E. coli infection disrupts the epithelial barrier and activates intrinsic neurosecretory reflexes in the pig colon.

This study aims to assess the barrier integrity and possible activation of enteric neural pathways associated with secretion and motility in the pig colon induced by an enterotoxigenic Escherichia coli (ETEC) challenge. 50 Danbred male piglets were used for this study. 16 were challenged with an oral dose of the ETEC strain F4+ 1.5 × 109 colony-forming unit. Colonic samples were studied 4- and 9-days post-challenge using both a muscle bath and Ussing chamber. Colonic mast cells were stained with methylene blue. In control animals, electrical field stimulation induced neurosecretory responses that were abolished by tetrodotoxin (10-6M) and reduced by the combination of atropine (10-4M) and α-chymotrypsin (10U/mL). Exogenous addition of carbachol, vasoactive intestinal peptide, forskolin, 5-HT, nicotine, and histamine produced epithelial Cl- secretion. At day 4 post-challenge, ETEC increased the colonic permeability. The basal electrogenic ion transport remained increased until day 9 post-challenge and was decreased by tetrodotoxin (10-6M), atropine (10-4M), hexamethonium (10-5M), and ondansetron (10-5M). In the muscle, electrical field stimulation produced frequency-dependent contractile responses that were abolished with tetrodotoxin (10-6M) and atropine (10-6M). Electrical field stimulation and carbachol responses were not altered in ETEC animals in comparison with control animals at day 9 post-challenge. An increase in mast cells, stained with methylene blue, was observed in the mucosa and submucosa but not in the muscle layer of ETEC-infected animals on day 9 post-challenge. ETEC increased the response of intrinsic secretory reflexes and produced an impairment of the colonic barrier that was restored on day 9 post-challenge but did not modify neuromuscular function.

Response Variability to Drug Testing in Two Models of Chemically Induced Colitis.

The lack of knowledge regarding the pathogenesis of IBD is a challenge for the development of more effective and safer therapies. Although in vivo preclinical approaches are critical for drug testing, none of the existing models accurately reproduce human IBD. Factors that influence the intra-individual response to drugs have barely been described. With this in mind, our aim was to compare the anti-inflammatory efficacy of a new molecule (MTADV) to that of corticosteroids in TNBS and DSS-induced colitis mice of both sexes in order to clarify further the response mechanism involved and the variability between sexes. The drugs were administered preventively and therapeutically, and real-time bioluminescence was performed for the in vivo time-course colitis monitoring. Morphometric data were also collected, and colonic cytokines and acute plasma phase proteins were analyzed by qRT-PCR and ELISA, respectively-bioluminescence images correlated with inflammatory markers. In the TNBS model, dexamethasone worked better in females, while MTADV improved inflammation in males. In DSS-colitis, both therapies worked similarly. Based on the molecular profiles, interaction networks were constructed to pinpoint the drivers of therapeutic response that were highly dependent on the sex. In conclusion, our results suggest the importance of considering sex in IBD preclinical drug screening.

Different responses of the blockade of the P2Y1 receptor with BPTU in human and porcine intestinal tissues and in cell cultures.

BACKGROUND: Gastrointestinal smooth muscle relaxation is accomplished by activation of P2Y1 receptors, therefore this receptor plays an important role in regulation of gut motility. Recently, BPTU was developed as a negative allosteric modulator of the P2Y1 receptor. Accordingly, the aim of this study was to assess the effect of BPTU on purinergic neurotransmission in pig and human gastrointestinal tissues. METHODS: Ca2+ imaging in tSA201 cells that express the human P2Y1 receptor, organ bath and microelectrodes in tissues were used to evaluate the effects of BPTU on purinergic responses. KEY RESULTS: BPTU concentration dependently (0.1 and 1 µmol L-1 ) inhibited the rise in intracellular Ca2+ evoked by ADP in tSA201 cells. In the pig small intestine, 30 µmol L-1 BPTU reduced the fast inhibitory junction potential by 80%. Smooth muscle relaxations induced by electrical field stimulation were reduced both in pig ileum (EC50  = 6 µmol L-1 ) and colon (EC50  = 35 µmol L-1 ), but high concentrations of BPTU (up to 100 µmol L-1 ) had no effect on human colonic muscle. MRS2500 (1 µmol L-1 ) abolished all responses. Finally, 10 µmol L-1 ADPßS inhibited spontaneous motility and this was partially reversed by 30 µmol L-1 BPTU in pig, but not human colonic tissue and abolished by MRS2500 (1 µmol L-1 ). CONCLUSIONS & INFERENCES: BPTU blocks purinergic responses elicited via P2Y1 receptors in cell cultures and in pig gastrointestinal tissue. However, the concentrations needed are higher in pig tissue compared to cell cultures and BPTU was ineffective in human colonic tissue.

