What’s intestinal flora?4

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The large intestine is by far the most heavily colonized region of the digestive tract, with up to 1012 bacteria for every gram of gut contents. 

In general, intestinal bacteria may be divided into species that exert either harmful or beneficial effects on the host. Health-promoting effects may be caused by the inhibition of growth of harmful bacteria, stimulation of immune functions, lowering of gas distention problems, improved digestion and absorption of essential nutrients, and synthesis of vitamins. Pathogenic effects include diarrhea, infections, liver damage, carcinogenesis and intestinal putrefaction.

An increase in numbers and activities of bacterial groups (such as Bifidobacterium and Lactobacillus) may have desirable health-promoting properties.

References:
4. Gibson GR, Roberfroid MB. Dietary modulation of the human colonic microbiota: introducing the concept of prebiotics. J Nutr  1995; 125 (6): 1401-12.