Cysteine Protease-Dependent Mucous Disruptions and Differential Mucin Gene Expression in Giardia duodenalis Infection

The intestinal mucous layer provides a critical host defense against pathogen exposure and epithelial injury, yet little is known about how enteropathogens may circumvent this physiologic barrier. Giardia duodenalis is a small intestinal parasite responsible for diarrheal disease and chronic postinfectious illness. This study reveals a complex interaction at the surface of epithelial cells, between G. duodenalis and the intestinal mucous layer. Here, we reveal mechanisms whereby G. duodenalis evades and disrupts the first line of host defense by degrading human mucin-2 (MUC2), depleting mucin stores and inducing differential gene expression in the mouse small and large intestines. Human colonic biopsy specimens exposed to G. duodenalis were depleted of mucus, and in vivo mice infected with G. duodenalis had a thinner mucous layer and demonstrated differential Muc2 and Muc5ac mucin gene expression. Infection in Muc2^-/- mice elevated trophozoite colonization in the small intestine and impaired weight gain. In vitro, human LS174T goblet-like cells were depleted of mucus and had elevated levels of MUC2 mRNA expression after G. duodenalis exposure. Importantly, the cysteine protease inhibitor E64 prevented mucous degradation, mucin depletion, and the increase in MUC2 expression. This article describes a novel role for Giardia's cysteine proteases in pathogenesis and how Giardia's disruptions of the mucous barrier facilitate bacterial translocation that may contribute to the onset and propagation of disease.

Giardia co-infection promotes the secretion of antimicrobial peptides beta-defensin 2 and trefoil factor 3 and attenuates attaching and effacing bacteria-induced intestinal disease

Our understanding of polymicrobial gastrointestinal infections and their effects on host biology remains incompletely understood. Giardia duodenalis is an ubiquitous intestinal protozoan parasite infecting animals and humans. Concomitant infections with Giardia and other gastrointestinal pathogens commonly occur. In countries with poor sanitation, Giardia infection has been associated with decreased incidence of diarrheal disease and fever, and reduced serum inflammatory markers release, via mechanisms that remain obscure. This study analyzed Giardia spp. co-infections with attaching and effacing (A/E) pathogens, and assessed whether and how the presence of Giardia modulates host responses to A/E enteropathogens, and alters intestinal disease outcome. In mice infected with the A/E pathogen Citrobacter rodentium, co-infection with Giardia muris significantly attenuated weight loss, macro- and microscopic signs of colitis, bacterial colonization and translocation, while concurrently enhancing the production and secretion of antimicrobial peptides (AMPs) mouse β-defensin 3 and trefoil factor 3 (TFF3). Co-infection of human intestinal epithelial cells (Caco-2) monolayers with G. duodenalis trophozoites and enteropathogenic Escherichia coli (EPEC) enhanced the production of the AMPs human β-defensin 2 (HBD-2) and TFF3; this effect was inhibited with treatment of G. duodenalis with cysteine protease inhibitors. Collectively, these results suggest that Giardia infections are capable of reducing enteropathogen-induced colitis while increasing production of host AMPs. Additional studies also demonstrated that Giardia was able to directly inhibit the growth of pathogenic bacteria. These results reveal novel mechanisms whereby Giardia may protect against gastrointestinal disease induced by a co-infecting A/E enteropathogen. Our findings shed new light on how microbial-microbial interactions in the gut may protect a host during concomitant infections.

Mixed species biofilms of Fusobacterium necrophorum and Porphyromonas levii impair the oxidative response of bovine neutrophils in vitro