ICLAS LAQ Network for the Promotion of Animal Quality in Research.

ICLAS Laboratory Animal Quality Network (LAQN) programs currently consist of the Performance Evaluation Program (PEP), which focuses on microbial monitoring by and for laboratory animal diagnostic laboratories, and the Genetic Reference Monitoring Program (GENRef), which provides assay-ready reference DNA for genetic testing of mouse strains. Since 2008, PEP has grown to become a truly international program with participating laboratories in 5 continents. Launched in 2016, GENRef currently distributes DNA from 12 common inbred mouse strains for use in genetic monitoring of locally inbred colonies as well as for genetic testing of stocks, particularly genetically engineered stocks, of uncertain origins. GENRef has the capacity to include additional strains as well as additional species. PEP and GENRef provide the reagents at cost, as a resource to the international scientific community, in the interest of improving research quality in an environment of growing concern for research quality, rigor, and reproducibility.

The asymmetric innervation of the circular and longitudinal muscle of the mouse colon differently modulates myogenic slow phasic contractions.

BACKGROUND: Neuromuscular transmission has been extensively studied in the circular layer of the mouse colon where a co-transmission of purines acting on P2Y1 receptors and NO has been previously described. However, the corresponding mechanisms in the longitudinal layer are less known. METHODS: Electrophysiological and myography techniques were used to evaluate spontaneous phasic contractions (SPC) and neural-mediated responses in the proximal, mid, and distal colon devoid of CD1 mice. Immunohistochemistry against c-kit and PDGFRα was performed in each colonic segment. KEY RESULTS: SPC were recorded in both muscle layers at a similar frequency being about four contractions per minute (c.p.m.) in the proximal and distal colon compared to the mid colon (2 c.p.m.). In non-adrenergic, non-cholinergic conditions, L-NNA (1 mmol/L) increased contractility in the circular but not in the longitudinal layer. In the longitudinal muscle, both electrophysiological and mechanical neural-mediated inhibitory responses were L-NNA and ODQ (10 µmol/L) sensitive. NaNP (1 µmol/L) caused cessation of SPC and the response was blocked by ODQ. Neither ADPßS (10 µmol/L) nor CYPPA (10 µmol/L), which both targeted the purinergic pathway, altered longitudinal contractions. PDGFRα + cells were located in both muscle layers and were more numerous compared with cKit + cells, which both formed a heterologous cellular network. A decreasing gradient of the PDGFRα labeling was observed along the colon. CONCLUSION: An inhibitory neural tone was absent in the longitudinal layer and neuronal inhibitory responses were mainly nitrergic. Despite the presence of PDGFRα + cells, purinergic responses were absent. Post-junctional pathways located in different cell types might be responsible for neurotransmitter transduction.

Mast cell-mediated splanchnic cholestatic inflammation.

INTRODUCTION: Splanchnic mast cells increase in chronic liver and in acute-on-chronic liver diseases. We administered Ketotifen, a mast cell stabilizer, and measured the mast cells in the splanchnic organs of cholestatic rats. MATERIAL AND METHODS: These groups were studied: sham-operated rats (S; n = 15), untreated microsurgical cholestasic rats (C; n = 20) and rats treated with Ketotifen: early (SK-e; n = 20 and CKe; n = 18), and late (SK-l; n = 15 and CK-l; n = 14). RESULTS: The cholestatic rats showed systemic and splanchnic impairments, such as ascites, portal hypertension, and biliary proliferation and fibrosis. The rats also showed a splanchnic increase of TNF-α, IL-1ß and MCP-1, and a reduction of IL-4, IL-10 and antioxidants. An increase of VEGF in the ileum and mesenteric lymphatic complex was associated with a liver reduction of TGF-ß1. Ketotifen reduces the degree of hepatic insufficiency and the splanchnic inflammatory mediators, as well as VEGF and TGF-ß1 levels. Ketotifen also reduces the connective tissue mast cells in the mesenteric lymphatic complex of cholestatic rats, while increases the hepatic mucosal mast cells. CONCLUSIONS: In cholestatic rats, Ketotifen improves liver function and ascites, and also reduces pro-inflammatory mediators in the splanchnic area. The decrease in connective tissue mast cells in the mesenteric lymphatic complex due to the administration of Ketotifen would lead to the improvement of the inflammatory splanchnic response, and consequently the abovementioned complications.

