Roles of motility, chemotaxis, and penetration through and growth in intestinal mucus in the ability of an avirulent strain of Salmonella typhimurium to colonize the large intestine of streptomycin-treated mice

Impact of enteric bacterial infections at and beyond the epithelial barrier

The mucosal lining of the gut has co-evolved with a diverse microbiota over millions of years, leading to the development of specialized mechanisms to actively limit the invasion of pathogens. However, some enteric microorganisms have adapted against these measures, developing ways to hijack or overcome epithelial micro-integrity mechanisms. This breach of the gut barrier not only enables the leakage of host factors out of circulation but can also initiate a cascade of detrimental systemic events as microbiota, pathogens and their affiliated secretions passively leak into extra-intestinal sites. Under normal circumstances, gut damage is rapidly repaired by intestinal stem cells. However, with substantial and deep perturbation to the gut lining and the systemic dissemination of gut contents, we now know that some enteric infections can cause the impairment of host regenerative processes. Although these local and systemic aspects of enteric disease are often studied in isolation, they heavily impact one another. In this Review, by examining the journey of enteric infections from initial establishment to systemic sequelae and how, or if, the host can successfully repair damage, we will tie together these complex interactions to provide a holistic overview of the impact of enteric infections at and beyond the epithelial barrier.

Cross-Talk Between the Intestinal Epithelium and Salmonella Typhimurium

Salmonella enterica serovars are invasive gram-negative bacteria, causing a wide range of diseases from gastroenteritis to typhoid fever, representing a public health threat around the world. Salmonella gains access to the intestinal lumen after oral ingestion of contaminated food or water. The crucial initial step to establish infection is the interaction with the intestinal epithelium. Human-adapted serovars such as S. Typhi or S. Paratyphi disseminate to systemic organs and induce life-threatening disease known as typhoid fever, whereas broad-host serovars such as S. Typhimurium usually are limited to the intestine and responsible for gastroenteritis in humans. To overcome intestinal epithelial barrier, Salmonella developed mechanisms to induce cellular invasion, intracellular replication and to face host defence mechanisms. Depending on the serovar and the respective host organism, disease symptoms differ and are linked to the ability of the bacteria to manipulate the epithelial barrier for its own profit and cross the intestinal epithelium.

This review will focus on S. Typhimurium (STm). To better understand STm pathogenesis, it is crucial to characterize the crosstalk between STm and the intestinal epithelium and decipher the mechanisms and epithelial cell types involved. Thus, the purpose of this review is to summarize our current knowledge on the molecular dialogue between STm and the various cell types constituting the intestinal epithelium with a focus on the mechanisms developed by STm to cross the intestinal epithelium and access to subepithelial or systemic sites and survive host defense mechanisms.

Microbial Imbalance Induces Inflammation by Promoting Salmonella Penetration through the Mucosal Barrier

The balance of microbial species in the intestine must be maintained to prevent inflammation and disease. Healthy bacteria suppress infection by pathogens and prevent disorders such as inflammatory bowel diseases (IBDs). The role of mucus in the relation between pathogens and the intestinal microbiota is poorly understood. Here, we hypothesized that healthy bacteria inhibit infection by preventing pathogens from penetrating the mucus layer and that microbial imbalance leads to inflammation by promoting the penetration of the mucosal barrier. We tested this hypothesis with an in vitro model that contains mucus, an epithelial cell layer, and resident immune cells. We found that, unlike probiotic VSL#3 bacteria, Salmonella penetrated the mucosal layers and induced the production of interleukin-8 (IL-8) and tumor necrosis factor (TNF)-α. At ratios greater than 10⁴:1, probiotic bacteria suppressed the growth and penetration of Salmonella and reduced the production of inflammatory cytokines. Counterintuitively, low densities of healthy bacteria increased both pathogen penetration and cytokine production. In all cases, mucus increased Salmonella penetration and the production of cytokines. These results suggest that mucus lessens the protective effect of probiotic bacteria by promoting barrier penetration. In this model, a more imbalanced microbial population caused infection and inflammation by selecting pathogens that are more invasive and immunogenic. Combined, the results suggest that the depletion of commensal bacteria or an insufficient dosage of probiotics could worsen an infection and cause increased inflammation. A better understanding of the interactions between pathogens, healthy microbes, and the mucosal barrier will improve the treatment of infections and inflammatory diseases.

Mucin-Degrading Microbes Release Monosaccharides That Chemoattract Clostridioides difficile and Facilitate Colonization of the Human Intestinal Mucus Layer

It is widely accepted that the pathogen Clostridioides difficile exploits an intestinal environment with an altered microbiota, but the details of these microbe-microbe interactions are unclear. Adherence and colonization of mucus has been demonstrated for several enteric pathogens and it is possible that mucin-associated microbes may be working in concert with C. difficile. We showed that C. difficile ribotype-027 adheres to MUC2 glycans and using fecal bioreactors, we identified that C. difficile associates with several mucin-degrading microbes. C. difficile was found to chemotax toward intestinal mucus and its glycan components, demonstrating that C. difficile senses the mucus layer. Although C. difficile lacks the glycosyl hydrolases required to degrade mucin glycans, coculturing C. difficile with the mucin-degrading Akkermansia muciniphila, Bacteroides thetaiotaomicron, and Ruminococcus torques allowed C. difficile to grow in media that lacked glucose but contained purified MUC2. Collectively, these studies expand our knowledge on how intestinal microbes support C. difficile.

Mucus blocks probiotics but increases penetration of motile pathogens and induces TNF-α and IL-8 secretion

The mucosal barrier in combination with innate immune system are the first line of defense against luminal bacteria at the intestinal mucosa. Dysfunction of the mucus layer and bacterial infiltration are linked to tissue inflammation and disease. To study host-bacterial interactions at the mucosal interface, we created an experimental model that contains luminal space, a mucus layer, an epithelial layer, and suspended immune cells. Reconstituted porcine small intestinal mucus formed an 880 ± 230 µm thick gel layer and had a porous structure. In the presence of mucus, sevenfold less probiotic and nonmotile VSL#3 bacteria transmigrated across the epithelial barrier compared to no mucus. The higher bacterial transmigration caused immune cell differentiation and increased the concentration of interleukin-8 (IL-8) and tumor necrosis factor-alpha (TNF-α; p < .01). Surprisingly, the mucus layer increased transmigration of pathogenic Salmonella and increased secretion of TNF-α and IL-8 (p < .05). Nonmotile, flagella knockout Salmonella had lower transmigration and caused lower IL-8 and TNF-α secretion (p < .05). These results demonstrate that motility enables pathogenic bacteria to cross the mucus and epithelial layers, which could lead to infection. Using an in vitro coculture platform to understand the interactions of bacteria with the intestinal mucosa has the potential to improve the treatment of intestinal diseases.

Recent progress in experimental and human disease-associated multi-species biofilms

Human bodies are colonized by trillions of microorganisms, which are often referred to as human microbiota and play important roles in human health. Next generation sequencing studies have established links between the genetic content of human microbiota and various human diseases. However, it remains largely unknown about the spatial organizations and interspecies interactions of individual species within the human microbiota. Bacterial cells tend to form surface-attached biofilms in many natural environments, which enable intercellular communications and interactions in a microbial ecosystem. In this review, we summarize the recent progresses on the experimental and human disease-associated multi-species biofilm studies. We hypothesize that engineering biofilm structures and interspecies interactions might provide a tool for manipulating the composition and function of human microbiota.

