The relationship with our intestinal microbes is symbiotic and mutualistic, the benefits for the host being largely due to the metabolic attributes of the resident bacteria that can derive energy from the fermentation of otherwise indigestible carbohydrates. In humans, this accounts for 6-9% of the total energy requirement whereas this can be as high as 44% for ruminants (McNeil 1984). Additional symbiotic functions include the competitive exclusion of pathogens and the production of essential vitamins and amino acids. The coevolution of mammals with intestinal bacteria had a strong impact on the host immune system, which needed to develop the ability to avoid excessive inflammatory responses to antigens and commensal bacteria while retaining the capacity to defend the body against infections of pathogenic microorganisms. Commensal bacteria are not ignored but dynamically controlled via many complex overlapping and intertwined mechanisms regulating innate defenses in response to signals from the microbiota (Wells 2010; Wells 2011).
The references (in blue) may be found below.