The gestational power of mast cells in the injured tissue.

The inflammatory response expressed after wound healing would be the recapitulation of systemic extra-embryonic functions, which would focus on the interstitium of the injured tissue. In the injured tissue, mast cells, provided for a great functional heterogeneity, could play the leading role in the re-expression of extra-embryonic functions, i.e., coelomic-amniotic and trophoblastic-vitelline. Moreover, mast cells would favor the production of a gastrulation-like process, which in certain tissues and organs would induce the regeneration of the injured tissue. Therefore, the engraftment of mesenchymal stem cells and mast cells, both with an extra-embryonic regenerative phenotype, would achieve a blastema, from the repaired and regenerated injured tissue, rather than by fibrosis, which is commonly made through wound-healing.

Effects of nerve growth factor antagonist K252a on peritoneal mast cell degranulation: implications for rat postoperative ileus.

Stabilization of mast cell (MC) degranulation has been proposed to prevent postoperative ileus (POI). Nerve growth factor (NGF) mediates MC degranulation. The aim of the study was to evaluate whether NGF receptor antagonist K252a acts as a MC stabilizer in vitro and in vivo model of POI. Peritoneal mast cells (PMCs) were obtained from Sprague-Dawley rats and were incubated with K252a and exposed to NGF or Compound 48/80 (C48/80). MC degranulation was assessed by ß-hexosaminidase assay. POI was induced in rats by intestinal manipulation (IM). Rats were pretreated with K252a (100 µg/kg sc) 20 min prior to POI induction. At 20 min after IM, release of rat mast cell protease 6 (RMCP-6) was evaluated in peritoneal lavage. At 24 h, intestinal transit (IT) and gastric emptying (GE) were evaluated. Ileal inflammation was assessed by myeloperoxidase (MPO) activity, expression of IL-6, NGF, TrkA, RMCP-2 and 6, and MC density within the full-thickness ileum. C48/80 and NGF evoked degranulation of PMCs in a dose-dependent manner. K252a prevented NGF-evoked, but not C48/80-evoked, MC degranulation. IM evoked the release of peritoneal RMCP-6 and subsequently delayed IT and GE. IM increased MPO activity and expression of IL-6. In IM rats, K252a prevented upregulation of IL-6 expression and reduced TrkA. IT, GE, and inflammation were not affected by K252a. K252a inhibited NGF-evoked degranulation of PMCs in vitro. In vivo, K252a decreased IL-6 and PMC degranulation. This may be of relevance for the development of new therapeutic targets for POI.

Fixation of Light Weight Polypropylene Mesh with n-Butyl-2-cyanocrylate in Pelvic Floor Surgery: Experimental Design Approach in Sheep for Effectiveness Evaluation.

OBJECTIVE: The aim of this study was to find a proper experimental design and to evaluate n-butyl-2-cyanoacrylate (Histoacryl) as a fixation method for a light-weight and large pore PP mesh (Synthetic PP Mesh-1) using the sheep as an animal model. METHODS: Posterior vaginal implantation by means of episiotomy was used to implant 8 ewes which were evaluated macroscopically and histologically at 3 months (n = 4) and 6 months (n = 4) post-surgery. In previous pilot studies anterior vaginal implantation was evaluated, as well as different synthetic mesh materials, sizes and fixation methods (n = 1 to 3) during three weeks. In all cases a clinical evaluation of the animal was performed. RESULTS: A reduction in the mesh size (Synthetic PP Mesh-1) together with precise application of the surgical glue Histoacryl to fix the mesh yielded significantly better histocompatibility results (P < 0.01) compared to larger size or other fixation methods. CONCLUSION: The combination of Synthetic PP Mesh-1 with Histoacryl offered a high degree of graft integration without vaginal ulceration and a minimal foreign body reaction, being the sheep a proper animal model to test these types of medical devices.

Lipopolysaccharides facilitate colonic motor alterations associated to the sensitization to a luminal antigen in rats.

