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 of 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 increases 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 the 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 ways 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.
The Terrain is decisive!
Why is there a need for probiotic bacteria today?
In modern times, we have dramatically changed the composition of bacteria entering our body via food and drinks. The intake of bacteria, preferably Lactobacillus species is highly reduced. In former times, fermentation was one of the main ways of preserving food, vegetables and dairy products. This assured a constant intake of lactic acid-producing bacteria. Today the situation has changed. Use of freezers, refrigerators and pasteurisation of feeds like dairy products and the use of different kinds of preservatives have replaced fermentation as a method for preservation of food. Today, most people do not get probiotic bacteria via the food in the same amount as in former times. As a matter of fact we get 2g faeces every day without knowing via contamination and by inhalation!
Uses of antibiotics in healthcare and agriculture, and the increased hygienic measures in modern society with antibacterial substances in cleaning agents, toothpaste, deodorants etc., have decreased the amount of total bacteria in our environment. Studies have shown differences between the intestinal microflora of people in developed and developing countries, both in infants and adults.
In developing countries, people have a more varied intestinal microflora with more strains and a higher turn-over of bacteria. In Western industrialised countries we have relatively few strains in the intestinal flora and we keep the same strains for longer periods maybe years. A higher turn-over of bacteria is certainly very important in keeping the immune system stimulated.
On top of this, we have today an increased interference on the bacterial ecosystem in our body compared to previous times, via a changed diet (rich in protein, trans fats and refined products like sugar and poor in fibres), stress, increased intake of medicines like antibiotics, and environmental toxins etc. The need of probiotic bacteria is thus due to the many factors in our society that can damage the intestinal health and the fact that the intake of probiotic bacteria via the food has decreased. A healthy intestinal milieu is crucial for our health.
We have since the discovery of bacteria and the invention of antibiotics had the general idea that the major part of the microbes are pathogenic but it is in fact only a minor part of the thousand of bacterial species that are harmful to humans.High strength probiotics must be seen as the primary food supplement.