Characterization of the virulence potential of Bacillus cereus and Bacillus thuringiensis in the Drosophila melanogaster model

President of the jury :                       

• Patricia Lepage, DR INRAE, Micalis

Rapporteurs :                     

• Romain Briandet, DR INRAE, Micalis
• Louis Coroller, Pr Univ. Brest, INRAE, ESIAB, LUBEM              

Examiners :

• Mathilde Bonis, Cheffe de projet Anses, LSAl
• Renata Matos, CR CNRS, IGFL
• Patricia Lepage, DR INRAE, Micalis

Thesis Director :

• Armel Gallet, DR CNRS, ISA
• Olivier Firmesse, Chef de projet Anses, LSAl

Abstract :

Bacillus cereus group (Bc) is composed of Gram-positive, aerobic and facultative anaerobic spore-forming bacteria. This group hosts opportunistic bacteria well known for their involvement in foodborne outbreaks (FBOs) and, more rarely, in extra-digestive infections. In 2022, Bc represented the leading cause of FBOs in France, and also the first cause in Europe of FBOS due to bacterial toxins. Bc can trigger digestive illnesses associated with two types of syndromes: diarrheal and emetic. The diarrheal syndrome is thought to result from ingestion of food contaminated with Bc spores, able to germinate in the gut and to produce the enterotoxins (Nhe, Hbl and CytK) responsible for gastrointestinal disorders.

Bacillus thuringiensis (Bt), a member of Bc, is widely used in both organic and conventional farming due to its ability to produce highly specific insecticidal crystals (Cry) during sporulation. However, in recent years, questions have been raised by health authorities and several scientific studies concerning the putative involvement of Bt products in FBOs. These concerns mainly relate to the genetic proximity between Bt and some members of the Bc group responsible for FBOs, the presence of genes encoding enterotoxins and the lack of available data to assess the pathogenic potential of the strains used in agriculture.

To address these concerns, genetic markers specific to the Bt subspecies used in commercial products were identified to facilitate their identification, by analyzing genomes of 286 Bc/Bt isolated from FBOs, pesticides and collections. In vitro tools using the Caco-2 cell model (for cytotoxicity and inflammation assessment) and in vivo tools using the Bt non-target insect model Drosophila melanogaster (to assess survival, intestinal permeability and diarrheal effects) were developed. These tools, tested on a panel of 48 Bacillus spp strains, revealed that the majority of the pesticidal Bt strains exhibited virulence potential in both models, in tested conditions. Complementary analyses also demonstrated for the first time in vivo that Bt was able to germinate in the Drosophila gut and to express enterotoxin genes, inducing diarrheal symptoms.

Finally, using in silico tools, I identified virulence factors potentially involved in Bc pathogenicity. These analyses revealed the presence of numerous virulence genes, in addition to those encoding enterotoxins, in the genome of Bc members and particularly Bt. Moreover, a transcriptomic analysis of Bt pesticide bacteria in contact with Caco-2 cells revealed the overexpression of most of the virulence and motility genes, while genes associated with fatty acids and biotin metabolism were found repressed.

My PhD work provided new data related to the enteropathogenic potential of pesticidal Bt strains, as well as tools for quantifying the risk associated with their use. My results can be further used to implement appropriate measures concerning the evaluation and use of these products in agriculture. 

Keywords : Bacillus cereus group, foodborne outbreak, spores, Drosophila, Caco-2

You will find below the teams link that will enable you to follow the presentation by videoconference.
Microsoft Teams Need help? 
Join the meeting now 
Meeting ID: 337 856 723 180 
Secret code: GB8PSP 
Participate by phone 
+33 1 73 00 69 51,,348244857# France, All locations 
Find a local number 
Conference call ID: 348 244 857#