Frontline defenders: goblet cell mediators dictate host-microbe interactions in the intestinal tract during health and disease

Goblet cells (GCs) are the predominant secretory epithelial cells lining the luminal surface of the mammalian gastrointestinal (GI) tract. Best known for their apical release of mucin 2 (Muc2), which is critical for the formation of the intestinal mucus barrier, GCs have often been overlooked for their active contributions to intestinal protection and host defense. In part, this oversight reflects the limited tools available to study their function but also because GCs have long been viewed as relatively passive players in promoting intestinal homeostasis and host defense. In light of recent studies, this perspective has shifted, as current evidence suggests that Muc2 as well as other GC mediators are actively released into the lumen to defend the host when the GI tract is challenged by noxious stimuli. The ability of GCs to sense and respond to danger signals, such as bacterial pathogens, has recently been linked to inflammasome signaling, potentially intrinsic to the GCs themselves. Moreover, further work suggests that GCs release Muc2, as well as other mediators, to modulate the composition of the gut microbiome, leading to both the expansion as well as the depletion of specific gut microbes. This review will focus on the mechanisms by which GCs actively defend the host from noxious stimuli, as well as describe advanced technologies and new approaches by which their responses can be addressed. Taken together, we will highlight current insights into this understudied, yet critical, aspect of intestinal mucosal protection and its role in promoting gut defense and homeostasis.

Gut biofilm forming bacteria in inflammatory bowel disease

Inflammatory bowel disease (IBD) symbolizes a group of intestinal disorders in which prolonged inflammation occur in the digestive tract (esophagus, large intestine, small intestine mouth, stomach). Both genetic and environmental factors (infections, stress, diet) are involved in the development of IBD. As we know that bacteria are found in the intestinal mucosa of human and clinical observations revealed bacterial biofilms associated with patients of IBD. Various factors and microbes are found to play an essential role in biofilm formation and mucosal colonization during IBD. Biofilm formation in the digestive tract is dependent on an extracellular matrix synthesized by the bacteria and it has an adverse effect on the immune response of the host. There is no satisfactory and safe treatment option for IBD. Therefore, the current research aims to disrupt biofilm in IBD and concentrates predominantly on improving the drug. Here, we review the literature on bacterial biofilm and IBD to gather new knowledge on the current understanding of biofilm formation in IBD, host immune deregulation and dysbiosis in IBD, molecular mechanism, bacteria involved in biofilm formation, current and future regimen. It is urgently required to plan new ways to control and eradicate bacteria in biofilms that will open up novel diagnostic and therapeutic avenues for IBD. This article includes the mechanism of signaling molecules with respect to the biofilm-related genes as well as the diagnostic methods and new technologies involved in the treatment of IBD.

Challenges in simulating the human gut for understanding the role of the microbiota in obesity

There is an elevated incidence of cases of obesity worldwide. Therefore, the development of strategies to tackle this condition is of vital importance. This review focuses on the necessity of optimising in vitro systems to model human colonic fermentation in obese subjects. This may allow to increase the resolution and the physiological relevance of the information obtained from this type of studies when evaluating the potential role that the human gut microbiota plays in obesity. In light of the parameters that are currently used for the in vitro simulation of the human gut (which are mostly based on information derived from healthy subjects) and the possible difference with an obese condition, we propose to revise and improve specific standard operating procedures.

Anti-infective activities of lactobacillus strains in the human intestinal microbiota: from probiotics to gastrointestinal anti-infectious biotherapeutic agents

A vast and diverse array of microbial species displaying great phylogenic, genomic, and metabolic diversity have colonized the gastrointestinal tract. Resident microbes play a beneficial role by regulating the intestinal immune system, stimulating the maturation of host tissues, and playing a variety of roles in nutrition and in host resistance to gastric and enteric bacterial pathogens. The mechanisms by which the resident microbial species combat gastrointestinal pathogens are complex and include competitive metabolic interactions and the production of antimicrobial molecules. The human intestinal microbiota is a source from which Lactobacillus probiotic strains have often been isolated. Only six probiotic Lactobacillus strains isolated from human intestinal microbiota, i.e., L. rhamnosus GG, L. casei Shirota YIT9029, L. casei DN-114 001, L. johnsonii NCC 533, L. acidophilus LB, and L. reuteri DSM 17938, have been well characterized with regard to their potential antimicrobial effects against the major gastric and enteric bacterial pathogens and rotavirus. In this review, we describe the current knowledge concerning the experimental antibacterial activities, including antibiotic-like and cell-regulating activities, and therapeutic effects demonstrated in well-conducted, placebo-controlled, randomized clinical trials of these probiotic Lactobacillus strains. What is known about the antimicrobial activities supported by the molecules secreted by such probiotic Lactobacillus strains suggests that they constitute a promising new source for the development of innovative anti-infectious agents that act luminally and intracellularly in the gastrointestinal tract.

The Serine Protease Autotransporter Pic Modulates Citrobacter rodentium Pathogenesis and Its Innate Recognition by the Host

Bacterial pathogens produce a number of autotransporters that possess diverse functions. These include the family of serine protease autotransporters of Enterobacteriaceae (SPATEs) produced by enteric pathogens such as Shigella flexneri and enteroaggregative Escherichia coli. Of these SPATEs, one termed "protein involved in colonization," or Pic, has been shown to possess mucinase activity in vitro, but to date, its role in in vivo enteric pathogenesis is unknown. Testing a pic null (ΔpicC) mutant in Citrobacter rodentium, a natural mouse pathogen, found that the C. rodentium ΔpicC strain was impaired in its ability to degrade mucin in vitro compared to the wild type. Upon infection of mice, the ΔpicC mutant exhibited a hypervirulent phenotype with dramatically heavier pathogen burdens found in intestinal crypts. ΔpicC mutant-infected mice suffered greater barrier disruption and more severe colitis and weight loss, necessitating their euthanization between 10 and 14 days postinfection. Notably, the virulence of the ΔpicC mutant was normalized when the picC gene was restored; however, a PicC point mutant causing loss of mucinase activity did not replicate the ΔpicC phenotype. Exploring other aspects of PicC function, the ΔpicC mutant was found to aggregate to higher levels in vivo than wild-type C. rodentium. Moreover, unlike the wild type, the C. rodentium ΔpicC mutant had a red, dry, and rough (RDAR) morphology in vitro and showed increased activation of the innate receptor Toll-like receptor 2 (TLR2). Interestingly, the C. rodentium ΔpicC mutant caused a degree of pathology similar to that of wild-type C. rodentium when infecting TLR2-deficient mice, showing that despite its mucinase activity, PicC's major role in vivo may be to limit C. rodentium's stimulation of the host's innate immune system.

Commensal and Pathogenic Escherichia coli Metabolism in the Gut

E. coli is a ubiquitous member of the intestinal microbiome. This organism resides in a biofilm comprised of a complex microbial community within the mucus layer where it must compete for the limiting nutrients that it needs to grow fast enough to stably colonize. In this article we discuss the nutritional basis of intestinal colonization. Beginning with basic ecological principles we describe what is known about the metabolism that makes E. coli such a remarkably successful member of the intestinal microbiota. To obtain the simple sugars and amino acids that it requires, E. coli depends on degradation of complex glycoproteins by strict anaerobes. Despite having essentially the same core genome and hence the same metabolism when grown in the laboratory, different E. coli strains display considerable catabolic diversity when colonized in mice. To explain why some E. coli mutants do not grow as well on mucus in vitro as their wild type parents yet are better colonizers, we postulate that each one resides in a distinct "Restaurant" where it is served different nutrients because it interacts physically and metabolically with different species of anaerobes. Since enteric pathogens that fail to compete successfully for nutrients cannot colonize, a basic understanding of the nutritional basis of intestinal colonization will inform efforts to develop prebiotics and probiotics to combat infection.

