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Poultry gut is a complex ecosystem consisting of several species of microbes and microbial interactions ensure the stability of the ecosystem and the health of the bird. Gut is virtually a battle field between the native organisms and the invading pathogens. It's a delicate balance between the normal microflora and the pathogens. Soon after hatching, gut is virtually sterile and chick start acquiring microflora from surrounding environment. Many times, as the pathogens multiply in a faster pace than beneficial bacteria, chick may get infected resulting in so called Early Chick Mortality. However, as the days progress, the microflora stabilizes and attains the balance between the 'favorable' and 'harmful'. Two types of beneficial microbes get established faster one in close association with epithelium like Lactobacillus acidophilus and the other - that exists freely in the gut lumen as Streptococcus faecium. As stated, this balance being delicate can get affected or influenced by the surrounding environment like infectious pressure or internal factors like stress. Hence, the concept of probiotics emerges.
'Probiotics' is derived from a Greek word 'probios' meaning 'for life'. The term is used to describe the living microorganism having positive influence in improving intestinal microbial balance. They act as 'watch dogs' by keeping an eye on, and effectively controlling, the spread of undesirable microorganisms. A probiotic is a microorganism or combination of microorganisms, which would quickly establish in the gut to suppress colonization and growth of harmful bacteria. To do so, it should be alive and viable culture of organisms, which can withstand the hassles of gut organisms should be nonpathogenic, grampositive, strain-specific, acid/bile resistant, viable/stable and in large numbers.
Probiotics in the form of substances containing Lactobacillus cultures have been used for centuries as food preservatives and natural sources to promote good health without specific knowledge of their active ingredients or how they work. Lactobacillus was first identified by Pasteur (1845-1895) in France, the pioneer of modern microbiology. A real understanding of how probiotics function began when the Nobel prize winning Russian physiologist, Metchnikoff (1845-1916), introduced his intoxication theory. He stated that the main cause of aging is 'toxicants' formed by intestinal putrefication and fermentation and suggested drinking beverages such as yogurt containing lactic acid bacteria would prevent aging. Lactobacilli suddenly attracted world attention.
Inspite of unknown data or limited understanding on composition and physiology of gut microflora, from the results obtained with probiotic use has provided more and more knowledge and experience atleast to understand how probiotics should be used to give reliable results. It is a starting point to achieve more desired results. Several microorganisms have been identified as could be used as probiotics - different types of Lactobacillus eg. L. acidiphilus, L. reuteri, Streptococus faecium, S. lactis, Saccharomyces cerevisiae, Bacillus coagulans, Bacillus subtilis and so on. Newer and newer species are emerging. Thanks to today's biotechnological development. Nearly 40 species of organisms are identified as 'generally recognized as safe' i.e., GRAS listed. Some of these are native organisms and rest support them in getting established in the Gut. Specific strains are genetically modified to make them suitable for animal feed. eg. Feed processing viability, lactic acid production, enzyme production, shelf life etc.
Lactobacilli were the first genus of bacteria proved to have health benefits. Lactobacilli have been shown to be present in the GI tract of most of animals and birds. Lactobacillus, one of many friendly species of intestinal microflora is considered as beneficial bacteria in its ability to aid in the breakdown of proteins, carbohydrate and fats in food, and help absorption of necessary elements and nutrients such as minerals, amino acids and vitamins by the host. They lower the pH of the gut by converting sugar to lactic acid, which inhibits the growth of enteropathogens. Lactobacillus sp. can get quickly colonized in the Gut epithelium to deprive sites for attachments of pathogens. They have immunoregulatory actions by increasing macrophage activity and also by enhancing the production of immunoglobulins globally.
Saccaromyces cerevisiae is one of the highly resistant bug. It neither get interfered nor interferes with the activity of antibiotics. It's a strain of Yeast, which releases the mannaoligosaccarides through its cell wall. MOS having mannan sugar can preferentially bind to bacterial lectins before they get attached to surface carbohydrates of Gut epithelia and literally carry them out of the gut. Hence, pathogens pass through the gut without getting colonized. It is a classic competitive exclusion. Several pathogenic E-coli and Salmonella sp. possess lectins which are specific to mannos and hence gets bound by MOS. Apart from that Yeast provides certain cofactors which are beneficial to birds. In addition, they are also proved to bind Mycotoxins present in feed.
Traditionally, live direct fed
probiotic contain Lactic acid bacteria which are relatively fragile and heat
liable. However, they undoubtedly have growth promoting effect
by virtue of producing Lactic acid. Recent studies have emerged with
more resistant bug. i.e., Bacillus coagulans which is commonly called
as Lactobacillus sporogens. They are spore forming bacteria
whose spores posess high degree of stability in pelleted and mash feed, during
long storage in different 
environmental
conditions. It is nature's own micro-encapsulation. Once they get in
to the gut of the bird, they undergo rapid germination in upper digestive
tract and get transformed into vegetative cells, which are metabolically
active. They are
referred to as 'live enzyme factory' as they produce wide range of
enzymes, which can break down even complex carbohydrates, hence beneficial
to the host.
How probiotics /DFMs work-
Establishing and maintaining normal gut microflora
Rapid colonization prevents the pathogens getting established in the gut
Efficient digestion and utilization of nutrients
Stimulating the immune system making birds less vulnerable to diseases
Recolonising the gut with beneficial bacteria which are depleted use of antibiotics, adverse effect of stress / diseases
Elaboration of antibacterial substances (eg. Lactocidin, organic acids etc.)
Offering digestible proteins, vitamins, enzymes and other important co-factors.
Decreasing gut pH by production of lactic acids
Absorption of toxins
The natural balance of indigenous microflora in gut of birds often gets disturbed, specially during change in feed / ingredients, contamination, environmental stress, vaccination, de-worming, house shifting, transportation, problems related with ventilation and lighting etc. This disturbance in natural balance facilitates proliferation of harmful bacteria like E-coli. Multi strains of probiotics are used in feed in order to maintain the balance and to enhance productivity. Day-old-chicks having sterile gut can easily get infected. Using probiotic during early stages helps the bird to establish gut microbial balance preventing morbidity and mortality. Secondly, whenever the antibiotics are used in therapeutic doses, can disturb the normal microflora. Probiotics in conjunction with antibiotics can maintain the gut health. Stress due to any reason can decrease normal microflora, which calls for the use of probiotics. However, the birds are always under stress and amidst of high infectious pressure, they are continuously challenged. So, it would be better to supplement probiotics continuously.
Few questions are frequently asked about the probiotics. Such as, species of organisms to be used, number of organisms, stability during feed processing, storage and lastly the consistency of results. It is always better to use the consistency of the results. Ideally multistrain probiotics can contain bacteria like Lactobacillus which can quickly get established in the gut and resistant organisms like Yeast and Bacillus which can give sustained benefits.
There is apparently no known standard as per the number of organisms are considered. However, they should be in sufficient number. Many research people consider that it should be in an order of 107 to 109 to obtain balance between probiotic and bacteria of resident flora. As far as the stability of organism is considered, many of today's used probiotic species are known to be stable in presence of commonly used antibacterial growth promoters. Yeast and Lactobacillus sporogens are highly resistant to pelleting temperatures and storage at different environmental condition. Nothing speaks better than a bird The usefulness of a chosen probiotics has to be evaluated by onfarm repeated testing.
More and more knowledge and experience is gained on probiotics. More understanding on exact mode of action of individual species would help in making further improvements. The science of probiotic is not yet complete. Generic manipulation of these microorganisms with mild challenge of pathogens in future may produce probiotics which will stop spread of pathogens.