Biofilms composed of anaerobic bacteria can result in persistent infections and chronic inflammation. Host immune cells have difficulties clearing biofilm-related infections and this can result in tissue damage. Neutrophils are a vital component of the innate immune system and help clear biofilms. The comparative neutrophilic response to biofilms versus planktonic bacteria remains incompletely understood, particularly in the context of mixed infections. The objective of this study was to generate mixed species anaerobic bacterial biofilms composed of two opportunistic pathogens, Fusobacterium necrophorum and Porphyromonas levii, and evaluate neutrophil responses to extracellular fractions from both biofilms and planktonic cell co-cultures of the same bacteria. Purified bovine neutrophils exposed to culture supernatants from mixed species planktonic bacteria showed elevated oxidative activity compared to neutrophils exposed to biofilms composed of the same bacteria. Bacterial lipopolysaccharide plays a significant role in the stimulation of neutrophils; biofilms produced substantially more lipopolysaccharide than planktonic bacteria under these experimental conditions. Removal of lipopolysaccharide significantly reduced neutrophil oxidative response to culture supernatants of planktonic bacteria. Oxidative responses to LPS-removed biofilm supernatants and LPS-removed planktonic cell supernatants were similar. The limited neutrophil response to biofilm bacteria observed in this study supports the reduced ability of the innate immune system to eradicate biofilm-associated infections. Lipopolysaccharide is likely important in neutrophil response; however, the presence of other extracellular, immune modifying molecules in the bacterial media also appears to be important in altering neutrophil function.

The nasopharyngeal microbiota of beef cattle before and after transport to a feedlot

Background

The nasopharyngeal (NP) microbiota plays an important role in bovine health, comprising a rich and diverse microbial community. The nasopharynx is also the niche for potentially pathogenic agents which are associated with bovine respiratory disease (BRD), a serious and costly illness in feedlot cattle. We used 14 beef heifers from a closed and disease-free herd to assess the dynamics of the NP microbiota of cattle that are transported to a feedlot. Cattle were sampled prior to transport to the feedlot (day 0) and at days 2, 7, and 14.

Results

The structure of the NP microbiota changed significantly over the course of the study, with the largest shift occurring between day 0 (prior to transport) and day 2 (P < 0.001). Phylogenetic diversity and richness increased following feedlot placement (day 2; P < 0.05). The genera Pasteurella, Bacillus, and Proteus were enriched at day 0, Streptococcus and Acinetobacter at day 2, Bifidobacterium at day 7, and Mycoplasma at day 14. The functional potential of the NP microbiota was assessed using PICRUSt, revealing that replication and repair, as well as translation pathways, were more relatively abundant in day 14 samples. These differences were driven mostly by Mycoplasma. Although eight cattle were culture-positive for the BRD-associated bacterium Pasteurella multocida at one or more sampling times, none were culture-positive for Mannheimia haemolytica or Histophilus somni.

Conclusions

This study investigated the effect that feedlot placement has on the NP microbiota of beef cattle over a 14-d period. Within two days of transport to the feedlot, the NP microbiota changed significantly, increasing in both phylogenetic diversity and richness. These results demonstrate that there is an abrupt shift in the NP microbiota of cattle after transportation to a feedlot. This may have importance for understanding why cattle are most susceptible to BRD after feedlot placement.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-0978-6) contains supplementary material, which is available to authorized users.

Giardia duodenalis induces pathogenic dysbiosis of human intestinal microbiota biofilms

Giardia duodenalis is a prevalent cause of acute diarrheal disease worldwide. However, recent outbreaks in Italy and Norway have revealed a link between giardiasis and the subsequent development of chronic post-infectious irritable bowel syndrome. While the mechanisms underlying the causation of post-infectious irritable bowel syndrome remain obscure, recent findings suggest that alterations in gut microbiota communities are linked to the pathophysiology of irritable bowel syndrome. In the present study, we use a laboratory biofilm system to culture and enrich mucosal microbiota from human intestinal biopsies. Subsequently, we show that co-culture with Giardia induces disturbances in biofilm species composition and biofilm structure resulting in microbiota communities that are intrinsically dysbiotic - even after the clearance of Giardia. These microbiota abnormalities were mediated in part by secretory-excretory Giardia cysteine proteases. Using in vitro cell culture and germ-free murine infection models, we show that Giardia-induced disruptions of microbiota promote bacterial invasion, resulting in epithelial apoptosis, tight junctional disruption, and bacterial translocation across an intestinal epithelial barrier. Additionally, these dysbiotic microbiota communities resulted in increased activation of the Toll-like receptor 4 signalling pathway, and overproduction of the pro-inflammatory cytokine IL-1beta in humanized germ-free mice. Previous studies that have sought explanations and risk factors for the development of post-infectious irritable bowel syndrome have focused on features of enteropathogens and attributes of the infected host. We propose that polymicrobial interactions involving Giardia and gut microbiota may cause persistent dysbiosis, offering a new interpretation of the reasons why those afflicted with giardiasis are predisposed to gastrointestinal disorders post-infection.

