Our gut is an ecosystem. An ecosystem can be defined as a geographical place where the constituents coexist living and inert. The gut, responds well to this definition.

The number of microorganisms in the final portion of the human gastrointestinal tract is approximately 1011 bacteria per gram of contents. This microflora is the result of interactions between bacteria, host and external environment and is of fundamental importance in the healthy individual as well as on the patient. Its development began in the time of birth and then continues slow and gradual process that is completed in several years.

The colonic microflora of infants fed breast milk contains a high percentage of Bifidobacterium. Over the years after stopping this type of feeding, the number of Bifidobacterium begins to decline.

The components of the intestinal flora vary from one adult to another and depending on the environment in which humans live, their diet and genetic heritage of each individual.

Gastric acid and normal peristaltic flow of small intestine bacterial population limits the upper gastrointestinal tract.

The small intestine is a transitional zone between the stomach and colon, in a gradual transition occurs flora Gram-positive and Gram-negative population. This flora varies with the intestinal segment by itself does not cause, constitute a real self-regulating culture.

There are two types of intestinal flora: the flora and native resident or temporary or transitory. The first adheres to the mucosal epithelial cells are fixed microorganisms multiply rapidly, they are well adapted and stable and safe. The transient flora is not fixed to the epithelium is not established in the intestine and is formed by non-pathogenic microorganisms from the upper digestive tract, food and environment.

Some of the effects of the intestinal flora are:

• Qualitative modification of the intestine.
• His role on the degradation of nutrients.
• The synthesis of vitamins
• The production of volatile fatty acids and resorption of bacterial metabolites.
• Synthesis of active amines and polyamines.
• The role of endogenous products.
• The production of gases.
• The action on the metabolism of xenobiotics.

There are certain features of the colonic microflora in which Bifidobacterium predominate among which are the production of short chain fatty acids and lactic acid from the fermentation of carbohydrates, which lower the pH in the colon by creating a milieu where potentially pathogenic bacteria can not grow and develop. Bacteriocins also produce calls that act as antibiotics and inhibit pathogenic bacteria. The stimulation of the immune system, especially the gut and the ability to synthesize certain B vitamins

Persistent infections in the intestinal tract cause significant loss of immunoglobulins, lymphocytes and other effector cells and molecules as well as nutrients that lead to a secondary immunodeficiency body, it develops a cycle that causes severe impairment of functioning. Secondary immunodeficiencies are frequently associated with diarrhea, these alterations disrupt temporarily or permanently some components of immunity and increase susceptibility to infections.

Malnutrition increases the infectious process and specifically infectious diarrhea increased frequency of infections in the mucosa and decreased intestinal motility is the loss of one of the most important functional characteristics for the control of bacterial growth. The overgrowth of bacteria in the intestinal tract causes a decrease in the micelle formation, increased bile acids results in increased permeability of the mucosa, allowing absorption of macromolecules including foreign antigens and toxins. The mitotic activity of cells decreases cryptic slowing production, migration and maturation of enterocysts and there is a defective repair mechanism in the intestinal mucosa.

The surface of the intestinal mucosa has defense mechanisms that adequately differentiate between the commensal flora, the symbiotic and exogenous pathogens.

A Brief History Of Probiotics

The role of fermented milks beneficial to health has been known for centuries, but it was not until 1908 when Russian scientist Ilya Metchnikoff emphasized the benefits of yogurt consumption provided the inhabitants of the Balkans, which teamed her great good physical health and longevity to the high consumption of yogurt, for his research received the Nobel Prize for medicine in that year.

Lilly and Stillwell in 1965 first used the term probiotic, to name the gastric fermentation products. This word is derived from two words, from the Latin "pro" meaning for or on behalf of, and Greek-bios-which means "life.

This definition was modified and redefined the term of probiotics as microorganisms and compounds involved in the intestinal microbial balance and development. At present the definition of probiotics has been given by R. Fuller in 1989 as "Those living microorganisms, mainly bacteria and yeasts, which are added as a dietary supplement that beneficially affect the development of microbial flora in the gut".

Probiotics are microorganisms that stimulate protective functions of the digestive tract, are also known as bio therapeutics or Bio, are used to prevent enteric infections and gastrointestinal.

For an organism to fulfill this function must possess protective features such as: Being normal inhabitant of the gut, have a short time of reproduction, be able to produce antimicrobial compounds and stable during the process of production, marketing and distribution that might be alive in the intestine.

