Pathogenesis of Listeria monocytogenes for gnotobiotic rats

Akkermansia muciniphila in infectious disease: A new target for this next-generation probiotic?

The common gastrointestinal commensal Akkermansia muciniphila is a mucin-degrading bacterium that is greatly reduced in individuals consuming a high-fat diet. Increasing evidence from a variety of clinical and pre-clinical studies suggests that oral supplementation with Akkermansia can improve metabolic health and moderate systemic inflammation. We and others have demonstrated a role for Akkermansia administration in protection against infectious disease and the outcome from sepsis. Very recent studies have indicated the molecular mechanisms by which A. muciniphila may interact with the host to influence systemic immune-regulation and control of microbial pathogenesis. Here we consider recent studies which demonstrate the efficacy of this potential next-generation probiotic in animal models of Salmonella Typhimurium, Listeria monocytogenes and Clostridioides difficile as well as influenza virus and phlebovirus. The potential mechanisms by which A. muciniphila may influence local and systemic immune responses are discussed.

Mouse models for bacterial enteropathogen infections: insights into the role of colonization resistance

Globally, enteropathogenic bacteria are a major cause of morbidity and mortality.1-3 Campylobacter, Salmonella, Shiga-toxin-producing Escherichia coli, and Listeria are among the top five most commonly reported zoonotic pathogens in the European Union.4 However, not all individuals naturally exposed to enteropathogens go on to develop disease. This protection is attributable to colonization resistance (CR) conferred by the gut microbiota, as well as an array of physical, chemical, and immunological barriers that limit infection. Despite their importance for human health, a detailed understanding of gastrointestinal barriers to infection is lacking, and further research is required to investigate the mechanisms that underpin inter-individual differences in resistance to gastrointestinal infection. Here, we discuss the current mouse models available to study infections by non-typhoidal Salmonella strains, Citrobacter rodentium (as a model for enteropathogenic and enterohemorrhagic E. coli), Listeria monocytogenes, and Campylobacter jejuni. Clostridioides difficile is included as another important cause of enteric disease in which resistance is dependent upon CR. We outline which parameters of human infection are recapitulated in these mouse models, including the impact of CR, disease pathology, disease progression, and mucosal immune response. This will showcase common virulence strategies, highlight mechanistic differences, and help researchers from microbiology, infectiology, microbiome research, and mucosal immunology to select the optimal mouse model.

Polymicrobial oral conventionalization model in mice

The aim of this study is to describe a new polymicrobial oral conventionalization protocol in mice. Oral biofilm samples were collected from wild C57 BL/6 mice (WG), which had not been previously submitted to any experimental procedure. The contents of these samples were used for inoculation in the oral cavity of specific pathogen free (SPF) animals. This inoculation was repeated 3 times. Qualitative cytological analyses were performed in the days 0, 16 and 80 of the experimental protocol, to check the presence or absence of microorganisms, their morphology and staining characteristics on the oral cavity of the animals. At the end of this study, was observed a combination of oral bacterial microbiota of SPF animals and wild animals in the conventionalized group (CONV). Samples collected from CONV mice on day 16, a period in which these animals had been previously inoculated 3 times with wild mouse microbiota, showed a greater amount of Gram-positive cocci, as seen in SPF animals. In addition, Gram-negative cocci were present, although in a much smaller proportion than previously seen in wild mice. On the 80th experimental day, CONV animals showed a predominance of Gram-positive cocci and bacilli. Filamentous bacteria were also seen in this group. The conventionalization of SPF animals using the technique with inoculum from the resident microbiota of wild mice proved to be an effective, low-cost and easily reproducible technique. The conventionalized animals showed the colonization of a microbiota similar to wild animals up to 80 days of experiment.

Early cellular responses of germ-free and specific-pathogen-free mice to Francisella tularensis infection

Bacteria that are highly virulent, expressing high infectivity, and able to survive nebulization, pose great risk to the human population. One of these is Francisella tularensis, the etiological agent of tularemia. F. tularensis is a subject of intense scientific interest due to the fact that vaccines for its immunoprophylaxis in humans are not yet routinely available. One of the substantial obstacles in developing such vaccines is our insufficient knowledge of processes that initiate and regulate the expression of effective protective immunity against intracellular bacteria. Here, we present data documenting the different pattern of cellular behavior occurring in an environment unaffected by microbiota using the model of germ-free mice mono-associated with F. tularensis subsp. holarctica strain LVS in comparison with a classic specific-pathogen-free murine model during early stages of infection.

A Comparison of Oral and Intravenous Mouse Models of Listeriosis

Listeria monocytogenes is one of several enteric microbes that is acquired orally, invades the gastric mucosa, and then disseminates to peripheral tissues to cause systemic disease in humans. Intravenous (i.v.) inoculation of mice with L. monocytogenes has been the most widely-used small animal model of listeriosis over the past few decades. The infection is highly reproducible and has been invaluable in deciphering mechanisms of adaptive immunity in vivo, particularly CD8⁺ T cell responses to intracellular pathogens. However, the i.v. model completely bypasses the gut phase of the infection. Recent advances in generating both humanized mice and murinized bacteria, as well as the development of a foodborne route of transmission has reignited interest in studying oral models of listeriosis. In this review, we analyze previously published reports to highlight both the similarities and differences in tissue colonization and host response to infection using either oral or i.v. inoculation.

