Symbiosis is an intimate relationship between two organisms where both organisms benefit directly from each other. Most symbiosis occurs between plants or animals and microbes. We have symbiotic bacteria in our guts that break down food into smaller molecules that our intestinal cells can absorb and the bacteria in return get food and a protective environment. Establishing a symbiotic relationship among many different animals (vertebrates and invertebrates) with their microbial counterparts is an important relationship that is detrimental to the fitness of the host.
Figure 1: Complexity of gut microbiota contributions to host physiology
Figure 2. Regression of ciliated epithelialium post V. fischeri infection (top) and remodeling of light organ (bottom).
Figure 4: Overview of some interactions with gut microbiota and host immune system