Diversity of bacteria within the human gut and its contribution to the functional unity of holobionts.

The composition of bacteria in the human colon has been a subject of interest since the beginning of microbiology. With the development of methods for culturing strict anaerobic bacteria under multiple culture conditions, it was shown the gut contained more than 400 bacterial species and different people harbor different abundant species. The term "gut microbiome" in this review refers to bacteria studied in stool samples. Molecular methods for determining the bacterial composition of human gut has revealed more than 3000 species and less than 130 genera, indicating that the diversity of human colonic bacteria is concentrated at the species and strain levels. This review concludes with a discussion of how diversity can lead to unity of individual holobionts, between holobionts, and between populations. One of the reasons for the unity is that different bacterial species can have similar functional genes.