Probiotics has been defined as live microbial feed supplements that beneficially affect the host by improving the intestinal bacterial balance. Bacterial species commonly referred to as probiotic are lactic-acid producing bacteria such as Bifidobacterium spp., Lactobacillus spp. and Streptococcus spp., but also other microorganisms like the fungi S. boulardii have been used. There are many properties of probiotic bacteria that may help to improve the intestinal milieu like their metabolism, cell-wall structures and intracellular components.

Suggested beneficial effect of probiotics:

• Increased nutritional value (better digestibility, increased absorption of minerals and vitamins).
• Promotion of intestinal lactose digestion.
• Positive influence on the intestinal microflora (antibiotics or radiation induced colitis).
• Prevention of intestinal tract infections (bacteria or virus induced, Candida enteritis, Helicobacter pylori ulcers/neoplasia).
• Regulation of gut motility.
• Improvement of the immune system
• Prevention of cancer.
• Reduction of catabolic products eliminated by kidney and liver.
• Prevention of atherosclerosis by the reduction of inflammation!
• Prevention of osteoporosis.
• Improved wellbeing.

Lactic-acid-producing bacteria metabolise nutrients by fermentation. During this process different organic acids are formed like (L+) lactic acid and butyric acid, which lowers the intestinal pH. The lowered pH has many beneficial effects on the intestinal milieu like inhibiting growth of putrefactive bacteria. Putrefactive bacteria produce nitrogen waste products during digestion of nutrients. The benefits of inhibiting growth of these bacteria are the reduction of blood ammonia, free serum phenols and free amino acids in the urine. This implies improved nitrogen retention. These characteristics could be responsible for the growth and improved body weight (muscle volume) seen in different animal studies. Reduced concentration of nitrogen related toxins play an important role in the detoxification of the liver. Uric acid, the natural antioxidant produced in the liver as a response to excess amounts of free radicals, is related to the level of dysbiosis in the intestinal tract. This explains why probiotics will stimulate liver regeneration and subsequently would be used in a “liver support program”.

It is often claimed that a probiotic culture has to be able to adhere to the intestinal epithelia and colonise the intestine to have any effect. This issue is controversial. First of all, what is the definition of a “colonising bacteria”? Colonising bacteria are sometimes referred to bacteria that are more permanently found in individuals flora but in other cases bacteria remaining in the intestinal flora for 1-3 days are considered as colonising. In constipated people transient bacteria can probably be found in faeces for longer periods than three days.

A method, often used to test bacteria’s ability to colonise the intestine is to see if they adhere to intestinal epithelial cells in vitro. This is not a fully adequate way of looking at colonisation ability since there are other habitats for colonising bacteria than the epithelia like the overlying mucosal layer and loose particles. Thus, bacteria with negative results in the test mentioned may anyway be able to colonise. Ninety-nine per cent of the intestinal microflora consists of anaerobic bacteria, which means that they die in the presence of oxygen. The area closest to the epithelia is probably not the preferable habitat for these bacteria since oxygen diffuses from underlying blood vessels to the epithelial cells and then to the supra-epitelial area. This supports the fact that bacteria must not be able to adhere to the intestinal epithelia to be able to colonise.
Lactobacillus GG have shown to reduce traveller’s diarrhoea, antibiotic-associated diarrhoea and relapsing Clostridium difficile colitis even though only “colonizing” for 1-3 or 7 days, which would be considered as transient.

Bacterial strains regarded as transient can certainly also have effect when administered in high counts by interaction with the immune system and by improving the intestinal environment, stimulating growth of the normal bacterial flora.

Nutritional aspects of probiotics

All mineral- and vitamin substitution will demand a healthy intestinal ecosystem. The microflora must be in a good shape and in balance for the nutritional system to be able to effectively utilise minerals and vitamins. This is often not the case. The metabolism of proteins, lipids and carbohydrates will be improved if the bacterial flora is balanced. The endocrine, the immune and the nutritional system are dependent on that both the external and the internal ecology are in balance.

