Résumé:
The naturally fermented dairy products are an interesting source of new strains of lactic acid bacteria (LAB) with proven technological potential. Some of them are capable of synthesizing exopolysaccharides (EPS) during fermentation, which improve the rheological properties of dairy matrices. This thesis work focuses on the selection of LAB producing EPS from local fermented dairy products. A total of 30 samples were collected in Eastern Algeria, from which 584 presumptive isolates of LAB were tested for the production of EPS. Only 18 isolates presented a productive phenotype, their polyphasic characterization confirmed that they belong to two species: Leuconostoc mesenteroides and Lactobacillus plantarum. A preselection based on the stability of EPS production character made it possible to retain 6 strains in addition to a control strain, all affiliated to the Lactobacillus plantarum species. Then macroscopic characterization of their lactic gels allowed to select two strains LBIO1 and LBIO28 producing viscosifying EPS in a dairy matrix. The study of their production revealed that the polymers produced are primary metabolites, with a high molecular weight. In parallel, a characterization of the macro and microstructure under confocal laser scanning microscopy of the two gels SMCLBIO1 and SMCLBIO28, compared to the control gel SMCLBIO14, concluded that the EPS of the two ropy strains contribute significantly to the improvement of the physical attributes of lactic gels. Furthermore, micrographs made by cryo-electron microscopy showed the existence of a dense web-like EPS attached to the cells for both ropy strains, which is not observed for LBIO14. Finally, a complete sequencing of the three genomes followed by an in silico genomic analysis confirmed the safety of the strains and the existence of genes coding for bacteriocins and vitamins. The study of the biosynthetic machinery of EPS in comparison with the reference strain Lactobacillus plantarum WCF1 revealed the presence of 2 complete eps clusters cps3 and cps4 in the three genomes with 100% similarity between them, all chromosomal. However, the two strains LBIO1 and LBIO28 have in addition genes belonging to cps2, which would therefore be involved in the synthesis of the ropy polymer with high molecular weight. The two ropy strains LBIO1 and LBIO28 can therefore be exploited in dairy fermentation, based on the techno-functional properties of their polymers, which contribute to the improvement of the rheological characteristics of the matrices generated.
