Bacterial adherence to the intestinal epithelium plays a role in niche establishment in the gut lumen. Through sampling natural populations of Caenorhabditis, we discovered several bacterial species that adhere to the intestinal epithelium via polar, intimate association, best described as attachment. These bacteria had varying effects on host fitness and physiology, with one species having negative effects, and the others exhibiting neutral effects. These bacteria can actively divide in the gut lumen, either replicating throughout the gut simultaneously or anteroposteriorly. In competition assays, animals pre-colonized with an attaching commensal bacteria reduced colonization by the pathogenic bacteria, but this effect was not seen when animals were colonized by both species simultaneously. Regardless of the colonization paradigm, populations exposed to both bacteria showed a near-identical mitigation of the pathogenic effects. Altogether, these strains illustrate the capacity of microbiome bacteria to attach, replicate, and establish a niche across the entire intestinal lumen.