Probiotics are commonly defined as viable, non-pathogenic microorganisms (including bacteria and/or yeast). When administered in adequate amounts, they are able to confer health benefits to the host. The main beneficial activities of probiotics intake include digestive tract health and immune modulation. In particular, probiotic consumption can regulate the composition of the microbiota and consequently make an influence on the balance of intestinal microbiota. The possible beneficial effects of probiotics are believed to be the process of production of antimicrobial substances (including organic acids, hydrogen peroxide, antifungal peptides, and bacteriocins), enhancement of host mucosal barrier integrity, competition with pathogens, and among others. It is widely known that the majority of species characterized by probiotic properties belong to the genera Lactobacillus and Bifidobacterium, which commonly exist in the gastrointestinal tract of humans and animals. Besides, there are also members of other bacterial genera that have been demonstrated to have health benefits, such as Enterococcus, Bacillus, and yeast Saccharomyces. The health benefits of probiotics are strictly strain-specific, despite belonging to the same species distinct strains may have different effects. Therefore, accurate characterization of novel potentially probiotic strains is required, and appears to be particularly important.
The free-living nematodes Caenorhabditis elegans reside in the soil and decaying organic matter, where they feed on bacteria and fungi. For C. elegans, microbes can be food (for example, as part of a beneficial gut microbiome), pathogens, parasites, and even competitors. They can interact with vector organisms that facilitate dispersal to new habitats, as well as compete with host for similar food environments. Given the high degree of exposure to microbes in the natural environment, the C. elegans intestine may be populated by plentiful various micro-organisms. It is reported that 18 species of bacteria were identified in the microbiota of C. elegans that had been fed on soil and rotting fruits, and the natural microbiota conferred protection from pathogenic P. aeruginosa infection. And similar habiting bacteria species were found in the C. elegans intestine, although the samples were from the distinct geographical origins over the world.
As a differentiated multi-cellular organism, C. elegans is similar to mammals in many aspects of physiology. The C. elegans intestine is composed of 20 non-renewable epithelial cells. Despite the simple composition, the nematode has polarized epithelial cells with microvilli that are structurally attached to a terminal web composed of actin and intermediate filaments underneath the apical membrane, which is a key similarity with the human intestine. The functions of C. elegans intestine include not only assimilate nutrients, but also enable to detoxify metabolites and toxins. Besides, as a second-largest surface area in contact with the environment, the intestinal lumen consists of the first line of defense to invading pathogenic microbes, thereby many host-microbe interactions occur in the intestine.
The C. elegans can be used in in vitro studies through culturing on a lawn of Escherichia coli in labs. The nematode has been widely applied as an experimental system for biological studies. In particular, the numerous advantages, including short reproductive cycle, the transparent gut, ease of handling, and the ability to create a defined gut microbiota make it an ideal candidate organism for testing the effects of ingested bacteria on host physiology. Previous studies had indicated that ingestion of Bifidobacterium and Lactobacillus spp. showed a positive impact on the health and lifespan of C. elegans and the Lactobacillus exhibits modification of host defense that protects against the foodborne pathogens Enterococcus faecalis and Staphylococcus aureus. These studies highlight the power of the worm in vivo model. Furthermore, it is notable that "germ-free" C. elegans can be easily generated by bleaching, which is a unique advantage over other experimental animal models. Therefore, C. elegans is becoming an increasingly valuable in vivo model for studying host-probiotic interactions and for providing mechanistic insights on the beneficial effects of probiotics.
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