Modification, breakdown or disregulation of the network of physical, physiological, immune and endogenous microflora leads to ineffective clearance, sequestration or degradation of harmful antigens and/or disruption of regulatory function, and results in mucosal damage, increased gut permeability and overgrowth of harmful pathogens. Factors which may contribute to this include unanswered pathogenic challenge, dietary toxins, antimicrobial compounds (antibiotics) and modifications due to age and/or overall immune competency.
1.4.1 Bacterial Pathogens
Many pathogens are able to subvert the gut defences and immune system in order to enhance their survival, resulting in persistent infections which, in some cases, subsist by upregulating host inflammatory responses in order to use host-derived material of inflammatory or tissue damage-origin as growth substrates. Other pathogens are able to downregulate host responses, ensuring their unhindered invasion of the epithelia, with subsequent risk of systemic infection.
1.4.1a Helicobacter pylori
Helicobacter pylori is a Gram-negative bacterium which has adapted to the harsh gastric
environment through expression of urease to neutralise the acidic pH through establishment of a higher local pH gradient, and by existing within the gastric and duodenal mucus through selective binding to exposed fucosylated or sialyl-dimeric-Lewis x blood group antigens using its BabA and SabA adhesins (Mahdavi et al., 2002). Such antigens are upregulated in gastric epithelial cells during inflammation, aided by the H. pylori cag pathogenicity island- encoded type IV secretion system which stimulates host cell receptors resulting in upregulated IL-8 production and maintenance of the inflammatory response (Fischer et al., 2001). Subsequent recruitment of polymorphonuclear cells (PMNs) and H. pylori urease- induced upregulation of the PMN nitric oxide production and release of more inflammatory mediators further contribute to mucosal damage (Gobert et al., 2002). Coupled with the H.
pylori ability to downregulate immune responses such as inhibition of T cell activation
(Gebert et al., 2003) and inhibition of APC antigen degradation/processing (Molinari et al., 1998), and the gastric mucosa’s lack of TLR4 (Backhed et al., 2003), this leads to the gastric mucosa being largely non responsive with regard to H. pylori removal, allowing chronic colonisation.
12 1.4.1b Escherichia coli
Many different types of Escherichia coli have been linked to intestinal diseases (Acheson and Luccioli, 2004), including enterohaemorrhagic (EHEC), enteropathogenic (EPEC), enterotoxigenic (ETEC) and enteroaggregative (Eagg) E. coli. Some of these produce stable virulence factors such as the Shiga toxins of EHEC, the heat stable, labile toxins of ETEC, and others. Briefly, these toxins/virulence factors lead to inhibition of IL-2, IL-4, IFN- γ and other regulatory cytokine production leading to a decreased host response (Malstrom and James, 1998), combined with upregulated chemokine production leading to increased toxin uptake by underlying host cells (Hurley et al., 2001) and further propagation of the condition.
1.4.1c Salmonella and Yersinia
Salmonella and Yersinia species have also adopted immunosuppresive strategies to ensure
survival, although their approach is somewhat different. Both cross the epithelia at the M cells. There, Salmonella adopts a long term survival within cells by means of altered PAMPs (Acheson and Luccioli, 2004) and an ability to downregulate NADPH oxidase and NO-mediated ROI formation in vacuoles (Hornef et al., 2002). Yersinia survives by impairing internalisation by disruption of host cytoskeletal structure and resisting opsonisation and complement-mediated phagocytosis (Hornef et al., 2002). Yersinia and some Salmonella are also able to inhibit inflammatory responses and upregulate IL-10 production with subsequent immunosuppressive and host tolerance effects (Sing et al., 2002; Rhen et al., 2003).
1.4.1d Listeria and Shigella
In contrast to Salmonella and Yersinia, Listeria monocytogenes and Shigella, both of which avoid host responses by existing within host phagocytes through disruption of the endosome or phagosome membranes resulting in cytosolic colonisation, activate NF-kB leading to increased IL-8 production and other inflammatory mediators (Dramsi and Cossart, 2002; Philpot et al., 2001). This benefits L. monocytogenes by recruiting more phagocytes to ensure a constant supply of host cells, and benefits Shigella species through the inflammatory disruption of epithelial membranes and further bacterial invasion.
13 1.4.1d Staphylococcus
Staphyloccocus aureus is a common or commensal skin or foodborne organism with
extreme medical significance due to the consequences of wound or nosocomial infection (Safdar and Maki, 2002). Its risks are due to multi-drug resistance, and because it is a producer of toxins including superantigens. Superantigens induce potent T-cell stimuli implicated in the pathophysiology of autoimmune and inflammatory disease (Benjamin et al., 1998). S. aureus is the test organism used by the Active Manuka Honey Association to assay manuka honeys for antimicrobial activity.