Gut-derived cholecystokinin contributes to visceral hypersensitivity via nerve growth factor-dependent neurite outgrowth

BACKGROUND AND AIM

Irritable bowel syndrome is characterized by abdominal pain and altered bowel habits and may occur following stressful events or infectious gastroenteritis such as giardiasis. Recent findings revealed a link between cholecystokinin (CCK), neurotrophin synthesis, and intestinal hyperalgesia. The aim was to investigate the role of CCK in visceral hypersensitivity using mouse models challenged with a bout of infection with Giardia lamblia or psychological stress, either alone or in combination.

METHODS

Abdominal pain was evaluated by visceromoter response to colorectal distension. Nerve fibers in intestinal tissues were stained using immunohistochemistry (PGP9.5). Human neuroblastoma SH-SY5Y cells incubated with bacterial-free mouse gut supernatant or recombinant CCK-8S were assessed for neurite outgrowth and nerve growth factor (NGF) production.

RESULTS

Intestinal hypersensitivity was induced by either stress or Giardia infection, and a trend of increased pain was seen following dual stimuli. Increased CCK levels and PGP9.5 immunoreactivity were found in colonic mucosa of mice following stress and/or infection. Inhibitors to the CCK-A receptor (L-364718) or CCK-B receptor (L-365260) blocked visceral hypersensitivity caused by stress, but not when induced by giardiasis. Nerve fiber elongation and NGF synthesis were observed in SH-SY5Y cells after incubation with colonic supernatants from mice given the dual stimuli, or after treatment with CCK-8S. Increased nerve fiber length by colonic supernatant and CCK-8S was attenuated by L-365260 or neutralizing anti-NGF.

CONCLUSIONS

This new model successfully recapitulates intestinal hypernociception induced by stress or Giardia. Colonic CCK contributes to visceral hypersensitivity caused by stress, but not by Giardia, partly via NGF-dependent neurite outgrowth.

Disruptions of Host Immunity and Inflammation by Giardia Duodenalis: Potential Consequences for Co-Infections in the Gastro-Intestinal Tract

Giardia duodenalis (syn. G. intestinalis, or G. lamblia) is a leading cause of waterborne diarrheal disease that infects hundreds of millions of people annually. Research on Giardia has greatly expanded within the last few years, and our understanding of the pathophysiology and immunology on this parasite is ever increasing. At peak infection, Giardia trophozoites induce pathophysiological responses that culminate in the development of diarrheal disease. However, human data has suggested that the intestinal mucosa of Giardia-infected individuals is devoid of signs of overt intestinal inflammation, an observation that is reproduced in animal models. Thus, our understanding of host inflammatory responses to the parasite remain incompletely understood and human studies and experimental data have produced conflicting results. It is now also apparent that certain Giardia infections contain mechanisms capable of modulating their host’s immune responses. As the oral route of Giardia infection is shared with many other gastrointestinal (GI) pathogens, co-infections may often occur, especially in places with poor sanitation and/or improper treatment of drinking water. Moreover, Giardia infections may modulate host immune responses and have been found to protect against the development of diarrheal disease in developing countries. The following review summarizes our current understanding of the immunomodulatory mechanisms of Giardia infections and their consequences for the host, and highlights areas for future research. Potential implications of these immunomodulatory effects during GI co-infection are also discussed.

