A model of Salmonella colitis with features of diarrhea in SLC11A1 wild-type mice

Akkermansia muciniphila isolated from forest musk deer ameliorates diarrhea in mice via modification of gut microbiota

BACKGROUND

The forest musk deer, a rare fauna species found in China, is famous for its musk secretion which is used in selected Traditional Chinese medicines. However, over-hunting has led to musk deer becoming an endangered species, and their survival is also greatly challenged by various high incidence and high mortality respiratory and intestinal diseases such as septic pneumonia and enteritis. Accumulating evidence has demonstrated that Akkermannia muciniphila (AKK) is a promising probiotic, and we wondered whether AKK could be used as a food additive in animal breeding programmes to help prevent intestinal diseases.

METHODS

We isolated one AKK strain from musk deer feces (AKK-D) using an improved enrichment medium combined with real-time PCR. After confirmation by 16S rRNA gene sequencing, a series of in vitro tests was conducted to evaluate the probiotic effects of AKK-D by assessing its reproductive capability, simulated gastrointestinal fluid tolerance, acid and bile salt resistance, self-aggregation ability, hydrophobicity, antibiotic sensitivity, hemolysis, harmful metabolite production, biofilm formation ability, and bacterial adhesion to gastrointestinal mucosa.

RESULTS

The AKK-D strain has a probiotic function similar to that of the standard strain in humans (AKK-H). An in vivo study found that AKK-D significantly ameliorated symptoms in the enterotoxigenic Escherichia coli (ETEC)-induced murine diarrhea model. AKK-D improved organ damage, inhibited inflammatory responses, and improved intestinal barrier permeability. Additionally, AKK-D promoted the reconstitution and maintenance of the homeostasis of gut microflora, as indicated by the fact that AKK-D-treated mice showed a decrease in Bacteroidetes and an increase in the proportion of other beneficial bacteria like Muribaculaceae, Muribaculum, and unclassified f_Lachnospiaceae compared with the diarrhea model mice.

CONCLUSION

Taken together, our data show that this novel AKK-D strain might be a potential probiotic for use in musk deer breeding, although further extensive systematic research is still needed.

Time resolved multi-omics reveals diverse metabolic strategies of Salmonella during diet-induced inflammation

With a rise in antibiotic resistance and chronic infection, the metabolic response of Salmonella enterica serovar Typhimurium to various dietary conditions over time remains an understudied avenue for novel, targeted therapeutics. Elucidating how enteric pathogens respond to dietary variation not only helps us decipher the metabolic strategies leveraged for expansion but also assists in proposing targets for therapeutic interventions. Here, we use a multi-omics approach to identify the metabolic response of Salmonella enterica serovar Typhimurium in mice on both a fibrous diet and high-fat diet over time. When comparing Salmonella gene expression between diets, we found a preferential use of respiratory electron acceptors consistent with increased inflammation of the high-fat diet mice. Looking at the high-fat diet over the course of infection, we noticed heterogeneity of samples based on Salmonella ribosomal activity, which separated into three infection phases: early, peak, and late. We identified key respiratory, carbon, and pathogenesis gene expression descriptive of each phase. Surprisingly, we identified genes associated with host-cell entry expressed throughout infection, suggesting sub-populations of Salmonella or stress-induced dysregulation. Collectively, these results highlight not only the sensitivity of Salmonella to its environment but also identify phase-specific genes that may be used as therapeutic targets to reduce infection.

Importance

Identifying novel therapeutic strategies for Salmonella infection that occur in relevant diets and over time is needed with the rise of antibiotic resistance and global shifts towards Western diets that are high in fat and low in fiber. Mice on a high-fat diet are more inflamed compared to those on a fibrous diet, creating an environment that results in more favorable energy generation for Salmonella . Over time on a high-fat diet, we observed differential gene expression across infection phases. Together, these findings reveal the metabolic tuning of Salmonella to dietary and temporal perturbations. Research like this, exploring the dimensions of pathogen metabolic plasticity, can pave the way for rationally designed strategies to control disease.

Exposing new taxonomic variation with inflammation - a murine model-specific genome database for gut microbiome researchers

BACKGROUND

The murine CBA/J mouse model widely supports immunology and enteric pathogen research. This model has illuminated Salmonella interactions with the gut microbiome since pathogen proliferation does not require disruptive pretreatment of the native microbiota, nor does it become systemic, thereby representing an analog to gastroenteritis disease progression in humans. Despite the value to broad research communities, microbiota in CBA/J mice are not represented in current murine microbiome genome catalogs.

