Intestinal ecosystem is composed by a few members, which interacts each other maintaining the balance of the microenvironment, such as microflora, mediators, immune cells, secretions and epithelial barriers. All these components are essential for a proper development and the functionality of the intestinal ecosystem. In case that one of the components disappears or is altered, that balance will get damaged so pathogenic agents may have access.

In order to keep this balance, during last 50 years there have been used a few products to try to achieve it. Some of them are interactive and act in the intestinal ecosystem to maintain homeostasis; others are therapeutical used to treat agents which cause diseases on digestive system such as coccidiostats. We propose the following products, which are some of the commonest ones, to maintain the balance of the digestive system in birds:

-Abiotics

-Acidifiers

-Competitive exclusion

-Enzymes

-Prebiotics

-Probiotics

-Antimicrobial

-Symbiotics and Eubiotics

1. Abiotics: Some purified substances, such as beta-glucans, have been defined as abiotic substances which can regulate or influence the innate immune response if added in feed, against some pathogens such as Salmonella. First report regarding the activity of these substances in birds was in 1989, and many studies in different species confirm its efficacy. These substances derive from the cell wall of some cereals (for example oats), fungi or yeasts, and enhance the immunity on digestive tract. Recently, it was demonstrated its efficacy in preventing the colonisation of organs by Salmonella Enteritidis in birds, so that they have become an efficient alternative to the traditional methodology to treat enteric diseases.

2. Acidifiers: are biodegradable active principles often used for breeding birds, despite of its controversial efficacy. Anyhow, the acidification of aliments by using a proper mixture permits to modulate positively the bacterial microflora of the intestine by reducing the pathogenic bacteria. In the market there are some different mixtures between organic acids and vegetable acids in order to enhance a gradual and controlled liberation of gastrointestinal tract of birds. Some studies have published its beneficial effects so that they have placed these products as an efficient alternative of the growth promoters. Recent studies indicate the possibility of stimulating the production “in situ” of organic acids with the use of probiotics and prebiotics, with a proper formulation adapted to portions, which will suppose the control of enteric pathogens, especially Salmonella, with a better rate cost/profit. They are broadly used to prevent contamination by Salmonella in laying and breeding hens. On the other hand, the use of acidifiers can cause some problems as some microorganisms could develop tolerance which could be linked to an increase of virulence. The acidifying capacity mainly depends on the dissociation constant, which it must be reduced, but it also depends on other factors such as the portion and intestinal environment. It is important to clarify that acids used are the organic ones because the inorganic ones do not pass through the cell walls so its antimicrobial activity is restricted. These acids act by two mechanisms:

• Reduction of Ph

• Antibacterial activity

3. Competitive Exclusion: Volterra in 1928 was the first person to suggest that the coexistence of two or more species is limited, and its location in the same place is impossible. The concept of competitive exclusion was described for the first time by Nurmi and Rantala in 1973, after an outbreak of Salmonella infantis in Finland. The concept implies the prevention of entering by an agent in a precolonised environment. This way, microflora of adult type is administered in one-day-old birds, so that chicks accelerate the maturing process of the microflora and increase its resistance to the colonisation by intestinal pathogens. The mechanisms of protector effect have not been fully explained; however it has been proposed that native microflora exerts a competitive exclusion, by reducing contaminant bacteria in intestinal tract of birds. Please note this mechanism is the most accepted. This mechanism is recognised as the precocious colonisation of the intestine, where microorganisms are excluded due to the competence exerted by the microflora already settled down. The concept can be summarised as it follows:

• One-day-old birds get infected by just one cell of Salmonella

• Adult birds are resistant to infections by the existence of native microflora of intestine

• Introduction of the adult bird’s microflora in a 1-day-old bird accelerates the maturing process of the microflora and increase the resistance of chicks to the colonisation.

The enterpathogen control can be as it follows:

• Chemical: There are bacterial groups that block locals of adhesion of some enteric pathogens through a fimbria net; others have the capacity to reduce the pH by the production of organic acids by inhibiting enteric pathogens.

