The Development of Our Organ of Other Kinds—The Gut Microbiota

Gut microbiota is a collection of microbial organisms that naturally exist within the gastrointestinal (GI) tract. It is clear that the gut microbiota is involved in host physiological development, defense against pathogens and diseases. Thus, gut microbiota is regarded as a hidden organ composed of other kinds (Bocci, 1992). A feature of the gut microbiota is that the compositional microbes are host species-specific. For a given host, a defined core microbes exist which distinguish one host from another (Kostic et al., 2013). For example, Hawaiian bobtail squid selectively acquires Vibrio. fischeri (V. fischeri) from environment and maintains them in its light organ. This symbiont is so selective that other V. fischeri strains, such as a fish-associated V. fischeri strain are not able to colonize the squid (Mandel et al., 2009; Kostic et al., 2013). Zebrafish gut is predominated by members of the γ-Proteobacteria and Fusobacteria classes (Kostic et al., 2013). Wild mouse gut is primarily composed of Bacteroidetes, Firmicutes, Tenericutes, and Proteobacteria (Hoy et al., 2015; Weldon et al., 2015; Xiao et al., 2015). Interestingly, transfer of the conventional zebrafish microbe to germ-free (GF) mice did not result in a zebrafish-like gut microbiota in the recipient mice; vice versa, conventional mouse gut microbe-transplanted GF zebrafish did not resemble the gut microbes of mice. Rather, the gut microbiota of both recipients after transplantation resembled the microbiota of their conventional species. Thus, the host gut predefined its microbiota (Rawls et al., 2006). While host-microbe interaction is a two-way crosstalk (Celluzzi and Masotti, 2016), in this opinion article, I wish to discuss intrinsic factors that contribute to the development of gut microbiota. Namely, where the microbes come from and how the host selects them.