Principles and clinical implications of the brain-gut-enteric microbiota axis

In the paper

“The brain can influence commensal organisms (enteric microbiota) indirectly, via changes in gastrointestinal motility and secretion, and intestinal permeability, or directly, via signaling molecules released into the gut lumen from cells in the lamina propria (enterochromaffin cells, neurons, immune cells).”

In the paper

“In maternal separation-induced, postnatal stress, reduction in lactobacilli was associated with the appearance of stress-indicative behaviors, and affected animals were more susceptible than unstressed controls to opportunistic infection.”

What the sources say

• Not checked yetcites [16] Bailey, M. T. & Coe, C. L. Maternal separation disrupts the integrity of the intestinal microflora in infant rhesus monkeys. Dev. Psychobiol. 35, 146–155 (1999). source

In the paper

“Vagal, afferent innervation of enterochromaffin cells provides a direct pathway for enterochromaffin-cell signaling to neuronal circuits, which may have an important role in pain and immune-response modulation, control of background emotions and other homeostatic functions.”

In the paper

“the role of the enteric microbiota in bidirectional gut–brain interactions in health and disease has received little attention until the past 5 years or so.”

In the paper

“Both pre natal17 and postnatal stress16 were associated with transient reductions in the levels of the enteric microbiota in rhesus monkeys.”

What the sources say

• Not checked yetcites [17] Bailey, M. T. et al. Prenatal stress alters bacterial colonization of the gut in infant monkeys. J. Pediatr. Gastroenterol. Nutr. 38, 414–421 (2004). source
• Not checked yetcites [16] Bailey, M. T. & Coe, C. L. Maternal separation disrupts the integrity of the intestinal microflora in infant rhesus monkeys. Dev. Psychobiol. 35, 146–155 (1999). source

In the paper

“In general, bacteria located in the biofilm seem to be less affected by environmental alterations, such as changes in intestinal transit rate and luminal contents, than luminal populations are, but seem to have increased involvement in bidirectional signaling with the host.19”

What the sources say

• Not checked yetcites [19] Macfarlane, S. & Dillon, J. F. Microbial biofilms in the human gastrointestinal tract. J. Appl. Microbiol. 102, 1187–1196 (2007). source

In the paper

“For example, several studies have demonstrated that stressful stimuli can enhance the permeability of the intestinal epithelium, which allows bacterial antigens to penetrate the gut epithelium and triggers an immune response in the intestinal mucosa.23-27”

What the sources say

• Not checked yetcites [23] Kiliaan, A. J. et al. Stress stimulates transepithelial macromolecular uptake in rat jejunum. Am. J. Physiol. 275, G1037–G1044 (1998).
• Not checked yetcites [24] Groot, J. et al. Stress-induced decrease of the intestinal barrier function. The role of muscarinic receptor activation. Ann. NY Acad. Sci. 915, 237–246 (2000). source
• Not checked yetcites [25] Yates, D. A., Santos, J., Soderholm, J. D. & Perdue, M. H. Adaptation of stress-induced mucosal pathophysiology in rat colon involves opioid pathways. Am. J. Physiol. Gastrointest. Liver Physiol. 281, G124–G128 (2001). source
• Not checked yetcites [26] Soderholm, J. D. et al. Neonatal maternal separation predisposes adult rats to colonic barrier dysfunction in response to mild stress. Am. J. Physiol. Gastrointest. Liver Physiol. 283, G1257–G1263 (2002). source
• Not checked yetcites [27] Jacob, C. et al. Mast cell tryptase controls paracellular permeability of the intestine. Role of protease-activated receptor 2 and beta-arrestins. J. Biol. Chem. 280, 31936–31948 (2005). source

In the paper

“The ANS also affects epithelial mechanisms involved in immune activation of the gut, either directly, through modulation of the response of the gut immune cells (for example, macrophages and mast cells) to luminal bacteria, or indirectly, through alteration of the access of luminal bacteria to gut immunocytes (Figure 2).”

In the paper

“Notably, most of the studies that have examined the influence of these systems on enteric microbiota have been performed on luminal bacterial populations from stool samples, whereas the influence of these systems on organisms contained in the biofilm adjacent to the intestinal mucosa are less well documented.”