Giardia duodenalis Surface Cysteine Proteases Induce Cleavage of the Intestinal Epithelial Cytoskeletal Protein Villin via Myosin Light Chain Kinase

Giardia duodenalis infections are among the most common causes of waterborne diarrhoeal disease worldwide. At the height of infection, G. duodenalis trophozoites induce multiple pathophysiological processes within intestinal epithelial cells that contribute to the development of diarrhoeal disease. To date, our understanding of pathophysiological processes in giardiasis remains incompletely understood. The present study reveals a previously unappreciated role for G. duodenalis cathepsin cysteine proteases in intestinal epithelial pathophysiological processes that occur during giardiasis. Experiments first established that Giardia trophozoites indeed produce cathepsin B and L in strain-dependent fashion. Co-incubation of G. duodenalis with human enterocytes enhanced cathepsin production by Assemblage A (NF and S2 isolates) trophozoites, but not when epithelial cells were exposed to Assemblage B (GSM isolate) trophozoites. Direct contact between G. duodenalis parasites and human intestinal epithelial monolayers resulted in the degradation and redistribution of the intestinal epithelial cytoskeletal protein villin; these effects were abolished when parasite cathepsin cysteine proteases were inhibited. Interestingly, inhibition of parasite proteases did not prevent degradation of the intestinal tight junction-associated protein zonula occludens 1 (ZO-1), suggesting that G. duodenalis induces multiple pathophysiological processes within intestinal epithelial cells. Finally, this study demonstrates that G. duodenalis-mediated disruption of villin is, at least, in part dependent on activation of myosin light chain kinase (MLCK). Taken together, this study indicates a novel role for parasite cathepsin cysteine proteases in the pathophysiology of G. duodenalis infections.

Deciphering the pathogenesis of NSAID enteropathy using proton pump inhibitors and a hydrogen sulfide-releasing NSAID

The small intestine is a significant site of ulceration and bleeding induced by nonsteroidal anti-inflammatory drugs (NSAIDs). The pathogenesis is poorly understood. The present study explored the roles of bile, bacteria, and enterohepatic circulation to NSAID enteropathy, using both a conventional NSAID (naproxen) and a gastrointestinal-safe naproxen derivative (ATB-346), as well as proton pump inhibitors (PPIs). Rats were treated orally with naproxen or equimolar doses of ATB-346 over a 5-day period, with or without PPI administration, and intestinal damage was quantified. The cytotoxicity of bile from the rats was evaluated in vitro. Biliary excretion of naproxen and ATB-346 was determined. The impact of the NSAIDs and of PPIs on the composition of the intestinal microbiota was examined by deep sequencing of 16s rRNA. Naproxen caused significant intestinal damage and inflammation, whereas ATB-346 did not. Naproxen, but not ATB-346, dose dependently increased the cytotoxicity of bile, and it was further increased by PPI coadministration. Whereas biliary excretion of naproxen was significant in naproxen-treated rats, it was greatly reduced in rats treated with ATB-346. The enteric microbiota of naproxen-treated rats was distinct from that in vehicle- or ATB-346-treated rats, and PPI administration caused significant intestinal dysbiosis. The increase in cytotoxicity of bile induced by naproxen and PPIs may contribute significantly to intestinal ulceration and bleeding. Some of these effects may occur secondary to significant changes in the jejunal microbiota induced by both naproxen and PPIs.

Brucella abortus Induces the Premature Death of Human Neutrophils through the Action of Its Lipopolysaccharide

