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

Clinical Findings of Listeria monocytogenes Infections with a Special Focus on Bone Localizations

Listeria monocytogenes is a Gram-positive pathogenic bacterium which can be found in soil or water. Infection with the microorganism can occur after ingestion of contaminated food products. Small and large outbreaks of listeriosis have been described in the past. L. monocytogenes can cause a number of different clinical syndromes, most frequently sepsis, meningitis, and rhombencephalitis, particularly in immunocompromised hosts. L. monocytogenes systemic infections can develop following tissue penetration across the gastrointestinal tract or to hematogenous spread to sterile sites, possibly evolving towards bacteremia. L. monocytogenes only rarely causes bone or joint infections, usually in the context of prosthetic material that can provide a site for bacterial seeding. We describe here the clinical findings of invasive listeriosis, mainly focusing on the diagnosis, clinical management, and treatment of bone and vertebral infections occurring in the context of invasive listeriosis.

Inactivation of the gene encoding the cationic antimicrobial peptide resistance factor MprF increases biofilm formation but reduces invasiveness of Listeria monocytogenes

AIMS

To understand the genetics involved in surface attachment and biofilm formation of Listeria monocytogenes.

METHODS AND RESULTS

An in vitro screen of a Himar1 transposon library of L. monocytogenes strain 15G01 identified three transposants that produced significantly different biofilm levels when compared to the wild-type strain; two mutants exhibited enhanced biofilm formation and one produced less biofilm biomass than the wild-type. The mutant 15G01 mprF::Himar1, which had a transposon insertion in the mprF gene, was selected for further analysis. The mutant produced a more densely populated biofilm on solid surfaces such as stainless steel and polystyrene, as determined using scanning electron and light microscopy. The 15G01 mprF::Himar1 mutant remained viable in biofilms, but showed an increase in sensitivity to the cationic antimicrobial gallidermin. The mutant also displayed reduced invasiveness in CaCo-2 intestinal cells, suggesting virulence properties are compromised by the inactivation of mprF.

CONCLUSIONS

Biofilm formation and gallidermin resistance of L. monocytogenes is influenced by mprF, but this trait is associated with a compromise in invasiveness.

SIGNIFICANCE AND IMPACT OF THE STUDY

The presence of pathogenic microorganisms in the food processing environment can cause a significant problem, especially when these microorganisms are established as biofilms. This study shows that the inactivation of the mprF gene results in enhanced biofilm formation and abiotic surface attachment of L. monocytogenes.

Epidemiology and Clinical Manifestations of Listeria monocytogenes Infection

Listeria monocytogenes is a Gram-positive pathogenic bacterium which can be found in soil or water. Infection with the organism can develop after ingestion of contaminated food products. Small and large outbreaks of listeriosis have been described. Listeria monocytogenes can cause a number of clinical syndromes, most frequently sepsis, meningitis, and rhombencephalitis, particularly in immunocompromised hosts. The latter syndrome mimics the veterinary infection in ruminants called "circling disease". Neonatal infection can occur as a result of maternal chorioamnionitis ("early onset" sepsis) or through passage through a birth canal colonized with Listeria from the gastrointestinal tract. ("late onset" meningitis). Treatment of listeriosis is usually with a combination of ampicillin and an aminoglycoside but other regimens have been used. The mortality rate is high, reflecting the combination of an immunocompromised host and an often delayed diagnosis.

Pathogenicity, genotyping and antibacterial susceptibility of the Listeria spp. recovered from stray dogs

The present study aimed to determine the prevalence of Listeria spp. in stray dogs in the Kayseri province of Turkey. In addition, serotyping, genotyping and virulence gene analysis of the isolated Listeria spp. were performed and their pathogenicity and antibacterial susceptibility were investigated. The study included 80 rectal swaps taken from 80 stray dogs of different ages and gender that were sheltered in the Kayseri Municipal Dog Shelter. Listeria selective broth and Listeria selective agar were used for the isolation of Listeria spp. and the isolates were identified using a Microbact 12L (Oxoid, England) identification test kit. 16S rDNA sequencing and species-specific polymerase chain reaction (PCR) were performed for molecular identification of the isolates, multiplex PCR and a serological test were performed for serotyping, and PCR was used for virulence gene analysis. For determining the pathogenicity of L. monocytogenes and L. innocua isolates, a total of 100 mice (50 pregnant and 50 non-pregnant) were used. The mice were infected intraperitoneally; the inoculation dose was 1 × 10⁹ CFU/mL and 0.2 mL was used for each animal. Tissue samples obtained from infected mice were processed for the re-isolation of the Listeria spp. and then stained with hematoxylin eosin and Brown-Brenn Gram stain. The antibiotic susceptibilities of the isolates were determined by the disk diffusion method. Listeria spp. were isolated from 5 (6.25%) of the 80 fecal samples. While 1 of the isolates was identified as L. monocytogenes, 4 of them were identified as L. innocua. Serotyping by serological and molecular methods revealed the isolate of L. monocytogenes to be serotype 1/2a. According to the phylogenetic trees, L. innocua and L. monocytogenes strains were clustered in different groups. The L. monocytogenes isolate was positive for all virulence genes tested. All L. innocua isolates were positive for the plcB gene. While all L. innocua isolates were negative for the lin1068 gene, 3 L. innocua isolates were found to be positive for the lin0558 gene. In mice infected with L. monocytogenes, pathological findings were observed in the uterus, intestines, pancreas, and heart. In mice infected with L. innocua, pathological findings were observed in the stomach, intestines and spleen. L. monocytogenes- or L. innocua-related infections or other inflammatory reactions were not observed in the brains of infected animals. On histopathological examination with Gram stain, an abundance of Listeria spp. was observed in the lesions of the liver, spleen, uterus, and kidney. Moreover, while abortion was observed in all animals infected with L. monocytogenes, it was not observed in any of the animals infected with L. innocua. Antibiotic susceptibility testing revealed that all 5 isolates were sensitive to ampicillin, amoxicillin/clavulanic acid, erythromycin, gentamicin, penicillin G, and trimethoprim-sulfamethoxazole and were resistant to nalidixic acid, streptomycin, and cefuroxime sodium. Considering also the pathogenicity of the isolated microorganisms, it can be suggested that stray dogs as carriers of Listeria spp. are a significant risk to public health. As L. innocua isolates, which are considered apathogenic, led to the occurrence of lesions similar to those caused by L. monocytogenes, detailed studies on the pathogenesis of L. innocua infections caused by L. innocua isolates recovered from various sources are required.