Pathogenic E. coli exploits SslE mucinase activity to translocate through the mucosal barrier and get access to host cells

SslE is a zinc-metalloprotease involved in the degradation of mucin substrates and recently proposed as a potential vaccine candidate against pathogenic E. coli. In this paper, by exploiting a human in vitro model of mucus-secreting cells, we demonstrated that bacteria expressing SslE have a metabolic benefit which results in an increased growth rate postulating the importance of this antigen in enhancing E. coli fitness. We also observed that SslE expression facilitates E. coli penetration of the mucus favouring bacteria adhesion to host cells. Moreover, we found that SslE-mediated opening of the mucosae contributed to the activation of pro-inflammatory events. Indeed, intestinal cells infected with SslE-secreting bacteria showed an increased production of IL-8 contributing to neutrophil recruitment. The results presented in this paper conclusively designate SslE as an important colonization factor favouring E. coli access to both metabolic substrates and target cells.

Bacterial flagella: twist and stick, or dodge across the kingdoms

The flagellum organelle is an intricate multiprotein assembly best known for its rotational propulsion of bacteria. However, recent studies have expanded our knowledge of other functions in pathogenic contexts, particularly adherence and immune modulation, e.g., for Salmonella enterica, Campylobacter jejuni, Pseudomonas aeruginosa, and Escherichia coli. Flagella-mediated adherence is important in host colonisation for several plant and animal pathogens, but the specific interactions that promote flagella binding to such diverse host tissues has remained elusive. Recent work has shown that the organelles act like probes that find favourable surface topologies to initiate binding. An emerging theme is that more general properties, such as ionic charge of repetitive binding epitopes and rotational force, allow interactions with plasma membrane components. At the same time, flagellin monomers are important inducers of plant and animal innate immunity: variation in their recognition impacts the course and outcome of infections in hosts from both kingdoms. Bacteria have evolved different strategies to evade or even promote this specific recognition, with some important differences shown for phytopathogens. These studies have provided a wider appreciation of the functions of bacterial flagella in the context of both plant and animal reservoirs.

From food to cell: nutrient exploitation strategies of enteropathogens

Upon entering the human gastrointestinal tract, foodborne bacterial enteropathogens encounter, among numerous other stress conditions, nutrient competition with the host organism and the commensal microbiota. The main carbon, nitrogen and energy sources exploited by pathogens during proliferation in, and colonization of, the gut have, however, not been identified completely. In recent years, a huge body of literature has provided evidence that most enteropathogens are equipped with a large set of specific metabolic pathways to overcome nutritional limitations in vivo, thus increasing bacterial fitness during infection. These adaptations include the degradation of myo-inositol, ethanolamine cleaved from phospholipids, fucose derived from mucosal glycoconjugates, 1,2-propanediol as the fermentation product of fucose or rhamnose and several other metabolites not accessible for commensal bacteria or present in competition-free microenvironments. Interestingly, the data reviewed here point to common metabolic strategies of enteric pathogens allowing the exploitation of nutrient sources that not only are present in the gut lumen, the mucosa or epithelial cells, but also are abundant in food. An increased knowledge of the metabolic strategies developed by enteropathogens is therefore a key factor to better control foodborne diseases.

Compound(s) secreted by Lactobacillus casei strain Shirota YIT9029 irreversibly and reversibly impair the swimming motility of Helicobacter pylori and Salmonella enterica serovar Typhimurium, respectively

We conducted experiments in order to examine whether the probiotic Lactobacillus casei strain Shirota YIT9029 (LcS) in vitro and in vivo antagonism of Helicobacter pylori and Salmonella, involves inhibition of the swimming motility of these pathogens. We report the irreversible inhibition of the swimming motility of H. pylori strain 1101 and reversible inhibition of Salmonella enterica serovar Typhimurium (S. Typhimurium) strain SL1344 by compound(s) secreted by LcS. In H. pylori 1101, irreversible inhibition results in the helical cells being progressively replaced by cells with 'c'-shaped and coccoid morphologies, accompanied by a loss of FlaA and FlaB flagellin expression. In S. Typhimurium SL1344, transient inhibition develops after membrane depolarization and without modification of expression of FliC flagellin. The inhibitory activity of strain LcS against both S. Typhimurium and H. pylori swimming motilities is linked with a small sized, heat-sensitive, and partially trypsin-sensitive, secreted compound(s), and needed the cooperation of the secreted membrane permeabilizing lactic acid metabolite. The inhibition of S. Typhimurium SL1344 swimming motility leads to delayed cell entry into human enterocyte-like Caco-2/TC7 cells and a strong decrease of cell entry into human mucus-secreting HT29-MTX cells.

Microbial biofilms and gastrointestinal diseases

The majority of bacteria live not planktonically, but as residents of sessile biofilm communities. Such populations have been defined as 'matrix-enclosed microbial accretions, which adhere to both biological and nonbiological surfaces'. Bacterial formation of biofilm is implicated in many chronic disease states. Growth in this mode promotes survival by increasing community recalcitrance to clearance by host immune effectors and therapeutic antimicrobials. The human gastrointestinal (GI) tract encompasses a plethora of nutritional and physicochemical environments, many of which are ideal for biofilm formation and survival. However, little is known of the nature, function, and clinical relevance of these communities. This review summarizes current knowledge of the composition and association with health and disease of biofilm communities in the GI tract.

Near surface swimming of Salmonella Typhimurium explains target-site selection and cooperative invasion

Targeting of permissive entry sites is crucial for bacterial infection. The targeting mechanisms are incompletely understood. We have analyzed target-site selection by S. Typhimurium. This enteropathogenic bacterium employs adhesins (e.g. fim) and the type III secretion system 1 (TTSS-1) for host cell binding, the triggering of ruffles and invasion. Typically, S. Typhimurium invasion is focused on a subset of cells and multiple bacteria invade via the same ruffle. It has remained unclear how this is achieved. We have studied target-site selection in tissue culture by time lapse microscopy, movement pattern analysis and modeling. Flagellar motility (but not chemotaxis) was required for reaching the host cell surface in vitro. Subsequently, physical forces trapped the pathogen for ∼1.5–3 s in “near surface swimming”. This increased the local pathogen density and facilitated “scanning” of the host surface topology. We observed transient TTSS-1 and fim-independent “stopping” and irreversible TTSS-1-mediated docking, in particular at sites of prominent topology, i.e. the base of rounded-up cells and membrane ruffles. Our data indicate that target site selection and the cooperative infection of membrane ruffles are attributable to near surface swimming. This mechanism might be of general importance for understanding infection by flagellated bacteria.

