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The fate of bacteria of the Bacillus cereus group in the environment : interaction with amoebas

Abstract : The Bacillus cereus sensu lato group consists of several closely related species, including B. anthracis (Ba), B. cereus sensu stricto (Bc), and B. thuringiensis (Bt). Ba is the causal agent of anthrax, Bc is involved in some food poisoning events and Bt strains are pathogens to insects. Spores of these pathogenic bacteria are commonly found in the soil. However, evidence suggests that they are unable to grow in such a natural environment without nutrient input.Amoebas have been reported to be an amplifier for several species of pathogenic bacteria and their potential involvement to explain the large amount of Bt and Bc spores in soil has been frequently proposed. We speculated that amoebas are an amplifier for Bacillus and studied the fate of Bacillus and amoebas when cultured together. My results show that the virulence factors produced by Bt and Bc do not affect the amoeba Acanthamoeba castellanii, which, on the contrary, can phagocytose and effectively digest vegetative Bacillus cells to grow and prevent the formation of cysts. Bacterial spores can germinate in the amoeba environment and the vegetative cells can then form chains or aggregates that appear to be less efficiently phagocytosed by the amoeba. The use of transcriptional fusions between fluorescent reporter genes and stationary phase- and sporulation-specific promoters showed that the sporulation process occurs more efficiently in the presence of amoebas than in their absence. Moreover, my results showed that the amoeba environment promotes spore germination and allows the bacteria to complete their developmental cycle. Overall, my study suggests that the amoeba-Bacillus interaction creates a virtuous circle in which each protagonist helps the other to develop.
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Submitted on : Wednesday, January 26, 2022 - 2:14:09 PM
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Haibo Chen. The fate of bacteria of the Bacillus cereus group in the environment : interaction with amoebas. Microbiology and Parasitology. Université Paris-Saclay, 2021. English. ⟨NNT : 2021UPASB044⟩. ⟨tel-03544129⟩

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