In the paper

“For example, serotonin secretion into the stomach lumen has been reported in response to an intrathecal injection into the cerebrospinal fluid (central injection) of TRP, an analog of thyrotropin-releasing hormone, which is a central mediator of the stress response to cold temperatures.31,32 This secretion is probably mediated by vagal activation of gastric enterochromaffin cells.”

What the sources say

• Not checked yetcites [31] Stephens, R. L. & Tache, Y. Intracisternal injection of a TRH analogue stimulates gastric luminal serotonin release in rats. Am. J. Physiol. 256, G377–G383 (1989).
• Not checked yetcites [32] Yang, H. et al. TRH analogue microinjected into specific medullary nuclei stimulates gastric serotonin secretion in rats. Am. J. Physiol. 262, G216–G222 (1992).

In the paper

“For example, secretion of antimicrobial peptides, such as defensins, by Paneth cells has an important role in host defense mechanisms against inflammatory and infectious diseases of the gut.29”

What the sources say

• Not checked yetcites [29] Salzman, N. H., Underwood, M. A. & Bevins, C. L. Paneth cells, defensins, and the commensal microbiota: a hypothesis on intimate interplay at the intestinal mucosa. Semin. Immunol. 19, 70–83 (2007). source

In the paper

“For example, Bifidobacterium bifidum and Lactobacillus acidophilus are able to promote motility, while Escherichia species can inhibit it.6”

What the sources say

• Not checked yetcites [6] Mazmanian, S. K. et al. A microbial symbiosis factor prevents intestinal inflammatory disease. Nature 453, 620–625 (2008). source

In the paper

“Several signaling molecules used by the host for neuronal and neuroendocrine signaling (for example, catecholamines, serotonin, dynorphin, and cytokines) are also likely to be secreted into the gut lumen by neurons, immune cells and enterochromaffin cells, and the CNS is likely to have an important role in the release of these molecules.”

In the paper

“Similarly to eukaryotes, prokaryotes communicate with each other through hormones and hormone-like compounds. This pattern of mutual bacterial inter action is called quorum sensing.41”

What the sources say

• Not checked yetcites [41] Nealson, K. H., Platt, T. & Hastings, J. W. Cellular control of the synthesis and activity of the bacterial luminescent system. J. Bacteriol. 104, 313–322 (1970). source

In the paper

“Autoinducer 3 binds to the bacterial membrane receptor QseC, which results in its autophosphorylation. QseC then phosphorylates its response regulator, QseB, to initiate a complex signaling cascade that activates the expression of bacterial genes associated with virulence and motility, including the gene that presides over flagellum development.34”

What the sources say

• Not checked yetcites [34] Hughes, D. T. & Sperandio, V. Inter-kingdom signalling: communication between bacteria and their hosts. Nat. Rev. Microbiol. 6, 111–120 (2008). source

In the paper

“Stress-induced changes in permeability involve activation of glial and mast cells in the gut, overproduction of interferon-γ and changes in the morphology of the colonic epithelium via reduced expression of tight junction protein 2 (zona occludens 2) and by occlusion of important components of the intestinal tight junction.28”

What the sources say

• Not checked yetcites [28] Demaude, J. et al. Phenotypic changes in colonocytes following acute stress or activation of mast cells in mice: implications for delayed epithelial barrier dysfunction. Gut 55, 655–661 (2006). source

In the paper

“In vitro norepinephrine stimulates the growth of several strains of enteric pathogens (reviewed elsewhere34) and magnifies the virulent properties of Campylobacter jejuni.36”

What the sources say

• Not checked yetcites [34] Hughes, D. T. & Sperandio, V. Inter-kingdom signalling: communication between bacteria and their hosts. Nat. Rev. Microbiol. 6, 111–120 (2008). source
• Not checked yetcites [36] Cogan, T. A. et al. Norepinephrine increases the pathogenic potential of Campylobacter jejuni. Gut 56, 1060–1065 (2007). source

In the paper

“For example, autoinducer 3 can stimulate nor epinephrine receptors on the surface of eukaryotic cells, and α-2 receptors are present on the brush border of human enterocytes.51”

What the sources say

• Not checked yetcites [51] Valet, P. et al. Characterization and distribution of alpha 2-adrenergic receptors in the human intestinal mucosa. J. Clin. Invest. 91, 2049–2055 (1993). source

In the paper

“Although the effects of enteric microbiota to host signaling on various gut functions, including motility, secretion and immune function, have been studied extensively in healthy and diseased states, the possible effect of such microbial signaling beyond the gastro intestinal tract and on the nervous system has received little attention.”

