The aim of this work was to investigate the potential antibacterial activity of probiotics and prebiotics and to unravel the mechanisms of this activity. Probiotics are live microorganisms, often strains of lactobacilli or bifidobacteria, that are added to food to confer a health benefit to the consumer. A positive property of probiotics is their capacity to inhibit pathogenic bacteria such as Escherichia coli and Salmonella. Several probiotic strains, including commercial ones, were tested and it was found that both bifidobacteria and lactobacilli inhibited harmful bacteria. The antibacterial activity of the bifidobacteria tested was due to the production of organic acids, such as acetic acid and lactic acid. The antibacterial activity of most lactobacilli tested was also correlated with the production of organic acids. Yet, certain lactobacilli, in addition to organic acids, produced hitherto unknown anti-Salmonella compounds.In the colon, organic acids are produced through fermentation of non-digestible ingredients such as prebiotics by bifidobacteria. Prebiotics are non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon that can improve the host’s health. Data on the growth of colon bacteria, other than bifidobacteria, on prebiotic inulin-type fructans remains scarce. The ability of lactobacilli to ferment inulin-type fructans was investigated. It was shown that the ability to ferment prebiotic inulin-type fructans is not a common characteristic of lactobacilli. Yet, a human colonic mucosa isolate, Lactobacillus paracasei subsp. paracasei 8700:2, rapidly degraded inulin-type fructans such as oligofructose and inulin. Moreover, a Lactobacillus strain from dairy origin was able to ferment oligofructose. Synbiotics combine the beneficial properties of probiotics and prebiotics. To develop synbiotics it is necessary to have information about how prebiotic ingredients interact with probiotic bacteria. Through in vitro co-culture fermentations it was demonstrated that both positive and negative interactions between probiotic lactobacilli and bifidobacteria occur in the presence of prebiotic inulin-type fructans. First, the interactions between two commercial probiotic bacteria in the presence of the prebiotic oligofructose were studied through co-culture fermentations. The degradation of oligofructose by a probiotic Bifidobacterium strain was beneficial for a probiotic Lactobacillus strain, of which the growth was enhanced. Further, the interactions between lactobacilli, bifidobacteria, and pathogenic bacteria in the presence of oligofructose-enriched inulin were investigated. A Lactobacillus strain was most competitive under those conditions and the growth of a Bifidobacterium strain was less pronounced. In contrast, the growth of pathogenic bacteria, such as Escherichia coli and Salmonella, was not inhibited. Knowledge on such interactions is important for a better understanding of the physiology of the microbiota of the human colon.