RESULTS

Here we present the first microbial and viral genomic catalog of the CBA/J murine gut microbiome. Using fecal microbial communities from untreated and Salmonella-infected, highly inflamed mice, we performed genomic reconstruction to determine the impacts on gut microbiome membership and functional potential. From high depth whole community sequencing (~ 42.4 Gbps/sample), we reconstructed 2281 bacterial and 4516 viral draft genomes. Salmonella challenge significantly altered gut membership in CBA/J mice, revealing 30 genera and 98 species that were conditionally rare and unsampled in non-inflamed mice. Additionally, inflamed communities were depleted in microbial genes that modulate host anti-inflammatory pathways and enriched in genes for respiratory energy generation. Our findings suggest decreases in butyrate concentrations during Salmonella infection corresponded to reductions in the relative abundance in members of the Alistipes. Strain-level comparison of CBA/J microbial genomes to prominent murine gut microbiome databases identified newly sampled lineages in this resource, while comparisons to human gut microbiomes extended the host relevance of dominant CBA/J inflammation-resistant strains.

CONCLUSIONS

This CBA/J microbiome database provides the first genomic sampling of relevant, uncultivated microorganisms within the gut from this widely used laboratory model. Using this resource, we curated a functional, strain-resolved view on how Salmonella remodels intact murine gut communities, advancing pathobiome understanding beyond inferences from prior amplicon-based approaches. Salmonella-induced inflammation suppressed Alistipes and other dominant members, while rarer commensals like Lactobacillus and Enterococcus endure. The rare and novel species sampled across this inflammation gradient advance the utility of this microbiome resource to benefit the broad research needs of the CBA/J scientific community, and those using murine models for understanding the impact of inflammation on the gut microbiome more generally. Video Abstract.

The Game for Three: Salmonella–Host–Microbiota Interaction Models

Colonization of the gastrointestinal (GI) tract by enteric pathogens occurs in a context strongly determined by host-specific gut microbiota, which can significantly affect the outcome of infection. The complex gameplay between the trillions of microbes that inhabit the GI tract, the host, and the infecting pathogen defines a specific triangle of interaction; therefore, a complete model of infection should consider all of these elements. Many different infection models have been developed to explain the complexity of these interactions. This review sheds light on current knowledge, along with the strengths and limitations of in vitro and in vivo models utilized in the study of Salmonella–host–microbiome interactions. These models range from the simplest experiment simulating environmental conditions using dedicated growth media through in vitro interaction with cell lines and 3-D organoid structure, and sophisticated “gut on a chip” systems, ending in various animal models. Finally, the challenges facing this field of research and the important future directions are outlined.

Mouse Model to Study Salmonella-Induced Colitis

Salmonella efficiently colonizes the cecum and proximal colon of mice where it induces inflammation resulting in colitis. To study intestinal infection of non-typhoidal Salmonella enterica serovars in mice, the colonization resistance of the intestine is overcome by transiently reducing the gut microbiota by an oral antibiotic treatment 1 day prior to infection with Salmonella. The in vivo colitis model is crucial for understanding the role of mucosal host defenses, analysis of histopathological changes, and the identification of host and bacterial factors leading to acute infections or facilitating bacterial persistence.

High-Fat Diet and Antibiotics Cooperatively Impair Mitochondrial Bioenergetics to Trigger Dysbiosis that Exacerbates Pre-inflammatory Bowel Disease

The clinical spectra of irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD) intersect to form a scantily defined overlap syndrome, termed pre-IBD. We show that increased Enterobacteriaceae and reduced Clostridia abundance distinguish the fecal microbiota of pre-IBD patients from IBS patients. A history of antibiotics in individuals consuming a high-fat diet was associated with the greatest risk for pre-IBD. Exposing mice to these risk factors resulted in conditions resembling pre-IBD and impaired mitochondrial bioenergetics in the colonic epithelium, which triggered dysbiosis. Restoring mitochondrial bioenergetics in the colonic epithelium with 5-amino salicylic acid, a PPAR-γ (peroxisome proliferator-activated receptor gamma) agonist that stimulates mitochondrial activity, ameliorated pre-IBD symptoms. As with patients, mice with pre-IBD exhibited notable expansions of Enterobacteriaceae that exacerbated low-grade mucosal inflammation, suggesting that remediating dysbiosis can alleviate inflammation. Thus, environmental risk factors cooperate to impair epithelial mitochondrial bioenergetics, thereby triggering microbiota disruptions that exacerbate inflammation and distinguish pre-IBD from IBS.

The SLC transporter in nutrient and metabolic sensing, regulation, and drug development

The prevalence of metabolic diseases is growing worldwide. Accumulating evidence suggests that solute carrier (SLC) transporters contribute to the etiology of various metabolic diseases. Consistent with metabolic characteristics, the top five organs in which SLC transporters are highly expressed are the kidney, brain, liver, gut, and heart. We aim to understand the molecular mechanisms of important SLC transporter-mediated physiological processes and their potentials as drug targets. SLC transporters serve as ‘metabolic gate’ of cells and mediate the transport of a wide range of essential nutrients and metabolites such as glucose, amino acids, vitamins, neurotransmitters, and inorganic/metal ions. Gene-modified animal models have demonstrated that SLC transporters participate in many important physiological functions including nutrient supply, metabolic transformation, energy homeostasis, tissue development, oxidative stress, host defense, and neurological regulation. Furthermore, the human genomic studies have identified that SLC transporters are susceptible or causative genes in various diseases like cancer, metabolic disease, cardiovascular disease, immunological disorders, and neurological dysfunction. Importantly, a number of SLC transporters have been successfully targeted for drug developments. This review will focus on the current understanding of SLCs in regulating physiology, nutrient sensing and uptake, and risk of diseases.