Multifaceted Defense against Listeria monocytogenes in the Gastro-Intestinal Lumen

Listeria monocytogenes is a foodborne pathogen that can cause febrile gastroenteritis in healthy subjects and systemic infections in immunocompromised individuals. Despite the high prevalence of L. monocytogenes in the environment and frequent contamination of uncooked meat and poultry products, infections with this pathogen are relatively uncommon, suggesting that protective defenses in the general population are effective. In the mammalian gastrointestinal tract, a variety of defense mechanisms prevent L. monocytogenes growth, epithelial penetration and systemic dissemination. Among these defenses, colonization resistance mediated by the gut microbiota is crucial in protection against a range of intestinal pathogens, including L. monocytogenes. Here we review defined mechanisms of defense against L. monocytogenes in the lumen of the gastro-intestinal tract, with particular emphasis on protection conferred by the autochthonous microbiota. We suggest that selected probiotic species derived from the microbiota may be developed for eventual clinical use to enhance resistance against L. monocytogenes infections.

Commensal microbes provide first line defense against Listeria monocytogenes infection

Becattini et al. provide evidence that a diverse gut microbiota antagonizes the foodborne pathogen Listeria monocytogenes in the intestinal lumen, thereby reducing bloodstream invasion. Microbiota perturbation by antibiotic treatment increases susceptibility to listeriosis, with dramatic effects in immunocompromised hosts.

Listeria monocytogenes is a foodborne pathogen that causes septicemia, meningitis and chorioamnionitis and is associated with high mortality. Immunocompetent humans and animals, however, can tolerate high doses of L. monocytogenes without developing systemic disease. The intestinal microbiota provides colonization resistance against many orally acquired pathogens, and antibiotic-mediated depletion of the microbiota reduces host resistance to infection. Here we show that a diverse microbiota markedly reduces Listeria monocytogenes colonization of the gut lumen and prevents systemic dissemination. Antibiotic administration to mice before low dose oral inoculation increases L. monocytogenes growth in the intestine. In immunodeficient or chemotherapy-treated mice, the intestinal microbiota provides nonredundant defense against lethal, disseminated infection. We have assembled a consortium of commensal bacteria belonging to the Clostridiales order, which exerts in vitro antilisterial activity and confers in vivo resistance upon transfer into germ free mice. Thus, we demonstrate a defensive role of the gut microbiota against Listeria monocytogenes infection and identify intestinal commensal species that, by enhancing resistance against this pathogen, represent potential probiotics.

Gnotobiotic mouse model's contribution to understanding host-pathogen interactions

This brief review is dedicated to the legacy of Prof. Jaroslav Šterzl and his colleagues, who laid the foundation for gnotobiology in the former Czechoslovakia 55 years. Prof. Sterzl became one of the founders of modern Czechoslovak immunology, which was characterized by work on a wide range of problems needing to be solved. While examining the mechanisms of innate immunity, he focused his studies on the induction of antibody production by immunocompetent cells involved in adaptive immune transmission while using the model of pig fetuses and germ-free piglets and characterizing immunoglobulins in the sera of these piglets. Although not fully appreciated to this day, his experimental proof of the hypothesis focused on the common precursor of cell-forming antibodies of different isotypes was later confirmed in experiments at the gene level. Prof. Sterzl's work represented a true milestone in the development of not solely Czechoslovak but also European and global immunology. He collaborated closely with the World Health Organization for many years, serving there as leader of the Reference Laboratory for Factors of Innate Immunity.

Blowing on embers: commensal microbiota and our immune system

Vertebrates have co-evolved with microorganisms resulting in a symbiotic relationship, which plays an important role in health and disease. Skin and mucosal surfaces are colonized with a diverse population of commensal microbiota, over 1000 species, outnumbering the host cells by 10-fold. In the past 40 years, studies have built on the idea that commensal microbiota is in constant contact with the host immune system and thus influence immune function. Recent studies, focusing on mutualism in the gut, have shown that commensal microbiota seems to play a critical role in the development and homeostasis of the host immune system. In particular, the gut microbiota appears to direct the organization and maturation of lymphoid tissues and acts both locally and systemically to regulate the recruitment, differentiation, and function of innate and adaptive immune cells. While the pace of research in the area of the mucosal–immune interface has certainly intensified over the last 10 years, we are still in the early days of this field. Illuminating the mechanisms of how gut microbes shape host immunity will enhance our understanding of the causes of immune-mediated pathologies and improve the design of next-generation vaccines. This review discusses the recent advances in this field, focusing on the close relationship between the adaptive immune system and commensal microbiota, a constant and abundant source of foreign antigens.

