The animal intestine is home to an abundance of microorganisms that play diverse roles in host health and disease. Characterizing the gut microbiota has become an active area of research, but the relative contribution of factors shaping microbiota composition is still unclear. Gut microbiota composition may be determined by environmental availability or shaped by host factors, enabling the selection of potentially beneficial taxa. Thus, it is of interest to understand the significance of those selected microbes, or those that are lost due to aberrant assembly, for their host. Using the genetically tractable Caenorhabditis elegans as a model host and 16S rDNA 'deep sequencing,' we are studying the assembly of the gut microbiota starting from diverse environmental communities. Comparisons of worm microbiotas to microbiotas in their soil environment revealed that worm microbiotas resembled each other, even when assembled from different microbial environments. Our results support a dominant contribution of the host niche in shaping the gut microbiota. Among the recurring taxa in the gut microbiota, we found Enterobacter and Pseudomonas, both of which provide services to their hosts. One resident, P. mendocina, is able to provide protection against the gram-negative pathogen P. aeruginosa by priming the
p38-oxidative stress pathway, while E. cloacae can extend survival of worms infected with the gram-positive pathogen, Enterococcus faecalis. Assessing these interactions between gut isolates and their host will fill in a gap of our understanding of C. elegans interactions with microbes, and will help establish C. elegans as a model for studying host-microbiota interactions.