Epithelial transport in digestive diseases: mice, monolayers, and mechanisms

The transport of electrolytes and fluid by the intestinal epithelium is critical in health to maintain appropriate levels of fluidity of the intestinal contents. The transport mechanisms that underlie this physiological process are also subject to derangement in various digestive disease states, such as diarrheal illnesses. This article summarizes the 2019 Hans Ussing Lecture of the Epithelial Transport Group of the American Physiological Society and discusses some pathways by which intestinal transport is dysregulated, particularly in the setting of infection with the diarrheal pathogen, Salmonella, and in patients treated with small-molecule inhibitors of the tyrosine kinase activity of the epidermal growth factor receptor (EGFr-TKI). The burdensome diarrhea in patients infected with Salmonella may be attributable to decreased expression of the chloride-bicarbonate exchanger downregulated in adenoma (DRA) that participates in electroneutral NaCl absorption. This outcome is possibly secondary to increased epithelial proliferation and/or decreased epithelial differentiation that occurs following infection. Conversely, the diarrheal side effects of cancer treatment with EGFr-TKI may be related to the known ability of EGFr-associated signaling to reduce calcium-dependent chloride secretion. Overall, the findings described may suggest targets for therapeutic intervention in a variety of diarrheal disease states.

NADPH oxidases and ROS signaling in the gastrointestinal tract

Reactive oxygen species (ROS), initially categorized as toxic by-products of aerobic metabolism, have often been called a double-edged sword. ROS are considered indispensable when host defense and redox signaling is concerned and a threat in inflammatory or degenerative diseases. This generalization does not take in account the diversity of oxygen metabolites being generated, their physicochemical characteristics and their production by distinct enzymes in space and time. NOX/DUOX NADPH oxidases are the only enzymes solely dedicated to ROS production and the prime ROS producer for intracellular and intercellular communication due to their widespread expression and intricate regulation. Here we discuss new insights of how NADPH oxidases act via ROS as multifaceted regulators of the intestinal barrier in homeostasis, infectious disease and intestinal inflammation. A closer look at monogenic VEOIBD and commensals as ROS source supports the view of H₂O₂ as key beneficial messenger in the barrier ecosystem.

Goblet cell associated antigen passages are inhibited during Salmonella typhimurium infection to prevent pathogen dissemination and limit responses to dietary antigens

Dietary antigen acquisition by lamina propria (LP) dendritic cells (DCs) is crucial to induce oral tolerance and maintain homeostasis. However, encountering innocuous antigens during infection can lead to inflammatory responses, suggesting processes may limit steady-state luminal antigen capture during infection. We observed that goblet cell (GC) associated antigen passages (GAPs), a steady-state pathway delivering luminal antigens to LP-DCs, are inhibited during Salmonella infection. GAP inhibition was mediated by IL-1β. Infection abrogated luminal antigen delivery and antigen-specific T cell proliferation in the mesenteric lymph node (MLN). Antigen-specific T cell proliferation to dietary antigen was restored by overriding GAP suppression; however, this did not restore regulatory T cell induction, but induced inflammatory T cell responses. Salmonella translocation to the MLN required GCs and correlated with GAPs. Genetic manipulations overriding GAP suppression, or antibiotics inducing colonic GAPs, but not antibiotics that do not, increased dissemination and worsened outcomes independent of luminal pathogen burden. Thus, steady-state sampling pathways are suppressed during infection to prevent responses to dietary antigens, limit pathogen entry, and lessen the disease. Moreover, antibiotics may worsen Salmonella infection by means beyond blunting gut microbiota colonization resistance, providing new insight into how precedent antibiotic use aggravates enteric infection.

Salmonella-Mediated Inflammation Eliminates Competitors for Fructose-Asparagine in the Gut

Salmonella enterica elicits intestinal inflammation to gain access to nutrients. One of these nutrients is fructose-asparagine (F-Asn). The availability of F-Asn to Salmonella during infection is dependent upon Salmonella pathogenicity islands 1 and 2, which in turn are required to provoke inflammation. Here, we determined that F-Asn is present in mouse chow at approximately 400 pmol/mg (dry weight). F-Asn is also present in the intestinal tract of germfree mice at 2,700 pmol/mg (dry weight) and in the intestinal tract of conventional mice at 9 to 28 pmol/mg. These findings suggest that the mouse intestinal microbiota consumes F-Asn. We utilized heavy-labeled precursors of F-Asn to monitor its formation in the intestine, in the presence or absence of inflammation, and none was observed. Finally, we determined that some members of the class Clostridia encode F-Asn utilization pathways and that they are eliminated from highly inflamed Salmonella-infected mice. Collectively, our studies identify the source of F-Asn as the diet and that Salmonella-mediated inflammation is required to eliminate competitors and allow the pathogen nearly exclusive access to this nutrient.