Listeria monocytogenes: survival and adaptation in the gastrointestinal tract

The foodborne pathogen Listeria monocytogenes has the capacity to survive and grow in a diverse range of natural environments. The transition from a food environment to the gastrointestinal tract begins a process of adaptation that may culminate in invasive systemic disease. Here we describe recent advances in our understanding of how L. monocytogenes adapts to the gastrointestinal environment prior to initiating systemic infection. We will discuss mechanisms used by the pathogen to survive encounters with acidic environments (which include the glutamate decarboxylase and arginine deiminase systems), and those which enable the organism to cope with bile acids (including bile salt hydrolase) and competition with the resident microbiota. An increased understanding of how the pathogen survives in this environment is likely to inform the future design of novel prophylactic approaches that exploit specific pharmabiotics; including probiotics, prebiotics, or phages.

Regulation of bacterial pathogenesis by intestinal short-chain Fatty acids

The human gut microbiota is inextricably linked to health and disease. One important function of the commensal organisms living in the intestine is to provide colonization resistance against invading enteric pathogens. Because of the complex nature of the interaction between the microbiota and its host, multiple mechanisms likely contribute to resistance. In this review, we dissect the biological role of short-chain fatty acids (SCFA), which are fermentation end products of the intestinal microbiota, in host-pathogen interactions. SCFA exert an extensive influence on host physiology through nutritional, regulatory, and immunomodulatory functions and can also affect bacterial fitness as a form of acid stress. Moreover, SCFA act as a signal for virulence gene regulation in common enteric pathogens. Taken together, these studies highlight the importance of the chemical environment where the biology of the host, the microbiota, and the pathogen intersects, which provides a basis for designing effective infection prevention and control.

Animal models of listeriosis: a comparative review of the current state of the art and lessons learned

Listeriosis is a leading cause of hospitalization and death due to foodborne illness in the industrialized world. Animal models have played fundamental roles in elucidating the pathophysiology and immunology of listeriosis, and will almost certainly continue to be integral components of the research on listeriosis. Data derived from animal studies helped for example characterize the importance of cell-mediated immunity in controlling infection, allowed evaluation of chemotherapeutic treatments for listeriosis, and contributed to quantitative assessments of the public health risk associated with L. monocytogenes contaminated food commodities. Nonetheless, a number of pivotal questions remain unresolved, including dose-response relationships, which represent essential components of risk assessments. Newly emerging data about species-specific differences have recently raised concern about the validity of most traditional animal models of listeriosis. However, considerable uncertainty about the best choice of animal model remains. Here we review the available data on traditional and potential new animal models to summarize currently recognized strengths and limitations of each model. This knowledge is instrumental for devising future studies and for interpreting current data. We deliberately chose a historical, comparative and cross-disciplinary approach, striving to reveal clues that may help predict the ultimate value of each animal model in spite of incomplete data.

The interaction between Listeria monocytogenes and the host gastrointestinal tract

Listeria monocytogenes is a ubiquitous bacterium that causes significant foodborne disease with high mortality rates in immunocompromised adults. In pregnant women foodborne infection can give rise to infection of the fetus resulting in miscarriage. In addition, the bacterium has recently been demonstrated to cause localized gastrointestinal symptoms, predominantly in immunocompetent individuals. The murine model of systemic L. monocytogenes infection has provided numerous insights into the mechanisms of pathogenesis of this organism. However, recent application of transcriptomic and proteomic approaches as well as the development of new model systems has allowed a focus upon factors that influence adaptation to gastrointestinal environments and adhesion to and invasion of the gastrointestinal mucosa. In addition, the availability of a large number of complete L. monocytogenes genome sequences has permitted inter-strain comparisons and the identification of factors that may influence the emergence of 'epidemic' phenotypes. Here we review some of the exciting recent developments in the analysis of the interaction between L. monocytogenes and the host gastrointestinal tract.

Role of the innate immune system in the pathogenesis of inflammatory bowel disease

Crohn disease and ulcerative colitis are chronic inflammatory diseases of the intestinal tract commonly denoted as inflammatory bowel diseases. It has been proposed that these diseases result from aberrant mucosal immune responses to nonpathogenic microbial residents of the intestines. Recently, it was established that continuous interactions between the innate and the adaptive intestinal immune cells and the microbiota are directly involved in maintaining the physiological noninflammatory state of the intestinal mucosa. In light of the complexity of this mucosal homeostasis, it is astonishing that the inflammatory bowel diseases are relatively rare. Recently, altered functions of the innate immune system have been identified. As such, both hyperresponsiveness and hyporesponsiveness of innate cells have been implicated in the pathogenesis of inflammatory bowel diseases.

Ecological and evolutionary forces shaping microbial diversity in the human intestine

The human gut is populated with as many as 100 trillion cells, whose collective genome, the microbiome, is a reflection of evolutionary selection pressures acting at the level of the host and at the level of the microbial cell. The ecological rules that govern the shape of microbial diversity in the gut apply to mutualists and pathogens alike.