The animal body is protected by physical, chemical and immunological barriers. Identification of “promising” target sites is therefore of importance for any pathogen. This crucial step of the infection is still poorly understood. Here, we have studied target site selection by the flagellated Gram-negative gut pathogen Salmonella Typhimurium. Using a well-established tissue culture model system, we found that flagella-driven motility forces the bacterium into a “near surface swimming” mode which facilitates “scanning” of the host cell surface. The near surface swimming was found to target the pathogen towards sites with particular topological features, i.e., rounded cells and membrane ruffles. This explains how S. Typhimurium “identifies” particular target sites and infects membrane ruffles in a cooperative manner. Interestingly, the near surface swimming is attributable to generic physical principles acting on moving particles. Therefore, our findings might be of general importance for the infection by motile pathogens.

Near surface swimming of Salmonella Typhimurium explains target-site selection and cooperative invasion

Targeting of permissive entry sites is crucial for bacterial infection. The targeting mechanisms are incompletely understood. We have analyzed target-site selection by S. Typhimurium. This enteropathogenic bacterium employs adhesins (e.g. fim) and the type III secretion system 1 (TTSS-1) for host cell binding, the triggering of ruffles and invasion. Typically, S. Typhimurium invasion is focused on a subset of cells and multiple bacteria invade via the same ruffle. It has remained unclear how this is achieved. We have studied target-site selection in tissue culture by time lapse microscopy, movement pattern analysis and modeling. Flagellar motility (but not chemotaxis) was required for reaching the host cell surface in vitro. Subsequently, physical forces trapped the pathogen for ∼1.5-3 s in "near surface swimming". This increased the local pathogen density and facilitated "scanning" of the host surface topology. We observed transient TTSS-1 and fim-independent "stopping" and irreversible TTSS-1-mediated docking, in particular at sites of prominent topology, i.e. the base of rounded-up cells and membrane ruffles. Our data indicate that target site selection and the cooperative infection of membrane ruffles are attributable to near surface swimming. This mechanism might be of general importance for understanding infection by flagellated bacteria.

Salmonella Interaction with and Passage through the Intestinal Mucosa: Through the Lens of the Organism

Salmonella enterica serotypes are invasive enteric pathogens spread through fecal contamination of food and water sources, and represent a constant public health threat around the world. The symptoms associated with salmonellosis and typhoid disease are largely due to the host response to invading Salmonella, and to the mechanisms these bacteria employ to survive in the presence of, and invade through the intestinal mucosal epithelia. Surmounting this barrier is required for survival within the host, as well as for further dissemination throughout the body, and subsequent systemic disease. In this review, we highlight some of the major hurdles Salmonella must overcome upon encountering the intestinal mucosal epithelial barrier, and examine how these bacteria surmount and exploit host defense mechanisms.

Host-bacterial symbiosis in health and disease

All animals live in symbiosis. Shaped by eons of co-evolution, host-bacterial associations have developed into prosperous relationships creating mechanisms for mutual benefits to both microbe and host. No better example exists in biology than the astounding numbers of bacteria harbored by the lower gastrointestinal tract of mammals. The mammalian gut represents a complex ecosystem consisting of an extraordinary number of resident commensal bacteria existing in homeostasis with the host's immune system. Most impressive about this relationship may be the concept that the host not only tolerates, but has evolved to require colonization by beneficial microorganisms, known as commensals, for various aspects of immune development and function. The microbiota provides critical signals that promote maturation of immune cells and tissues, leading to protection from infections by pathogens. Gut bacteria also appear to contribute to non-infectious immune disorders such as inflammatory bowel disease and autoimmunity. How the microbiota influences host immune responses is an active area of research with important implications for human health. This review synthesizes emerging findings and concepts that describe the mutualism between the microbiota and mammals, specifically emphasizing the role of gut bacteria in shaping an immune response that mediates the balance between health and disease. Unlocking how beneficial bacteria affect the development of the immune system may lead to novel and natural therapies based on harnessing the immunomodulatory properties of the microbiota.

Salmonella pathogenesis and processing of secreted effectors by caspase-3

The enteric pathogen Salmonella enterica serovar Typhimurium causes food poisoning resulting in gastroenteritis. The S. Typhimurium effector Salmonella invasion protein A (SipA) promotes gastroenteritis by functional motifs that trigger either mechanisms of inflammation or bacterial entry. During infection of intestinal epithelial cells, SipA was found to be responsible for the early activation of caspase-3, an enzyme that is required for SipA cleavage at a specific recognition motif that divided the protein into its two functional domains and activated SipA in a manner necessary for pathogenicity. Other caspase-3 cleavage sites identified in S. Typhimurium appeared to be restricted to secreted effector proteins, which indicates that this may be a general strategy used by this pathogen for processing of its secreted effectors.

The blessings and curses of intestinal inflammation

The intestinal immune system has to strike a delicate balance between initiating inflammatory responses against invading bacterial pathogens and avoiding their induction against microbiota colonizing the lumen. Adequate inflammatory responses against bacterial invasion result in the lumenal secretion of antimicrobial peptides, as well as the release of cytokines in tissue that recruit and activate phagocytes. However, pathogens have evolved to utilize these environmental changes in the inflamed intestine to promote colonization. This review focuses on the costs and benefits of intestinal inflammation and the fine interplay between the host, its microbiota, and enteric pathogens.

Interaction between host cells and septicemic Salmonella enterica serovar typhimurium isolates from pigs

Salmonella enterica serovar Typhimurium is an important pathogen in swine and is also a frequently reported zoonotic agent. The objective of this study was to characterize isolates of S. enterica serovar Typhimurium associated with septicemia in swine and to compare them to isolates recovered from clinically healthy pigs. We were particularly interested in comparing the two groups of isolates for their ability to adhere to and invade host cells, to be phagocytized and survive in monocyte cells, to induce apoptosis, and to adhere to intestinal mucus. Their surface properties were also evaluated by interactions with solvents. The isolates recovered from diseased animals were shown to invade intestinal epithelial cell lines at a higher rate (P = 0.003) than isolates from healthy pigs. Septicemic isolates were phagocytized by human monocytes at a higher rate than isolates from healthy pigs (P = 0.009). The mean percentages of phagocytosis were significantly lower for human monocytes than for porcine monocytes (P = 0.02 and P = 0.008, respectively) for isolates from both diseased and healthy animals. Healthy animal isolates were phagocytized more by porcine monocytes at 15 min (P = 0.02) than septicemic isolates. No difference between isolates from septicemic pigs and isolates from healthy pigs was detected for other tested parameters. These results suggest that septicemic isolates have a particular pattern of invasion.

Role of deoxyribose catabolism in colonization of the murine intestine by pathogenic Escherichia coli strains

We previously suggested that the ability to metabolize deoxyribose, a phenotype encoded by the deoK operon, is associated with the pathogenic potential of Escherichia coli strains. Carbohydrate metabolism is thought to provide the nutritional support required for E. coli to colonize the intestine. We therefore investigated the role of deoxyribose catabolism in the colonization of the gut, which acts as a reservoir, by pathogenic E. coli strains. Molecular and biochemical characterization of 1,221 E. coli clones from various collections showed this biochemical trait to be common in the E. coli species (33.6%). However, multivariate analysis evidenced a higher prevalence of sugar-metabolizing E. coli clones in the stools of patients from countries in which intestinal diseases are endemic. Diarrhea processes frequently involve the destruction of intestinal epithelia, so it is plausible that such clones may be positively selected for in intestines containing abundant DNA, and consequently deoxyribose. Statistical analysis also indicated that symptomatic clinical disorders and the presence of virulence factors specific to extraintestinal pathogenic E. coli were significantly associated with an increased risk of biological samples and clones testing positive for deoxyribose. Using the streptomycin-treated-mouse model of intestinal colonization, we demonstrated the involvement of the deoK operon in gut colonization by two pathogenic isolates (one enteroaggregative and one uropathogenic strain). These results, indicating that deoxyribose availability promotes pathogenic E. coli growth during host colonization, suggest that the acquisition of this trait may be an evolutionary step enabling these pathogens to colonize and persist in the mammalian intestine.

