Caenorhabditis elegans-microbe interactions are here broadly defined and encompass the associations with all microbes that are temporarily or permanently living in or on this nematode. The microbes might engage in a commensal, mutualistic or pathogenic interaction with the host and include bacteria, viruses, unicellular eukaryotes, and fungi. In nature C. elegans harbours a variety of different microbes. In contrast, C. elegans strains that are cultivated in laboratories for research purposes have lost their naturally associated microbial communities and are commonly maintained on a single bacterial strain, Escherichia coli' OP50.