A good understanding of how microbes interact with hosts has a direct bearing on our capability of fighting infectious microbial pathogens and making good use of beneficial ones. Among the model organisms used to study reciprocal actions among microbes and hosts, C. elegans may be the most advantageous in the context of its unique attributes such as the short life cycle, easiness of laboratory maintenance, and the availability of different genetic mutants. This review summarizes the recent advances in understanding host-microbe interactions in C. elegans. Although these investigations have greatly enhanced our understanding of C. elegans-microbe relationships, all but one of them involve only one or few microbial species. We argue here that more research is needed for exploring the evolution and establishment of a complex microbial community in the worm’s intestine and its interaction with the host. 1. Introduction Host-microbe symbiosis exists in almost all animals, and the symbiotic bacteria can be profitable, harmful, or of no effect to the host. For example, the harmless Escherichia coli strains commonly found in organismal intestine are a normal part of the gut flora and can advantage their hosts by producing vitamin K [1] and by keeping pathogenic bacteria from colonizing the intestine [2, 3]. By contrast, some others like E. coli strain O26 can cause diseases in its hosts [4]. The interactions between host and microbe form complicated networks. Understanding these interactions can help us effectively cure the diseases caused by pathogenic microbes and promote good health in animals by benign microbes. A number of model organisms, for example, C. elegans, Saccharomyces cerevisiae, Drosophila melanogaster, Arabidopsis, zebrafish, and mice, have been used to study the mechanisms involved in host-microbe interactions. Among these organisms, C. elegans has its unique characteristics that can be regarded as its advantages being a model animal. The nematode C. elegans is a free living, multicellular invertebrate. Its two original strains were isolated from soil in France [5] and mushroom compost in England [6] more than a half-century ago. Naturally C. elegans lives on microbes including bacteria and fungi, and it is also a natural host of some pathogenic microorganisms such as the Gram-negative bacterium Microbacterium nematophilum, the fungus Drechmeria coniospora, the microsporidian parasite Nematocida parisii, and the Orsay virus [7, 8]. Major attributes of C. elegans as a model include its tractability to be easily maintained in the laboratory, the
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