Listeria monocytogenes associated with New Zealand seafood production and clinical cases: unique sequence types, truncated InlA, and attenuated invasiveness

Listeriosis is caused by the food-borne pathogen Listeria monocytogenes, which can be found in seafood and processing plants. To evaluate the risk to human health associated with seafood production in New Zealand, multi-virulence-locus sequence typing (MVLST) was used to define the sequence types (STs) of 31 L. monocytogenes isolates collected from seafood-processing plants, 15 from processed foods, and 6 from human listeriosis cases. The STs of these isolates were then compared with those from a collection of seafood isolates and epidemic strains from overseas. A total of 17 STs from New Zealand clustered into two lineages: seafood-related isolates in lineages I and II and all human isolates in lineage II. None of the New Zealand STs matched previously described STs from other countries. Isolates (belonging to ST01-N and ST03-N) from mussels and their processing environments, however, were identical to those of sporadic listeriosis cases in New Zealand. ST03-N isolates (16 from mussel-processing environments, 2 from humans, and 1 from a mussel) contained an inlA premature stop codon (PMSC) mutation. Therefore, the levels of invasiveness of 22 isolates from ST03-N and the three other common STs were compared using human intestinal epithelial Caco-2 cell lines. STs carrying inlA PMSCs, including ST03-N isolates associated with clinical cases, had a low invasion phenotype. The close relatedness of some clinical and environmental strains, as revealed by identical MVLST profiles, suggests that local and persistent environmental strains in seafood-processing environments pose a potential health risk. Furthermore, a PMSC in inlA does not appear to give L. monocytogenes a noninvasive profile.

Pathogenesis of listeriosis during pregnancy

Listeria monocytogenes causes several clinical manifestations in humans and domestic animals. This bacterium is a saprophyte in soil and ensiled feeds, which are sources of infection for food producing animals (i.e. ruminants). The most common route of infection for people is via ingestion of contaminated ready-to-eat food products such as produce, soft cheeses and deli meats. In the United States, L. monocytogenes causes relatively few cases of clinical disease compared to other food-borne pathogens. However, clinical listeriosis is associated with high mortality, especially in immunocompromised patients, pregnant women, neonates, and the elderly. Listeria is an intracellular pathogen, which has been widely used in basic research to elucidate mechanisms of molecular pathogenesis and protective cell-mediated immunity. Despite the sizeable knowledge on L. monocytogenes pathogenesis, key points regarding listeriosis during pregnancy and the perinatal period remain unknown. This review summarizes listeriosis in humans and domestic animals during pregnancy, and animal models used to study the pathogenesis and immune response to L. monocytogenes infection during these periods.

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.

Genome sequence of Listeria monocytogenes Scott A, a clinical isolate from a food-borne listeriosis outbreak

Listeria monocytogenes is an opportunistic food-borne pathogen and the causative agent of listeriosis in animals and humans. We present the genome sequence of Listeria monocytogenes Scott A, a widely distributed and frequently used serovar 4b clinical isolate from the 1983 listeriosis outbreak in Massachusetts.

Significant shift in median guinea pig infectious dose shown by an outbreak-associated Listeria monocytogenes epidemic clone strain and a strain carrying a premature stop codon mutation in inlA

Listeria monocytogenes contains (i) epidemic clone (EC) strains, which have been linked to the majority of listeriosis outbreaks worldwide and are overrepresented among sporadic cases in the United States, and (ii) strains commonly isolated from ready-to-eat foods that carry a mutation leading to a premature stop codon (PMSC) in inlA, which encodes the key virulence factor internalin A (InlA). Internalin A binds certain isoforms of the cellular receptor E-cadherin to facilitate crossing the intestinal barrier during the initial stages of an L. monocytogenes infection. Juvenile guinea pigs, which express the human isoform of E-cadherin that binds InlA, were intragastrically challenged with a range of doses of (i) an EC strain associated with a listeriosis outbreak or (ii) a strain carrying a PMSC mutation in inlA. Recovery of L. monocytogenes from tissues (i.e., liver, spleen, mesenteric lymph nodes, and ileum) was used to develop strain-specific dose-response curves on the basis of individual and combined organ data. Modeling of individual and combined organ data revealed an approximate 1.2 to 1.3 log(10) increase in the median infectious dose for the strain carrying a PMSC in inlA relative to that for the EC strain. Inclusion of the strain parameter significantly improved the goodness of fit for individual and combined organ models, indicating a significant shift in median infectious dose for guinea pigs challenged with an inlA PMSC strain compared to that for guinea pigs challenged with an EC strain. Results from this work provide evidence that the L. monocytogenes dose-response relationship is strain specific and will provide critical data for enhancement of current risk assessments and development of future risk assessments.

Rhombencephalitis Caused by Listeria monocytogenes in Humans and Ruminants: A Zoonosis on the Rise?