1.4.1e Clostridium
Clostridium difficile is a Gram-positive sporulating anaerobe which predominantly exists in
spore form in the colon but can enter its vegetative state upon perturbations of the gut microflora, whereupon toxic forms can produce an enterotoxin (toxin A) and cytotoxin (toxin B). Toxin A can cause epithelial cell apoptosis (Brito et al., 2005), whilst both toxins can result in upregulated IL-8 and ICAM-1 expression, leading to inflammation, necrosis and protein loss, whilst increasing peristalsis and capillary permeability, which collectively may result in diarrhoea and intestinal perforation (Durai, 2007). Multiple drug-resistant C. difficile is an increasingly prevalent pathogen worldwide. Its adaptability and drug resistance can be correlated to a highly mobile and mosaic genome (Sebaihia et al., 2006). This versatile genome content includes conjugative transposons, phage, IS elements and IStrons, providing resistance, virulence factors and a very adaptable metabolic capability with a preference for carbohydrate growth substrates allowing the vegetative form to effectively compete for GI environmental niches (Sebaibia et al., 2006).
1.4.2 Parasites
Major parasites of the GI tract which damage the mucosal epithelia upon infection include helminths and protozoans. Helminths attach to the mucosal epithelia by buccal attachment (hookworms)(Loukas and Prociv, 2001) or by lectin-mediated binding (nematodes)(Loukas and Maizels, 2000), and generally stimulate a TH2 response similar to an allergy (Maizels and Yazdanbakhsh, 2003). Hookworms appear to be resistant to proteolytic host defences (Loukas and Prociv, 2001), whilst nematodes inhibit inflammatory responses and mimic
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immunosuppresive cytokines (Maizels and Yazdanbakhsh, 2003; Schonemeyer et al., 2001; Gomez-Escobar et al., 1998; Pastrana et al., 1998), thus engendering stable infection. Protozoan infection begins upon cyst digestion by host digestive enzymes to release trophozoites, which adhere to the epithelia (Katz and Taylor, 2001) and cause mucosal damage. Giardia resist host defences by expression of variant-specific surface proteins (VSPs) which are protease-resistant and impair immunoglobulin recognition (Nash, 2002), whilst Entamoeba evade complement-mediated assault by mimicking the membrane attack complex (MAC) of the complement cascade (Braga et al., 1992). Cryptosporidium is unusual amongst protozoa in that it remains bound at the lumenal suface, and is commonly found in immunocompromised patients (Acheson and Luccioli, 2004).
1.4.3 Dietary Compounds
A vast number of harmful and potentially harmful compounds are ingested alongside compounds of nutritious value in food, and their effects depend largely upon the dose of available compound within that food. For example, plants contain a broad spectra of toxins and antinutrients including cyanogenic glycosides, glucosinolates, glykoalkaloids, lectins, oxalate, phenolics, phytate, protease inhibitors, saponins and tannins, many of which may lead to gastrointestinal inflammation and damage, as well as myriad of other ill-effects (Novak and Haslberger, 2000). In addition, plant material may contain contaminants such as quinolones, tetracyclines, sulfonamides, dithiocarbamates, polar-P containing compounds (glyphosate etc), organophosphorus, urea-derived compounds, triazine and acidic herbicides (Juan-Garcia et al., 2005).
Some plant compounds, such as lectins, are ingested daily in appreciable amounts, survive digestion, and bind to gastrointestinal epithelial membrane glycosyl groups and initiate a series of harmful local and systemic reactions (Vasconcelos and Oliveira, 2004). Lectin- induced reactions which harm the gastrointestinal epithelia are as follows: Lectins cause inhibition of parietal secretion of gastric acid (Kordas et al., 2000; Ibid, 2001). They cause shedding of brush border membranes, with accelerated cell loss and shortened, sparse and irregular enterocyte microvilli (Bardocz et al., 1995; Herzig et al., 1997). Lectins stimulate alterations in the microbial balance by reducing efficiency of food digestion whilst increasing mucin secretion, epithelial cell loss and serum protein leakage, which has the result of providing a source of nutrients for overgrowth of coliform bacteria such as E. coli (Grant, 1999). Lectins also modulate the GI immune system by triggering antibody production, histamine secretion by basophils, and upregulation of the inflammatory TH2 response by
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stimulating IL-4 and IL-13 secretion (Haas et al., 1999). They might also be implicated in facilitation of translocation of both dietary and gut-derived pathogenic antigens to peripheral tissues (Cordain et al., 2000).
1.4.4 Antibiotics
Antibiotics may contribute to the breakdown of GI defensive function by eliminating vulnerable organisms from the commensal flora which act to inhibit or suppress the growth of pathogenic organisms. Antibiotics which preferentially target anaerobes compared to facultative organisms pose a higher risk of promoting pathogenic overgrowth and thus a higher prevalence of infection. Furthermore, prolonged use of antibiotics may favour colonisation by antibiotic-resistant strains, which can pose serious problems if those strains are pathogenic.
1.4.5 Alterations in Immune Competency
Immune competency can be altered by drugs such as steroids, extreme youth or old age, acquired disease states such as infections or malignancies, stressors such as trauma or surgery, and germline immunodeficiency. The end result is naïve, delayed or defective IgA, phagocytic or T cell effector immune responses with commensurate increases in penetration of pathogen into the epithelia, decreased elimination of penetrative pathogens and reduced elaboration of cytokines and other chemokines which regulate the complex cell-cell interactions, respectively, leading to microbial disease and chronic persistence of tissue pathogens resulting in malignancies, inflammatory disorders and other systemic illness (Acheson and Luccioli, 2004).