Listeria monocytogenes and listeriosis: a review of hazard characterisation for use in microbiological risk assessment of foods

Considerable effort has been put into the application of quantitative microbiological risk assessment for Listeria monocytogenes, and data are available for England and Wales (probably more so than most other countries) on the adverse health effects, together with incidence data on different age and risk groups for human L. monocytogenes infections. This paper reviews aspects of Listeria and human listeriosis, especially from a public health perspective and provide hazard characterisation data, i.e. the qualitative and/or quantitative evaluation of the adverse health effect associated with the hazard, which is the relationship between exposure levels (dose) and frequency of illness. The majority of cases of human listeriosis are food-borne; however, the disease process is complex with multiple routes of infection. The dose-response relationship is poorly understood, and data from human volunteer studies are not available and would be unethical to produce. Data are available from a range of different animal and in vitro models, although these poorly mimic the natural disease process in route of infection, end point, host and history of prior exposure to the bacterium. Epidemiological data provide some information on infective doses and dose responses, but because of the characteristics of the disease (the hugely variable and potentially very long incubation periods, the low attack rates and the rarity of identification of specific food vehicles), this also provides limited data for calculation of dose responses. There is some, albeit limited, evidence for strain variation, but this is an area of considerable uncertainty despite great advances in the genetic basis of the virulence of this bacterium, and almost all strains seem capable of causing serious disease. A variety of mathematical approaches have been used to model dose responses. The review is written to provide a clinical and epidemiological background to the mathematically oriented, as well as to outline the mathematical approaches to those interested in food-borne infection.

Antagonistic activity of Lactobacillus casei strain shirota against gastrointestinal Listeria monocytogenes infection in rats

In the present study, the effect of ingested viable Lactobacillus casei Shirota strain YIT9029 on oral infection with the enteric pathogen Listeria monocytogenes in Wistar rats was investigated. Rats were orally infected with 10(9) viable L. monocytogenes. Starting 3 days before the infection, rats received a daily dosage of 10(9) viable L. casei. It was shown that supplementation of L. casei significantly reduced the numbers of L. monocytogenes in stomach, caecum, faeces, spleen and liver, 2 days after L. monocytogenes infection. The number of L. monocytogenes in the mesenteric lymph nodes was not affected by the ingestion of L. casei. In comparison with control animals, the levels of the liver-specific alanine aminotransferase were lower in L. casei-fed rats. Histological analysis of spleen and liver revealed no differences between the experimental and control animals. In a parallel study with orally L. monocytogenes infected rats, it was shown that L. casei was able to increase cellular immunity significantly as determined with the delayed-type hypersensitivity response against heat-killed L. monocytogenes. In conclusion, in the present study it was shown that orally administered L. casei is able to enhance host resistance against oral L. monocytogenes infection. In the gastrointestinal tract, as well as in the spleen and liver, L. monocytogenes numbers were reduced. Furthermore, it is concluded that the enhancement of this anti-Listeria activity might be, at least partly, due to increased cell-mediated immunity.

Detection of anti-listeriolysin O and Listeria monocytogenes in experimentally infected buffaloes (Bubalus bubalis)

The kinetics of antibody production against listeriolysin O (ALLO) and the recovery pattern of Listeria monocytogenes from bacteriological samples were studied following oral infection of buffalo calves with 3 x 10(9) cells each of pathogenic L. monocytogenes. Antibodies to LLO appeared by 7-10 days post infection (PI), with a shallow peak between days 16 and 36 PI, when tested by indirect plate-ELISA. The titres of ALLO in all the animals then declined slowly but remained detectable up to day 70 PI. In dot-ELISA, ALLO could be detected by days 5 to 7 PI, and with higher titres than with the plate-ELISA. The pathogen was recovered at low rates as ALLO first appeared but was absent in the faecal, nasal and blood cultures as production of ALLO peaked.

The mucosal phase of Listeria infection

Listeria monocytogenes is an enteroinvasive bacterial pathogen of man and animals. Listeriae have been shown capable of infecting the host by translocating from the intestinal lumen through Peyer's Patches (PP), however, results of experiments now indicate that these facultative intracellular parasites may also translocate through PP-independent routes. With regards to this, on occasion we observed that listeriae were absent from the PP of mice inoculated intragastrically with L. monocytogenes, but were present in the mesenteric lymph nodes of these same mice. These observations suggested that PP were not necessary for listerial translocation from the intestinal lumen. Two experimental approaches were used to determine whether luminal listeriae could indeed infect the host through PP-independent routes. First, since it is known that: 1) following the intragastric inoculation of L. monocytogenes, listeriae rapidly transit the length of the gastrointestinal tract and reside in the colonic lumen for up to a week, 2) the colon lacks PP, and 3) the descending colon and rectum are drained exclusively by the caudal lymph node (CLN), it was determined whether colonic listeriae could access the CLN. Inoculation of listeriae into the rectum of mice resulted in the infection of the CLN which indicated that PP were not required for listerial translocation. Second, since germfree SCID mice lack PP, it was determined whether listeriae could translocate from the intestinal lumen and infect these immunoincompetent mice. Shortly after the intragastric inoculation of L. monocytogenes into germfree SCID mice, listeriae were found in the mesenteries, livers and spleens. These results also indicate that PP are not required for listerial translocation from the intestinal lumen. One possible route of translocation from the intestinal lumen might occur by listeriae entering enterocytes. Results were obtained showing that listeriae were capable of entering cultured mouse small intestine enterocytes. Internalized listeriae were observed to multiply and spread intracellularly between enterocytes.

