Dietary effects and host-microbe interactions. General principles.
Elisabeth Norin
Microbiology and Tumor Biology Center, Karolinska Institute, Stockholm, Sweden.
The number of cells in the intestinal flora in man is very high. Estimates tell us that the microbial community account for 90% of 'our' total number of cells in the body and only 10% are host originating cells. Thus, the intestinal flora should be regarded as one of the largest organ in the body - a metabolic active `organ� - which conduct a multitude of biochemical reactions most often beneficial for the host. The microbes, to a large extent, serve the host by degrading dietary substances, otherwise non-digestible by the host, and at the same time the microbes themselves obtain specified nutrient-rich niches where they get energy for surviving and multiplying. An example of this is that conventionally raised laboratory animals require 30% less caloric intake to maintain their body weight than their germ-free counterparts.
By applying the GAC/MAC consept, (Germ-free Animal Characteristic and Microflora Associated Characteristic) in nutritional studies and by using germ-free, gnotobiotic and conventional animals, series of intestinal co-functions can be evaluated. The interactions between the normally occurring intestinal bacteria and their host is most often mutually beneficial for the partners, while a pathogenic relationship benefit just one partner on the expense of the other.
I will exemplify by talking about polysaccharides, which form the bulk of most ingested diets. Mammals have almost no capacity to utilise these polymers in the intestine. However, microbial digestion will benefit the host by hydrolysing these carbohydrates to smaller, absorbable molecules, and at the same time, the microbes also obtain niches for their survival and multiplication.
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