Species-specific 16S rRNA-targeted, Cy3 (indocarbocyanine)-labeled oligonucleotide probes were designed and validated to quantify different Eubacterium species in human fecal samples. Probes were directed at Eubacterium barkeri, E. biforme, E. contortum, E. cylindroides (two probes), E. dolichum, E. hadrum, E. lentum, E. limosum, E. moniliforme, and E. ventriosum. The specificity of the probes was tested with the type strains and a range of common intestinal bacteria. With one exception, none of the probes showed cross-hybridization under stringent conditions. The species-specific probes were applied to fecal samples obtained from 12 healthy volunteers. E. biforme, E. cylindroides, E. hadrum, E. lentum, and E. ventriosum could be determined. All other Eubacterium species for which probes had been designed were under the detection limit of 10 7 cells g (dry weight) of feces ؊1 . The cell counts obtained are essentially in accordance with the literature data, which are based on colony counts. This shows that whole-cell in situ hybridization with species-specific probes is a valuable tool for the enumeration of Eubacterium species in feces.The genus Eubacterium (41) contains anaerobic, non-sporeforming, gram-positive rods which are distinguished from other genera mainly on the basis of negative metabolic characteristics (37). In the human intestinal tract, Eubacterium is the second most common genus after the genus Bacteroides and is more common than the genus Bifidobacterium (16). The importance of members of the genus Eubacterium has been reported previously (10,24,38,48). Since the identification of Eubacterium species based on phenotypic traits requires experience and is time-consuming (5,15,16,36,44), many studies involving human fecal flora composition have refrained from looking at this genus (13,20,34,42).Considerable effort has been invested in the application of molecular techniques such as PCR (19,27,50) and hybridization (12, 25, 52) for the identification of fecal bacteria. However, the extraction, purification, and amplification of nucleic acids by PCR from fecal samples are often selective and limited. In contrast, whole-cell hybridization with fluorescently labeled, 16S rRNA-targeted oligonucleotide probes allows the determination of the numerical abundance of bacteria, including unculturable strains (39), in various ecosystems such as water (1,26,32), sludge (23), and fecal samples (17,18,29).In contrast to the genus Bifidobacterium, for example, which is a phylogenetically and phenotypically well-defined taxon, Eubacterium species (9, 31) are phylogenetically diverse and thus it is not possible to design a genus-specific probe for Eubacterium spp. Therefore, probes for phylogenetic clusters or species have to be considered. The development and validation of a probe for the detection of a Eubacterium species has been reported recently (45).The purpose of this study was to develop and apply 16S rRNA-targeted oligonucleotide probes to human feces for the detection of numerically dominant fecal Eubact...