Most bacterial infections induce the activation of polymorphonuclear neutrophils (PMNs), enhance their microbicidal function, and promote the survival of these leukocytes for protracted periods of time. Brucella abortus is a stealthy pathogen that evades innate immunity, barely activates PMNs, and resists the killing mechanisms of these phagocytes. Intriguing clinical signs observed during brucellosis are the low numbers of Brucella infected PMNs in the target organs and neutropenia in a proportion of the patients; features that deserve further attention. Here we demonstrate that B. abortus prematurely kills human PMNs in a dose-dependent and cell-specific manner. Death of PMNs is concomitant with the intracellular Brucella lipopolysaccharide (Br-LPS) release within vacuoles. This molecule and its lipid A reproduce the premature cell death of PMNs, a phenomenon associated to the low production of proinflammatory cytokines. Blocking of CD14 but not TLR4 prevents the Br-LPS-induced cell death. The PMNs cell death departs from necrosis, NETosis and classical apoptosis. The mechanism of PMN cell death is linked to the activation of NADPH-oxidase and a modest but steadily increase of ROS mediators. These effectors generate DNA damage, recruitments of check point kinase 1, caspases 5 and to minor extent of caspase 4, RIP1 and Ca⁺⁺ release. The production of IL-1β by PMNs was barely stimulated by B. abortus infection or Br-LPS treatment. Likewise, inhibition of caspase 1 did not hamper the Br-LPS induced PMN cell death, suggesting that the inflammasome pathway was not involved. Although activation of caspases 8 and 9 was observed, they did not seem to participate in the initial triggering mechanisms, since inhibition of these caspases scarcely blocked PMN cell death. These findings suggest a mechanism for neutropenia in chronic brucellosis and reveal a novel Brucella-host cross-talk through which B. abortus is able to hinder the innate function of PMN.

The absence of obvious clinical symptoms during the early stages of brucellosis is linked to the Brucella stealthy strategy and its non-canonical PAMPs, which are low PRRs agonists. Still, there are clinical profiles that require explanation. For instance ‒despite the fact that neutrophils readily ingest Brucella during the onset of infection, brucellosis courses without neutrophilia, and just a low number of infected neutrophils are present in target organs. In the chronic phases, a significant proportion of the patients display absolute neutropenia and bone marrow pancytopenia linked to the myeloid cell linage. Examination of the Brucella infected bone marrow reveals granulomas and phagocytosis of myeloid cells. Based on these observations we explored the fate of native neutrophils during their interaction with Brucella. We found that the bacterium induces the premature cell death of neutrophils without inducing proinflammatory phenotypic changes. This event was reproduced by the lipid A of the Brucella LPS and depends on NADPH-oxidase activation and low ROS formation. We believe that this phenomenon explains ‒at least in part‒ the hematological and histological profiles observed during brucellosis. In addition, it may be that dying Brucella-infected neutrophils serve as “Trojan horse” vehicles for infecting phagocytic cells without promoting activation.

Giardia duodenalis-induced alterations of commensal bacteria kill Caenorhabditis elegans: a new model to study microbial-microbial interactions in the gut

Giardia duodenalis is the most common cause of parasitic diarrhea worldwide and a well-established risk factor for postinfectious irritable bowel syndrome. We hypothesized that Giardia-induced disruptions in host-microbiota interactions may play a role in the pathogenesis of giardiasis and in postgiardiasis disease. Functional changes induced by Giardia in commensal bacteria and the resulting effects on Caenorhabditis elegans were determined. Although Giardia or bacteria alone did not affect worm viability, combining commensal Escherichia coli bacteria with Giardia became lethal to C. elegans. Giardia also induced killing of C. elegans with attenuated Citrobacter rodentium espF and map mutant strains, human microbiota from a healthy donor, and microbiota from inflamed colonic sites of ulcerative colitis patient. In contrast, combinations of Giardia with microbiota from noninflamed sites of the same patient allowed for worm survival. The synergistic lethal effects of Giardia and E. coli required the presence of live bacteria and were associated with the facilitation of bacterial colonization in the C. elegans intestine. Exposure to C. elegans and/or Giardia altered the expression of 172 genes in E. coli. The genes affected by Giardia included hydrogen sulfide biosynthesis (HSB) genes, and deletion of a positive regulator of HSB genes, cysB, was sufficient to kill C. elegans even in the absence of Giardia. Our findings indicate that Giardia induces functional changes in commensal bacteria, possibly making them opportunistic pathogens, and alters host-microbe homeostatic interactions. This report describes the use of a novel in vivo model to assess the toxicity of human microbiota.

Anti-inflammatory and cytoprotective actions of hydrogen sulfide: translation to therapeutics

SIGNIFICANCE

There is a rapidly expanding body of evidence for important roles of hydrogen sulfide in protecting against tissue injury, reducing inflammation, and promoting repair. There is also growing evidence that H2S can be successfully exploited in drug development.

