The bacteria maintain a complex and generally beneficial relationship with the host when retained in the gut, but when they escape this environment they can cause significant pathology, including bacteremia and abscess formation in multiple body sites.

Carbohydrate fermentation by Bacteroides and other intestinal bacteria results in the production of a pool of volatile fatty acids that are reabsorbed through the large intestine and utilized by the host as an energy source, providing a significant proportion of the host's daily energy requirement (118).

Across multiple studies, the ratio of Firmicutes/Bacteroidetes (two major bacteria phyla) has been correlated with obesity, with a higher ratio (more Bacteroidetes) associated with leanness and a lower ratio (more Firmicutes) associated with higher body fat (Kasai et al., 2015).

Firmicutes, due to their negative influence on glucose and fat metabolism, are commonly referred to as bad gut microbes¹⁰.