• Biological: due to the existence of several microbial genders in the EC that colonize and develop themselves, they produce an exclusion environment nearly permanent, which confers to the mucosa a structural identity of the bacterial composition.

• Physical: Products of competitive exclusion create a system of spatial integrity which does not allow intestinal pathogens to find a place to establish.

• Biochemical: many microorganisms produce inhibitory substances which allow keeping a determinate group of bacteria in a determinate spatial and nutritional place in an intestinal environment.

4. Enzymes: β-Glucans arabinoxylans are the main non amylum polysaccharides existing in cell’s walls of cereals (wheat, barley and other). The chemical structure of β-Glucans is similar to cellulose’s, excepting the type of bonds. Β (1-3) bonds break the linearity of the molecule by introducing irregularities, prevent the formation of fibrils and enhance its solubility and formation of viscose solutions. The enzymes used in the feed of monogastrics have to resist and keep a considerable enzymatic activity after the manufacturing and digestion process. Factors which can influence on its stability are as follows: origin or microorganism’ origin, type of activity, its covering and conditions during digestive process and endogen enzymes. Most of enzymes used are of fungal origin, stable at room temperature although they are inactivated at temperatures over 60ºC.The activity of β-Glucans does not get as damaged as xylanase’s activity. The recovering of enzymes is a method used to avoid problems due to high temperatures usually reached during granulation process. The use of high levels of cereals can reduce the productive parameters due to an increase of intestinal viscosity which alters the velocity of intestinal transit. This process alters the secretion of digestive enzymes and modifies the bacterial microflora. The efficacy of enzymes are still depending on a series of factors:

• The enzymatic supplement must contain the proper spectrum of enzymes to neutralise the anti nutritive effects of the specific substratum.

• Supplement must contain a proper level of enzymatic activity.

• Different varieties of cereals and crops contain different levels of non amylum polysaccharides. For that, the answer to a treatment with enzymes varies into the same cereal. Results vary with the level of cereal and type of fat added.

• They must not be inactivated for the processed ration, the intestinal pH or the pancreatic enzymes.

In a future, enzymes will have to be designed to induce specific effects in the Microbiota. The AGV resulting of the Arabinoxylans enhance the growth of bifidbacteria and other species, so that the rate of Campylobacter and Clostridium are reduced. Several essays have demonstrated that xylanases reduce the intestinal population of Clostridium perfringens and coliforms, enterococci and bacteria in general. Some researchers found that the addition of xylanases was as efficient as avilamycin to improve productive rates and reduce population of coliforms; they get same effects when combining these enzymes with oligosaccharides or organic acids with products of yeast or oligosaccharides. They have also been found synergic effects among xylanases, organic acids and products of the cell wall of yeasts.

5. Prebiotics: The concept was introduced one decade ago by Gibson and Roberfroid, and was defined as a non digestible feed ingredient which has beneficial effects in host due to the selective stimulation of the growth or the activity of a bacteria or only a few of them. So that an ingredient can be considered as prebiotic has to meet the following criteria:

• It cannot be hydrolysed in the first part of the gastrointestinal tract.

• It has to be a selective substratum for one bacteria or only a few of them

• As consequence of the alteration of microflora, it has to be capable of returning to the healthiest environment. The importance of using these substances lies on its viability, as they tolerate variations of gastrointestinal ecosystem.

The most studied are fructooligosaccharides (FOS), manano-oligosaccharides (MOS) and xylo-oligosaccharides (XOS). Oligosaccharides can be of natural origin, but most of them are obtained by synthesis or enzymatic hydrolysis. FOS are obtained industrially by saccharose or by hydrolysis of fructans such as inulin. XOS are obtained by enzymatic hydrolysis of xylans, while MOS are mainly obtained by cell walls of yeasts, although they can be obtained from other origins. Basically these non digestive products are the substratum for the beneficial microflora in the intestine, block the pathogen bacteria preventing its adherence to the surface of CEI, stimulate some communities of beneficial microorganisms and can modulate the immune response.