Systemic dissemination by intrarectal infection with Listeria monocytogenes in mice

Orally ingested Listeria monocytogenes is known to penetrate into Peyer's patches (PP) and translocate to the spleen and liver. Herein, extraintestinal dissemination of the bacterium independent of PP was investigated. Dissemination of Listeriae to the spleen and liver was observed in intrarectally infected mice as well as in intragastrically infected animals in spite that no Listeriae were detected in the small intestines of mice infected intrarectally. Decreased numbers of intestinal intraepithelial lymphocytes (iIEL) and increased numbers of lymphocytes in the contents of the small and large intestines were observed after intragastric infection and in the large intestine after intrarectal infection, giving the assumption that the leakage of iIEL caused by injury of epithelial layers in intestines might occur during infection. These results suggest that L. monocytogenes might be able to disseminate through small and large intestines in part by a PP-independent mechanism.

Prolonged persistence of Listeria monocytogenes after intragastric infection in corticosteroid-treated mice

In an attempt to obtain a model more closely resembling natural listeriosis, we studied the course of infection in mice inoculated by the intragastric route with Listeria monocytogenes. Corticosteroid-treated, and untreated mice both developed subclinical infection without mortality, but faecal shedding and persistence of bacteria in the liver and spleen of corticosteroid-treated mice were significantly more protracted than in untreated mice. Untreated mice cleared the bacteria from their livers and spleens by day 5 postinfection (PI), whereas treated mice did not clear the organisms until 8-9 days PI. In untreated mice faecal shedding lasted 5 days PI, whereas in treated mice the organisms were recovered at significantly higher levels until day 9 PI. The only intestinal lesions observed were mild pyogranulomatous changes in the dome area of some Peyer's patches in treated mice.

Non-specific resistance mechanisms to listeriosis: implications for experimental and naturally occurring infection

Use of murine listeriosis as an experimental model has greatly increased our understanding of the complex interplay of cells and mediators in non-specific antibacterial resistance (innate immunity). Important contributions made with this experimental model include demonstrating the ability of inflammatory cytokines (i.e. IFN-gamma, IL-1 alpha, TNF-alpha) to protect against bacterial infection, and illustrating the rapidity of the host cytokine response (detectable within 1 h of challenge) during bacterial infection. Most experimental studies of host defense against Listeria monocytogenes (L. monocytogenes) have used a parenteral challenge (i.v. or i.p.). This ignores the pathogenesis of naturally occurring listeriosis, which usually results from ingestion of Listeria-contaminated food products. In this review, we will include consideration of the host-pathogen interactions that occur when L. monocytogenes invades through its natural portal of entry, the gastrointestinal tract. Although resistance to facultative intracellular pathogens, such as L. monocytogenes, was formerly thought to revolve exclusively around the T helper cell/macrophage axis, more recent evidence indicates that neutrophils are able to ingest and kill L. monocytogenes and prevent the unrestricted multiplication of listeriae in parenchymal cells. Exploring the mechanisms involved in this process will provide new insights into the communication between leukocytes and tissue cells in host defense.

Human HLA-B27 gene enhances susceptibility of rats to oral infection by Listeria monocytogenes

Germfree rats transgenic for the human genes HLA-B27 and beta 2-microglobulin were colonized with hemolysin-positive (Hly+) or hemolysin-negative (Hly-) strains of Listeria monocytogenes. HLA-B27 rats were very susceptible to infection with Hly+ L monocytogenes none survived beyond 6 days. Conversely, nontransgenic control rats survived alimentary tract colonization with the Hly+ strain, and both transgenic and nontransgenic rats survived colonization with the Hly- strain of L monocytogenes. After colonization with Hly+ L monocytogenes, both transgenic and nontransgenic rats developed severe bowel inflammation which consisted histologically of microab scesses, granulomatous lesions, and ulcers; however, whereas the transgenic rats died within 6 days, only very mild intestinal lesions were seen in nontransgenic rats 10 to 42 days after colonization. Liver and splenic lesions were small and transient in nontransgenic rats. Transgenic and nontransgenic control rats infected with Hly- Listeria developed mild transient diarrhea but showed no histological changes in the intestine. This study thus documents an association between a particular bacterial product (hemolysin produced by L monocytogenes) and the induction of severe inflammatory disease and death in rats expressing HLA-B27 and beta 2-microglobulin.

Host resistance to an intragastric infection with Listeria monocytogenes in mice depends on cellular immunity and intestinal bacterial flora

Suckling and adult mice were infected intragastrically with different doses of viable Listeria monocytogenes. The 50% lethal dose for the intragastric infection was 10(3.7) CFU for suckling mice, while adult mice were highly resistant and the 50% lethal dose was more than 10(9.3) CFU. When adult mice were infected intragastrically with 5 x 10(8) CFU of L. monocytogenes, no mice died. However, 35% of adult mice died when they were treated with cyclosporin A 1 day before infection. Although mice did not die when treated with an L. monocytogenes-resistant broad-spectrum cephalosporin, sodium cefbuperazone, before and during infection, the number of L. monocytogenes bacteria increased in the feces. The sodium cefbuperazone treatment of mice resulted in superinfection, i.e., a marked decrease of Escherichia coli and an increase of Enterococcus spp. in the intestines. Furthermore, host resistance against the intragastric infection markedly decreased when the mice were treated with both drugs. The growth of L. monocytogenes was augmented in the spleens, mesenteric lymph nodes, Peyer's patches, and feces, and the mortality of the mice was 65%. These results suggest that both cellular immunity and the intestinal bacterial flora are required for host resistance against oral L. monocytogenes infection.