RECENT ADVANCES

H2S synthesis and degradation are regulated in circumstances of inflammation and injury so as to promote repair and re-establish homeostasis. Novel H2S-releasing drugs exhibit enhanced anti-inflammatory and pro-restorative effects, while having reduced adverse effects in many tissues.

CRITICAL ISSUES

H2S is a pleiotropic mediator, having effects on many elements in the inflammatory cascade and promoting the resolution of inflammation and injury. It also contributes significantly to mucosal defence in the gastrointestinal tract, and in host defence against infection. There is strong evidence that novel, H2S-based therapeutics are safe and effective in animal models, and several are progressing through human trials.

FUTURE DIRECTIONS

A better understanding of the physiological and pathophysiological roles of H2S continues to be restrained by the lack of simple, reliable methods for measurement of H2S synthesis, and the paucity of highly selective inhibitors of enzymes that participate in endogenous H2S synthesis. On the other hand, H2S donors show promise as therapeutics for several important indications.

Proresolution effects of hydrogen sulfide during colitis are mediated through hypoxia-inducible factor-1α

During a course of colitis, production of the gaseous mediator hydrogen sulfide (H2S) is markedly up-regulated at sites of mucosal damage and contributes significantly to healing and resolution of inflammation. The signaling mechanisms through which H2S promotes resolution of colitis are unknown. We hypothesized that the beneficial effects of H2S in experimental colitis are mediated via stabilization of hypoxia-inducible factor (HIF)-1α. The hapten dinitrobenzene sulfonic acid was used to induce colitis in rats and mice. This resulted in an elevated expression of the H2S-producing enzyme, cystathionine γ-lyase (CSE), and HIF-1α at sites of mucosal ulceration, and the expression of these 2 enzymes followed a similar pattern throughout the course of colitis. This represented a functionally important relationship because the loss of CSE-derived H2S production led to decreased HIF-1α stabilization and exacerbation of colitis. Furthermore, application of an H2S-releasing molecule, diallyl disulfide (DADS), stabilized colonic HIF-1α expression, up-regulated hypoxia-responsive genes, and reduced the severity of disease during peak inflammation. Importantly, the ability of DADS to promote the resolution of colitis was abolished when coadministered with an inhibitor of HIF-1α in vivo (PX-478). DADS was also able to maintain HIF-1α expression at a later point in colitis, when HIF-1α levels would have normally returned to control levels, and to enhance resolution. Finally, we found that HIF-1α stabilization inhibited colonic H2S production and may represent a negative feedback mechanism to prevent prolonged HIF-1α stabilization. Our findings demonstrate an important link between H2S and HIF-1α in the resolution of inflammation and injury during colitis and provide mechanistic insights into the therapeutic value of H2S donors.

Giardia duodenalis infection reduces granulocyte infiltration in an in vivo model of bacterial toxin-induced colitis and attenuates inflammation in human intestinal tissue

Giardia duodenalis (syn. G. intestinalis, G. lamblia) is a predominant cause of waterborne diarrheal disease that may lead to post-infectious functional gastrointestinal disorders. Although Giardia-infected individuals could carry as much as 10⁶ trophozoites per centimetre of gut, their intestinal mucosa is devoid of overt signs of inflammation. Recent studies have shown that in endemic countries where bacterial infectious diseases are common, Giardia infections can protect against the development of diarrheal disease and fever. Conversely, separate observations have indicated Giardia infections may enhance the severity of diarrheal disease from a co-infecting pathogen. Polymorphonuclear leukocytes or neutrophils (PMNs) are granulocytic, innate immune cells characteristic of acute intestinal inflammatory responses against bacterial pathogens that contribute to the development of diarrheal disease following recruitment into intestinal tissues. Giardia cathepsin B cysteine proteases have been shown to attenuate PMN chemotaxis towards IL-8/CXCL8, suggesting Giardia targets PMN accumulation. However, the ability of Giardia infections to attenuate PMN accumulation in vivo and how in turn this effect may alter the host inflammatory response in the intestine has yet to be demonstrated. Herein, we report that Giardia infection attenuates granulocyte tissue infiltration induced by intra-rectal instillation of Clostridium difficile toxin A and B in an isolate-dependent manner. This attenuation of granulocyte infiltration into colonic tissues paralled decreased expression of several cytokines associated with the recruitment of PMNs. Giardia trophozoite isolates that attenuated granulocyte infiltration in vivo also decreased protein expression of cytokines released from inflamed mucosal biopsy tissues collected from patients with active Crohn’s disease, including several cytokines associated with PMN recruitment. These results demonstrate for the first time that certain Giardia infections may attenuate PMN accumulation by decreasing the expression of the mediators responsible for their recruitment.