6. Probiotics: It was first introduced by Lilly and Stilwell in 1965. The commonest definition was made by Fuller (1989) who defined Probiotics as those microorganisms which added to feed affect beneficially the balance of the intestinal ecosystem, contributing with the protection against gastrointestinal infections and inflammatory diseases of the intestine. The genders Enterococcus, Bacteroides, Eubacterium and especially Lactobacillus and Bifidobacterium can be found in mixtures of defined cultures. Among the action mechanisms described for these substances are the following:

• Production of antimicrobial substances (bacteriocine and volatile fat acids (AGV)) which suppress populations or pathogen species.

• Immune stimulus of resident cells, especially cells presenters of antigens (CPA’s).

• Competitive exclusion associated to the competence by places of adhesion in the mucosa.

• Competence of nutrients.

• Protection of hairiness and absorbing surfaces against irritating toxins produced by pathogen microorganisms permitting the regeneration of the affected mucosa.

They can also be used in order to restore the intestinal microflora after the antibiotic therapy. The use of probiotics as growth promoters are increasing, with positive results in the gain of weight and the feed conversion.

Several microorganisms have been used such as Lactobacillus, Saccharomyces cerevisiae, Aspergillus oryzae, Enterococcus faecium, Bifidobacterium bifidum and others. In laying hens it has demonstrated the increase of resistance to diseases and the positive effect on the production of eggs. Probiotics present beneficial effects. Among the most important ones we can mention the following:

• Positive influence on the intestinal microflora by enhancing the growth of bacteria of the normal Microbiota of the intestine maintaining the balance

• Prevention of intestinal infections: interfere with pathogen bacteria, preventing an adherence and colonisation of the intestinal tract.

• Stimulate the local immunity: stimulate the production of Ig secretor in the intestine, which prevents the colonisation of pathogen bacteria.

• Reduction of inflammatory reactions

• Regulation of the intestinal motility: due to the balance among solutions and liquids in the intestine, regulate the intestinal motility enhancing the absorption of nutrients.

7. Symbiotics and Eubiotics: When probiotics and prebiotics are presented simultaneously in a product are called symbiotic or eubiotic. The word reminds a synergism and must be used for products in which the prebiotic compound enhances selectively the prebiotic already contained in the formula. Therefore it is enhanced the survival of prebiotic contained in the formula because prebiotic is a specific substratum.

8. Antimicrobial: The promoter effect of growth of antibiotics was discovered in 40’s, when it was observed that animals fed with dry mycelium of Streptomyces aureofaciens which contain residues of Chlortetracycline, enhancing its growth. This mechanism of action is related to its interactions with microbial populations. In the US, the FDA approved the use of antibiotics as additives for animals without veterinary prescription in 1951. During 50’s and 60’s in Europe, every member of the EU approved their own national regulations for the use of antibiotics for animal feed. Currently in the US they have approved for commercial laying hens the use of bacitracin in water and portion and the use of chlortetracycline and tylosin in portion. Enrofloxacin was forbidden in the US for the avian production although it is still being used in Latin America. In the EU all the antimicrobial (with the exception of some coccidiostats) have been withdrawn since January 2006. Anticoccidian drugs will also be forbidden before 2013. In an egg, residues of many products used in laying hens can be easily found both in white and yolk. Literature has demonstrated that can be found residues such as sulfonamides, amprolio, decoquinate, dimetridazole, olaquindox, tetracycline, nitrofurans, quinolone, macrolide, ionóforos, cloranfenicol and other components. Obviously the prerequisite to find these products in the egg is their intestinal absorption and the physic-chemical properties of the drugs, among others. In indispensable cases, the use of antimicrobial as therapy must respect the periods of rest, with the aim of not generating residues in avian products. This is important because an active ingredient can have several periods of rest, depending on the vehicles used in its formulation and the animal specie where are administered.

A parallel activity to the antimicrobial rest (therapeutical or growth promoters) is the improvement of the hygienic conditions in farms, as well as its handling inside them in order to improve the quality and digestibility of portions by modifying, in necessary cases, its composition not only in ingredients but in its nutritional levels. Nowadays exists a wide range of alternative additives (mentioned above), with different ways of action and levels of efficacy, which can be converted shortly into a viable and efficient alternative, if they begin working from the point of view of the preventive medicine.