Bacillus cereus (Bc) is a wide group of Gram-positive and spore-forming bacteria, known to be the etiological agents of various human infections, primarily food poisoning. The Bc group includes enteropathogenic strains able to germinate in the digestive tract and to produce enterotoxins such as Nhe, Hbl, and CytK. One species of the group, Bacillus thuringiensis (Bt), has the unique feature of producing insecticidal crystals during sporulation, making it an important alternative to chemical pesticides to protect crops from insect pest larvae. Nevertheless, several studies have suggested a link between the ingestion of pesticide strains and human cases of food poisoning, calling their safety into question. Consequently, reliable tools for virulence assessment are worth developing to aid decision making in pesticide regulation. Here, we propose complementary approaches based on two biological models, the human intestinal Caco-2 cell line and the insect Drosophila melanogaster, to assess and rank the enteric virulence potency of Bt strains in comparison with other Bc group members. Using a dataset of 48 Bacillus spp. strains, we showed that some Bc group strains, including Bt, were able to induce cytotoxicity in Caco-2 cells with concomitant release of IL-8 cytokine, a landmark of pro-inflammatory response. In the D. melanogaster model, we were able to sort a panel of 39 strains into four different classes of virulence, ranging from no virulence to strong virulence. Importantly, for the most virulent strains, mortality was associated with a loss of intestinal barrier integrity. Interestingly, although strains can share a common toxinotype, they display different degrees of virulence, suggesting the existence of specific mechanisms of virulence expression in vivo in the intestine.

Bacillus thuringiensis (Bt), belonging to the Bacillus cereus (Bc) group, is commonly used as a biopesticide worldwide due to its ability to produce insecticidal crystals during sporulation. The use of Bt, especially subspecies aizawai and kurstaki, to control pests such as Lepidoptera, generally involves spraying mixtures containing spores and crystals on crops intended for human consumption. Recent studies have suggested that the consumption of commercial Bt strains may be responsible for foodborne outbreaks (FBOs). However, its genetic proximity to Bc strains has hindered the development of routine tests to discriminate Bt from other Bc, especially Bacillus cereus sensu stricto (Bc ss), well known for its involvement in FBOs. Here, to develop tools for the detection and the discrimination of Bt in food, we carried out a genome-wide association study (GWAS) on 286 complete genomes of Bc group strains to identify and validate in silico new molecular markers specific to different Bt subtypes. The analyses led to the determination and the in silico validation of 128 molecular markers specific to Bt, its subspecies aizawai, kurstaki and four previously described proximity clusters associated with these subspecies. We developed a command line tool based on a 14-marker workflow, to carry out a computational search for Bt-related markers from a putative Bc genome, thereby facilitating the detection of Bt of interest for food safety, especially in the context of FBOs.

Bacillus cereus sensu lato (Bcsl) is a group of closely related bacterial species known for their resistant spores, enabling them to persist in a dormant state and thereby colonize and adapt across diverse environments. Bcsl is known for its harmful impact on human health, producing toxins that cause emetic and diarrheal syndromes or provoking extradigestive infections. Importantly, Bcsl is the most frequent confirmed or presumptive causative agent associated with foodborne outbreaks (FBOs) in France. In our study, we assessed the population structure of a large collection of Bcsl isolated during FBOs investigation in France between 2004 and 2023, focusing on the association between distinct populations and food categories. Using 294 genomes from 183 FBOs, we applied genomic clustering and phylogenomic analysis and then identified three predominant Bcsl populations. B. cereus sensu stricto (17.0 %) prevailed in composite dishes, B. paranthracis (16.1 %) was positively associated with cereals, and B. thuringiensis subsp. kurstaki (7.6 %) was predominantly found in vegetable-based salads. Some strains were phylogenetically closely related to clinical isolates, highlighting the need to assess the antibiotic susceptibility of Bcsl. Notably, one Bcsl clade, B. cytotoxicus, lacking beta-lactamase-encoding genes showed a greatly increased sensitivity to ampicillin than other Bcsl considered to be naturally resistant to beta-lactams. Additionally, various strains from distinct populations showed reduced susceptibility to macrolides and cyclins. Finally, accurately differentiated populations will be used in further epidemiological studies and in dose-response modeling.