A model of food-borne Listeria monocytogenes infection in the Sprague-Dawley rat using gastric inoculation: development and effect of gastric acidity on infective dose

Recent epidemiological evidence suggests that Listeria monocytogenes (LM) is a food-borne pathogen in humans. A model of LM infection was developed using the Sprague-Dawley (SD) rat to study the interaction of LM with gastrointestinal epithelium as the first step in the pathogenesis of invasive listeriosis. Conventionally raised, juvenile female SD rats were given 10(2)-10(9) virulent L. monocytogenes, serotype 4b or nonpathogenic Listeria species. Only rats given virulent LM developed dose-dependent invasive infection of the liver and spleen. Light and electron microscopic studies suggested attachment to and invasion of the gastrointestinal mucosa by virulent LM. Because the development of invasive listeriosis in humans has been epidemiologically associated with a decrease in gastric acidity, the effect of decreasing gastric acidity on dose-dependent infection was studied. Rats were pretreated with cimetidine (50 mg/kg) by intraperitoneal injection prior to oral inoculation of 10(2)-10(9) virulent L. monocytogenes. Cimetidine significantly lowered the infective dose of virulent L. monocytogenes (P < 0.05). This oral model should allow further study of host and organism-specific virulence factors mediating the gastrointestinal phase of invasive LM infection, an increasingly important public health problem.

Determination of virulence of different strains of Listeria monocytogenes and Listeria innocua by oral inoculation of pregnant mice

A pregnant mouse model was developed to follow the course of infection after peroral inoculation with six different strains of Listeria monocytogenes and one strain of Listeria innocua. Tissues were sampled and analyzed by microbiologic and histologic methods for 5 days postinoculation. In gnotobiotic pregnant BALB/c mice, L. monocytogenes Scott A (SA), serotype 4b, colonized the gastrointestinal tract, translocated to the livers and spleens of mice by day 1 postinoculation, and multiplied in these tissues until day 4. Infection of the placental tissues occurred by days 3 and 4 and was followed by infection of the fetuses. Little damage of colonic and cecal tissues was evident by histologic examination. Livers and spleens showed a cellular immune response; a similar immune response was not detected in the placentas or fetuses. A rough variant of L. monocytogenes SA which was as virulent as the parent strain in mice when injected intraperitoneally was less virulent perorally and did not consistently infect the fetuses. L. monocytogenes ATCC 19113, serotype 3a, did not colonize the gastrointestinal tract, nor was it isolated from any internal tissue. L. monocytogenes strains of serotypes 1/2a and 1/2b behaved like the SA strain in this mouse model. L. innocua colonized the gastrointestinal tract and translocated to the livers and spleens but did not survive in these organs and rapidly disappeared without infecting placental and fetal tissues. In comparison with gnotobiotic mice, conventional pregnant mice inoculated with L. monocytogenes strains showed less consistent infection. These results suggest that the gnotobiotic pregnant mouse is a useful model for detecting differences in virulence relating to colonization, invasiveness, and uteroplacental infection which cannot be detected by intraperitoneal inoculation of mice.

A microbiological, histopathological and immunohistological study of the intragastric inoculation of Listeria monocytogenes in mice

The course of murine infection after intragastric inoculation of L. monocytogenes was investigated by immunocytochemical, histopathological and microbiological techniques. L. monocytogenes antigen was observed in epithelial cells of intestinal mucosa overlying Peyer's patches, but not in mucosa devoid of them. This suggests that penetration of L. monocytogenes into the host organism may take place through epithelium overlying Peyer's patches. The efficiency of bacterial penetration appeared to be low, as shown by the small amounts of L. monocytogenes antigen detected and the low counts of bacteria in organs. Gross or histopathological lesions in the intestinal tract were not observed. The presence of L. monocytogenes in spleen, liver and in maxillary and mesenteric lymph nodes, confirmed that the systemic course of infection by this route of inoculation is similar to that of the parenteral routes. The results emphasize the subclinical character of murine listeriosis by the oral route.

Hemolysin is required for extraintestinal dissemination of Listeria monocytogenes in intragastrically inoculated mice

In this study we demonstrated that a hemolytic strain of Listeria monocytogenes, but not a nonhemolytic mutant derived from it, translocated in substantial numbers to the mesenteric lymph nodes, spleen, and liver after intragastric inoculation of mice. Growth at 4 degrees C prior to inoculation did not increase the virulence of the nonhemolytic mutant. These results indicate that hemolytic activity is required for the virulence of L. monocytogenes via the gastrointestinal tract, as has been shown previously for parenteral challenge.