The role of microbiota in infectious disease

The intestine harbors an ecosystem composed of the intestinal mucosa and the commensal microbiota. The microbiota fosters development, aids digestion and protects host cells from pathogens - a function referred to as colonization resistance. Little is known about the molecular basis of colonization resistance and how it can be overcome by enteropathogenic bacteria. Recently, studies on inflammatory bowel diseases and on animal models for enteric infection have provided new insights into colonization resistance. Gut inflammation changes microbiota composition, disrupts colonization resistance and enhances pathogen growth. Thus, some pathogens can benefit from inflammatory defenses. This new paradigm will enable the study of host factors enhancing or inhibiting bacterial growth in health and disease.

Motility allows S. Typhimurium to benefit from the mucosal defence

The mammalian intestine is colonized by a dense bacterial community, called microbiota. The microbiota shields from intestinal infection (colonization resistance). Recently, we have shown that enteropathogenic Salmonella spp. can exploit inflammation to compete with the intestinal microbiota. The mechanisms explaining the enhanced pathogen growth in the inflamed intestine are elusive. Here, we analysed the function of bacterial flagella in the inflamed intestine using a mouse model for acute Salmonella Typhimurium enterocolitis. Mutations affecting flagellar assembly (Fla(-)) and chemotaxis (Che(-)) impaired the pathogen's fitness in the inflamed intestine, but not in the normal gut. This was attributable to a localized source of high-energy nutrients (e.g. galactose-containing glyco-conjugates, mucin) released as an element of the mucosal defence. Motility allows Salmonella Typhimurium to benefit from these nutrients and utilize them for enhanced growth. Thus, nutrient availability contributes to enhanced pathogen growth in the inflamed intestine. Strategies interfering with bacterial motility or nutrient availability might offer starting points for therapeutic approaches.

Adherence, anti-adherence, and oligosaccharides preventing pathogens from sticking to the host

For many pathogenic bacteria, infections are initiated only after the organism has first adhered to the host cell surface. If adherence can be inhibited, then the subsequent infection can also be inhibited. This approach forms the basis of anti-adherence strategies, which have been devised to prevent a variety of bacterial infections. In this chapter, the molecular basis by which respiratory, urinary, and gastrointestinal tract pathogens adhere to host cells will be described. The five general types of anti-adherence agents will also be reviewed. The most well-studied are the receptor analogs, which include oligosaccharides produced synthetically or derived from natural sources, including milk, berries, and other plants. Their ability to inhibit pathogen adherence may lead to development of novel, food-grade anti-infective agents that are inexpensive and safe.

Intestinal innate immunity and the pathogenesis of Salmonella enteritis

Acute gastroenteritis caused by Salmonella typhimurium infection is a clinical problem with significant public health impact. The availability of several experimental models of this condition has allowed detailed investigation of the cellular and molecular interactions involved in its pathogenesis. Such studies have shed light on the roles played by bacterial virulence factors and host innate immune mechanisms in the development of intestinal inflammation.

L-fucose stimulates utilization of D-ribose by Escherichia coli MG1655 DeltafucAO and E. coli Nissle 1917 DeltafucAO mutants in the mouse intestine and in M9 minimal medium

Escherichia coli MG1655 uses several sugars for growth in the mouse intestine. To determine the roles of L-fucose and D-ribose, an E. coli MG1655 DeltafucAO mutant and an E. coli MG1655 DeltarbsK mutant were fed separately to mice along with wild-type E. coli MG1655. The E. coli MG1655 DeltafucAO mutant colonized the intestine at a level 2 orders of magnitude lower than that of the wild type, but the E. coli MG1655 DeltarbsK mutant and the wild type colonized at nearly identical levels. Surprisingly, an E. coli MG1655 DeltafucAO DeltarbsK mutant was eliminated from the intestine by either wild-type E. coli MG1655 or E. coli MG1655 DeltafucAO, suggesting that the DeltafucAO mutant switches to ribose in vivo. Indeed, in vitro growth experiments showed that L-fucose stimulated utilization of D-ribose by the E. coli MG1655 DeltafucAO mutant but not by an E. coli MG1655 DeltafucK mutant. Since the DeltafucK mutant cannot convert L-fuculose to L-fuculose-1-phosphate, whereas the DeltafucAO mutant accumulates L-fuculose-1-phosphate, the data suggest that L-fuculose-1-phosphate stimulates growth on ribose both in the intestine and in vitro. An E. coli Nissle 1917 DeltafucAO mutant, derived from a human probiotic commensal strain, acted in a manner identical to that of E. coli MG1655 DeltafucAO in vivo and in vitro. Furthermore, L-fucose at a concentration too low to support growth stimulated the utilization of ribose by the wild-type E. coli strains in vitro. Collectively, the data suggest that L-fuculose-1-phosphate plays a role in the regulation of ribose usage as a carbon source by E. coli MG1655 and E. coli Nissle 1917 in the mouse intestine.

Adhesion of vancomycin-resistant enterococcus to human intestinal mucus

The intestinal mucus layer provides a potential niche for colonization by vancomycin-resistant Enterococcus faecium (VREF). We therefore examined the ability of six VREF strains to adhere to human intestinal mucus and determined binding kinetics. Four of six (67%) VREF strains demonstrated significant adhesion to immobilized intestinal mucus compared with a Salmonella typhimurium-negative control strain, but the level of adherence was low compared with Lactobacillus rhamnosus GG. Binding kinetics studies demonstrated that the maximum number of these four VREF strains that could adhere to a unit surface area of immobilized mucus was similar to or higher than the maximum number of L. rhamnosus GG that could adhere; however, L. rhamnosus GG demonstrated 20- to 130-times higher affinity than the VREF strains. These results demonstrate that VREF strains may adhere to human intestinal mucus and suggest that L. rhamnosus GG might be able to displace VREF strains.

Mouse intestine selects nonmotile flhDC mutants of Escherichia coli MG1655 with increased colonizing ability and better utilization of carbon sources

D-gluconate which is primarily catabolized via the Entner-Doudoroff (ED) pathway, has been implicated as being important for colonization of the streptomycin-treated mouse large intestine by Escherichia coli MG1655, a human commensal strain. In the present study, we report that an MG1655 Deltaedd mutant defective in the ED pathway grows poorly not only on gluconate as a sole carbon source but on a number of other sugars previously implicated as being important for colonization, including L-fucose, D-gluconate, D-glucuronate, N-acetyl-D-glucosamine, D-mannose, and D-ribose. Furthermore, we show that the mouse intestine selects mutants of MG1655 Deltaedd and wild-type MG1655 that have improved mouse intestine-colonizing ability and grow 15 to 30% faster on the aforementioned sugars. The mutants of MG1655 Deltaedd and wild-type MG1655 selected by the intestine are shown to be nonmotile and to have deletions in the flhDC operon, which encodes the master regulator of flagellar biosynthesis. Finally, we show that DeltaflhDC mutants of wild-type MG1655 and MG1655 Deltaedd constructed in the laboratory act identically to those selected by the intestine; i.e., they grow better than their respective parents on sugars as sole carbon sources and are better colonizers of the mouse intestine.