Listeriosis is an emerging zoonotic infection of humans and ruminants worldwide caused by Listeria monocytogenes (LM). In both host species, CNS disease accounts for the high mortality associated with listeriosis and includes rhombencephalitis, whose neuropathology is strikingly similar in humans and ruminants. This review discusses the current knowledge about listeric encephalitis, and involved host and bacterial factors. There is an urgent need to study the molecular mechanisms of neuropathogenesis, which are poorly understood. Such studies will provide a basis for the development of new therapeutic strategies that aim to prevent LM from invading the brain and spread within the CNS.

Contributions of six lineage-specific internalin-like genes to invasion efficiency of Listeria monocytogenes

Listeria monocytogenes strains are divided into at least three lineages, which seem to differ in virulence. Internalins are surface-attached or secreted proteins that encode leucine-rich repeats, and L. monocytogenes encodes species-specific as well as lineage-specific internalin and internalin-like genes. Internalins A and B have previously been shown to be critical for L. monocytogenes host cell invasion. Transcription of selected internalins is regulated by the virulence gene regulator PrfA and/or the stress-responsive alternative sigma factor sigma(B). We hypothesized that lineage-specific internalin-like genes may contribute to differential virulence and niche adaptation of the L. monocytogenes lineages. Initial quantitative real time, reverse transcriptase PCR (RT-PCR) showed that the six selected lineage-specific internalin-like genes were transcribed in cells grown at 16 degrees and 37 degrees C. Lineage-specific internalin-like gene, lineage II (lsiIIX) showed significantly higher transcript levels in log-phase cells grown at 37 degrees C as compared to 16 degrees C. The gene lsiIA was preceded by a putative sigma(B)-dependent promoter and showed sigma(B)-dependent transcription. None of the null mutants in lineage-specific internalin-like genes differed from their respective parent strain in ability to invade either human intestinal epithelial or hepatocyte-like cell lines. All three mutants in lineage I-specific internalin-like genes exhibited the same growth condition-dependent invasion phenotype as their parent strain ( approximately 1.5 log higher invasion efficiency when grown at 30 degrees C without aeration versus 37 degrees C with aeration). Despite structural similarities to internalins with known roles in host cell attachment and invasion, none of the six lineage-specific internalin-like genes characterized here appear to contribute to invasion. Combined with the observation that some nonpathogenic Listeria species also carry internalin genes, our findings suggest a broad role of Listeria internalins, not limited to attachment and invasion of human cells. Due to the wide host range of L. monocytogenes and the fact that transcription of internalin-like genes can differ considerably depending on growth condition, elucidating the function of different internalins and internalin-like genes will remain a challenge.

The Key Events Dose-Response Framework: its potential for application to foodborne pathogenic microorganisms

The Key Events Dose-Response Framework (KEDRF) is an analytical approach that facilitates the use of currently available data to gain insight regarding dose-response relationships. The use of the KEDRF also helps identify critical knowledge gaps that once filled, will reduce reliance on assumptions. The present study considers how the KEDRF might be applied to pathogenic microorganisms, using fetal listeriosis resulting from maternal ingestion of food contaminated with L. monocytogenes as an initial example. Major biological events along the pathway between food ingestion and the endpoint of concern are systematically considered with regard to dose (i.e., number of organisms), pathogen factors (e.g., virulence), and protective host mechanisms (e.g., immune response or other homeostatic mechanisms). It is concluded that the KEDRF provides a useful structure for systematically evaluating the complex array of host and pathogen factors that influence the dose-response relationship. In particular, the KEDRF supports efforts to specify and quantify the sources of variability, a prerequisite to strengthening the scientific basis for food safety decision making.

Low prevalence of Listeria monocytogenes in foods from Italy

Listeria monocytogenes is an important foodborne pathogen that causes gastrointestinal disorders, and, especially in immunocompromised people, serious extraintestinal diseases, such as septicemia and meningitis, as well as abortion in pregnant women. Many foods, from both plant and animal origin, have been involved in listeriosis outbreaks. This article reports the results of a 12-year survey (1993 through 2004) on the presence of L. monocytogenes in several kinds of food marketed in Italy. Of 5,788 analyzed samples, 121 (2.1%) were contaminated with L. monocytogenes. The highest prevalence was found in smoked salmon (10.6%) and in poultry meat samples (8.5%) and the lowest in red meat (0.3%). L. monocytogenes was not found in 154 samples of fresh seafood products. Fifty-two isolates were also serotyped by the agglutination method. The most common serotypes detected in the 52 strains tested were 1/2a (36.5%), followed by 1/2c (32.8%), 1/2b (13.5%), 4b (11.5%), 3a (3.8%), and 3b (1.9%). The results of the present study showed low levels of L. monocytogenes in the analyzed samples. A total of 61.5% of the 52 L. monocytogenes strains analyzed belonged to serotypes 1/2a, 1/2b, and 4b, namely the serovars that are most commonly involved in extraintestinal human listeriosis outbreaks. In the ready-to-eat samples, these three serotypes were 40.0% (1/2a), 17.1% (1/2b), and 14.3% (4b). This finding highlights the need to implement strict hygienic measures during the production, distribution, and sale of foods to reduce the risk of foodborne listeriosis in humans to an acceptable level.

