without which its life would be in peril systematically after food ingestion. This local defense results in the expression and secretion of antimicrobial peptides (AMP) by the enterocytes in insects or Paneth cells in mammals, which will kill the pathogen.
Bt is considered as entomopathogenic because when it penetrates inside the body of an insect (whatever it is), it causes septicemia and kills its host quickly. When Bt is ingested by a "target" (or "sensitive") organism, Cry toxins are necessary to damage the intestinal epithelium of the host. Consequently, the intestine no longer fulfills its role as a barrier and Bt enters the inside the body, spreads out via the hemolymph, proliferates and kills its host in 2-3days. In non-target organisms (such as Drosophila or human), Cry toxins can not damage the intestine either because Cry toxins are ineffective or because the intestine is repaired faster than toxins damage it. In parallel, the secretion of PAM by the enterocytes eliminates Bt. The bacterium does not penetrate inside the body and therefore does not kill its host. However, Bt is considered to be an opportunistic bacterium in mammals (Bt belongs to the group of B. cereus and B. anthracis), meaning that it can cause diseases (pneumonia and eye infections for examples) in immune-weakened individuals (immunocompromised).
Therefore, we are studying, in Drosophila, the impact of the ingestion of Bt bioinsecticides on the immune response and its impacts on individuals weakened by stress or aggression prior to the ingestion of Bt.
Scientific and social issues
- Scientific issues: Although the acute targets of Bt bioinsecticides and the underlying cellular/molecular mechanisms are fairly well characterized, targets and modes of action during chronic intoxication remain unexplored. Our complementary ecotoxicological and mechanistic approaches will allow to link finalized and fundamental researches. The more we understand the foundations of a problem, at best we will assess the risks to anticipate them.
- Social and economic issues: Bioinsecticides will be increasingly used and Bt currently represents the majority of the indications ("organic" farming; mosquito control, transgenic crops). Sources of contact with these bioinsecticides toxins are multiple for human (food, air, water). Anticipating the risks associated with an increasing use of Bt bioinsecticides should avoid the renewal of a posteriori disappointments encountered with the chemical pesticides. Our work will help to establish preventive measures for users and consumers and allow adapting the mode of use of Bt bioinsecticides. Our work will also allow to optimize the compositions and production of Bt bioinsecticides. As Cry toxins are also broadly used in transgenic plants, our work will help to expertise the sanitary and environmental impacts of such crops.