Colonization of mucin by human intestinal bacteria and establishment of biofilm communities in a two-stage continuous culture system

The human large intestine is covered with a protective mucus coating, which is heavily colonized by complex bacterial populations that are distinct from those in the gut lumen. Little is known of the composition and metabolic activities of these biofilms, although they are likely to play an important role in mucus breakdown. The aims of this study were to determine how intestinal bacteria colonize mucus and to study physiologic and enzymatic factors involved in the destruction of this glycoprotein. Colonization of mucin gels by fecal bacteria was studied in vitro, using a two-stage continuous culture system, simulating conditions of nutrient availability and limitation characteristic of the proximal (vessel 1) and distal (vessel 2) colon. The establishment of bacterial communities in mucin gels was investigated by selective culture methods, scanning electron microscopy, and confocal laser scanning microscopy, in association with fluorescently labeled 16S rRNA oligonucleotide probes. Gel samples were also taken for analysis of mucin-degrading enzymes and measurements of residual mucin sugars. Mucin gels were rapidly colonized by heterogeneous bacterial populations, especially members of the Bacteroides fragilis group, enterobacteria, and clostridia. Intestinal bacterial populations growing on mucin surfaces were shown to be phylogenetically and metabolically distinct from their planktonic counterparts.

The Life of Commensal Escherichia coli in the Mammalian Intestine

In this chapter we review the literature with respect to what is known about how Escherichia coli colonizesthe mammalian intestine. We begin with a brief discussion of the mammalian large intestine, the major site that commensal strains of E. coli colonize. Next, evidence is discussed showing that, in order to colonize, E. coli must be able to penetrate and grow in the mucus layer of the large intestine. This is followed by discussions of colonization resistance, i.e., factors that are involved in the ability of a complete microbiota (microflora) to resist colonization by an invading bacterium, the advantages and disadvantages of the in vivo colonization models used in colonization research, the initiation and maintenance stages of E. coli colonization, and the rate of E. coli growth in the intestine. The next two sections of the chapter discuss the role of motility in colonization and how adhesion to mucosal receptors aids or inhibits penetration of the intestinal mucus layer and thereby either promotes or prevents E. coli colonization. Finally, the contribution of nutrition to the ability of E. coli to colonize is discussed based on the surprising finding that different nutrients are used by E. coli MG1655, a commensal strain, and by E. coli EDL933, an enterohemorrhagic strain, to colonize the intestine.

An Escherichia coli MG1655 lipopolysaccharide deep-rough core mutant grows and survives in mouse cecal mucus but fails to colonize the mouse large intestine

The ability of E. coli strains to colonize the mouse large intestine has been correlated with their ability to grow in cecal and colonic mucus. In the present study, an E. coli MG1655 strain was mutagenized with a mini-Tn5 Km (kanamycin) transposon, and mutants were tested for the ability to grow on agar plates with mouse cecal mucus as the sole source of carbon and nitrogen. One mutant, designated MD42 (for mucus defective), grew poorly on cecal-mucus agar plates but grew well on Luria agar plates and on glucose minimal-agar plates. Sequencing revealed that the insertion in MD42 was in the waaQ gene, which is involved in lipopolysaccharide (LPS) core biosynthesis. Like "deep-rough" E. coli mutants, MD42 was hypersensitive to sodium dodecyl sulfate (SDS), bile salts, and the hydrophobic antibiotic novobiocin. Furthermore, its LPS core oligosaccharide was truncated, like that of a deep-rough mutant. MD42 initially grew in the large intestines of streptomycin-treated mice but then failed to colonize (<10(2) CFU per g of feces), whereas its parent colonized at levels between 10(7) and 10(8) CFU per g of feces. When mouse cecal mucosal sections were hybridized with an E. coli-specific rRNA probe, MD42 was observed in cecal mucus as clumps 24 h postfeeding, whereas its parent was present almost exclusively as single cells, suggesting that clumping may play a role in preventing MD42 colonization. Surprisingly, MD42 grew nearly as well as its parent during growth in undiluted, highly viscous cecal mucus isolated directly from the mouse cecum and, like its parent, survived well after reaching stationary phase, suggesting that there are no antimicrobials in mucus that prevent MD42 colonization. After mini-mariner transposon mutagenesis, an SDS-resistant suppressor mutant of MD42 was isolated. The mini-mariner insertion was shown to be in the bipA gene, a known regulator of E. coli surface components. When grown in Luria broth, the LPS core of the suppressor mutant remained truncated; however, the LPS core was not truncated when the suppressor mutant was grown in the presence of SDS. Moreover, when the suppressor mutant was grown in the presence of SDS and fed to mice, it colonized the mouse large intestine. Collectively, the data presented here suggest that BipA may play a role in E. coli MG1655 LPS core biosynthesis and that because MD42 forms clumps in intestinal mucus, it is unable to colonize the mouse large intestine.

Interactions of Salmonella enterica subsp. enterica serovar Muenchen with intestinal explants of the turtle Trachemys scripta scripta

Salmonella infections in reptiles, in contrast to those in birds and mammals, are limited to the intestinal tract. In this study, interactions of a strain of Salmonella enterica subsp. enterica serovar Muenchen (SEEM) with intestinal explants of the turtle Trachemys scripta scripta were examined by scanning electron microscopy (SEM). Adhesion and invasion in the chelonian intestinal explants at 30 degrees C and 37 degrees C were evaluated quantitatively. For purposes of comparison, the invasive capacity of SEEM in the continuous avian epithelial cell line DIV-1 at 30 degrees C and 37 degrees C was determined. Small numbers of M-like cells were found in the ileum of the turtles. The bacteria adhered mainly to the mucus of the intestinal explants. Only small numbers of salmonellae were associated with epithelial cells. Higher numbers of bacteria adhered at 30 degrees C than at 37 degrees C. Epithelial damage, embedding of bacteria in the epithelial surface and a ruffling-like process were noted only at 37 degrees C. Minimal numbers of salmonellae invaded the explants at 30 degrees C and 37 degrees C. Invasion of DIV-1 cells was greater at 37 degrees C than at 30 degrees C. The study suggested that the intestinal mucous layer provides an important site of colonization for salmonellae in the chelonian host and protects the underlying epithelial cells.

Characterization of the properties of human- and dairy-derived probiotics for prevention of infectious diseases in fish

The present study aimed to investigate the potential probiotic properties of six lactic acid bacteria (LAB) intended for human use, Lactobacillus rhamnosus ATCC 53103, Lactobacillus casei Shirota, Lactobacillus bulgaricus, L. rhamnosus LC 705, Bifidobacterium lactis Bb12, and Lactobacillus johnsonii La1, and one for animal use, Enterococcus faecium Tehobak, for use as a fish probiotic. The strains for human use were specifically chosen since they are known to be safe for human use, which is of major importance because the fish are meant for human consumption. The selection was carried out by five different methods: mucosal adhesion, mucosal penetration, inhibition of pathogen growth and adhesion, and resistance to fish bile. The adhesion abilities of the seven LAB and three fish pathogens, Vibrio anguillarum, Aeromonas salmonicida, and Flavobacterium psychrophilum, were determined to mucus from five different sites on the surface or in the gut of rainbow trout. Five of the tested LAB strains showed considerable adhesion to different fish mucus types (14 to 26% of the added bacteria). Despite their adhesive character, the LAB strains were not able to inhibit the mucus binding of A. salmonicida. Coculture experiments showed significant inhibition of growth of A. salmonicida, which was mediated by competition for nutrients rather than secretion of inhibitory substances by the probiotic bacteria as measured in spent culture liquid. All LAB except L. casei Shirota showed tolerance against fish bile. L. rhamnosus ATCC 53103 and L. bulgaricus were found to penetrate fish mucus better than other probiotic bacteria. Based on bile resistance, mucus adhesion, mucus penetration, and suppression of fish pathogen growth, L. rhamnosus ATCC 53103 and L. bulgaricus can be considered for future in vivo challenge studies in fish as a novel and safe treatment in aquaculture.