Salmonella Typhimurium PagP- and UgtL-dependent resistance to antimicrobial peptides contributes to the gut colonization

Mucosal barrier formed by cationic antimicrobial peptides (CAMPs) is believed to be crucial for host protection from pathogenic gut infection. However, some pathogens can develop resistance to the CAMPs to survive in hosts. Salmonella enterica is a common cause of acute diarrhea. During the course of this disease, the pathogen must continuously colonize the gut lumen, which contains CAMPs. However, it is incompletely understood whether the resistance of Salmonella strains to CAMPs contributes to the development of gut infections. PhoPQ two-component system-dependent lipid A modifications confer resistance to CAMPs in S. enterica serovar Typhimurium. Therefore, we introduced mutations into the PhoPQ-regulated genes in an S. Typhimurium strain, obtaining pagP ugtL and pmrA mutant strains. Each mutant strain demonstrated a distinct spectrum of the resistance to CAMPs. Using streptomycin mouse model for Salmonella diarrhea, we show that the pagP ugtL, but not pmrA, mutant strain had a gut colonization defect. Furthermore, the pagP ugtL, but not pmrA, mutant strain had decreased outer membrane integrity and susceptibility to magainin 2, an alpha-helical CAMP. Taken together, the PagP- and UgtL-dependent resistance to CAMPs was demonstrated to contribute to sustained colonization in the gut. This may be due to the robust outer membrane of S. Typhimurium, inducing the resistance to alpha-helical CAMPs such as α-defensins. Our findings indicate that the development of resistance to CAMPs is required for the S. Typhimurium gut infection.

CXCR3 plays a critical role for host protection against Salmonellosis

CXCR3 and its ligands are heavily associated with inflammation and have been implicated in numerous inflammatory diseases. CXCR3 plays an important role in recruiting pro-inflammatory cells, specifically neutrophils, in a model of sterile colitis whereby CXCR3^−/− mice showed an attenuated course of colitis with markedly reduced host-tissue damage in the inflamed caecum. The role of CXCR3 during infectious colitis, however, is unclear and therefore in this study, we investigated the role of CXCR3 in the regulation of the immune response during acute and chronic gastrointestinal infection, using a murine model of Salmonella enterica serovar Enteritidis. During acute infection with Salmonella, we observed significantly increased Salmonella loading in the caecum and dissemination to the spleen and liver in CXCR3^−/− mice, but not in Wt counterparts. During chronic infection, increased pathological features of inflammation were noted in the spleen and liver, with significantly increased levels of apoptosis in the liver of CXCR3^−/− mice, when compared to Wt counterparts. In addition, compromised intestinal IgA levels, CD4⁺ helper T cells and neutrophil recruitment were observed in CXCR3^−/− challenged with Salmonella, when compared to Wt counterparts. Our data suggests that CXCR3 is a key molecule in host intestinal immunity against Salmonellosis via regulating neutrophils chemotaxis.

Use of Attenuated but Metabolically Competent Salmonella as a Probiotic To Prevent or Treat Salmonella Infection

Salmonella enterica is among the most burdensome of foodborne disease agents. There are over 2,600 serovars that cause a range of disease manifestations ranging from enterocolitis to typhoid fever. While there are two vaccines in use in humans to protect against typhoid fever, there are none that prevent enterocolitis. If vaccines preventing enterocolitis were to be developed, they would likely protect against only one or a few serovars. In this report, we tested the hypothesis that probiotic organisms could compete for the preferred nutrient sources of Salmonella and thus prevent or treat infection. To this end, we added the fra locus, which encodes a utilization pathway for the Salmonella-specific nutrient source fructose-asparagine (F-Asn), to the probiotic bacterium Escherichia coli Nissle 1917 (Nissle) to increase its ability to compete with Salmonella in mouse models. We also tested a metabolically competent, but avirulent, Salmonella enterica serovar Typhimurium mutant for its ability to compete with wild-type Salmonella The modified Nissle strain became more virulent and less able to protect against Salmonella in some instances. On the other hand, the modified Salmonella strain was safe and effective in preventing infection with wild-type Salmonella While we tested for efficacy only against Salmonella Typhimurium, the modified Salmonella strain may be able to compete metabolically with most, if not all, Salmonella serovars, representing a novel approach to control of this pathogen.

