The cell-free supernatant of Streptococcus salivarius M18,investigation of anti-bacterial effects on pathogenic bacteria and anti-cancer effects on colon cancer cells

In recent years, there is an increasing attempt to develop potential approaches as an alternative to the use of live probiotic microorganisms. In the case of cell-free supernatants, postbiotics include the secreted metabolic products such as enzymes, vitamins, bio-surface-active molecules, amino acids, peptides, and organic acids. Streptococcus salivarius, found in human saliva, breast milk, intestine, and stool samples, is a Gram-positive and facultative anaerobic bacteria. Being the dominant type of oral mucosa, S. salivarius is commonly found in the dorsum of the tongue and pharyngeal mucosa. Streptococcus salivarius M18 is a probiotic strain isolated from the oral cavity of a healthy adult. The probiotic strain has been shown to reduce dental plaque formation, moderate and severe gingivitis, and periodontitis and prevent the occurrence of new tooth decay through the production of some bacteriocins as well as dextranase and urease enzymes that help reduce dental plaque formation and acidification, respectively. In this thesis, the anti-pathogenic effects of S. salivarius M18 cell-free supernatant on two Gram-negative pathogens, Pseudomonas aeruginosa and Klebsiella pneumonia, which are responsible for many different human infections as well as food borne-diseases, were investigated. The probiotic supernatant was shown to inhibit the growth of the pathogens and biofilm formation and sensitize the pathogens to the antibiotic treatment. In this study, the anti-cancer effects of S.salivarius M18 cell-free supernatant on colorectal cells were also investigated in vitro, and the cell-free supernatant of the M18 strain was shown to inhibit cancer cell viability and promote apoptosis. This effect was enhanced by the use of the prebiotic inulin. Most importantly, the observed anti-cancer effect was specific for the cancer microenvironment. In summary, the data obtained through this thesis reveal the anti-pathogenic and anti-cancer effects of the S.salivarius M18 cell-free supernatant. These results are important when evaluated in terms of current literature highlighting that colon microbiota dysbiosis, as a contributing factor for colorectal cancer, is affected by oral Microbiota.