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22 Concise Monograph SeriesVaccinationsAnimal studies have convincingly demonstratedthat certain probiotic strains can both enhance theimmune response to a vaccine and reduce the risk ofsubsequent infection. Human studies are much fewer,but an increasing number of well-controlled trials arebeing conducted. Preliminary evidence indicates thatthe response to vaccines against influenza, choleraor childhood diseases can be enhanced by selectedprobiotics, as measured by the number of subjects whorespond to the vaccine or an increase in the level of serumimmunoglobulins. The effects are strain-specific in termsof efficacy of the probiotics and, in the case of influenza,specific to the pathogen strains. In one study, there waslimited evidence that subsequent risk of infection withthe influenza virus was reduced. In the case of prebiotics,although evidence in animals seemed promising, clinicalstudies have not yet been supportive of an effect.Allergic conditionsAllergy can be defined in simple terms as an inappropriateimmune reaction or over-reaction to an otherwiseharmless foreign antigen (mostly proteins or peptides).In medical terms, it is described as a hypersensitivityreaction mediated by specific antibodies (IgE) or cellbasedmechanisms. Common allergies include reactionsto certain food proteins (e.g. milk, eggs, peanuts, treenuts, soy, wheat gluten, fish, shellfish and shrimp) or toenvironmental allergens such as pollen (hay fever), housedust mites and pet hair. Food allergies are more commonin infants and children than in adults. The most seriousform of allergy resulting in anaphylaxis (which can be fatalwhen the throat and respiratory tract swell and restrictbreathing) is rare, albeit a lifelong concern. Less severesymptoms of allergies are more common (prevalence isabout 2% for food allergies and up to 30% for respiratoryallergies) and can substantially reduce the quality of lifefor allergic subjects.As noted, the prevalence of allergy has increased inwesternised societies. There is growing evidence thatthe nature of microbiota acquired by the infant in thepostnatal period has an important bearing on maturationof the immune system. There is some indication thatatopic children tend to have a degree of dysbiosis, withmore clostridia and fewer bifidobacteria than non-atopicindividuals. In addition, data suggests that breast-fedinfants are less prone to allergic conditions. It has thusbeen suggested that prebiotics may help reduce the riskof developing atopy or reduce the associated symptomsof atopic eczema or allergic rhinitis. There is promisingevidence, based on a follow-up of one intervention,that not only can prebiotic-supplemented infant formulareduce susceptibility to atopy but that the benefits persistup to 2 years of age. Furthermore, studies in infants at highrisk of allergy who were fed supplemented formulas for 6months had reduced levels of IgE and some IgG types.There have been several studies on the impact ofprobiotics on the development of allergic symptoms ininfants at high risk of developing atopic disease. In mostof these studies, the mother consumed the probiotic priorto birth and the infant was administered the probioticafter birth. Results showed a decreased risk of eczema at2 years of age and beyond. Overall, results of the studiespoint towards strain-specificity and also hint towardstwo separate windows of opportunity: first, the maternalconsumption of probiotics during the perinatal periodand, second, the use of probiotics during weaning. Pastand ongoing studies have also targeted the managementor reduction of allergic symptoms such as those linked toatopic eczema or allergic rhinitis; results are promisingbut not yet conclusive. This probably reflects thecomplexity of the allergic diseases spectrum and the factthat a range of different clinical designs was used.
Probiotics, Prebiotics and the Gut Microbiota 23PROBIOTICS ANDPREBIOTICS:MECHANISMS OF ACTIONOverall mechanismBoth probiotics and prebiotics are thought to work largelythrough direct or indirect effects on the gut microbiotaand environment and/or on host function. In the caseof probiotics, a live micro-organism is consumed, in arange of dosages, spanning from ~10 8 to 10 12 cells/day,depending on the product. This large number of microbeshas the potential for a greater impact in the upper GI tractwhere lower densities of micro-organisms are found, butis also thought to impact the colon. Prebiotics enhancethe growth of the endogenous microbiota or possiblystimulate the growth of probiotics when providedconcurrently. Thus, probiotics and prebiotics share manycommon mechanisms of action mediated through animpact of microbes on the host and these are discussedbelow. In the case of health effects that relate only toprebiotics or only to probiotics, the mechanisms are lesswell known and have been alluded to in the section onhealth effects.Probiotics and prebiotics (via their stimulation ofcommensal organisms) act on and interact with the hostby two main modes of action, or a combination of actions(Figure 6 – see page 24):• Impact of micro-organisms or their metabolites/enzymes on the host’s GI tract and its microbiota• Interaction with the host’s cells and immune systemGI tract and its microbiotaAs noted, bifidobacteria and lactobacilli in the colonpreferentially ferment carbohydrates that escapedigestion in the upper GI tract, resulting in a reduced pHof the colon. Bifidobacteria can ferment fructans becausethey have an enzyme, β-fructofuranosidase, that otherbacteria either lack or have present at a lower activity,thus giving them a competitive advantage when exposedto fructans in the human gut. Similarly, the presenceof β-galactosidase in lactobacilli or streptococci exertsa competitive advantage in GOS fermentation. Themetabolism of prebiotic fructans by bifidobacteria yieldsmainly the acidic compounds acetate and lactate. Crossfeedingof these fermentation products to other speciesgives rise to butyrate and propionate. Butyrate andpropionate are also formed from the direct fermentationof other dietary carbohydrates.The benefits of a lower pH in the colon are that itencourages the multiplication and survival of commensalorganisms that prefer acidic conditions and generallyinhibits the ability of some pathogens to adhere, grow,translocate across the epithelium or colonise the GI tract.Furthermore, butyrate has long been known, from invitro studies on fermentable dietary fibres, to enhancemucosal cell differentiation and this may also promotethe barrier function of the epithelium.Saccharolytic fermentation concomitantly reduces thepotentially adverse effects of protein fermentation andother processes, which give rise to nitrogen and sulphurcontainingcompounds such as ammonia, N-nitroso- andazo- compounds as well as sulphides.Many bacteria produce bacteriocins, which are peptidesor proteins that are intended to reduce the survival ofcompeting organisms. Bacteriocins produced by probioticbacteria have been observed in in vitro studies to decreasethe ability of pathogens such as E. coli O157:H7 toadhere to and invade cultured intestinal cells. Bacteriocin
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