Endogenous and exogenous control of gastrointestinal epithelial function: building on the legacy of Bayliss and Starling

Transport of fluid and electrolytes in the intestine allows for appropriate adjustments in luminal fluidity while reclaiming water used in digesting and absorbing a meal, and is closely regulated. This article discusses various endogenous and exogenous mechanisms whereby transport is controlled in the gut, placing these in the context of the ideas about the neurohumoral control of alimentary physiology that were promulgated by William Bayliss and Ernest Starling. The article considers three themes. First, mechanisms that intrinsically regulate chloride secretion, centred on the epidermal growth factor receptor (EGFr), are discussed. These may be important in ensuring that excessive chloride secretion, with the accompanying loss of fluid, is not normally stimulated by intestinal distension as the meal passes through the gastrointestinal tract. Second, mechanisms whereby probiotic microorganisms can impart beneficial effects on the gut are described, with a focus on targets at the level of the epithelium. These findings imply that the commensal microbiota exert important influences on the epithelium in health and disease. Finally, mechanisms that lead to diarrhoea in patients infected with an invasive pathogen, Salmonella, are considered, based on recent studies in a novel mouse model. Diarrhoea is most likely attributable to reduced expression of absorptive transporters and may not require the influx of neutrophils that accompanies infection. Overall, the goal of the article is to highlight the many ways in which critical functions of the intestinal epithelium are regulated under physiological and pathophysiological conditions, and to suggest possible targets for new therapies for digestive disease states.

Symbiotic maple saps minimize disruption of the mice intestinal microbiota after oral antibiotic administration

This study was undertaken to evaluate the in vivo impact of new symbiotic products based on liquid maple sap or its concentrate. Sap and concentrate, with or without inulin (2%), were inoculated with Bifidobacterium lactis Bb12 and Lactobacillus rhamnosus GG valio at initial counts of 2-4 × 10(8) cfu mL(-1). The experiments started with intra-gastric administration of antibiotic (kanamycin 40 mg in 0.1 cc) (to induce microbiota disturbance and/or diarrhea) to 3-to-5-week-old C57BL/6 female mice followed by a combination of prebiotic and probiotics included in the maple sap or its concentrate for a week. The combination inulin and probiotics in maple sap and concentrate appeared to minimize the antibiotic-induced breakdown of mice microbiota with a marked effect on bifidobacterium and bacteroides levels, thus permitting a more rapid re-establishment of the baseline microbiota levels. Results suggest that maple sap and its concentrate represent good candidates for the production of non-dairy functional foods.

Regulation of bacterial virulence by Csr (Rsm) systems

Most bacterial pathogens have the remarkable ability to flourish in the external environment and in specialized host niches. This ability requires their metabolism, physiology, and virulence factors to be responsive to changes in their surroundings. It is no surprise that the underlying genetic circuitry that supports this adaptability is multilayered and exceedingly complex. Studies over the past 2 decades have established that the CsrA/RsmA proteins, global regulators of posttranscriptional gene expression, play important roles in the expression of virulence factors of numerous proteobacterial pathogens. To accomplish these tasks, CsrA binds to the 5' untranslated and/or early coding regions of mRNAs and alters translation, mRNA turnover, and/or transcript elongation. CsrA activity is regulated by noncoding small RNAs (sRNAs) that contain multiple CsrA binding sites, which permit them to sequester multiple CsrA homodimers away from mRNA targets. Environmental cues sensed by two-component signal transduction systems and other regulatory factors govern the expression of the CsrA-binding sRNAs and, ultimately, the effects of CsrA on secretion systems, surface molecules and biofilm formation, quorum sensing, motility, pigmentation, siderophore production, and phagocytic avoidance. This review presents the workings of the Csr system, the paradigm shift that it generated for understanding posttranscriptional regulation, and its roles in virulence networks of animal and plant pathogens.

Salmonella-induced Diarrhea Occurs in the Absence of IL-8 Receptor (CXCR2)-Dependent Neutrophilic Inflammation

BACKGROUND

Gastroenteritis is the most common manifestation of nontyphoidal Salmonella enterica infections, but little is known about the pathogenesis of diarrhea in this infection

METHODS

To determine whether polymorphonuclear neutrophils (PMNs) are required for diarrhea for Salmonella colitis, we infected kanamycin-pretreated interleukin 8R (IL-8R) mutant mice and controls, both with nonmutant Slc11a1 (Nramp1, ItyR). We compared the 2 mouse strains for increases in fecal water content (diarrhea) 3 days after infection, changes in expression of ion transporters in colonic epithelial cells, proliferation of epithelial cells, and severity of infection as measured by colony-forming units (CFUs).

RESULTS

The IL-8R knockout mice had fewer PMNs in the colon but the other variables we measured were unaffected except for an increase in CFUs in the colon. The pathologic changes in the cecum were similar in both groups except for the lack of PMNs in the IL-8R knockout mice. There was minimal damage to the colon more distally.

CONCLUSIONS

In the early stage of Salmonella colitis, PMNs are not required for diarrhea or for the decrease in expression of colonic epithelial cell apical ion transporters. They contribute to defense against infection in the cecum but not extracolonically at this stage of Salmonella colitis.

