Résumé:
The aim of this work is to select a set of new strains of non-pathogenic thermophilic streptococci namely S. thermophilus and S. macedonicus from traditional Algerian fermented milk samples (lben and rayeb) and to characterize some properties prior to a probiotic effect (abilities to resist environments simulating the conditions of the gastrointestinal tract [GIT] and to adhere to intestinal epithelial cells in vitro). The methodology adopted in this work consists of phenotypic (growth in Bile Esculin Azide, 6.5% w/v NaCl, pH 9.6) and genotypic identification (sequencing of 16S rRNA, STIs and RAPD and MLST techniques) of the isolates from lben and rayeb, as well as an assessment of their acidification capacity, thus allowing a first characterization of the strains. Then, in a second step, the selected strains are studied for their ability to survive for 2 hours at low pH values and at different concentrations of an equimolar mixture of bile salts (MBS). Three cellular models Caco-2 TC7, HT29-MTX and HT29-CL16E were used for the study of adhesion. Results were obtained from four strains of S. macedonicus genetically distinct from the reference strain S. macedonicus ACA-DC-198 and from one strain of S. thermophilus also different from the strain S. thermophilus LMD-9. These strains are capable of surviving down to pH 3 and tolerate high concentrations of bile salts (10 mM) in opposition to reference strains Streptococcus thermophilus LMD-9 and Lactobacillus rhamnosus GG. In addition, the studied strains have adhesion ability similar to that of Lactobacillus rhamnosus GG on Caco-2 TC7. on the other hand, strains S. macedonicus PA and S. thermophilus LF9 have good adhesion qualities to HT29-MTX and HT29-CL16E mucus cells. This first characterization makes it possible to consider the strains of S. macedonicus and Streptococcus thermophilus as potential candidates for prospective probiotic effects which deserve to be studied further more.