BACKGROUND/AIMS: Enteric dysbiosis is a risk factor for dietary proteins-associated intestinal alterations, contributing to the development of food allergies and the symptomatology of functional gastrointestinal disorders, mainly irritable bowel syndrome (IBS). We explored if a dysbiotic-like state, simulated by intraperitoneal administration of bacterial lipopolysaccharides (LPS), facilitates the sensitiza-tion to a luminal antigen, ovalbumin (OVA), in rats. METHODS: Rats were exposed to oral OVA for 1 week, alone or with LPS. Thereafter, colonic histology, goblet cell density, mucosal eosi-nophils and mucosal mast cell (MMC) and connective tissue mast cell (CTMC) were evaluated. Colonic expression (real-time quantitative polymerase chain reaction) of interleukins, IFN-α1 and integrins was assessed to determine local immune responses. Luminal and wall adhered microbiota were characterized by fluorescence in situ hybridization. Colonic contractility (in vitro) served to assess functional changes associated to OVA and/or LPS. RESULTS: Neither OVA nor LPS, alone or combined, lead to structural alterations, except for a reduced goblet cell density in OVA-LPS- treated rats. MMC density was unaffected, while CTMC counts increased within the submucosa of OVA-LPS-treated animals. Marginal immune activation (IFN-α1 up-regulation) was observed in OVA-LPS-treated rats. LPS induced a dysbiotic-like state characterized by decreased luminal bacterial counts, with a specific loss of clostridia. LPS facilitated Clostridium spp. wall adherence, an effect prevented by OVA. Colonic contractility was altered in OVA-LPS-treated animals, showing increased basal activity and enhanced motor responses to OVA. CONCLUSIONS: Changes in gut microbiota and/or direct effects of LPS might enhance/facilitate local neuroimmune responses to food antigens leading to motor alterations similar to those observed in IBS.

Roles of the International Council for Laboratory Animal Science (ICLAS) and International Association of Colleges of Laboratory Animal Medicine (IACLAM) in the Global Organization and Support of 3Rs Advances in Laboratory Animal Science.

Practical implementation of the 3Rs at national and regional levels around the world requires long-term commitment, backing, and coordinated efforts by international associations for laboratory animal medicine and science, including the International Association of Colleges of Laboratory Animal Medicine (IACLAM) and the International Council for Laboratory Animal Science (ICLAS). Together these organizations support the efforts of regional organization and communities of laboratory animal science professionals as well as the development of local associations and professional colleges that promote the training and continuing education of research facility personnel and veterinary specialists. The recent formation of a World Organization for Animal Health (OIE) Collaborating Center for Laboratory Animal Science and Welfare emphasizes the need for research into initiatives promoting laboratory animal welfare, particularly in emerging economies and regions with nascent associations of laboratory animal science.

Changes in Epithelial Barrier Function in Response to Parasitic Infection: Implications for IBD Pathogenesis.

BACKGROUND AND AIMS: Mast cells [MCs] are implicated in epithelial barrier alterations that characterize inflammatory and functional bowel disorders. In this study, we describe mast cell proteinases [chymases and tryptases] and tight junction [TJ] proteins kinetics in a rat model of postinfectious gut dysfunction. METHODS: Jejunal tissues of control and -infected rats were used. Inflammation-related changes in MCs and the expression of TJ-related proteins were evaluated by immunostaining and reverse transcription-quantitative polymerase chain reaction. Epithelial barrier function was assessed in vitro (Ussing chambers) and in vivo. RESULTS: After infection, intestinal inflammation was associated with a generalized overexpression of MC chymases, peaking between Days 6 and 14. Thereafter, a mucosal MC hyperplasia and a late increase in connective tissue MC counts were observed. From Day 2 post-infection, TJ proteins occludin and claudin-3 expression was down-regulated whereas the pore-forming protein claudin-2 was overexpressed. The expression of proglucagon, precursor of the barrier-enhancing factor glucagon-like peptide-2, was reduced. These changes were associated with an increase in epithelial permeability, both in vitro and in vivo. CONCLUSIONS: Proteinases expression and location of mucosal and connective tissue MCs indicate a time-related pattern in the maturation of intestinal MCs following infection. Altered expression of TJ-related proteins is consistent with a loss of epithelial tightness, and provides a molecular mechanism for the enhanced epithelial permeability observed in inflammatory conditions of the gut.

Plasticity of dorsal root ganglion neurons in a rat model of post-infectious gut dysfunction: potential implication of nerve growth factor.

