They reside in very specific ecological niches in the gastrointestinal tract allowing them to closely interact with the host (us) biology. The presence of the gut microflora is critical for the proper absorption of nutrients, vitamin synthesis (including B and K) and the break-down of fiberous tissues for the production of health-promoting metabolic products like the short-chain fatty acids (SCFAs). The SCFAs are the fermentation products of certain classes of bacteria that contain enzymes to breakdown fibers indigestible to humans. Butyrate, propionate and acetate are the major health-promoting SCFAs and their sustained concentrations in human blood circulation have been linked to reduced gastrointestinal and metabolic diseases.

Each human being has a unique microflora signature. The relative populations of our gut microflora is dependent on several aspects including: birth method (vaginal vs. cesarean), length of time breast-feeding, exposure to microbes during childhood, antibiotic usage and dietary habits. Based on the latter (diet), three distinct classifications based on the types of of bacteria present have been made. These are known as enterotypes and they are age, gender, body weight and location-independent.

Class I: high Bacteriodes (high protein and animal fats)
Class II: few Bacteriodes but elevated Prevotella (high carbohydrates)
Class III: high Ruminococcus