Comparison of Listeria monocytogenes virulence in a mouse model

Listeriosis results from exposure to the foodborne pathogen Listeria monocytogenes. Although many different strains of L. monocytogenes are isolated from food, no definitive tests currently predict which isolates are most virulent. The objectives of this study were to address two major data gaps for risk assessors, variability among L. monocytogenes strains in pathogenicity and virulence. Strains used in our monkey clinical trial or additional food isolates were evaluated for their virulence and infectivity in mice. All strains were equally pathogenic to immunocompromised mice, causing deaths to 50% of the population 3 days after exposure to doses ranging from 2 to 3 log CFU. Doses resulting in 50% deaths on the fifth day after administration were 1 to 2 log lower than those on the third day, indicating that the full course of pathogenicity exceeds the 3-day endpoint in immunocompromised mice. Three strains were chosen for further testing for their virulence and infectivity in liver and spleen in normal (immunocompetent) mice. Virulence was not significantly different (P > 0.05) among the three strains, all resulting in deaths to 50% of mice at 5 to 7 log CFU by 5 days after administration. All strains were equally infective in liver or spleen, with higher numbers of L. monocytogenes directly correlated with higher doses of administration. In addition, there was no preference of organs by any strains. The lack of strain differences may reflect the limitation of the mouse model and suggests the importance of using various models to evaluate the pathogenicity and virulence of L. monocytogenes strains.

Effect of fat content on infection by Listeria monocytogenes in a mouse model

An estimated 2,500 cases of listeriosis occur annually in the United States. Listeriosis is particularly severe among pregnant women and immunocompromised individuals. Little is known regarding the effect of the food matrix on the ability of L. monocytogenes to survive in the gastrointestinal tract and cause systemic infection. Mice were inoculated with various doses of L. monocytogenes in skim milk, Half & Half, or whipping cream to determine whether differences in milk fat content influence the ability of L. monocytogenes to survive passage through the gut and infect the liver or spleen. The number of fecal samples positive for L. monocytogenes increased with increasing doses of L. monocytogenes for all three vehicles. The number of L. monocytogenes cells isolated from liver or spleen of mice dosed with L. monocytogenes was not significantly different among treatment vehicles. Dose-response models revealed that as the dosage of L. monocytogenes was increased in different milk vehicles, the number of L. monocytogenes cells in liver or spleen also increased. Although fat content of food had no dose-dependent effect on L. monocytogenes infection in the murine gastrointestinal tract, we cannot discount the possibility that it may be a factor in L. monocytogenes infections of humans because of differences in the physiology of gastrointestinal tracts of mice and humans.

Invasion of the placenta during murine listeriosis

Feto-placental infections due to Listeria monocytogenes represent a major threat during pregnancy, and the underlying mechanisms of placental invasion remain poorly understood. Here we used a murine model of listeriosis (pregnant mice, infected at day 14 of gestation) to investigate how this pathogen invades and grows within the placenta to ultimately infect the fetus. When L. monocytogenes is injected intravenously, the invasion of the placenta occurs early after the initial bacteremia, allowing the placental growth of the bacteria, which is an absolute requirement for vertical transmission to the fetus. Kinetically, bacteria first target the cells lining the central arterial canal of the placenta, which stain positively with cytokeratin, demonstrating their fetal trophoblast origin. Bacteria then disseminate rapidly to the other trophoblastic structures, like syncytiotrophoblast cells lining the villous core in the labyrinthine zone of placenta. Additionally, we found that an inflammatory reaction predominantly constituted of polymorphonuclear cells occurs in the villous placenta and participates in the control of infection. Altogether, our results suggest that the infection of murine placenta is dependent, at the early phase, on circulating bacteria and their interaction with endovascular trophoblastic cells. Subsequently, the bacteria spread to the other trophoblastic cells before crossing the placental barrier.

Nationwide outbreak of listeriosis due to contaminated meat

We used molecular subtyping to investigate an outbreak of listeriosis involving residents of 24 US states. We defined a case as infection with Listeria monocytogenes serotype 4b yielding one of several closely related patterns when subtyped by pulsed-field gel electrophoresis. Patients infected with strains yielding different patterns were used as controls. A total of 108 cases were identified with 14 associated deaths and four miscarriages or stillbirths. A case-control study implicated meat frankfurters as the likely source of infection (OR 17.3, 95% CI 2.4-160). The outbreak ended abruptly following a manufacturer-issued recall, and the outbreak strain was later detected in low levels in the recalled product. A second strain was recovered at higher levels but was not associated with human illness. Our findings suggest that L. monocytogenes strains vary widely in virulence and confirm that large outbreaks can occur even when only low levels of contamination are detected in sampled food. Standardized molecular subtyping and coordinated, multi-jurisdiction investigations can greatly facilitate detection and control of listeriosis outbreaks.

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.

Pathogenicity of food and clinical Listeria monocytogenes isolates in a mouse bioassay

Serotype distributions of Listeria monocytogenes in clinical samples and foods often differ. It is unknown whether such differences reflect a variation in the virulence of strains or are due to other factors that are not directly related to the strains' ability to cause illnesses. Fifty-two food and eight clinical isolates of L. monocytogenes were obtained from France, Japan, and the United States. Their pathogenicity in nonimmunocompromised female ICR mice was determined by intraperitoneal (i.p.) injection of the mice with test strains at 10(8) to 10(9) CFU per mouse. Five mice were injected with each Listeria strain and observed for 5 days. Listeria isolates that caused at least one death in 5 days were considered pathogenic. Isolates that caused no deaths in 5 days were considered nonpathogenic. All strains except Listeria innocua and one L. monocytogenes serotype 4b strain (RM3-1) isolated from bovine raw milk were pathogenic to nonimmunocompromised mice. Three food isolates of L. monocytogenes serotype 1/2c were weakly pathogenic to nonimmunocompromised mice, killing a maximum of 50% of mice at 10(8) CFU. Strains with no pathogenicity or reduced pathogenicity were further tested for their pathogenicity to immunocompromised mice. Each strain was inoculated i.p. into five mice at 10(3) to 10(10) CFU per mouse. No deaths of immunocompromised mice inoculated with 10(8) CFU were observed, but 20 to 40% of the mice died when inoculated with 10(9) CFU of L. monocytogenes RM3-1. The three L. monocytogenes serotype 1/2c isolates were also weakly pathogenic to immunocompromised mice, with two of the three isolates killing < or = 60% of mice at doses of < or = 10(8) CFU. The hemolytic activity of the three weakly pathogenic serotype 1/2c isolates was similar to that of pathogenic strains. However, the nonpathogenic strain RM3-1 was not found to be hemolytic on horse blood agar. We have identified several L. monocytogenes strains with reduced virulence levels. Further characterization of such isolates may aid in understanding factors affecting the variation in virulence among strains.