In the paper

“Microbial signaling molecules could interact directly with afferent nerve terminals in situations where intestinal permeability is enhanced (for example, during inflammation or stress) or their signal could be relayed to neurons within the intestinal wall by ‘transducer cells’ in the epithelium.”

In the paper

“That signaling molecules are released by the host into the lumen of the gastrointestinal tract during stress and that receptors and intercellular signaling mechanisms for these same molecules are present on certain luminal microbes34 strongly suggests that the nervous system can also directly modulate microbial behavior.”

What the sources say

• Not checked yetcites [34] Hughes, D. T. & Sperandio, V. Inter-kingdom signalling: communication between bacteria and their hosts. Nat. Rev. Microbiol. 6, 111–120 (2008). source

In the paper

“Enterochromaffin cells are distributed throughout the intestinal tract,52 and, like intestinal epithelial cells, those located in the intestinal mucosa are accessible to the enteric microbiota on the intestinal lumen side, and are in contact with afferent and efferent nerve terminals located on the lamina propria.”

What the sources say

• Not checked yetcites [52] Walsh, J. H. & Mayer, E. A. Gastrointestinal peptide hormones and signal transduction. In Gastrointestinal Disease: Pathophysiology, Diagnosis, Management 5th edn (Eds Sleizenger, M. & Fordtran, J.) 18–44 (WB Saunders, Philadelphia, 1993).

In the paper

“Similar mechanisms could have a role in the patho physiology of alterations in bowel habits for patients with IBS.”

In the paper

“Thus, whereas the bacterial adrenergic agonist autoinducer 3 may mediate inhibition of intestinal fluid secretion—and thus delay transit rate—by binding to α-2 receptors on epithelial cells, other microbial factors may stimulate serotonin release by enterochromaffin cells to enhance fluid secretion, and thus accelerate transit rate.”

In the paper

“This increase in luminal fluid in turn stimulates bowel motility, which ultimately helps remove intestinal contents, including pathogenic bacteria.”

In the paper

“Interestingly, olfactory and taste receptors are expressed in enterochromaffin cells, and odorants present in the luminal environment (probably produced by the enteric microbiota) are also able to effect serotonin release by these cells.53”

What the sources say

• Not checked yetcites [53] Braun, T., Voland, P., Kunz, L., Prinz, C. & Gratzl, M. Enterochromaffin cells of the human gut: sensors for spices and odorants. Gastroenterology 132, 1890–1901 (2007). source

In the paper

“Nerve terminals of vagal afferents are located in close proximity to enterochromaffin cells, and these terminals express the serotonin-specific receptor 5-HT3R.67”

What the sources say

• Not checked yetcites [67] Raybould, H. E., Cooke, H. J. & Christofi, F. L. Sensory mechanisms: transmitters, modulators and reflexes. Neurogastroenterol. Motil. 16 (Suppl. 1), 60–63 (2004). source

In the paper

“A mechanism of this type might explain the reported upregulation of mucosal serotonin in a mouse model of postinfectious bowel dysfunction.66”

What the sources say

• Not checked yetcites [66] Wheatcroft, J. et al. Enterochromaffin cell hyperplasia and decreased serotonin transporter in a mouse model of postinfectious bowel dysfunction. Neurogastroenterol. Motil. 17, 863–870 (2005). source

In the paper

“The cholera toxin, a secretory enterotoxin from Vibrio cholerae, is known to trigger intestinal-fluid secretion by binding to the GM1 ganglioside on the surface of enterochromaffin cells. This interaction results in serotonin-mediated activation of secretomotor reflexes.61”

What the sources say

• Not checked yetcites [61] Lencer, W. I. Microbes and microbial toxins: paradigms for microbial-mucosal toxins. V. Cholera: invasion of the intestinal epithelial barrier by a stably folded protein toxin. Am. J. Physiol. Gastrointest. Liver Physiol. 280, G781–G786 (2001). source

In the paper

“Inflammation-induced upregulation of serotonin signaling persists after the inflammation has receded, and, in this post inflammatory, upregulated state, microbe to entero chromaffin cell signaling may result in persistent symptoms of bowel dysfunction in human, post infectious IBS.”

