By Lennart Cedgård MD and CEO Wasa Medicals

There is a whole world of "microlife" in our intestines. The normal intestinal microflora build up a fantastic ecosystem, which is extremely complex and impossible to understand completely. What we know today is that our intestinal microflora consists of about E14 cells, which is ten times the number of human cells in our body. The normal microflora, sometimes called the biggest “organ” in the body has a weight of approximately 0,5-1,0 kg and at present it is estimated to consist of 1000 different species. However, recent genetic research shows that, when mapping the genome of the colonic bacteria only 35 per cent in previous times were to be identified and assigned to known bacteria. The remainder were unknown! We know today that the amount intestinal viruses are 10 times that of bacteria still to be defined.

The stomach and the upper part of the small intestine contain low counts of bacteria (E3–5/ml gastric juice) due to the gastric- and bile acid. Further down in the small intestine the number of bacteria increase to E6-7 and in the colon the counts are as high as E11/g faeces.

The intestinal microflora consists of both aerobic bacteria, which need oxygen to survive, and anaerobic bacteria that will die in the presence of oxygen. There are also facultative anaerobic bacteria, which can live, in both aerobic and anaerobic milieus.Immediately after birth, bacteria start colonising the skin and the mucosal membranes like the respiratory tract and the intestine. During the first days of life the milieu in the intestine is rich in oxygen. Therefore the first colonisers are aerobic and facultative anaerobic bacteria like E.coli and other Enterobacteriaceae, Enterococcus spp and Staphylococcus spp.. When these bacteria start proliferating they consume oxygen so the intestinal milieu will become more and more anaerobic, which in turn makes it possible for anaerobic bacteria like Bacteroides, Bifidobacterium, Clostridium and Lactobacillus to start colonising. Within a few days anaerobic bacteria will dominate the intestinal microflora of the infant and in adults more than 99% of the bacteria in the intestine are anaerobic. The intestinal microflora of infants are much simpler and much more liable to fluctuate than the one of the adults, which is generally very “stable”. During the weaning period the infants' bacterial flora starts to resemble the one of the adult but may not be fully developed until 4-8 years of age.

The dominant species in colon of an adult are Bacteroides and Bifidobacterium whereas Lactobacillus spp. and Streptococcus spp. dominate the small intestinal microflora. It is possible that the flora in the proximal parts of the gastrointestinal tract, though quantitatively much smaller than that in the colon, is most important since its products pass the absorptive part of the intestine. The bacteria in the small intestine may also have the strongest influence on the immune system since the gut-associated-lymphoid-tissue (GALT) is situated mainly in the small intestinal wall estimated to contain approximately 80% of the total immune system.

Some bacteria are said to be colonising, which means that they are able to colonise, proliferate and remain in the intestine for longer periods, weeks, months or years. Other bacteria are referred to as transient. These bacteria enter the gastrointestinal tract via food or drinks and are in transit from the mouth to the anus. Most probiotics are transient even if said to be colonizing as they remain in the colon up to 2 weeks after intake.

The habitats of the normal bacterial flora in the intestine are shredded intestinal cells or food particles, the mucosa, or rather the overlying mucous layer and the Lieberkühn's crypts. The crypts have shown to contain a very specific flora, often consisting of a single species. Both the crypt flora and the mucosal flora may be physiologically important but not reflected in faecal cultures. Another fact to consider is that the cell turnover may be great although the culture counts are not impressive.

Even though, the intestinal flora of adults is considered as stable as to the composition of species, new strains are entering the intestinal tract all the time via food and beverages or in other way swallowed and other strains leave via the faeces. Thus, there is a constant turnover of bacteria in the intestine, which is very important for the stimulation of the immune system.

In 1972 van der Waaij introduced the term “competitive colonisation” which describes the interference between the normal intestinal microflora and the invading pathogens. By competing about nutrients and space and by secretion of antibacterial substances the normal intestinal microflora can prevent pathogenic bacteria from colonising. In germ free animals only 10-100 salmonella bacteria is needed for infection to occur while in a conventional animal it takes 1.000.000 bacteria. The prerequisite for a bacterial infection to occur is that the normal microflora is disturbed.

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