What is a Poultry Probiotic?
Poultry probiotics or direct fed microbials (DFM) are live microorganisms that can be incorporated in diets in order to:
- Populate the intestine with beneficial bacteria
- Modulate the conditions within the gastrointestinal tract
The effects of poultry probiotics are particularly important in young animals in which stable intestinal bacteria have not yet been established.
By adding probiotics to feed or water, the intestine is populated with beneficial bacteria avoiding or decreasing the extent of pathogen colonization (Nurmi and Rantala, 1973).
The efficacy of different probiotics has been demonstrated in modern poultry systems. Because antibiotics are being removed from the routine practices of animal husbandry, probiotics are now considered a promising tool to prevent pathogens from causing health and disease challenges.
Probiotic, prebiotic and synbiotic
Probiotics can be combined with prebiotics in order to nourish the beneficial bacteria and achieve better results. The probiotic plus prebiotic combination is known as a synbiotic.
A prebiotic is a non-digestible additive often consisting of natural dietary fibers such as fructooligsaccharides (FOS) that stimulate the growth and activity of beneficial bacteria in the colon, thus improving host health (Gibson and Roberfroid, 1995).
A synbiotic is a combination of probiotic and prebiotic products (Patterson and Burkholder, 2003), often with the aim of improving efficacy. PoultryStar® is an EU-authorized synbiotic (probiotic plus prebiotic).
Table 1. Common definitions of probiotics (direct-fed microbials) –related terms
|Probiotic||Beneficial bacteria supplemented to a diet|
|Prebiotic||Substances delivered to nourish beneficial bacteria|
|Synbiotic||A combination of probiotics and prebiotics|
How probiotics work against pathogens
Several proposed mechanisms explain the mode of action of probiotics against pathogens, namely:
- Competitive exclusion
- Bacteriocin production
- Immune stimulation
- Improvement on gut health and integrity
Probiotics competitively exclude pathogens
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 (Pascual et al., 1999; Ibnou-Zekri et al., 2002).
Remarkably, the ability to attach to the surface of intestinal cells varies among different strains of the same species of bacteria (Ibnou-Zekri et al., 2002). Competitive exclusion also considers the consumption of available nutrients by beneficial bacteria limiting resources and space for pathogenic bacteria.
Probiotics produce bacteriocins that target pathogens
Another mechanism that reduces bacterial viability is the production of harmful substances that specifically target pathogens, like H2O2 and bacteriocins (Oh et al., 2000; Gillor et al., 2008).
Bacteriocins are amino acidic molecules that have bactericidal properties on genetically related organisms. Several bacteriocins have been identified. Small bacteriocins tend to be heat-stable whereas large bacteriocins tend to be heat-labile.
While described bacteriocins are mostly effective against Gram-positive bacteria, there are some bacteriocins already described which are effective against Gram-negative organisms (Ralph et al., 1995; Servin, 2004).
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‚ Enterobacteriaceae‚ Pseudomonas‚ Staphylococcus‚ and Streptococcus (Silva et al., 1987).
The right probiotics support the immune system
Stimulation of the immune system, or immunomodulation, is another theory that explains the efficacy of probiotics. The intestinal tract of newborns is basically sterile. Bacteria that first colonize the gut influence the gene expression of epithelial cells influencing in turn the subsequent bacterial colonization of the intestine.
As an immune organ, the intestine has a large component of lymphoid tissue (GALT, or gut-associated lymphoid tissue) which also needs proper stimulation from commensal microorganisms for maturation.
Chickens that have been immune stimulated with probiotics in the diet have shown increased secretion of anti-clostridial IgA antibodies (Hamid et al., 2006). On the other hand, the intestine must also peacefully coexist with commensal bacteria and antigens of alimentary origin (oral tolerance). In addition, non pathogenic bacteria are able to send stimulatory signals to the enterocytes which limit the production of pro-inflammatory cytokines while promoting the production of anti-inflammatory cytokines (Neish et al., 2000). This observation can be supported by germ-free mice that show continuous inflammation and inadequate immune responses against normal dietary antigens (Servin, 2004).
It should be noted that the immune-stimulatory function of commensal bacteria is strain specific and even closely related bacteria stimulate the immune system in different ways (Ibnou-Zekri et al., 2002). Theoretically, probiotics could achieve benefits by either pro- or anti-inflammatory effects. For example, in human medicine it could be desired to reduce inflammation in patients undergoing chronic inflammation (Crohn’s disease). On the other hand, enhanced inflammation and direction of the immune system towards the cellular component of the immune response may help fighting coccidia in poultry.
Probiotics support epithelial cells
In addition to the anti-pathogenic activity that probiotics have, it has been demonstrated that indigenous bacteria of the intestine also contribute to the healthy development of epithelial cells. Actually, indigenous bacteria can stimulate enterocytes to produce and release active gastrointestinal peptides that impact the regulation of epithelial structure and intestinal endocrine cells (Servin, 2004). It is also becoming clear that commensal bacteria modulate gene expression of epithelial cells influencing nutrient absorption, intestinal maturation and improvement of the mucosal barrier (Servin, 2004).
Some strains of Lactobacillus are able to reduce the epithelial invasion of enterohemorrhagic E. coli (EHEC) without decreasing the viability of the pathogen. Since this effect is only observed with live Lactobacillus, it is thought that it is the result of the interaction of commensal bacteria and intestinal epithelium that induces protective changes on the enterocytes interfering with the internalization process of EHEC (Hirano et al., 2003). There is increasing evidence indicating that probiotics exert selective activation of certain epithelial genes. Similarly, the modulation of immune response obtained with probiotics seems to be strain-dependent (Didierlaurent et al., 2002).
Probiotics may do even more
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 lactic acid (Fayol-Messaoudi et al., 2005). 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 (Alakomi et al., 2000). In addition, production and release of other endogenous metabolites that may bring positive benefits yet to discover.
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