Using phenotypic approaches, we have detected that 17% of human intestinal Lactobacillus and Bifidobacterium strains could be exopolysaccharide (EPS) producers. However, PCR techniques showed that only 7% harbored genes related to the synthesis of heteropolysaccharides. This is the first work to screen the human intestinal ecosystem for the detection of EPS-producing strains.The human intestinal microbiota has around 10 times more cells than the human body (2), and its genome ("microbiome") harbors at least 100 times more genes than our own genome (5). However, a significant fraction of intestinal bacteria has not been described yet, making it difficult to understand the mechanisms of communication among the microbiota, host cells, and intestinal environment (21). Bifidobacterium and Lactobacillus species are common inhabitants of the gastrointestinal tract, and they have received special attention because of their long history of safe use in foods and probiotic effect (11). Some probiotic strains are able to adhere to intestinal mucus, and we have postulated that exocellular polysaccharides isolated from lactic acid bacteria (LAB) and bifidobacteria interfere with the adhesion of probiotics and pathogens to human intestinal mucus (9, 10). Therefore, the production of exopolysaccharides (EPS) could also be an interesting property to consider for the selection of putative probiotic strains (17). Currently, the most suitable approach to the search for novel EPS-producing (EPSϩ) strains is the exploration of wild bacteria (6). The aim of this study was to investigate the EPSϩ capabilities among Lactobacillus and Bifidobacterium strains isolated from the human intestinal ecosystem as the initial step for further investigation of the roles of EPS in bacterium-host interactions and in human health.We have employed 362 Lactobacillus and Bifidobacterium strains, previously isolated from fecal and mucosal samples of healthy adult volunteers (3, 4), and 4 reference EPSϩ bifidobacteria (Bifidobacterium animalis subsp. lactis IPLA-R1, Bifidobacterium longum NB667 [10], B. longum BL1, and B. longum 667Co) for screening of EPS production in solid media. Strains were grown at 37°C for 72 h under anaerobic conditions on the surface of MRS agar containing 0.25% L-cysteine (MRSC agar) and supplemented with 2% glucose, fructose, lactose, or sucrose added separately. Sixty putative EPSϩ strains were detected, most of them being mucoid (92%) and only 5 (8%) having a "ropy" character. The percentage of putative EPSϩ strains (17%) was similar to that reported in the literature for EPSϩ LAB isolated from food environments (1, 6, 12) and from animal origins (14). Amplification, sequencing, and comparison with database sequences for the V1-V2 variable region of the 16S rRNA gene (7, 19) identified 35 out of our 60 putative EPSϩ strains as belonging to the genus Bifidobacterium and 25 as belonging to Lactobacillus (with nucleotide identity at the species level higher than 98%). The EPSϩ bifidobacteria were Bifidobacterium pseudocatenulatu...