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Abbreviations: FISH, fluorescent in situ hybridisation; TTGE, temporal temperature gradient gel electrophoresis analysis
Analysis of the human intestinal microbiota has rapidly evolved during the past 10 years (Tannock et al. 2000). Molecular techniques, based on detecting nucleotide sequences of 16S rRNA using fluorescent in situ hybridisation (FISH) and PCR amplification combined with separation by denaturing gel electrophoresis, have enabled a good assessment of the composition of the microbiota and the complexity of the dominant species present (Blaut et al. 2002). Moreover, these methods provide an opportunity to estimate the transient presence of food-borne bacteria within the predominant intestinal bacterial population (Alander et al. 1999; Tannock et al. 2000; Rochet et al. 2006).
Intestinal microbiota can induce both positive and negative effects on host physiology (Gorbach, 1986; Macfarlane & Cummings, 1991) and there has been great interest in the feasibility of modulating intestinal microbiota in a beneficial way in order to improve health. In this field, lactic acid bacteria with probiotic properties have been considered as potentially useful (Shortt, 1999; Teitelbaum & Walker, 2002; Guarner et al. 2005). Lactic acid bacteria have been used traditionally in food fermentation for thousands of years. Yogurt is milk fermented using a combination of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus salivarius ssp. thermophilus . More recently, other bacteria, mainly lactobacilli or bifidobacteria, have been combined with yogurt starters to produce fermented milks with specific functional properties related to health. Fermented milks deliver a large number of lactic acid bacteria into the gastrointestinal tract. These micro-organisms are capable of partially resisting gastric and bile secretions in vitro and in vivo (Mater et al. 2005) and can deliver enzymes and other substances into the intestines (Marteau & Rambaud, 1993; Oozeer et al. 2002, 2005). The ability of yogurt to provide β-galactosidase and reduce symptoms of lactose intolerance has been well documented (Marteau & Rambaud, 1993; Sanders, 1993). Lactic acid bacteria have also been claimed to modify the intestinal environment. Some studies have shown that L. acidophilus , L. casei and Bifidobacterium bifidum can decrease potentially harmful enzymes of the resident microbiota such as β-glucuronidase and nitroreductase (Goldin, 1986; Marteau et al. 1990; Guerin-Danan et al. 1998). These results were obtained during clinical trials involving volunteers who for a few weeks consumed...