Figure 1.

The gastrointestinal tract has a variety of favorable or less favorable habitats for micro-organisms, extending from the oral cavity to the anus. Furthermore, differences in conditions, due to the physiology or the substrates present (e.g., those deriving from food), can act as microbial selection “forces” for each environment. Along the gastrointestinal tract, micro-organisms are predominantly found in the large intestine and ileum, while a smaller number are observed in the jejunum. The stomach and duodenum are colonized by unique species [36,37]. Despite various difficulties, studies have shown that the human gastrointestinal system is dominated by two phyla: Bacillota and Bacteroidota [38]. Credit: Original figure by I.A. Charitos.