Growth at reduced temperatures increases the virulence of Listeria monocytogenes for intravenously but not intragastrically inoculated mice

Growth of three clinical isolates (Scott A, Murray B, and F5380) and one laboratory strain (EGD) of L. monocytogenes at 4 degrees C significantly increased their virulence for intravenously injected mice. Using the EGD strain for subsequent experiments, we determined that growth at either 4 degrees or 22 degrees C enhanced the growth of listeria in the spleen and liver. Similar numbers of listeriae were recovered from the spleens and livers of mice during the first 48 h after i.v. injection of strain EGD grown at 37 degrees C or 4 degrees C. At later timepoints (3-6 days), significantly more listeriae were recovered from the spleens and livers of mice injected i.v. with strain EGD grown at 4 degrees C. In contrast, L. monocytogenes EGD grown at 37 degrees C and 4 degrees C demonstrated similar abilities to survive in the gastrointestinal tract, to translocate to the mesenteric lymph nodes, and to disseminate to the spleen and liver in intragastrically inoculated mice. Listeria monocytogenes EGD grown at 4 degrees C released less hemolysin into the culture medium than did this strain when grown at 22 degrees C and 37 degrees C. Transfer to fresh broth and incubation at 37 degrees C for 2 h increased the release, to similar levels, of hemolysin from L. monocytogenes EGD grown at 4 degrees, 22 degrees, and 37 degrees C. Temperature-induced differences in virulence, therefore, may not reflect the amount of hemolysin released.

Effects of growth temperature on the ingestion and killing of clinical isolates of Listeria monocytogenes by human neutrophils

In this study, we compared three human isolates (F5380, Scott A, and Murray B) and one laboratory strain (EGD) of Listeria monocytogenes for their resistance to ingestion and killing by human neutrophils. We observed no substantial difference in killing among these strains when they were grown at 37 degrees C. Because it is likely that listerial growth occurs at lower temperatures during food-borne outbreaks of listeriosis, we also compared these strains after they were grown at 22 and 4 degrees C. A general reduction in the ability of human neutrophils to kill L. monocytogenes was observed as the temperature at which the listeriae were grown decreased. Growth at 4 degrees C significantly decreased the killing of all four strains of L. monocytogenes by human neutrophils; two strains (EGD and F5380) were more resistant to killing than were the other two strains (Scott A and Murray B). No obvious relationship was noted between the chemiluminescence response of neutrophils to opsonized listeriae and the ability of the neutrophils to kill listeriae in vitro. Growth at 4 degrees C, however, significantly increased the resistance of L. monocytogenes to killing by hydrogen peroxide.

Intestinal multiplication of Mycobacterium paratuberculosis in athymic nude gnotobiotic mice

In this study gnotobiotic mice were inoculated with a human isolate of Mycobacterium paratuberculosis (strain Linda; ATCC 43015) in an attempt to investigate the pathogenesis of intestinal paratuberculosis. Mycobacterium paratuberculosis-monoassociated nu/+ mice developed a persistent low-level intestinal infection but did not support progressive bacillary multiplication. In contrast, monoassociated nu/nu mice eventually harbored approximately 10(7) M. paratuberculosis per g of intestinal tissue. Acid-fast bacilli and granulomas were observed in the intestinal mucosa and livers of nu/nu but not nu/+ mice. Similar results were obtained after intragastric inoculation of M. paratuberculosis into nu/+ and nu/nu flora-defined mice. These observations suggest that the presence of an intact cellular immune system is important for limiting intestinal multiplication of M. paratuberculosis. The results of this study may be relevant to our understanding of the pathogenesis of Johne's disease in ruminants and of human inflammatory bowel diseases that have a mycobacterial etiology (e.g., some cases of Crohn's disease and Mycobacterium avium-Mycobacterium intracellulare enteritis in patients with acquired immunodeficiency syndrome).

Pathogenicity test for Listeria monocytogenes using immunocompromised mice

The lethality of Listeria isolates was determined with normal adult mice and mice that were immunocompromised by treatment with 20 mg of carrageenan per kg. The mean 50% lethal doses (LD50s) of the pathogenic isolates were significantly lower (alpha = 0.05) in the immunocompromised mice than in the untreated mice, with an average reduction of 5.8 log10 units. In contrast, the mean LD50s of the nonpathogenic isolates were lower in the immunocompromised mice by an average of only 0.4 log10 unit, a difference that was not significant (alpha = 0.05). When immunocompromised mice were used, the LD50s of pathogenic Listeria monocytogenes isolates were lower than those of nonpathogenic L. innocua and L. seeligeri isolates by greater than or equal to 6 log10 units and lower than those of nonpathogenic L. ivanovii isolates by greater than or equal to 4 log10 units. Pathogenic L. monocytogenes isolates could be distinguished from nonpathogenic isolates by their ability to cause deaths in immunocompromised mice in 3 days at a dose of approximately 10(4) CFU per mouse. An alternative procedure using iron-overloaded mice failed to effectively differentiate pathogenic Listeria isolates.

