Flow cytometric sorting based on its large cell size enabled an enriched fraction of Oscillospira guillermondii cells to be obtained from the rumen contents of a sheep. Phylogenetic analysis based on cloned 16S rDNA sequences indicated that the bacterium is a member of the low-G+C Gram-positive bacterial cluster. Sporobacter termitidis and Papillibacter cinnamivorans were the most closely related known species, with sequence similarities of only 86?3-88?1 %. Fluorescently labelled 16S rRNA-targeted oligonucleotide probes specific for Oscillospira were designed and applied to the rumen sample from which the enriched fraction was obtained. The probes hybridized specifically with the large, morphologically conspicuous Oscillospira cells.Oscillospira guillermondii is a large bacterium (3-66 10-40 mm), often observed in the rumen contents of sheep and cattle as well as the alimentary tract of other herbivorous animals. Oscillospira is characterized as a Grampositive bacterium, with closely spaced transverse septa and endospores (Gibson, 1974(Gibson, , 1986Grain & Senaud, 1976;Stewart et al., 1997). Although this bacterium was first described almost a century ago (Chatton & Pérard, 1913), growth in pure culture has not been reported (Gibson, 1974(Gibson, , 1986Stewart et al., 1997), hence, little is known of its ecological role and physiological properties in the intestinal tract. Clarke (1979) found that Oscillospira and other large bacteria attached rapidly to the cuticular surface of clover and grass leaves in the rumen, suggesting that the cuticle of green leaves constitutes a specific niche for these bacteria. Warner (1966) investigated the relationship between grazing behaviour and changes in rumen microbial populations in sheep and found that the population of Oscillospira in the rumen fluctuated and the length of the trichome also changed depending upon the amount of feed consumed. Kurihara et al. (1968) reported that total counts of Oscillospira in the sheep rumen tended to decrease in the presence of ciliates. Although the number of Oscillospira cells is relatively small compared with other bacterial cells, they may make a significant contribution to rumen fermentation because of their large biomass, roughly equivalent to that of ruminal ciliate protozoa (Clarke, 1979;Williams & Coleman, 1997).To date, culture-based techniques have not enabled the phylogenetic relatedness of Oscillospira to known bacteria to be studied. Flow cytometry (FCM), a well-established technology in cell biology and medical microbiology, is now being applied to microbial ecology studies. FCM allows the rapid analysis of bacterial communities and enables single cells to be detected, quantified and sorted according to differences in size, DNA content or phylogenetic affiliation as assessed by fluorescently labelled rRNA-targeted oligonucleotide probes (Amann et al., 1990;Button et al., 1996;Davey & Kell, 1996;Jansson & Prosser, 1997;Wallner et al., 1997). Very recently, Zoetendal et al. (2002) have detected and enumerated uncultured R...
