Probiotics is a term generally used for living microorganisms that upon ingestion exert beneficial health effects. Probiotics consist of one or several strains, mostly from species of the bacterial genera Lactobacillus, Lactococcus or Bifidobacterium. The yeast Saccharomyces is also used. With the exception of Bifidobacterium, these are microorganisms which spontaneously multiply to high numbers in traditional fermented foods. When live bacteria are eaten, they will come into contact with the gastro-intestinal (GI) tract, where they can target different functions from the mouth to the anus: the immune system can be targeted through different receptors in the mucosa. Physiological events can be initiated, for example, by stimulating the epithelial cells of the mucosa to produce more mucin (Mack et al.,1999; Mack et al., 2003). Probiotics can interfere with food components, e.g. by fermenting dietary fibres in the colon to carboxylic acids, or splitting up tannins to flavonoids (Osawa et al., 2000; Vaquero et al., 2004) and phenolic acids (Barthelmebs et al., 2000).

Probiotics have by tradition mostly been linked to gut health, and especially well proven for several different probiotics are the beneficial effects against antibiotic associated diarrhoea, and diarrhoea caused by Clostridium difficile, which make up the main part of the antibiotic associated diarrhoeas (meta-analysis, D'Souza et al., 2002). This agrees with the traditional explanation of the beneficial health effects of probiotics, i.e. that probiotics maintain the microbial balance in the gut (fig. 1). Thus, probiotics support beneficial components of the microbiota and suppress adverse ones, e.g. C. difficile. In contrast, probiotics, and particularly Lactobacillus rhamnosus GG, have been shown to suppress symptoms of acute, nonbacterial diarrhoea in small children, primarily rotavirus diarrhoea, where the probiotics appear to activate the immune defence in a supportive fashion (meta-analysis, Huang et al., 2002).
Different probiotics have also shown convincing effects against irritable bowel syndrome (IBS; Camilleri, 2006), e.g.

Lactobacillus plantarum 299v (Nobaek et al., 2000; Niedzielin et al., 2001), Bifidobacterium infantis 35624 (O'Mahony et al., 2005) and Bifidobacterium animalis DN-173 010 (Guyonnet et al., 2007). The alleviated IBS-symptoms by B. infantis 35624 were associated with normalisation of the cytokine-ratio IL-10/IL-12, suggesting an immune-modulating role for this organism in IBS (O'Mahony et al., 2005).

A Systemic Error in Modern Diet?

Diseases with a background of dysfunctional immune reactions are increasing in urban societies, but also so are diseases linked to the metabolic syndrome and characterised by a low-grade systemic inflammation. The increase in incidence is often blamed on urban life-style and diet. The fact that bacterial consumption and the resident gut microbiota can influence immune functions has often been neglected. Has urban living lead to an adversely altered bacterial flora in the gut, and can this change become a driving force for an over-reacting, dysfunctional immune system?

Humans have from the start of existence consumed large amounts of live bacteria by eating fermented foods as this is the simplest way to preserve sensitive raw-material for food. Depending on how the fermentation is carried out and what raw materials are being fermented, the product will contain varying concentrations of carboxylic acids (mainly lactic acid) and ethanol. Fermentation occurs spontaneously if food materials are stored with limited access to air and at temperatures above refrigeration. Certain types of bacteria, but also fungi such as baker's yeast, will grow in the product and convert some of the carbohydrates of the raw material to lactic acid, and often also to ethanol, acetic acid and carbon dioxide. The pH will go down to 3.5-4.0. The low pH together with organic acids and the high level of live bacteria keep away harmful microorganisms and protect from lipidoxidation. Due to the production of lactic acid, these types of bacteria which are dominant in lactic acid fermented foods are called lactic acid bacteria (LAB) which, hence, refers to a functional group of organisms, and not to a group of phylogenetic relationships.

When fermented products are consumed without heating, a large amount of living microorganisms are consumed. This, presumably, more or less daily supply of LAB that our ancestors have been exposed to through the centuries, has successively declined in industrialised countries. The food industry has preferred other means to process and secure the shelf-life of foods. Even several of the still remaining lactic acid fermented food products that today can be found in the shops are heat treated after the fermentation and, hence, do not contain any live LAB. LAB dominating the bacterial flora in fermented plant-based products as, for example sauerkraut, olives and capers are, typically, Lactobacillus plantarum, Lactobacillus fermentum, Lactobacillus pentosus and Pediococcus species. In milk-based products, however, Lactobacillus paracasei and Lactobacillus rhamnosus, together with different species of Lactococcus and milk-streptococci, are often found after spontaneous fermentation while Lactococcus lactis, Streptococcus thermophilus and Lactobacillus delbrueckii often are used as starter cultures in dairy products.