A functional cra gene is required for Salmonella enterica serovar typhimurium virulence in BALB/c mice

A minitransposon mutant of Salmonella enterica serovar Typhimurium SR-11, SR-11 Fad(-), is unable to utilize gluconeogenic substrates as carbon sources and is avirulent and immunogenic when administered perorally to BALB/c mice (M. J. Utley et al., FEMS Microbiol. Lett., 163:129-134, 1998). Here, evidence is presented that the mutation in SR-11 Fad(-) that renders the strain avirulent is in the cra gene, which encodes the Cra protein, a regulator of central carbon metabolism.

Questions about the behaviour of bacterial pathogens in vivo

Bacterial pathogens cause disease in man and animals. They have unique biological properties, which enable them to colonize mucous surfaces, penetrate them, grow in the environment of the host, inhibit or avoid host defences and damage the host. The bacterial products responsible for these five biological requirements are the determinants of pathogenicity (virulence determinants). Current knowledge comes from studies in vitro, but now interest is increasing in how bacteria behave and produce virulence determinants within the infected host. There are three aspects to elucidate: bacterial activities, the host factors that affect them and the metabolic interactions between the two. The first is relatively easy to accomplish and, recently, new methods for doing this have been devised. The second is not easy because of the complexity of the environment in vivo and its ever-changing face. Nevertheless, some information can be gained from the literature and by new methodology. The third aspect is very difficult to study effectively unless some events in vivo can be simulated in vitro. The objectives of the Discussion Meeting were to describe the new methods and to show how they, and conventional studies, are revealing the activities of bacterial pathogens in vivo. This paper sets the scene by raising some questions and suggesting, with examples, how they might be answered. Bacterial growth in vivo is the primary requirement for pathogenicity. Without growth, determinants of the other four requirements are not formed. Results from the new methods are underlining this point. The important questions are as follows. What is the pattern of a developing infection and the growth rates and population sizes of the bacteria at different stages? What nutrients are present in vivo and how do they change as infection progresses and relate to growth rates and population sizes? How are these nutrients metabolized and by what bacterial mechanisms? Which bacterial processes handle nutrient deficiencies and antagonistic conditions that may arise? Conventional and new methods can answer the first question and part of the second; examples are described. The difficulties of trying to answer the last two are discussed. Turning to production in vivo of determinants of mucosal colonization, penetration, interference with host defence and damage to the host, here are the crucial questions. Are putative determinants, which have been recognized by studies in vitro, produced in vivo and are they relevant to virulence? Can hitherto unknown virulence determinants be recognized by examining bacteria grown in vivo? Does the complement of virulence determinants change as infection proceeds? Are regulatory processes recognized in vitro, such as ToxR/ToxS, PhoP/PhoQ, quorum sensing and type III secretion, operative in vivo? What environmental factors affect virulence determinant production in vivo and by what metabolic processes? Examples indicate that the answers to the first four questions are 'yes' in most but not all cases. Attempts to answer the last, and most difficult, question are also described. Finally, sialylation of the lipopolysaccharide of gonococci in vivo by host-derived cytidine 5'-mono-phospho-N-acetyl neuraminic acid, and the effect of host lactate are described. This investigation revealed a new bacterial component important in pathogenicity, the host factors responsible for its production and the metabolism involved.

rpoS gene function is a disadvantage for Escherichia coli BJ4 during competitive colonization of the mouse large intestine

The ability of Escherichia coli to survive stress during growth in different environments is, in large part, dependent on rpoS and the genes that comprise the rpoS regulon. E. coli BJ4 and an isogenic BJ4 rpoS mutant were used to examine the influence of the rpoS gene on E. coli colonization of the streptomycin-treated mouse large intestine. Colonization experiments in which the wild-type E. coli BJ4 and its rpoS mutant were fed individually as well as simultaneously to mice suggested that E. coli BJ4 does not face prolonged periods of nutrient starvation in the mouse large intestine and that the rpoS regulon is not expressed during long-term colonization after adaptation of the bacteria to the gut environment.

Intestinal mucins: the binding sites for Salmonella typhimurium

Mucus-bacterial interactions in the gastrointestinal tract and their impact on subsequent enteric infections are poorly delineated. In the present study, we have examined the binding of Salmonella typhimurium to rat intestinal mucus and characterized a mucus protein (Mucus-Rs) which specifically binds to S. typhimurium. Both virulent (1402/84), and avirulent (SF 1835) S. typhimurium were observed to bind to crude mucus, however, the virulent strain showed 6 fold more binding as compared to avirulent strain. Fractionation of crude mucus on sepharose CL-6B resolved it into three major peaks. Maximal bacterial binding was observed with a high mol. wt. glycoprotein corresponding to neutral mucin. SDS-PAGE of purified protein (termed Mucus-Rs) under non reducing conditions showed it to be a homogenous glycoprotein (mol. wt. 250 kDa), while under reducing conditions, three bands corresponding to mol. wt. of 118,75 and 60 kDa were observed. Pretreatment of Mucus-Rs with pronase, trypsin and sodium metaperiodate markedly inhibited bacterial binding. GLC analysis of Mucus-Rs showed it to contain Mannose, Glucose, Galactose, Glucosamine, Galactosamine and Sialic acid as main sugars. Competitive binding in the presence of various sugars and lectins indicated the involvement of mannose in the mucus-bacterial interactions. The Mucus-Rs binding was highly specific for S. typhimurium; no significant binding was seen with E. coli and V. cholerae. Thus, we conclude that S. typhimurium specifically binds to a 250 kDa neutral mucin of intestinal tract. This binding appears to occur via specific adhesin-receptor interactions involving bacterial pili and mannose of neutral mucin.

Plasmid transfer in the animal intestine and other dynamic bacterial populations: the role of community structure and environment

The transfer of the R1drd19 plasmid between isogenic strains of Escherichia coli BJ4 in batch cultures of laboratory media and intestinal extracts was compared. Using an estimate of plasmid transfer rate that is independent of cell density, of donor:recipient ratios and of mating time, it was found that transfer occurs at a much lower rate in intestinal extracts than in laboratory media. Furthermore, the results suggest that the majority of intestinal plasmid transfer takes place in the viscous mucus layer covering the epithelial cells. Investigation of plasmid transfer in different flow systems harbouring a dynamic, continuously growing population of constant size showed that transfer kinetics were strongly influenced by bacterial biofilm formation. When donor and recipient populations were subjected to continuous mixing, as in a chemostat, transfer continued to occur at a constant rate. When donor and recipient populations retained fixed spatial locations, as in a biofilm, transfer occurred very rapidly in the initial phase, after which no further transfer was detected. From in vivo studies of plasmid transfer in the intestine of streptomycin-treated mice, results were obtained which were similar to those obtained in the biofilm, but differed markedly from those obtained in the chemostat. In spite of peristaltic movements in the gut, and of apparently even distribution of E. coli as single cells in the intestinal mucus, the intestinal environment displays transfer kinetics different from those expected of a mixed, liquid culture, but quite similar to those of a biofilm.