Same species, different diseases: how and why typhoidal and non-typhoidal Salmonella enterica serovars differ

Human infections by the bacterial pathogen Salmonella enterica represent major disease burdens worldwide. This highly ubiquitous species consists of more than 2600 different serovars that can be divided into typhoidal and non-typhoidal Salmonella (NTS) serovars. Despite their genetic similarity, these two groups elicit very different diseases and distinct immune responses in humans. Comparative analyses of the genomes of multiple Salmonella serovars have begun to explain the basis of the variation in disease manifestations. Recent advances in modeling both enteric fever and intestinal gastroenteritis in mice will facilitate investigation into both the bacterial- and host-mediated mechanisms involved in salmonelloses. Understanding the genetic and molecular mechanisms responsible for differences in disease outcome will augment our understanding of Salmonella pathogenesis, host immunity, and the molecular basis of host specificity. This review outlines the differences in epidemiology, clinical manifestations, and the human immune response to typhoidal and NTS infections and summarizes the current thinking on why these differences might exist.

Fructose-asparagine is a primary nutrient during growth of Salmonella in the inflamed intestine

Salmonella enterica serovar Typhimurium (Salmonella) is one of the most significant food-borne pathogens affecting both humans and agriculture. We have determined that Salmonella encodes an uptake and utilization pathway specific for a novel nutrient, fructose-asparagine (F-Asn), which is essential for Salmonella fitness in the inflamed intestine (modeled using germ-free, streptomycin-treated, ex-germ-free with human microbiota, and IL10-/- mice). The locus encoding F-Asn utilization, fra, provides an advantage only if Salmonella can initiate inflammation and use tetrathionate as a terminal electron acceptor for anaerobic respiration (the fra phenotype is lost in Salmonella SPI1- SPI2- or ttrA mutants, respectively). The severe fitness defect of a Salmonella fra mutant suggests that F-Asn is the primary nutrient utilized by Salmonella in the inflamed intestine and that this system provides a valuable target for novel therapies.

Altered expression and localization of ion transporters contribute to diarrhea in mice with Salmonella-induced enteritis

BACKGROUND & AIMS

Salmonella enterica serovar Typhimurium is an enteropathogen that causes self-limiting diarrhea in healthy individuals, but poses a significant health threat to vulnerable populations. Our understanding of the pathogenesis of Salmonella-induced diarrhea has been hampered by the lack of a suitable mouse model. After a dose of oral kanamycin, Salmonella-infected congenic BALB/c.D2(NrampG169) mice, which carry a wild-type Nramp1 gene, develop clear manifestations of diarrhea. We used this model to elucidate the pathophysiology of Salmonella-induced diarrhea.

METHODS

BALB /c.D2(NrampG169) mice were treated with kanamycin and then infected with wild-type or mutant Salmonella by oral gavage. Colon tissues were isolated and Ussing chambers, quantitative polymerase chain reaction, immunoblot, and confocal microscopy analyses were used to study function and expression of ion transporters and cell proliferation.

RESULTS

Studies with Ussing chambers demonstrated reduced basal and/or adenosine 3',5'-cyclic monophosphate-mediated electrogenic ion transport in infected colonic tissues, attributable to changes in chloride or sodium transport, depending on the segment studied. The effects of infection were mediated, at least in part, by effector proteins secreted by the bacterial Salmonella pathogenicity island 1- and Salmonella pathogenicity island-2-encoded virulence systems. Infected tissue showed reduced expression of the chloride-bicarbonate exchanger down-regulated in adenoma in surface colonic epithelial cells. Cystic fibrosis transmembrane conductance regulator was internalized in colonic crypt epithelial cells without a change in overall expression levels. Confocal analyses, densitometry, and quantitative polymerase chain reaction revealed that expression of epithelial sodium channel β was reduced in distal colons of Salmonella-infected mice. The changes in transporter expression, localization, and/or function were accompanied by crypt hyperplasia in Salmonella-infected mice.

CONCLUSIONS

Salmonella infection induces diarrhea by altering expression and/or function of transporters that mediate water absorption in the colon, likely reflecting the fact that epithelial cells have less time to differentiate into surface cells when proliferation rates are increased by infection.

Yersinia enterocolitica inhibits Salmonella enterica serovar Typhimurium and Listeria monocytogenes cellular uptake

Yersinia enterocolitica biovar 1B employs two type three secretion systems (T3SS), Ysa and Ysc, which inject effector proteins into macrophages to prevent phagocytosis. Conversely, Salmonella enterica serovar Typhimurium uses a T3SS encoded by Salmonella pathogenicity island 1 (SPI1) to actively invade cells that are normally nonphagocytic and a second T3SS encoded by SPI2 to survive within macrophages. Given the distinctly different outcomes that occur with regard to host cell uptake of S. Typhimurium and Y. enterocolitica, we investigated how each pathogen influences the internalization outcome of the other. Y. enterocolitica reduces S. Typhimurium invasion of HeLa and Caco-2 cells to a level similar to that observed using an S. Typhimurium SPI1 mutant alone. However, Y. enterocolitica had no effect on S. Typhimurium uptake by J774.1 or RAW264.7 macrophage-like cells. Y. enterocolitica was also able to inhibit the invasion of epithelial and macrophage-like cells by Listeria monocytogenes. Y. enterocolitica mutants lacking either the Ysa or Ysc T3SS were partially defective, while double mutants were completely defective, in blocking S. Typhimurium uptake by epithelial cells. S. Typhimurium encodes a LuxR homolog, SdiA, which detects N-acylhomoserine lactones (AHLs) produced by Y. enterocolitica and upregulates the expression of an invasin (Rck) and a putative T3SS effector (SrgE). Two different methods of constitutively activating the S. Typhimurium SdiA regulon failed to reverse the uptake blockade imposed by Y. enterocolitica.