Colonization and infection of athymic and euthymic germfree mice by Campylobacter jejuni and Campylobacter fetus subsp. fetus

Human clinical strains of Campylobacter jejuni and Campylobacter fetus subsp. fetus colonized the gastrointestinal tracts of both athymic (nu/nu) and euthymic (+/nu) germfree mice (BALB/c). Viable Campylobacter spp. (10(9) to 10(10) CFU/g [dry weight] of cecum and colon contents) were isolated on day 3 after oral challenge, and similar large numbers of viable cells were evident at several intervals during a 10-month experiment. The stomachs and upper small intestines of nu/nu and +/nu mice that were monoassociated for 224 days with C. jejuni 45100 contained 3 to 4 logs fewer viable bacteria than did their ceca or colons. Athymic mice that were monoassociated for 224 days with C. fetus subsp. fetus had 2 to 3 logs more viable Campylobacter spp. in their upper gastrointestinal tracts than did their +/nu littermates. Large viable populations (approximately 10(9)/g of contents) of C. fetus subsp. fetus were in the ceca and colons of both nu/nu and +/nu mice. All C. jejuni strains used in this study chronically infected the mesenteric lymph nodes of both nu/nu and +/nu mice. C. jejuni strains 24 and INN 73-83, which were cytotoxic for Chinese hamster ovary cells in vitro, were also more frequently isolated from the livers, spleens, and kidneys of nu/nu mice than was the weak cytotoxin-producing strain 45100. Additionally, heat-labile-enterotoxin-producing C. jejuni INN 73-83 was recovered more frequently from the internal organs of monoassociated +/nu mice than were any other Campylobacter spp. tested. Natural gastrointestinal colonization of neonatal nu/nu and +/nu mice (born to Campylobacter-colonized mothers) with Campylobacter spp. appeared to be delayed until approximately 1 to 2 weeks after birth. Conventionalization of C. jejuni 45100-monoassociated BALB/c mice with a complex mouse fecal microflora eliminated viable C. jejuni from the mesenteric lymph nodes by day 14 and from the cecum by day 78. These findings show that the gnotobiotic BALB/c mouse is a new model for studying acute and chronic host-Campylobacter sp. interactions.

Colonization of congenitally athymic, gnotobiotic mice by Candida albicans

Colony counts, scanning electron microscopy, and light microscopy were used to assess the capacity of Candida albicans to colonize (naturally) and infect the alimentary tract of adult and neonatal (athymic [nu/nu] or heterozygous [+/nu] littermates) germfree BALB/c mice. When exposed to yeast-phase C. albicans, the alimentary tract of adult germfree mice (nu/nu or +/nu) is quickly (within 24 to 48 h) colonized with yeast cells. Neither morbidity nor mortality was evident in any mice that were colonized with a pure culture of C. albicans for 6 months. Yeast cells of C. albicans predominated on mucosal surfaces in the oral cavities and vaginas of adult athymic and heterozygous mice. In both genotypes, C. albicans hyphae were observed in keratinized tissue on the dorsal posterior tongue surface and in the cardial-atrium section of the stomach. Conversely, neonatal athymic or heterozygous mice, born to germfree or C. albicans-colonized mothers, do not become heavily colonized or infected with C. albicans until 11 to 15 days after birth. Although yeast cells adhered to some mucosal surfaces in vivo, neither widespread mucocutaneous candidiasis, i.e., invasion of mucosal surfaces with C. albicans hyphae, nor overwhelming systemic candidiasis was evident in neonatal (nu/nu or +/nu) mice. Thus, even in the absence of functional T-cells and a viable bacterial flora, athymic and heterozygous littermate mice (adult or neonatal BALB/c) that are colonized with a pure culture of C. albicans manifest resistance to extensive mucocutaneous and systemic candidiasis.

Pseudomembranous colitis in Clostridium difficile-monoassociated rats

Germfree rats were monoassociated with either a toxin-producing strain of Clostridium difficile (Tox+) or a variant of this strain (ToxR) which produced much less toxin (1/10,000) in vivo and in vitro. Monoassociation of germfree rats with C. difficile Tox+ resulted in mortality (17%) and in pathology to the small and large intestines, livers, and lungs. Cecal filtrates from the Tox+-monoassociated rats were cytotoxic for tissue culture cells. The cytotoxicity of cecal filtrates could be blocked by sera from Tox+-monoassociated rats. Monoassociation of rats with C. difficile ToxR resulted in no deaths or pathology, and much less toxin was detected in the cecal filtrates of these animals than in those of rats colonized with the Tox+ strain. This gnotobiotic model may be useful for investigating the etiology, prophylaxis, therapy, and exacerbation of C. difficile-induced pseudomembranous colitis.

Killing of Listeria monocytogens by conventional and germfree rat sera

Serum from both germfree and conventional rats, but not plasma or plasma serum, killed Listeria monocytogenes in vitro by a calcium-dependent mechanism that was independent of either complement or lysozyme and was not inhibited by the addition of iron. The listericidin was purified by passing either rat serum or platelet lysate through a nitrocellulose filter (0.2 micrometer) and eluting the activity from the filter with 0.02 N HCl. The partially purified listericidin was heat stable (56 degrees C for 30 min), removed by absorption with zymosan or bentonite, sensitive to treatment with trypsin or pronase, and inhibited by the addition of citrate (0.045 M), suggesting that the serum listericidin is a cationic protein. The development of serum listericidal activity, which could be important in the innate resistance of rats to L. monocytogenes, was dependent on both age and microbial status. Although some discrepancies exist between the serum listericidin and previous descriptions of serum beta-lysin, we believe that the rat serum listericidin is a similar cationic protein.