Peptidoglycan is a meshwork macromolecule, made of polysaccharide strands cross-linked by short peptides, which encases the cytoplasmic membrane of bacteria and protects them against turgor pressure. Peptidoglycan peptidases are membrane or periplasmic enzymes that cleave these peptides, either lowering the cross-linking level of peptidoglycan to sculpt bacterial shape or allowing cell elongation by making space for the insertion of neosynthesized glycan strands. In the pathogen Helicobacter pylori , shape is important for virulence, and transition to a coccoid form after prolonged growth enables immune evasion. One particular endopeptidase, HdpA, is known to be involved in the maintenance of cell shape and in the transition to coccoids. Here we show that along growth, HdpA is constantly associated with LhiA, an inner membrane chaperone lipoprotein that keeps it in check while protecting it from fast proteolysis. The crystal structure of the HdpA-LhiA complex suggests that this interaction freezes the autoinhibitory interaction between the first domain of HdpA and the third, catalytic domain. Analysis of the evolution of the HdpA and LhiA protein levels over growth suggests that transition to coccoids is not triggered by a burst in HdpA activity but rather by a gradual weakening of the sacculus caused by the small fraction of free HdpA in equilibrium with LhiA-sequestered HdpA. SIGNIFICANCE STATEMENT: The cell wall, the exoskeleton of bacteria, is the target of numerous antibiotics. Its principal component, peptidoglycan, is remodeled by an interplay between peptidoglycan synthases and hydrolases to accomodate bacterial growth. Because their activity can be harmful, hydrolases have to be tightly regulated. We discovered the dedicated inhibitory chaperone of a H. pylori peptidoglycan hydrolase and showed how the chaperone keeps the hydrolase in check in the cytoplasmic membrane and releases it slowly to allow it to perform its task, ultimately triggering a shape transition to a spherical form important for immune evasion. The crystal structure of the complex gives clues to the mechanism of peptidase activation, suggesting strategies to design agonists that could be used as antibacterials.

Bacillus cereus sensu lato ( Bcsl ) is a group of closely related bacterial species known for their resistant spores, enabling them to persist in a dormant state and thereby colonize and adapt across diverse environments. Bcsl is known for its harmful impact on human health, producing toxins that cause emetic and diarrheal syndromes or provoking opportunistic infections in hospitals. Importantly, Bcsl is the most frequent confirmed or presumptive causative agent associated with foodborne outbreaks (FBOs) in France. In our study, we assessed the population structure of a large collection of Bcsl isolated during FBOs investigation in France between 2004 and 2023, focusing on the association between distinct populations and food categories. Using 294 genomes from 183 FBOs, we applied genomic clustering and phylogenomic analysis to accurately identify predominant Bcsl populations: B. cereus sensu stricto (17.0%), B. paranthracis (16.1%), and B. thuringiensis subsp. kurstaki (7.6%), which were positively associated with composite dishes, cereals, and vegetable-based salads, respectively. Some strains were phylogenetically closely related to clinical isolates, highlighting the need to assess the antibiotic susceptibility of Bcsl . Notably, one Bcsl clade, B. cytotoxicus , lacking beta-lactamase-encoding genes showed a greatly increased sensitivity to ampicillin than other Bcsl considered to be naturally resistant to beta-lactams. Additionally, some strains showed reduced susceptibility to macrolides and cyclins. Finally, we identified several signatures of recombination and horizontal transfer, suggesting the ability of Bcsl to share fitness-enhancing alleles and acquire virulence and antibiotic resistance genes by non-pathogenic strains. These findings would be valuable for improving the surveillance of Bcsl . HIGHLIGHTS: Bcsl associated with French food poisonings consist of 14 clusters. Bcss, the most frequent clade, is mainly found in composite dishes. Bcsl toxin genes are present in various clades, suggesting horizontal gene transfer. Some Bcsl linked to food poisonings or septicemia belong to the same populations. B. cytotoxicus, lacking bla genes, showed increased sensitivity to ampicillin.