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By Biomin, Austria
Probiotics are live microorganisms that can be incorporated in animal or human diets in order to populate the intestine and/or modulate the conditions within the gastrointestinal tract. This is particularly important in young animals in which stable intestinal bacteria have not yet been established. By adding probiotics in feed or water the intestine is populated with beneficial bacteria avoiding or decreasing the extent of pathogen colonisation. The efficacy of different probiotics has been demonstrated in humans, fish and in animals such as poultry. Because antibiotics are being removed from the routine practices of animal husbandry, probiotics are now being considered as promising tools to fight pathogens.Proposed mechanisms
There are several proposed mechanisms that explain the mode of action of probiotics against pathogens. Among them, competitive exclusion has obtained much popularity over the last years. Competitive exclusion refers to the blockage of cellular receptors on the luminal surface of epithelial cells, mechanically avoiding the entrance of pathogens. This can be supported by in vitro assays that show the capacity of selected probiotic bacteria to adhere to intestinal cells. Remarkably, the ability to attach to the surface of intestinal cells varies among different strains of the same species of bacteria. Competitive exclusion also considers the consumption of available nutrients by beneficial bacteria limiting resources and space for pathogenic bacteria.
There are several proposed mechanisms that explain the mode of action of probiotics against pathogens. Among them, competitive exclusion has obtained much popularity over the last years. Competitive exclusion refers to the blockage of cellular receptors on the luminal surface of epithelial cells, mechanically avoiding the entrance of pathogens. This can be supported by in vitro assays that show the capacity of selected probiotic bacteria to adhere to intestinal cells. Remarkably, the ability to attach to the surface of intestinal cells varies among different strains of the same species of bacteria. Competitive exclusion also considers the consumption of available nutrients by beneficial bacteria limiting resources and space for pathogenic bacteria.
Intestinal pH reduction
Along with the popular theory of competitive exclusion, other mechanisms for the probiotic-induced inhibition of pathogens have been studied. This is the case of intestinal pH reduction by the production and secretion of metabolites such as acetic and lactic acids. It has been suggested that lactic acid produced by probiotic strains increases permeability in the outer membrane of Gram-negative bacteria facilitating the diffusion of antimicrobial compounds produced by probiotics and by the host’s epithelium. Another mechanism that reduces bacterial viability is the production of harmful substances that specifically target pathogens, like H2O2 and bacteriocins. Bacteriocins are amino acidic molecules that have bactericidal properties on genetically related organisms. Several bacteriocins have been identified already; small bacteriocins tend to be heat-stable whereas large bacteriocins tend to be heat-labile.
Along with the popular theory of competitive exclusion, other mechanisms for the probiotic-induced inhibition of pathogens have been studied. This is the case of intestinal pH reduction by the production and secretion of metabolites such as acetic and lactic acids. It has been suggested that lactic acid produced by probiotic strains increases permeability in the outer membrane of Gram-negative bacteria facilitating the diffusion of antimicrobial compounds produced by probiotics and by the host’s epithelium. Another mechanism that reduces bacterial viability is the production of harmful substances that specifically target pathogens, like H2O2 and bacteriocins. Bacteriocins are amino acidic molecules that have bactericidal properties on genetically related organisms. Several bacteriocins have been identified already; small bacteriocins tend to be heat-stable whereas large bacteriocins tend to be heat-labile.
While the currently described bacteriocins are mostly effective against Gram-positive bacteria there are some bacteriocins already described which are effective against Gram-negative organisms. Because of their amino acidic origin, bacteriocins are susceptible to proteolytic enzymes. There is another group of non-acid substances that are resistant to heat and proteolytic enzymes and thus belong to a different category of inhibitory compounds produced by commensal bacteria. Most of these are not fully identified compounds but with established inhibitory activity against Clostridium, Bacteroides, Bifidobacterium, Enterobacteriaceae, Pseudomonas, Staphylococcus, and Streptococcus.
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Source: World Poultry, Vol. 27, No. 2, 2011
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