Influence of pregnancy on the pathogenesis of listeriosis in mice inoculated intragastrically

Pregnancy increases the risk of listeriosis, a systemic disease caused by Listeria monocytogenes. However, there is incomplete agreement on the reasons for this increased risk. We examined two features of listeriosis in gravid and nongravid female mice following intragastric (gavage) inoculation, namely, (i) disease severity (measured by lethality) and (ii) listerial infectivity (measured by liver and spleen colonization levels up to 120 h postinoculation). Two listerial strains of differing serotype (1/2a and 4nonb) were initially employed. Neither strain produced a lethal infection in nonpregnant female mice (dose range, 10(6) to 10(9) CFU/mouse), and only the 4nonb strain produced lethalities in pregnant mice (dose range, 10(6) to 10(8) CFU/mouse). The 4nonb strain also produced a higher level of liver and spleen colonization than the 1/2a strain following gavage administration. (The two strains showed similar levels of colonization if parenterally administered.) Both strains were equally capable of binding to and forming plaques upon cultured mouse enterocytes. The ability of the 4nonb strain to produce a lethal infection in pregnant animals did not correlate with an increased incidence or level of liver and spleen colonization over that in nonpregnant females. However, the lethality rate did correlate well with the rate at which embryos and their surrounding decidual covering became infected, suggesting that intrauterine infection could be responsible for the increased disease severity in the gravid females.

Sodium pentobarbital anesthesia transiently enhances the severity of infection following intragastric, but not intravenous, inoculation of Listeria monocytogenes in mice

In the present study we observed that mice anesthetized with sodium pentobarbital were far more susceptible to gastrointestinal challenge with Listeria monocytogenes than were unanaesthetized mice. The effect of pentobarbital anesthesia was transient (gone within 2 h) and did not alter the severity of infection following i.v. challenge with L. monocytogenes. Treatment with pharmacological inhibitors of gastric acidity (i.e. cimetidine and omeprazole), or intestinal motility (loperamide), did not duplicate the effect of pentobarbital on gastrointestinal listeriosis. These findings suggest that sodium pentobarbital anesthesia causes a short-lived but striking diminution in resistance to gastrointestinal listeriosis in mice, via an undefined mechanism.

Adhesion, invasion, and translocation characteristics of Listeria monocytogenes serotypes in Caco-2 cell and mouse models

Adhesion is a crucial first step in Listeria monocytogenes pathogenesis. In this study, we examined how the adhesion properties of serotypes correlate with their invasion efficiencies in a cell culture model (Caco-2) and in a mouse model. Adhesion characteristics of all 13 serotypes of L. monocytogenes (25 strains) were analyzed, which yielded three distinct groups (P < 0.05) with high-, medium-, and low-level-adhesion profiles. The efficiency of these strains in invading the Caco-2 cell line was analyzed, which produced two groups; however, the overall correlation (R(2)) was only 0.1236. In the mouse bioassay, all selected strains, irrespective of their adhesion profiles, translocated to the liver and the spleen with almost equal frequencies that did not show any clear relationship with adhesion profiles. However, the serotypes with increased adhesion showed a slightly increased translocation to the brain (R(2) = 0.3371). Collectively, these results indicate that an in vitro adhesion profile might not be an accurate assessment of a strain's ability to invade a cultured cell line or organs or tissues in a mouse model.

Expression of truncated Internalin A is involved in impaired internalization of some Listeria monocytogenes isolates carried asymptomatically by humans

Fourteen human carriage Listeria monocytogenes isolates were compared to sporadic and epidemic-associated human strains in order to ascertain the pathogenic behavior of these unrecognized asymptomatic strains. Experimental infection of 14-day-old chick embryos revealed that the majority of the carriage strains were attenuated for virulence. Of the 10 attenuated carriage strains, 5 were affected in their invasion capacities in vitro. Western blot analysis with antibody directed against InlA, the surface protein implicated in the internalization in host cells, allowed correlation between the ability of the carriage strains to enter Caco-2 cells and InlA expression. Indeed, these five carriage strains produced truncated forms of InlA. Four of the five truncated forms of InlA had an apparent molecular mass of 47 kDa. In order to assess the existence of a genetic lineage, partial sequences of inlA gene of these four strains were compared and revealed that they had a high degree of sequence conservation at the gene (99.86%) and amino acid (100%) levels. Comparison of their nucleotide sequences with that of the corresponding segment of inlA from EGD-e and Scott A strains, taken as epidemic references, showed more divergence. Taken together, these observations suggest the presence of specific traits that characterize L. monocytogenes strains isolated during asymptomatic carriage. Some of these traits could provide some explanations about the determinants that make them unable to cause systemic human infection.