OBJECTIVE: Intestinal infections are suggested as a risk factor for the development of irritable bowel syndrome (IBS)-like visceral hypersensitivity. The mechanisms implicated might involve long-term changes in visceral afferents, with implication of nerve growth factor (NGF). We explored plastic changes in dorsal root ganglia (DRGs) receiving innervation from the gut and the potential implication of NGF in a rat model of IBS-like post-infectious gut dysfunction. MATERIALS AND METHODS: Rats were infected with Trichinella spiralis larvae. Thirty days post-infection, inflammatory markers, including interleukins (ILs) and mucosal mast cell infiltration (rat mast cell protease II [RMCPII]), and NGF and TrkA expression was determined in the jejunum and colon (RT-qPCR). In the same animals, morphometry (neuronal body size) and NGF content (immunofluorescence) were assessed in thoracolumbar DRG neurons. RESULTS: In infected animals, a low-grade inflammatory-like response, characterized by up-regulated levels of RMCPII and IL-6, was observed in the jejunum and colon. TrkA expression was increased in the jejunum, whereas the colon showed a slight reduction. NGF levels remained unaltered regardless the gut region. Overall, the mean cross-sectional area of DRG neurons was increased in T. spiralis-infected animals, with a reduction in both TrkA and NGF staining. CONCLUSIONS: Results suggest that during T. spiralis infection in rats, there is a remodeling of sensory afferents that might imply a NGF-mediated mechanism. Plastic changes in sensory afferents might mediate the long-lasting functional alterations that characterize this model of IBS. Similar mechanisms might be operating in patients with post-infectious-IBS.

Colonic smooth muscle cells and colonic motility patterns as a target for irritable bowel syndrome therapy: mechanisms of action of otilonium bromide.

Otilonium bromide (OB) is a spasmolytic compound of the family of quaternary ammonium derivatives and has been successfully used in the treatment of patients with irritable bowel syndrome (IBS) due to its specific pharmacodynamic effects on motility patterns in the human colon and the contractility of colonic smooth muscle cells. This article examines how. OB inhibits the main patterns of human sigmoid motility in vitro, which are spontaneous rhythmic phasic contractions, smooth muscle tone, contractions induced by stimulation of excitatory motor neurons and contractions induced by direct effect of excitatory neurotransmitters. It does this mainly by blocking calcium influx through L-type calcium channels and interfering with mobilization of cellular calcium required for smooth muscle contraction, thereby limiting excessive intestinal contractility and abdominal cramping. OB also inhibits T-type calcium channels and muscarinic responses. Finally, OB inhibits tachykinin receptors on smooth muscle and primary afferent neurons which may have the joint effect of reducing motility and abdominal pain. All these mechanisms mediate the therapeutic effects of OB in patients with IBS and might be useful in patients with other spastic colonic motility disorders such as diverticular disease.

Specific changes of gut commensal microbiota and TLRs during indomethacin-induced acute intestinal inflammation in rats.

BACKGROUND AND AIMS: Gut microbiota is a contributing factor in the development and maintenance of intestinal inflammation, although precise cause-effect relationships have not been established. We assessed spontaneous changes of gut commensal microbiota and toll-like receptors (TLRs)-mediated host-bacterial interactions in a model of indomethacin-induced acute enteritis in rats. METHODS: Male Spague-Dawley rats, maintained under conventional conditions, were used. Enteritis was induced by systemic indomethacin administration. During the acute phase of inflammation, animals were euthanized and ileal and ceco-colonic changes evaluated. Inflammation was assessed through disease activity parameters (clinical signs, macroscopic/microscopic scores and tissue levels of inflammatory markers). Microbiota (ileal and ceco-colonic) was characterized using fluorescent in situ hybridization (FISH) and analysis of 16s rDNA polymorphism. Host-bacterial interactions were assessed evaluating the ratio of bacterial adherence to the intestinal wall (FISH) and expression of TLRs 2 and 4 (RT-PCR). RESULTS: After indomethacin, disease activity parameters increased, suggesting an active inflammation. Total bacterial counts were similar in vehicle- or indomethacin-treated animals. However, during inflammation the relative composition of the microbiota was altered. This dysbiotic state was characterized by an increase in the counts of Bacteroides spp., Enterobacteriaceae (in ileum and cecum-colon) and Clostridium spp. (in ileum). Bacterial wall adherence significantly increased during inflammation. In animals with enteritis, TLR-2 and -4 were up-regulated both in the ileum and the ceco-colonic region. CONCLUSIONS: Gut inflammation implies qualitative changes in GCM, with simultaneous alterations in host-bacterial interactions. These observations further support a potential role for gut microbiota in the pathophysiology of intestinal inflammation.

PAR-2-mediated control of barrier function and motility differs between early and late phases of postinfectious gut dysfunction in the rat.

