According to the classical definition probiotic organisms regulate the microbial colonization in the digestive tract with a beneficial effect on human health (Gedek 1991) In relation to current and future probiotic products this traditional definition seems to be too narrow (Jakobsen and Narvhus 1996). The definition of a probiotic starter culture should be added a third-dimension, i.e., the extra nutritional-physiological values leading to a more general improvement of human health (Kurmann 1993; Lambelet et al. 1992). As such, the role of yeast as probioticum in dairy products has been overlooked. Studies of probiotic yeast as starter culture in dairy product, so far, has been very limited even though yeast occur in many dairy-related products. Historically, yeast as a probioticum has been linked with livestock feed and to the genus Saccharomyces. As reviewed by Gedek (1991) S. boulardii is a nonpathogenic yeast and is used both as preventive and therapeutic agent for the treatment of different diarrhoeal diseases (McFarland and Bernasconi 1993; Surawicz et al. 1989). S. boulardii was first isolated in Indonesia in the 1950s. In the fourth edition of the taxonomical study "The Yeasts" edited by Kurtzman and Fell (1998). S. boulardii is taxonomically classified under the name S. cerevisiae.
Among few studies on the use of probiotic yeast in dairy products to be mentioned is that Lourens-Hattingh and Viljoen (2001) investigated the potential of adding S. boulardii to yoghurt. The result showed that S. boulardii was able to survive four weeks at 4°C and even multiply to a level of about 106 in plain and fruit yoghurt. Gas and alcohol production were not observed, but it might be a constraint for incorporating S. boulardii into retail bio-yoghurt (Lourens-Hattingh and Viljoen 2001).
Several specific interactions between S. cerevisiae and enteric pathogens, e.g., Escherichia coli, Salmonella, and Shigella have been reported. Further, S. cerevisia is reported to bind enterotoxin from enterobacteria to the surface of the yeast through a mannose-specific reaction (Gedek 1991). It has been reported that S. cerevisiae is able to survive the passage through the intestinal tract, with live cells detectable in the small intestine, which emphasize its potential as a probiotic.
Exploitation of killer factors as anti-microbial compounds could lead to another possibility for using yeast as probiotic. Killer toxins are proteins or glycoproteins secreted by the yeast cell and toxic to specific range of micro-organisms, e.g., enterobacteria (Brugier and Patte 1975; Polonelli and Morace 1986). Many yeast also produces metabolites, e.g., short-chain fatty acids known to be toxic against undesired micro-organisms in the intestinal tract (Gedek 1991).
Even though most yeast strains are considered to be safe, it is noteworthy that some strains found in dairy products, e.g., C. catenulata, G. geotrichum, and C. kefyr are seen as opportunistic pathogens (Radosavljevic et al. 1999), and could represent a risk for human health especially for immunocompromised persons (Minervini et al. 2001; Radosavljevic et al. 1999).
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