Shaping the (auto)immune response in the gut: the role of intestinal immune regulation in the prevention of type 1 diabetes

The pathogenesis of organ-specific autoimmune diseases such as Type 1 Diabetes (T1D) is regulated by genetic and environmental factors. There is increasing evidence that environmental factors acting at the intestinal level, with a special regard to the diverse bacterial species that constitute the microbiota, influence the course of autoimmune diseases in tissues outside the intestine both in humans and in preclinical models. In this review we recapitulate current knowledge on the intestinal immune system, its role in local and systemic immune responses and how multiple environmental factors can shape these responses with pathologic or beneficial outcomes for autoimmune diseases such T1D.

Diarrhea and colitis in mice require the Salmonella pathogenicity island 2-encoded secretion function but not SifA or Spv effectors

We investigated the roles of Salmonella pathogenicity island 2 (SPI-2) and two SPI-2 effectors in Salmonella colitis and diarrhea in genetically resistant BALB/c.D2(Slc11a1) congenic mice with the wild-type Nramp1 locus. Wild-type Salmonella enterica serovar Typhimurium 14028s caused a pan-colitis, and the infected mice developed frank diarrhea with a doubling of the fecal water content. An ssaV mutant caused only a 26% increase in fecal water content, without producing the pathological changes of colitis, and it did not cause weight loss over a 1-week period of observation. However, two SPI-2 effector mutants, the spvB and sifA mutants, and a double spvB sifA mutant caused diarrhea and colitis, even though the sifA mutant was sensitive to killing by bone marrow-derived macrophages from BALB/c.D2 mice and was severely impaired in extraintestinal growth but not in growth in the cecum. These results demonstrate that systemic S. enterica infection and diarrhea/colitis are distinct pathogenic processes and that only the former requires spvB and sifA.

The streptomycin mouse model for Salmonella diarrhea: functional analysis of the microbiota, the pathogen's virulence factors, and the host's mucosal immune response

The mammalian intestine is colonized by a dense microbial community, the microbiota. Homeostatic and symbiotic interactions facilitate the peaceful co-existence between the microbiota and the host, and inhibit colonization by most incoming pathogens ('colonization resistance'). However, if pathogenic intruders overcome colonization resistance, a fierce, innate inflammatory defense can be mounted within hours, the adaptive arm of the immune system is initiated, and the pathogen is fought back. The molecular nature of the homeostatic interactions, the pathogen's ability to overcome colonization resistance, and the triggering of native and adaptive mucosal immune responses are still poorly understood. To study these mechanisms, the streptomycin mouse model for Salmonella diarrhea is of great value. Here, we review how S. Typhimurium triggers mucosal immune responses by active (virulence factor elicited) and passive (MyD88-dependent) mechanisms and introduce the S. Typhimurium mutants available for focusing on either response. Interestingly, mucosal defense turns out to be a double-edged sword, limiting pathogen burdens in the gut tissue but enhancing pathogen growth in the gut lumen. This model allows not only studying the molecular pathogenesis of Salmonella diarrhea but also is ideally suited for analyzing innate defenses, microbe handling by mucosal phagocytes, adaptive secretory immunoglobulin A responses, probing microbiota function, and homeostatic microbiota-host interactions. Finally, we discuss the general need for defined assay conditions when using animal models for enteric infections and the central importance of littermate controls.

Chronic Salmonella infected mouse model

The bacterial infected mouse model is a powerful model system for studying areas such as infection, inflammation, immunology, signal transduction, and tumorigenesis. Many researchers have taken advantage of the colitis induced by Salmonella typhimurium for the studies on the early phase of inflammation and infection. However, only few reports are on the chronic infection in vivo. Mice with Salmonella persistent existence in the gastrointestinal tract allow us to explore the long-term host-bacterial interaction, signal transduction, and tumorigenesis. We have established a chronic bacterial infected mouse model with Salmonella typhimurium colonization in the mouse intestine over 6 months. To use this system, it is necessary for the researcher to learn how to prepare the bacterial culture and gavage the animals. We detail a methodology for prepare bacterial culture and gavage mice. We also show how to detect the Salmonella persistence in the gastrointestinal tract. Overall, this protocol will aid researchers using the bacterial infected mouse model to address fundamentally important biological and microbiological questions.