Proteinase-activated receptor-2 (PAR-2) and mast cell (MC) mediators contribute to inflammatory and functional gastrointestinal disorders. We aimed to characterize jejunal PAR-2-mediated responses and the potential MC involvement in the early and late phases of a rat model of postinfectious gut dysfunction. Jejunal tissues of control and Trichinella spiralis-infected (14 and 30 days postinfection) rats, treated or not with the MC stabilizer, ketotifen, were used. Histopathology and immunostaining were used to characterize inflammation, PAR-2 expression, and mucosal and connective tissue MCs. Epithelial barrier function (hydroelectrolytic transport and permeability) and motility were assessed in vitro in basal conditions and after PAR-2 activation. Intestinal inflammation on day 14 postinfection (early phase) was significantly resolved by day 30 (late phase) although MC counts and epithelial permeability remained increased. PAR-2-mediated ion transport (Ussing chambers, in vitro) and epithelial surface PAR-2 expression were reduced in the early phase, with a trend toward normalization during the late phase. In control conditions, PAR-2 activation (organ bath) induced biphasic motor responses (relaxation followed by excitation). At 14 days postinfection, spontaneous contractility and PAR-2-mediated relaxations were enhanced; motor responses were normalized on day 30. Postinfectious changes in PAR-2 functions were not affected by ketotifen treatment. We concluded that, in the rat model of Trichinella spiralis infection, alterations of intestinal PAR-2 function and expression depend on the inflammatory phase considered. A lack of a ketotifen effect suggests no interplay between MCs and PAR-2-mediated motility and ion transport alterations. These observations question the role of MC mediators in PAR-2-modulating postinfectious gut dysfunction.

Liver impairment after portacaval shunt in the rat: the loss of protective role of mast cells?

Mast cells are involved in various liver diseases and appear to play a broader pathogenic role than originally thought. They may participate in the splanchnic alterations related to a porto-systemic shunt. To verify this hypothesis we studied the serum and hepatic histological changes in rats four weeks after an end-to-side portacaval shunt. In this experimental model of chronic liver insufficiency we also assessed the mucosal mast cells (MMC) and connective tissue mast cells (CTMC) in the liver, mesenteric lymph nodes and small intestine, as well as the serum levels of rat mast cell protease-II (RMCP-II). The results show liver and testes atrophy, with hypoalbuminemia (p=0.0001), hyperbilirubinemia (p=0.0001) and increase in aspartate aminotransferase (p=0.004) and alanine aminotransferase (p=0.0001). Hepatic histopathology demonstrates hepatocytic necrosis and apoptosis, portal inflammation, biliary proliferation, steatosis and fibrosis. There is a decrease of MMCs and CTMCs in the liver, while in the ileum CTMCs increase and MMCs decrease. These results suggest the involvement of mast cells in the pathophysiological splanchnic impairments in this experimental model. In particular, the decreased number of liver mast cells may be associated with the hepatic atrophy. If this is the case, we propose that the disruption of the hepato-intestinal axis after a portocaval shunt in the rat could inhibit the ability of the liver to developing an appropriate repair response mediated by mast cells.

Characterization of housing-related spontaneous variations of gut microbiota and expression of toll-like receptors 2 and 4 in rats.

Gut microbiota has been suggested as a key component of gut homeostasis, affecting immune responses within the gut. We determined changes in intestinal commensal bacteria and expression of toll-like receptors (TLR) 2 and 4 in rats bred under microbiologically controlled conditions (barrier), under standard conditions (conventional), and in barrier animals adapted to standard conditions (barrier/conventional). Cecal microbiota was analyzed by plate culture, and fluorescence in situ hybridization and microbial profiles were assessed by terminal restriction fragment length polymorphism. Cecal expression of TLR-2 and TLR-4 was determined by reverse transcription polymerase chain reaction (PCR). Total number of cecal bacteria was similar in the three groups. However, the barrier group showed a higher number of strict anaerobic bacteria (Bacteroides spp. and Clostridium spp.) while Bifidobacterium spp. were scarce. Re-housing the barrier-bred rats into conventional conditions led to a microbiota with intermediate characteristics between the barrier and conventional groups. Richness of the cecal microbial ecosystem was similar in the three groups, although a relative time-dependent variation, with highest homogeneity in the barrier group, was observed. Expression levels of TLR-2 and TLR-4 had no clear correlation with the microbiota. These results show that the relative composition of the cecal microbiota in rats varies spontaneously with changes in the environmental conditions, with minor impact in the expression of TLR-2 and TLR-4. These observations might be important in the understanding of variability in animal responses, particularly to immune-related stimuli, when assessed in the context of the environmental/microbiological conditions.