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Etude de Eps1 et Eps2, deux exopolysaccharides du biofilm chez Bacillus thuringiensis

Abstract : Exopolysaccharides - polymers of exported sugars - are involved in essential functions of bacterial physiology. Exopolysaccharides are in fact major components, of the bacterial wall, secondary polymers attached to this wall, the capsules, and finally the biofilm’s matrix. Bacillus thuringiensis is an entomopathogenic bacterium of the cereus group, capable of forming a biofilm at the air-liquid interface. This biofilm has two distinct structures: a floating pellicle on the culture medium and, at the periphery, in continuity with the pellicle, a ring adhering to the solid surfaces. To identify the exopolysaccharides, which constitute the biofilm matrix in B. thuringiensis, I investigated the various chromosomal loci in the sequenced genome of B. thuringiensis strain 407. Two loci have been identified and were called eps1 and eps2. To date, no role in the formation of biofilms in B. thuringiensis had been attributed to eps1 locus. Our data showed that the exopolysaccharide Eps1, depending on the eps1 locus, forms a capsule in the stationary phase and in hypoxic conditions. This capsule, which has significant adhesive properties on biotic and abiotic surfaces, allows adhesion of the biofilm to the solid surfaces, thus forming of the biofilm ring. Consistently with these results, we observed that Eps1 is present only in the biofilm ring. We found that Eps2 exopolysaccharide depending on the eps2 locus is an essential element of the biofilm matrix and is necessary for the formation of the pellicle. We have shown using fluorescent markers that two mutant strains capable of producing only type of exopolysaccharides Eps1 or Eps2 are distributed heterogeneously in the biofilm when they are cocultured. The mutant strain producing only Eps1 is localized in the ring while the mutant strain producing only Eps2 is located in the pellicle. Our data show that the transcription of eps1 and eps2 loci is regulated identically by the same set of regulators. The SinR repressor, which controls the formation of the protein component of the biofilm’s matrix in B. thuringiensis, has no effect on the transcription of eps1 and eps2 in this bacterium. The transcription is activated by Spo0A and repressed by AbrB. The CodY regulator represses the expression of these loci in exponential phase, but activates it in the late stationary phase. Our results also show a negative feedback from Eps2 on the production of Eps1, suggesting the existence of a switch, allowing only one of these exopolysaccharides to be produced in an isolated cell.
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Submitted on : Tuesday, January 2, 2018 - 2:25:58 AM
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Racha Majed. Etude de Eps1 et Eps2, deux exopolysaccharides du biofilm chez Bacillus thuringiensis. Bactériologie. Université Paris-Saclay; Université Saint-Joseph (Beyrouth). Faculté des Sciences, 2017. Français. ⟨NNT : 2017SACLS107⟩. ⟨tel-01674224v2⟩



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