A/J mice are susceptible and C57BL/6 mice are resistant to Listeria monocytogenes infection by intragastric inoculation

Previous studies demonstrated that the innate resistance of mice to Listeria monocytogenes infection by intravenous or intraperitoneal inoculation is regulated principally by the Hc locus on mouse chromosome 2. The A/J and C57BL/6 mouse strains were identified as prototype L. monocytogenes-susceptible and -resistant strains, respectively. In the present study, we compared the relative susceptibilities of A/J and C57BL/6 mice to intragastric (i.g.) inoculation with L. monocytogenes. The results of our study indicate that A/J mice are significantly more susceptible than C57BL/6 mice to an i.g. challenge with L. monocytogenes. This was reflected in the estimated 50% lethal doses for the two strains (10(6) and 10(8) CFU for A/J and C57BL/6 mice, respectively) and a more rapid and severe dissemination of the infection to the spleen and liver in A/J mice than in C57BL/6 mice. Histopathological examination of tissues from the infected mice confirmed the greater severity of disease in A/J mice. Clearance of a primary infection enhanced the resistance of both A/J and C57BL/6 mice to reinfection with L. monocytogenes via the gastrointestinal tract. However, the relative difference in susceptibility between the two strains was evident even after immunization. The A/J mouse holds promise as a model for investigating the pathogenesis of gastrointestinal listeriosis because of its ability to develop systemic infection following challenge with numbers of organisms similar to those recovered from some L. monocytogenes-contaminated food products.

Listeria monocytogenes virulence and pathogenicity, a food safety perspective

Several virulence factors of Listeria monocytogenes have been identified and extensively characterized at the molecular and cell biologic levels, including the hemolysin (listeriolysin O), two distinct phospholipases, a protein (ActA), several internalins, and others. Their study has yielded an impressive amount of information on the mechanisms employed by this facultative intracellular pathogen to interact with mammalian host cells, escape the host cell's killing mechanisms, and spread from one infected cell to others. In addition, several molecular subtyping tools have been developed to facilitate the detection of different strain types and lineages of the pathogen, including those implicated in common-source outbreaks of the disease. Despite these spectacular gains in knowledge, the virulence of L. monocytogenes as a foodborne pathogen remains poorly understood. The available pathogenesis and subtyping data generally fail to provide adequate insight about the virulence of field isolates and the likelihood that a given strain will cause illness. Possible mechanisms for the apparent prevalence of three serotypes (1/2a, 1/2b, and 4b) in human foodborne illness remain unidentified. The propensity of certain strain lineages (epidemic clones) to be implicated in common-source outbreaks and the prevalence of serotype 4b among epidemic-associated stains also remain poorly understood. This review first discusses current progress in understanding the general features of virulence and pathogenesis of L. monocytogenes. Emphasis is then placed on areas of special relevance to the organism's involvement in human foodborne illness, including (i) the relative prevalence of different serotypes and serotype-specific features and genetic markers; (ii) the ability of the organism to respond to environmental stresses of relevance to the food industry (cold, salt, iron depletion, and acid); (iii) the specific features of the major known epidemic-associated lineages; and (iv) the possible reservoirs of the organism in animals and the environment and the pronounced impact of environmental contamination in the food processing facilities. Finally, a discussion is provided on the perceived areas of special need for future research of relevance to food safety, including (i) theoretical modeling studies of niche complexity and contamination in the food processing facilities; (ii) strain databases for comprehensive molecular typing; and (iii) contributions from genomic and proteomic tools, including DNA microarrays for genotyping and expression signatures. Virulence-related genomic and proteomic signatures are expected to emerge from analysis of the genomes at the global level, with the support of adequate epidemiologic data and access to relevant strains.

Bactericidal effect of gentamicin-induced membrane vesicles derived from Pseudomonas aeruginosa PAO1 on gram-positive bacteria

Previous studies have shown that gentamicin-induced membrane vesicles (g-MVs) from Pseudomonas aeruginosa PAO1 possess both the antibiotic (gentamicin) and a potent peptidoglycan hydrolase (PGase; autolysin) that is effective in killing gram-negative pathogens. This present study evaluated the therapeutic potential of g-MVs against four gram-positive bacteria. Bactericidal assays and electron microscopy of thin sections revealed that Bacillus subtilis 168 and Staphylococcus aureus D2C were susceptible to killing mediated by g-MVs, Listeria monocytogenes ATCC 19113 was slightly susceptible, whereas Enterococcus hirae ATCC 9790 was unaffected. g-MVs were generally more effective against the bacteria than was soluble gentamicin, suggesting they could have more killing power than natural membrane vesicles containing no antibiotic. Electron microscopy and hydrophobic interaction chromatography showed that more membrane vesicles (MVs) initially attached to B. subtilis (hydrophilic) than to predominantly hydrophobic E. hirae, L. monocytogenes, and S. aureus. Zymograms containing murein sacculi as an enzyme substrate illustrated that all organisms except E. hirae were sensitive to the 26-kDa autolysin to varying degrees. Peptidoglycan O-acetylation did not influence susceptibility to MV-mediated lysis. Though not universally effective, the g-MV delivery system remains a promising therapeutic alternative for specific gram-positive infections.

Gene fragments distinguishing an epidemic-associated strain from a virulent prototype strain of Listeria monocytogenes belong to a distinct functional subset of genes and partially cross-hybridize with other Listeria species

Most major food-borne outbreaks of listeriosis in Europe and in the United States have been caused by genetically closely related Listeria monocytogenes strains of serotype 4b. In order to assess whether genomic loci exist that could underlie this increased epidemic potential, we subtracted the genome of the virulent prototype L. monocytogenes strain EGD from a prototype epidemic strain. A total of 39 DNA fragments corresponding to 20% of an estimated total of 150 to 190 kb of differential genome material were isolated. For 21 of these fragments, no function on the basis of homology could be predicted. Of the remaining 18 fragments, 15 had homologies to bacterial surface proteins, some of which have been implicated in virulence mechanisms such as cell invasion, adhesion, or immune escape. Southern hybridization of arrays containing the epidemic-clone-specific DNA segments with genomic DNA of different L. monocytogenes strains was consistent with the current lineage division. Surprisingly, however, some of the fragments hybridized in a mosaic-like fashion to genomes of two other Listeria species, the animal pathogen L. ivanovii and the nonpathogen L. innocua. Taken together, our results provide a starting point for the identification of epidemic-trait-associated genes.