Daily supplementation of friendly bacteria, probiotics is needed to replace the bacteria we are supposed to get and that we did get earlier via the food but is deficient today due to modern technique of preserving food.

All mineral- and vitamin supplementation demands a healthy intestinal ecosystem.

Bacteria consume vitamins. When there is an overgrowth of colonic bacteria in the small intestine the loss of vitamins due to bacterial consumption can be remarkable and lead to vitamin deficiency. Probiotic bacteria inhibit overgrowth of colonic bacteria in the small intestine and therefore reduce the loss of vitamins. Compare to imbalances in spite of additional intake of vitamins.

An improved effect on the mineral absorption due to:

• Osmotic effects
• Increased production of organic acids due to fermentation
• Increased formation of soluble salts of these acids
• Proliferation of the intestinal wall

Enzyme induction or enzyme support will improve the absorption of nutrients.

The stimulated bifidobacteria increase the production of the vitamins B1, B2, B3, B6, B7, B9 and B12. These vitamins are absorbed by the intestine and utilised by the different tissues of the human body.

Reduced production of toxins or so-called free radicals, which consumes vitamins, minerals and antioxidants.

To be noted: most production of free radicals is to be seen in the intestine. This will increase if the microflora is dysbiotic. Bifidobacteria and lactobacilli produce organic acids, which will work as anti-oxidatives. Studies by Kaizu et. al. show that there is a reduction of approximately 70% of the oxidative activity in the intestine during supplementation of lactic-acid producing bacteria, which also implied a better status of vitamin E.

Immunological aspects of probiotics

Probiotics has been defined as a live microbial feed supplement that beneficially affects the host by improving the intestinal bacterial balance.  Probiotics have a balancing effect on the immune system, both the innate and specific immunity. It is though important to notice that different probiotic bacteria may elicit different kinds of immunological responses, differently strong.

The administration route of bacteria is also important to consider when testing the immunological response to probiotics. Many bacterial species are compromised or damaged during the pathway through the stomach and upper small intestine. Using acid and bile acid resistant strains and by using protective tableting matrix technique assures that the main part of the supplemented bacteria reach the intestine and can be immunologically active.

The normal intestinal microflora is part of the innate immune system. By  “competitive colonisation” commensal bacteria inhibit pathogenic bacteria from colonising the mucosal tissue, which is the initial step of infection. Probiotic bacteria inhibit colonisation of pathogens by producing organic acids like lactic acid and butyric acid, which lowers the intestinal pH and in that way inhibit growth of potentially pathogenic microorganisms. They also produce other bactericides that inhibit growth of certain bacteria. Competition about nutrients and space are also involved in the term competitive colonisation. The antimicrobial activity of probiotic bacteria explains the usefulness of such bacteria for conservation of food.

Regarding the influence of probiotics on the cell-mediated immunity, research has shown that intake of probiotic bacteria increases the production of IgA antibodies and increases the activity of macrophages and NK-cells, which leads to an increased killing of bacteria. It has also been shown that probiotic bacteria modulate the cytokine activity. Cytokines are signal substances in the immune system, which regulates the activity of the immune cells. Probiotics stimulates the immune system in an “anti-allergic” direction with decreased production of IgE antibodies as a result. Cytokines also work as a link between the immune system and the nervous system.

The intestinal barrier

The mucosa of the intestines is a selective barrier which, when functioning normally, prevents the penetration of potentially noxious agents into the bloodstream, while still enabling the passage of useful substances, such as nutrients. The health of the intestinal wall is totally crucial for our health.Probiotic bacteria have a healing effect on the intestinal mucosal membrane by stimulation of the formation of epithelial cells and by decreasing inflammation in the intestine, shown by a decreased production of inflammatory cytokines during intake of probiotic bacteria. A prerequisite for development of allergy is that substances that in normal cases do not pass the intestinal barrier leak out into the blood and come in touch with the immune system. It has been shown that atopic eczema is associated with intestinal inflammation and increased antigen transfer over the intestinal barrier. Treatment of food-allergies with probiotics has shown to have positive effects.

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