Chronic effects of a Salmonella type III secretion effector protein AvrA in vivo

BACKGROUND

Salmonella infection is a common public health problem that can become chronic and increase the risk of inflammatory bowel diseases and cancer. AvrA is a Salmonella bacterial type III secretion effector protein. Increasing evidence demonstrates that AvrA is a multi-functional enzyme with critical roles in inhibiting inflammation, regulating apoptosis, and enhancing proliferation. However, the chronic effects of Salmonella and effector AvrA in vivo are still unknown. Moreover, alive, mutated, non-invasive Salmonella is used as a vector to specifically target cancer cells. However, studies are lacking on chronic infection with non-pathogenic or mutated Salmonella in the host.

METHODS/PRINCIPAL FINDINGS

We infected mice with Salmonella Typhimurium for 27 weeks and investigated the physiological effects as well as the role of AvrA in intestinal inflammation. We found altered body weight, intestinal pathology, and bacterial translocation in spleen, liver, and gallbladder in chronically Salmonella-infected mice. Moreover, AvrA suppressed intestinal inflammation and inhibited the secretion of cytokines IL-12, IFN-gamma, and TNF-alpha. AvrA expression in Salmonella enhanced its invasion ability. Liver abscess and Salmonella translocation in the gallbladder were observed and may be associated with AvrA expression in Salmonella.

CONCLUSION/SIGNIFICANCE

We created a mouse model with persistent Salmonella infection in vivo. Our study further emphasizes the importance of the Salmonella effector protein AvrA in intestinal inflammation, bacterial translocation, and chronic infection in vivo.

Molecular mechanisms of Salmonella virulence and host resistance

Salmonella species can cause typhoid fever and gastroenteritis in humans and pose a global threat to human health. In order to establish a successful infection, Salmonella utilize a large number of genes encoding a variety of virulence factors. Different animal models of infection have been used to better understand the mechanisms underlying each disease including cattle, rodents, and nematodes. To date, a number of different bacterial virulence factors have been identified using such animal models, most of which are secreted by two type three secretion systems (T3SS) encoded within Salmonella pathogenicity islands (SPI) 1 and 2. These proteins alter various host cell pathways, facilitating the invasion of epithelial cells during infection, as well as the survival and replication of Salmonella inside phagocytic cells. On the other hand, host genetics and resistance also play a role in the susceptibility to Salmonella infection. The natural resistance-associated macrophage protein 1 (Nramp1), for example, is critical for host defense, since mice lacking Nramp1 fail to control bacterial replication and succumb to low doses of S. Typhimurium. In this chapter, we analyze the different pathogen and host factors that play a role in the dynamic interaction between Salmonella and its host and their impact on disease.

The role of the intestinal microbiota in enteric infection

The consortia of microorganisms inhabiting the length of the gastrointestinal tract, the gastrointestinal microbiota, are vital to many aspects of normal host physiology. In addition, they are an active participant in the progression of many diseases, among them enteric infections. Healthy intestinal microbiota contribute to host resistance to infection through their involvement in the development of the host immune system and provision of colonization resistance. It is not surprising then that disruptions of the microbial community translate into alterations of host susceptibility to infection. Additionally, the process of the infection itself results in a disturbance to the microbiota. This disturbance is often mediated by the host inflammatory response, allowing the pathogen to benefit from the inflammation at the intestinal mucosa. Uncovering the mechanisms underlying the host-pathogen-microbiota interactions will facilitate our understanding of the infection process and promote design of more effective and focused prophylactic and therapeutic strategies.

Nramp1 drives an accelerated inflammatory response during Salmonella-induced colitis in mice

A recently developed model for enterocolitis in mice involves pre-treatment with the antibiotic streptomycin prior to infection with Salmonella enterica serovar Typhimurium (S. Typhimurium). The contribution of Nramp1/Slc11a1 protein, a critical host defence mechanism against S. Typhimurium, to the development of inflammation in this model has not been studied. Here, we analysed the impact of Nramp1 expression on the early development of colitis using isogenic Nramp1(+/+) and Nramp1(-/-) mice. We hypothesized that Nramp1 acts by rapidly inducing an inflammatory response in the gut mucosa creating an antibacterial environment and limiting spread of S. Typhimurium to systemic sites. We observed that Nramp1(+/+) mice showed lower numbers of S. Typhimurium in the caecum compared with Nramp1(-/-) mice at all times analysed. Acute inflammation was much more pronounced in Nramp1(+/+) mice 1 day after infection. The effect of Nramp1 on development of colitis was characterized by higher secretion of the pro-inflammatory cytokines IFN-gamma, TNF-alpha and MIP-1alpha and a massive infiltration of neutrophils and macrophages, compared with Nramp1(-/-) animals. These data show that an early and rapid inflammatory response results in protection against pathological effects of S. Typhimurium infection in Nramp1(+/+) mice.