Gut colonization of mice with actA-negative mutant of Listeria monocytogenes can stimulate a humoral mucosal immune response

We used Listeria monocytogenes, a gram-positive, facultative intracellular bacterium, to study the gut mucosal immune responses following oral infection. We employed a germfree (GF) mouse model to try to accentuate the development of a humoral mucosal immune response in the gut, and we used oral colonization with one of the mutants, actA-negative (DeltaactA) L. monocytogenes, to restrict infection largely to the gut. The DeltaactA mutant was able to colonize the intestinal mucosa of formerly GF mice for long periods of time without causing disease while eliciting secretory immunoglobulin A (IgA) responses, as evidenced by gut tissue fragment culture assays. Flow cytometric analyses and immunohistochemical methods showed the development of only minimal germinal center reactions (GCR) in Peyer's patches and more robust GCR in mesenteric lymph nodes. Pronounced increases in total (natural) IgA production occurred in gut tissues by day 7 and were maintained for up to 90 days. Levels of specific IgA were modest in gut tissues on day 14, increased until day 76, and stabilized at day 90. We also observed a significant rise in serum IgA and IgG1 levels following oral infection by listeriae. Upon colonization, the organisms mainly infected the intestines and intestinal lumen, and we only sporadically observed few colony-forming bacteria in the liver and spleen. We observed a marked rise in IgA-secreting cells, including listeria-specific IgA antibody-secreting cells, in the lamina propria of the small intestine by enzyme-linked immunospot assays. To ascertain whether some of the IgA was specific for listeriae, we performed Western blot analysis to test the reactivity of IgA from fragment cultures to antigens in sonicates of L. monocytogenes. We detected IgA binding to antigenic proteins with molecular masses of 96, 60, 40, and 14 kDa in the Listeria sonicates.

Genomic typing of Listeria monocytogenes strains by automated laser fluorescence analysis of amplified fragment length polymorphism fingerprint patterns

The genetic relationship between isolates of Listeria monocytogenes belonging to different serotypes was determined and the suitability of automated laser fluorescent analysis (ALFA) of amplified fragment length polymorphism (AFLP) fingerprints was assessed by genomic typing of 106 L. monocytogenes isolates belonging to serotypes 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4ab, 4b, 4c, 4d, 4e, 1, and 7. Digitised AFLP fingerprints were obtained that showed approximately 50 clearly distinguishable selectively amplified EcoRI/MseI bands for each strain. The coefficient of similarity between the profiles was determined by simple matching (Ssm). Based on these coefficients of similarity the investigated strains clustered in two genomic groups. The first group consisted of strains belonging to serotype 1/2a, 1/2c, 3a and 4a, while the second group was comprised of strains belonging to serotypes 1/2b, 3b, 4ab, 4b, 4e and 1. The average simple matching coefficient of similarity between strains of the second group was 92%, which was 4% higher than within group 1. Hence, the serotypes which are responsible for the majority of the listeriosis cases, 1/2a, 1/2b and 4b, fall into two distinct genetic groups, in concordance with their flagellar antigen type. The discriminatory power of AFLP in combination with automation of the analysis of the fingerprint profiles by ALFA makes AFLP-ALFA highly suitable for typing L. monocytogenes.

Docosahexaenoic acid, a constituent of fetal and neonatal serum, inhibits nitric oxide production by murine macrophages stimulated by IFN gamma plus LPS, or by IFN gamma plus Listeria monocytogenes

Murine macrophage activation is deficient in the fetus and the neonate, and in areas of the placenta perfused by the fetal circulation. Fetal and neonatal serum concentrations of docosahexaenoic acid (DHA) are 150 microM, or approximately 50-fold higher than in the adult. We previously showed that DHA inhibits activation of the gene for inducible nitric oxide synthase (iNOS) in murine macrophages stimulated in vitro with interferon gamma (IFN gamma) plus lipopolysaccharide (LPS). We have now pursued these observations in greater depth. An assay system was developed which separated the stimulation of macrophages by IFN gamma plus LPS, and the actual production of nitric oxide (NO). It was found that macrophages do not produce NO until they have been stimulated by IFN gamma plus LPS for a period of 10 h. NO is produced during the subsequent 10 h, even though IFN gamma plus LPS are not longer present. DHA, if present, inhibited only during the initial 10 h stimulation; DHA did not inhibit the production of NO by macrophages which had previously been stimulated by IFN gamma plus LPS, and were already producing NO. It was also found that DHA was less inhibitory if given prior to the IFN gamma plus LPS stimulation. In a dose-responsive manner, DHA inhibited the increased abundance of iNOS mRNA by macrophages stimulated by IFN gamma plus LPS. NO contributes to the host defense against Listeria monocytogenes and other intracellular pathogens. We therefore investigated the ability of DHA to inhibit NO production by macrophages stimulated by IFN gamma plus Listeria monocytogenes in vitro; DHA inhibited transcription of the iNOS gene and also the listeriocidal activity of activated macrophages. Inhibition of NO production by DHA may contribute to the increased susceptibility of the fetoplacental unit and neonate to intracellular infections.

Penetration of Listeria monocytogenes in mice infected by the oral route

In this study, it is suggested that the Peyer's patches are the most important point of entry of Listeria monocytogenes in the host after subclinical infection by the oral route. Microbiological, histopathological and ultrastructural evidence of infection was obtained in mice inoculated with a sublethal dose of 10(9) cfu. No mortality was observed. L. monocytogenes was isolated from the mesenteric lymph nodes from 6 hours post infection (hpi) through day 7 p.i. and from the liver and spleen from 24 h p.i. until days 5 and 7 p.i. respectively. Lesions were mainly restricted to the dome area of Peyer's patches and consisted of a purulent to pyogranulomatous inflammatory reaction. Scarce and minor lesions were also observed in the mesenteric lymph nodes and liver. L. monocytogenes was detected by immunohistochemistry in the Peyer's patches from 12 h p. i. to day 6 p.i. Ultrastructural study of Peyer's patches showed that the majority of Listeria cells were free within the cytoplasm of neutrophils and macrophages, not surrounded by a phagosomal membrane, and some of them were dividing.