Inhibition of Escherichia coli precursor-16S rRNA processing by mouse intestinal contents

The correlation between ribosome content and growth rate found in many bacterial species has proved useful for estimating the growth activity of individual cells by quantitative in situ rRNA hybridization. However, in dynamic environments, the stability of mature ribosomal RNA causes problems in using cellular rRNA contents for direct monitoring of bacterial growth activity in situ. In a recent paper, Cangelosi and Brabant suggested monitoring the content of precursors in rRNA synthesis (pre-rRNAs) as an alternative approach. These are rapidly broken down after the cessation of bacterial growth. We have applied fluorescence in situ hybridization of pre-16S rRNA to Escherichia coil cells growing in vitro in extracts from two different compartments of the mouse intestine: the caecal mucus layer, where E. coli grew rapidly, and the contents of the caecum, which supported much slower bacterial growth. The amounts of 23S rRNA and pre-16S rRNA measured for E. coil growing in intestinal mucus corresponded to that expected for bacteria with the observed growth rate. In contrast, the slow-growing E. coli cells present in intestinal contents turned out to have an approximately ninefold higher content of pre-16S rRNA than cultures of the same strain growing rapidly in rich media. We present results suggesting that the mouse intestinal contents contain an agent that inhibits the growth of E. coli by disturbing its ability to process pre-16S rRNA.

Interaction of antigens and antibodies at mucosal surfaces

Infections often involve the mucosal surfaces of the body, which form a boundary with the outside world. This review focuses on immunoglobulin A (IgA) antibodies because IgA is the principal mucosal antibody class. IgA is synthesized by local plasma cells and has a specific polymeric immunoglobulin receptor-mediated transport mechanism for entry into the secretions. By serving as an external barrier capable of inhibiting attachment of microbes to the luminal surface of the mucosal epithelial lining, IgA antibodies form the first line of immune defense. In addition to this traditional mode of extracellular antibody function, recent evidence suggests that IgA antibodies can also function in a nontraditional fashion by neutralizing viruses intracellularly, if a virus is infecting an epithelial cell through which specific IgA antibody is passing on its way to the secretions. IgA antibodies are also envisaged as providing an internal mucosal barrier beneath the mucosal lining. Antigens intercepted by IgA antibodies here can potentially be ferried through the epithelium and thereby excreted. In addition to the polymeric immunoglobulin receptor on mucosal epithelial cells, IgA antibodies can bind to receptors on a variety of leukocytes and have been shown, in some experimental systems, to be capable of activating the alternative complement pathway, making IgA antibodies potential participants in inflammatory reactions. Although the relationship of IgA antibodies to inflammation is not entirely clear, the bias presented is that IgA is basically noninflammatory, perhaps even anti-inflammatory. According to this view, the major role of the Fc portion of IgA antibodies is to transport IgA across mucosal epithelial cells and not, as in the case of the other classes of antibody, to activate secondary phenomena of the kind that contribute to inflammation. Because of IgA's key role as an initial barrier to infection, much current research in mucosal immunology is directed toward developing new vectors and adjuvants that can provide improved approaches to mucosal vaccines. Finally, because of advances in monoclonal antibody technology, topical application of antibodies to mucosal surfaces has significant potential for preventing and treating infections.

The early dynamic response of the calf ileal epithelium to Salmonella typhimurium

Ileal loops including Peyer's patch were prepared in five 28-day-old calves and infused Salmonella typhimurium strain ST4/74. Loops were fixed 5 minutes to 2 hours after inoculation, and the mucosa was examined by light and electron microscopy. Within 5 minutes, the bacteria were interacting with the follicle-associated epithelium (FAE); the surface of M cells changed to lamellipodia, engulfing many bacteria. This process proceeded rapidly to 30 minutes, involving most M cells above crypt level. Most cells were exfoliated, and many were packed with bacteria, and the domed villi became stunted. There was a rapid migration of neutrophils through the FAE into the lumen by 15 minutes. By 60 minutes, there was no further interaction between the bacteria and the FAE; at this time bacteria were present in macrophages in the lamina propria. Restitution of the FAE was complete by 2 hours in spite of the many bacteria in the cell debris overlying the epithelium. Interaction of bacteria with the absorptive villi was delayed compared with interaction with the FAE. After 15 minutes, bacteria were seen adhering to some enterocytes of the upper third of the villi; many bacteria were adhering to the surface of the enterocytes at 20 and 30 minutes, but few were seen thereafter. Adherence was patchy and largely confined to cells whose surfaces were depressed relative to others. The microvillous surface of these enterocytes was extensively remodelled. Tissue response, with uptake of bacteria into vacuoles, exfoliation of enterocytes containing bacteria, and subsequent stunting of the villi, began at 30 minutes and was severe and progressive to 2 hours. Following the initial attachment and uptake of the bacteria loss of enterocytes progressed from these initial sites; bacteria were associated with the lateral cell membrane of cells adjacent to cells being extruded and not with the microvilli of cells at new sites. In a calf 4 hours after dosing orally with the same strain, M cells were engulfing bacteria and their cell surface was changed as seen in the inoculated loops; absorptive enterocytes were also taking up bacteria as seen in the ileal loops, indicating the process seen in the loops and after oral dosage was similar. For this strain of S typhimurium, there was an initial concentration of bacilli around the domed villus epithelium. This distribution was not random but may have resulted from a specific attraction to the FAE.

Role of lipopolysaccharide in colonization of the mouse intestine by Salmonella typhimurium studied by in situ hybridization

An avirulent, streptomycin-resistant Salmonella typhimurium strain, SL5319, and its lipopolysaccharide (LPS)-deficient mutant strain, SL5325, differ in their ability to colonize the large intestines of streptomycin-treated mice. When fed to mice independently, the strains colonize equally well, but when fed together, the LPS-deficient mutant is outcompeted by the wild-type strain during establishment in the gut (J.J. Nevola, B.A.D. Stocker, D.C. Laux, and P.S. Cohen, Infect. Immun. 50:152-159, 1985). In the present study, the spatial distribution in the intestinal mucosal layer of the two strains was visualized by specific hybridization to bacterial rRNA in histological sections of mouse colon and cecum. The first day after infection, 9.8% of the smooth SL5319 cells observed in mucus were found to be associated with the mouse epithelial cells, but three days after infection, the corresponding fraction of adhering bacteria was reduced to 2.1%. The LPS-deficient S. typhimurium strain was confined to the part of the mucosal layer closest to the colonic lumen and was not observed to adhere to the epithelium either at day 1 or 3 after infection. Quantitative determinations of the distance from the S. typhimurium cells to the epithelial wall confirmed that the average distance for the rough S. typhimurium SL5325 was much larger than for its smooth counterpart, S. typhimurium SL5319. Quantification of the hybridization signal from bacteria isolated from the cecal mucus revealed that the two strains had the same ribosome concentration, indicating that they have the same potential for growth in the intestinal environment. On the basis of these observations, we suggest that the better colonization ability of the strain carrying wild-type LPS is due to the better abilities to penetrate the intestinal mucosal layer and to subsequently bind to the epithelial cells in vivo.