The protection of these organisms is carried out through two mechanisms: The antagonism that prevents the multiplication of pathogens and production of toxins that prevent their pathogenic action. This antagonism is given by the competition for nutrients or adhesion sites. Immunomodulation by protecting the host from infection by inducing increased production immunoglobulins, increased activation of mononuclear cells and lymphocytes.

Lactic acid bacteria can transiently colonize the gut and survive the intestinal transit also for its adherence to the epithelium, modify the local immune response of the host.

It has been tested in vitro or in vivo the effect of probiotics in disease states such as diarrhea, vaginitis, urinary tract infections, immune disorders, lactose intolerance, hypercholesterolemia and food allergy.

The Immune System

It is the body's defense system that would put in place several mechanisms to cope with the massive invasion of foreign substances (antigens) to it. The type of immune response depends on the nature of the antigen (virus, bacteria, parasites, fungi, pollens, certain food proteins) as well as their gateway to the body (skin, blood, respiratory tract mucosa, gastrointestinal tract epithelium).

The first line of defense in preventing most infectious diseases and is constituted by physical-chemical barriers such as skin and mucous layer (eg in the nose and intestinal).

The secretory of mucosal immunity is the mechanism best-known protection against enteropathogens. Secretory IgA in the intestinal lumen reacts with specific antigens preventing their attack on the mucosal surface. This protective effect depends on the antigen-binding capacity and has called immuno-exclusion.

The immune response involves a complex interplay between its components. There are mainly three phases in this response: identification of the foreign particle, destroying it and regulating the immune response through various feedback mechanisms or "feedback".

The intestinal immune system remains "reactive" to the microflora, which is interpreted as a manifestation of immunological tolerance. This process is of vital importance in the integrity of the gut, a failure of this mechanism can lead to pathological inflammatory processes. The mechanisms by which indigenous microorganisms contribute to the modulation of reactivity in the intestinal defense against pathogens to preserve the integrity of the gut, is called barrier effect.

Food & Immune System

Food consumption is related to the immune system in various aspects. All food intake causes an immune response that usually develops tolerance to what in theory could be a foreign substance to the body. In fact, food allergies or hypersensitivity reactions include all those reactions involving the establishment of immunological mechanisms in this sense, differ from food intolerances where the immune system does not.

Likewise, we must take into account the importance of maintaining good nutritional status to ensure a properly functioning immune system, because food provides the nutrients essential for the synthesis of elements (substances and immunocompetent cells) which constitute the system.

However, we must not forget that along with food, eat a lot of bacteria, most of whom die when they pass through the stomach wall, due to its low pH. Indeed, the recent interest focuses on those bacteria that are able to survive once they have passed through the gastrointestinal tract. Theoretically, these organisms might interact with the microflora bacteria and/or intestinal mucosal cells, inducing or modulating different biological activities that could be beneficial it is because of microorganisms capable of surviving through the digestive tract, have a beneficial effect on bowel function and promote health. Indeed, LAB constitute a large proportion of the probiotic cultures used in developed countries.

Probiotics and Immune System

At the beginning of the last decade, pointed out the influence of probiotics on the immune response. It is essential that live LAB survive after passing through the gastrointestinal tract, and to express their immunomodulatory properties. In this sense, it has been observed that certain strains of LAB act on delayed hypersensitivity reactions, antibody production, functional activation of macrophages, it has been demonstrated further that some are able to prevent enteric infections and exercise antitumor action by inhibiting carcinogenic chemicals.

The immunomodulatory properties of lactic acid bacteria in humans have been described by several groups of researchers recently tested in a group of healthy volunteers supplemented a milk fermented with Lactobacillus acidophilus La1 or Bifidobacterium bifidum BB12 and measured the phagocytic activity of leukocytes in blood This was increased in both groups coincided with fecal colonization by lactic acid bacteria that remained in the intestine 6 weeks after ingestion of the product.

Recent studies say the mechanism is activated and increases phagocytosis in the treatments with enriched milk drink containing Lactobacillus and this is accompanied by the production of several cytokines such as interferon-g, Interleukin 12 and Interleukin 10.

Have been frequently cited in the literature some immunomodulatory properties of LAB, although still not understood exactly which mechanisms are involved. However, it has been reported in animal models a protective effect exerted by LAB against intracellular pathogens and could be associated with activation of the reticuloendothelial system.