Recombinant Listeria monocytogenes as a live vaccine vehicle and a probe for studying cell-mediated immunity

The ability of Listeria monocytogenes (L. monocytogenes) to enter the cytosol of host cells allows secreted proteins to efficiently enter the endogenous antigen-processing pathway leading to presentation by MHC class I molecules. L. monocytogenes has recently been exploited as a live vaccine vehicle for the induction of immunological memory against heterologous antigens. We have established a genetic system for site-specific integration of antigen expression cassettes into the Listeria genome which allows regulated expression and secretion of heterologous proteins. The ability of recombinant strains to stimulate long-term immunological memory and CD8+ T-cell-mediated protective immunity was investigated using the lymphocytic choriomeningitis virus (LCMV) murine infection model. Vaccination of mice with recombinant Listeria strains expressing LCMV antigens induced LCMV-specific CD8+ T cells which protected mice against LCMV challenge. We have also used a cottontail rabbit papillomavirus model to test the ability of recombinant Listeria strains to stimulate protective antitumor immunity in domestic rabbits. These studies have demonstrated the protective efficacy of recombinant L. monocytogenes vaccines and have established an experimental system for systematic analysis of cytotoxic T-cell induction by an intracellular bacterium.

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.

Primary Listeria monocytogenes infection in gestating mice

The facultative intracellular Gram-positive bacterium Listeria monocytogenes is a food-borne pathogen of frequently underestimated importance. Pregnant women represent the high-risk group for L. monocytogenes infection. Abortion, stillbirth or neonatal infection can be the serious outcome of such an infection. Recovery from listeriosis, resistance mechanisms of the host and the effect of L. monocytogenes on fetal development still remain to be fully understood. The results of our experiments showed an increased susceptibility of gestating BALB/c mice to primary L. monocytogenes infection. The duration of listeriosis in gestating animals was almost twice longer than in the control group. Furthermore, it was clearly shown that the detrimental effect of L. monocytogenes on fetal development was more pronounced if the infection was acquired earlier during gestation.

Treatment with the antigranulocyte monoclonal antibody RB6-8C5 impairs resistance of mice to gastrointestinal infection with Listeria monocytogenes

Treatment with the antigranulocyte monoclonal antibody (MAb) RB6-8C5 increased the severity of infection in mice intragastrically inoculated with Listeria monocytogenes. Most MAb RB6-8C5-treated mice died when inoculated intragastrically with as few as 4 x 10(4) L. monocytogenes bacteria, whereas most control mice survived intragastric inoculation with 4 x 10(8) L. monocytogenes bacteria. The increased severity of infection in MAb RB6-8C5-treated mice appeared to result from listerial multiplication in the spleen and liver rather than from local proliferation in the intestinal tract or mesenteric lymph nodes.

Listeriolysin O production and pathogenicity of non-growing Listeria monocytogenes stored at refrigeration temperature

Three haemolytic, pathogenic strains of Listeria monocytogenes (a reference strain NCTC 7973, a food-derived strain L70 and a human strain L94) and a control strain of Listeria innocua L27 were held in phosphate-buffered saline (PBS) of pH 7.0 or 5.5 at 4 degrees C for 4 weeks. The number of viable cells did not change significantly during this storage (the cells were non-growing). Titers of Listeria listeriolysin O (LLO) activity against washed human erythrocytes and the pathogenicity of non-growing bacterial cells for 14-day-old chick embryos were determined before storage and after 1, 2, 3 and 4 weeks of storage. Prolonged storage at 4 degrees C affected both LLO production and pathogenicity of the non-growing cells, but effects were strain- and pH-dependent. At pH 7.0, all three L. monocytogenes strains had lost LLO activity after 2 weeks of storage. At pH 5.5, the reference and the food strains lost LLO activity 1 week later than when stored at neutral pH, and the human strain maintained LLO activity throughout the 4-week period. Pathogenicity of the reference strain stored at pH 7.0 and 5.5 and that of the food strain stored at pH 7.0 decreased during storage at 4 degrees C. However, the human strain stored at pH 7.0 and 5.5, and the food strain stored at pH 5.5, maintained their pathogenicity throughout the 4-week period. In all cases, non-growing L. monocytogenes cells that had ceased LLO production and/or had a reduced pathogenicity, recovered these characteristics after growth in media at 37 degrees C. This study indicates that prolonged storage of chilled-foods in which L. monocytogenes is present, but not growing may have the desirable result that the L. monocytogenes has a reduced ability to cause illness in humans. As well, pathogenicity testing involving growth of L. monocytogenes in laboratory media may not reflect the actual pathogenicity of the organism in the food as eaten.

Experimental infection in mice with Bacillus licheniformis

The pathogenicity of Bacillus licheniformis was assessed in normal and immunodepressed BALB/c mice. The animals were challenged intravenously with 4 x 10(7) colony forming units of B. licheniformis (ATCC 14580) and both normal and immunodepressed mice were susceptible. However, the infection was more severe in the immunosuppressed animals. In normal mice, lesions were restricted to the liver and kidneys, while lesions also occurred in other organs of immunodepressed mice. By crossed immunoelectrophoresis it was shown that antigens of B. licheniformis are potent immunogens, and the bacteria could be identified in tissue sections by immunostaining. Immunohistochemically, B. licheniformis was demonstrated in hepatic and pulmonic macrophages, and from some animals the bacteria were also reisolated.