Probiotics, proven to be useful in certain situations and accepted by science.
The World Health Organization deemed probiotics to be an important immune defence system.
Lactobacilli are known to produce bacteriocines which have an immuno-relevant, health supporting function against potential pathogenic species.
L. reuteri displays tremendously various effects. Especially important is the synthesis of reuterine. This substance inhibits growth of gram negative and gram positive bacteria, yeasts, fungi as well as protozoa, thus it represses pathogens in the gut. Furthermore, L. reuteri is a producer of short chain fatty acids, which are antimicrobial agents themselves. Finally, it also produces hydrogenperoxide.



All these anitimicrobial agents do not display their bacteriocidal and bacteriostatic effects on commons. Competitive inhibition for bacterial adhesion sites on intestinal epithelial surfaces is another mechanism of action for probiotics. Consequently, some probiotic strains have been chosen for their ability to adhere to epithelial cells.
Competition for nutrients has been proposed as a mechanism for probiotics. Probiotics may utilize nutrients otherwise consumed by pathogens.
Another postulated mechanism is through degradation of a toxin receptor on the intestinal mucosa.

In a recent report the DGE (Deutsche Gesellschaft für Ernährung) confirms the health promoting effect of Lactobacilli and Bifidobacteria, present in fermented, not heat treated food products. A study at the Institute for Nutritional Science, University of Vienna, compared the stimulating effect on cellular immunity of conventional and probiotic yoghurt. No significant difference was found between the samples.



Critical aspects
"They sell well, but it is very doubtful that probiotic yoghurts are purchased entirely for health reasons. Consumers may be purchasing probiotics with the vague idea that it is "good for them".
Even advocates of the probiotic concept must admit that the "science" associated with probiotics over many decades has been remarkably weak. The probiotics industry is burdened with myth and a reliance on in vitro experimentation.
Much effort has been devoted to screening bacterial isolates for properties deemed appropriate for a "probiotic" strain, mostly characteristics that might enable the microbes to at least survive passage through the digestive tract. There must be millions of such strains to choose from, because the intestinal milieu of humans is already the home to bacteria with these properties.
Probably the biggest obstacle to allaying scepticism is that the probiotic concept is based on a very poor understanding of the intestinal microflora. The concept concentrates essentially on two groups of bacteria, lactobacilli and bifidobacteria, while practically ignoring the vast array of other species that inhabit the intestinal tract of humans". (Tannock)




Mixtures of probiotic strains
The GI tract is colonized with hundreds of microbial organisms, where our methods do not even allow to analyse many of them. Modern research analyses if genetic factors might contribute to aspects of colonization. Most classical probiotic strains ingested as probiotic do not really colonies the gut. They stay only for a very limited time and will be rapidly degraded or excreted. This is why some might assume that mixtures of different probiotic strains might better reflect the situation in the gut and might be ore likely to elicit beneficial reactions.






Probiotics and the immune system
The immune system is a key player in the prevention of infections by innate defense systems or the direction of specific immune responses. It also provides surveillance against developing malignant cells. Misguided immune responses such as allergies, autoimmune reactions or hypersensitivity reactions are underlying many diseases. Some of these diseases are rapidly increasing, possibly because of changing environmental factors, altered nutrition or lifestyle. The hygiene theory blames reduced interactions with microorganisms in childhood, typical in many areas, for misguided immune reactions.

The objective in prevention of atopic diseases is to direct the immune response into a tolerant state. Stimulation of the developing immune system is - following the hygiene hypothesis - a key measure to reach this objective. It could be shown that probiotic bacteria can stimulate components of the immune system and might even be capable of reprogramming overreacting immune responses. Furthermore there is strong scientific evidence that probiotics can protect against invading pathogens.




Probiotics in protection against pathogens
The need for a reduced use of antibiotics because of increasing problems with the development of resistances in many areas such as food production has resulted in the search for alternative ways to combat with pathogenic microorganisms. In food and feed production and distribution probiotic approaches are under development where non pathogenic microorganisms are exploited for their use in the inhibition of the growth of pathogenic microorganisms. For example, in farming and transport of shrimps probiotic approaches for protection against pathogens are of increasing importance as unsustainable ways of farming have resulted in ecological crises and food safety hazards.

Furthermore, some bacterial strains are used for topical application and protection of the human skin, claiming additional beneficial activities for the skin.


Reference:
Flinterman AE et al. Int Arch Allergy Immunol. 2007 Feb 9;143(3):237-244



For further reading visit:
Bergonzelli GE et al. Probiotics as a Treatment Strategy for Gastrointestinal Diseases? Digestion 2005;72:57–68.
Bischoff S and Crowe SE. Food allergy and the gastrointestinal tract. Curr Opin Gastroenterol 20:156–161
Probiotics for the Prevention of Beta Cell Autoimmunity in Children at Genetic Risk of Type 1 Diabetes
Soomro AH et al. Role of Lactic Acid Bacteria (LAB) in Food Preservation and Human Health – A Review. Pakistan Journal of Nutrition 1(1): 20-24, 2002