In the paper

“The influence on the brain of stimuli such as cytokines and vagal signaling has been well characterized in animal models of inflammation, and the constellation of psychological symptoms associated with inflammation (fatigue, social withdrawal and loss of appetite) has been labeled the sickness behavior syndrome.72”

What the sources say

• Not checked yetcites [72] Watkins, L. R. & Maier, S. F. The pain of being sick: implications of immune-to-brain communication for understanding pain. Annu. Rev. Psychol. 51, 29–57 (2000). source

In the paper

“As afferent nerves are not exposed to the luminal side of the intestine, under normal circumstances sensory neurons are indirectly activated by stimuli in the intestinal lumen via paracrine signaling, which is mediated by compounds such as serotonin, cholecystokinin, histamine, secretin, somatostatin, melatonin, uroguanylin, and corticotrophin-releasing factor, all of which can be released from ne”

What the sources say

• Not checked yetcites [68] Schonhoff, S. E., Giel-Moloney, M. & Leiter, A. B. Minireview: development and differentiation of gut endocrine cells. Endocrinology 145, 2639–2644 (2004). source
• Not checked yetcites [69] O'Hara, J. R., Ho, W., Linden, D. R., Mawe, G. M. & Sharkey, K. A. Enteroendocrine cells and 5-HT availability are altered in mucosa of guinea pigs with TNBS ileitis. Am. J. Physiol. Gastrointest. Liver Physiol. 287, G998–G1007 (2004). source

In the paper

“As discussed earlier, in the presence of increased intestinal permeability (caused, for instance, by stress or gut inflammation), access of various bacterial products and inflammatory mediators to nerve endings in the mucosa would be a plausible mechanism for microbe to brain communication.”

In the paper

“Possible mechanisms underlying such changes include effects secondary to the production of gas and short-chain fatty acids by enteric microbiota, and direct enteric microbiota to host signaling.”

In the paper

“Evidence of activation of the afferent vagal system was reported, which suggests a possible vagus-mediated microbe to brain signaling pathway.”

What the sources say

• Not checked yetcites [73] Lyte, M., Li, W., Opitz, N., Gaykema, R. P. & Goehler, L. E. Induction of anxiety-like behavior in mice during the initial stages of infection with the agent of murine colonic hyperplasia Citrobacter rodentium. Physiol. Behav. 89, 350–357 (2006). source

In the paper

“However, in healthy organisms, signaling via epithelial transducer cells, such as enterochromaffin cells, may be the predominant mechanism.”

In the paper

“In postinfectious IBS, major psychological stress around the time of gastroenteritis, trait anxiety, as well as extended duration of the infection, have been identified as risk factors for the persistence of symptoms.81”

What the sources say

• Not checked yetcites [81] Gwee, K. A. et al. The role of psychological and biological factors in postinfective gut dysfunction. Gut 44, 400–406 (1999). source

In the paper

“One way to explain this resilience would be to consider the epithelium-associated biofilm as a permanent bacterial reservoir whose presence allows prompt reconstitution of the physiological microbiota profile in the gut lumen following a decline in luminal microbiota.”

In the paper

“This reduction in abdominal girth was associ ated with an acceleration of orocecal as well as colonic transit and with an overall improvement in symptoms.”

What the sources say

• Not checked yetcites [13] Agrawal, A. et al. Clinical trial: the effects of a fermented milk product containing Bifidobacterium lactis DN-173-010 on abdominal distension and gastrointestinal transit in irritable bowel syndrome with constipation. Aliment. Pharmacol. Ther. (In press).

In the paper

“Inflammation-induced upregulation of the mucosal serotonin signaling system would be expected to enhance the ability of the enteric microbiota to modulate gut motility through activation of peristaltic and secretomotor reflexes.”

In the paper

“Evidence suggests that modification of the enteric microbiota by administration of antibiotics, probiotic bacteria or prebiotic substances (for example, certain fibers or lactulose) are beneficial in the treatment of IBD14 and improve some symptoms of IBS.12”

What the sources say

• Not checked yetcites [14] Vanderpool, C., Yan, F. & Polk, D. B. Mechanisms of probiotic action: implications for therapeutic applications in inflammatory bowel diseases. Inflamm. Bowel Dis. 14, 1585–1596 (2008). source
• Not checked yetcites [12] Spiller, P. Review article: probiotics and prebiotics in irritable bowel syndrome (IBS). Aliment. Pharmacol. Ther. (2008).