Post-infectious irritable bowel syndrome: Mechanistic insights into chronic disturbances following enteric infection

Irritable bowel syndrome (IBS) is a commonly encountered chronic functional gastrointestinal (GI) disorder. Approximately 10% of IBS patients can trace the onset of their symptoms to a previous a bout of infectious dysentery. The appearance of new IBS symptoms following an infectious event is defined as post-infectious-IBS. Indeed, with the World Health Organization estimating between 2 and 4 billion cases annually, infectious diarrheal disease represents an incredible international healthcare burden. Additionally, compounding evidence suggests many commonly encountered enteropathogens as unique triggers behind IBS symptom generation and underlying pathophysiological features. A growing body of work provides evidence supporting a role for pathogen-mediated modifications in the resident intestinal microbiota, epithelial barrier integrity, effector cell functions, and innate and adaptive immune features, all proposed physiological manifestations that can underlie GI abnormalities in IBS. Enteric pathogens must employ a vast array of machinery to evade host protective immune mechanisms, and illicit successful infections. Consequently, the impact of infectious events on host physiology can be multidimensional in terms of anatomical location, functional scope, and duration. This review offers a unique discussion of the mechanisms employed by many commonly encountered enteric pathogens that cause acute disease, but may also lead to the establishment of chronic GI dysfunction compatible with IBS.

Persistent gut barrier damage and commensal bacterial influx following eradication of Giardia infection in mice

Background

Recent studies of Giardia lamblia outbreaks have indicated that 40–80% of infected patients experience long-lasting functional gastrointestinal disorders after parasitic clearance. Our aim was to assess changes in the intestinal barrier and spatial distribution of commensal bacteria in the post-clearance phase of Giardia infection.

Methods

Mice were orogastrically inoculated with G. lamblia trophozoites (strain GS/M) or pair-fed with saline and were sacrificed on post-infective (PI) days 7 (colonization phase) and 35 (post-clearance phase). Gut epithelial barrier function was assessed by Western blotting for occludin cleavage and luminal-to-serosal macromolecular permeability. Gut-associated, superficial adherent, and mucosal endocytosed bacteria were measured by agar culturing and were examined by fluorescence in situ hybridization. Intracellular bacteria cultured from isolated mucosal cells were characterized by 16S rDNA sequencing. Neutrophil-specific esterase staining, a myeloperoxidase activity assay, and enzyme-linked immunosorbent assays for cytokine concentrations were used to verify intestinal tissue inflammation.

Results

Tight junctional damage was detected in the intestinal mucosa of Giardia-infected mice on PI days 7 and 35. Although intestinal bacterial overgrowth was evident only during parasite colonization (PI day 7), enhanced mucosal adherence and endocytosis of bacteria were observed on PI days 7 and 35. Multiple bacterial strains, including Bacillus, Lactobacillus, Staphylococcus, and Phenylobacterium, penetrated the gut mucosa in the post-infective phase. The mucosal influx of bacteria coincided with increases in neutrophil infiltration and myeloperoxidase activity on PI days 7 and 35. Elevated intestinal IFNγ, TNFα, and IL-1β levels also were detected on PI day 35.

Conclusions

Giardia-infected mice showed persistent tight junctional damage and bacterial penetration, accompanied by mucosal inflammation, after parasite clearance. These novel findings suggest that the host’s unresolved immune reactions toward its own microbiota, due to an impaired epithelial barrier, may partly contribute to the development of post-infective gut disorders.