Definition of genetically distinct attenuation mechanisms in naturally virulence-attenuated Listeria monocytogenes by comparative cell culture and molecular characterization

Epidemiological study on Listeria monocytogenes in Egyptian dairy cattle farms' insights into genetic diversity of multi-antibiotic-resistant strains by ERIC-PCR

Listeria monocytogenes (L. monocytogenes) is frequently detected in ruminants, especially dairy cattle, and associated with the sporadic and epidemic outbreak of listeriosis in farms. In this epidemiological study, the prevalence, virulence, antibiotic resistance profiles, and genetic diversity of L. monocytogenes in three Egyptian dairy cattle farms were investigated. The risk factors associated with the fecal shedding of L. monocytogenes were analyzed. The L. monocytogenes strains from the three farms were categorized into distinct genotypes based on sampling site and sample type through enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC-PCR). A total of 1896 samples were collected from animals, environments, and milking equipment in the three farms. Results revealed that 137 (7.23%) of these samples were L. monocytogenes positive. The prevalence of L. monocytogenes in the animal samples was high (32.1%), and the main environmental source of prevalent genotypes in the three farms was silage. For all sample types, L. monocytogenes was more prevalent in farm I than in farms II and III. Risk factor analysis showed seasonal variation in production hygiene. For all sample types, L. monocytogenes was significantly more prevalent in winter than in spring and summer. The level of L. monocytogenes fecal shedding was high likely because of increasing age, number of parities, and milk yield in dairy cattle. Two virulence genes, namely, hlyA & prfA, were also detected in 93 strains, whereas only one of these genes was found in 44 residual strains. Conversely, iap was completely absent in all strains. The strains exhibited phenotypic resistance to most of the tested antibiotics, but none of them was resistant to netilmicin or vancomycin. According to sample type, the strains from the animal samples were extremely resistant to amoxicillin (95.2%, 80/84) and cloxacillin (92.9%, 78/84). By comparison, the strains from the environmental samples were highly resistant to cefotaxime (86.95%, 20/23). Furthermore, 25 multi-antibiotic resistance (MAR) patterns were observed in L. monocytogenes strains. All strains had a MAR index of 0.22-0.78 and harbored antibiotic resistance genes, including extended-spectrum β-lactamase (blaCTX-M [92.7%] and blaDHA-1 [66.4%]), quinolones (qnrS [91.2%], qnrA [58.4%], parC [58.4%], and qnrB [51%]), macrolides (erm[B] [76.6%], erm(C) [1.5%], and msr(A) [27%]), trimethoprim (dfrD [65.7%]), and tetracyclines (tet(M) [41.6%], tet(S) [8%], and int-Tn [26.3%]). ERIC-PCR confirmed that the strains were genetically diverse and heterogeneous. A total of 137 isolated L. monocytogenes strains were classified into 22 distinct ERIC-PCR groups (A-V). Among them, ERIC E (10.2%) was the most prevalent group. These results indicated that environment and milking equipment served as reservoirs and potential transmission ways of virulent and multidrug-resistant L. monocytogenes to dairy animals, consequently posing threats to public health. Silage is the main environmental source of prevalent genotypes on all three farms. Therefore, hygienic measures at the farm level should be developed and implemented to reduce L. monocytogenes transmission inside dairy cattle farms.

Nonhemolytic Listeria monocytogenes-Prevalence Rate, Reasons Underlying Atypical Phenotype, and Methods for Accurate Hemolysis Assessment

Listeria monocytogenes is a foodborne pathogen that typically presents β-hemolytic activity. However, there are literature reports indicating that L. monocytogenes strains are sometimes nonhemolytic or their zones of hemolysis are perceivable only after removal of the colonies from the agar plate. Nonhemolytic L. monocytogenes are most commonly encountered in food products, but some have also been detected in clinical samples. Usually, atypical bacteria of this species belong to serotype 1/2a. Mutations of the prfA gene sequence are the most common reason for changed phenotype, and mutations of the hly gene are the second most common cause. There are also reports that the methodology used for detecting hemolysis may influence the results. Sheep or horse blood, although most commonly used in modern studies, may not allow for the production of clear hemolytic zones on blood agar, whereas other types of blood (guinea pig, rabbit, piglet, and human) are more suitable according to some studies. Furthermore, the standard blood agar plate technique is less sensitive than its modifications such as bilayer or top-layer (overlay) techniques. The microplate technique (employing erythrocyte suspensions) is probably the most informative when assessing listerial hemolysis and is the least susceptible to subjective interpretation.

Virulence Characterization of Listeria monocytogenes, Listeria innocua, and Listeria welshimeri Isolated from Fish and Shrimp Using In Vivo Early Zebrafish Larvae Models and Molecular Study

Listeriosis is one of the most notable foodborne diseases and is characterized by high rates of mortality. L. monocytogenes is the main cause of human listeriosis outbreaks, however, there are isolated cases of disease caused by other species of the genus Listeria. The aim of this study was to evaluate strains of L. monocytogenes (n = 7), L. innocua (n = 6), and L. welshimeri (n = 2) isolated from fish and shrimps for their virulence based on the presence of virulence genes and the in vivo Danio rerio (zebrafish) larvae models. A total of 15 strains were analyzed. The zebrafish larvae model showed that the larvae injected with L. monocytogenes strains were characterized by the lowest survival rate (46.5%), followed by L. innocua strains (64.2%) and L. welshimeri (83.0%) strains. Multiplex PCRs were used for detection of selected virulence genes (luxS, actA2, prfA, inlB, rrn, iap, sigB, plcB, actA, hlyA), the majority of which were present in L. monocytogenes. Only a few virulence-related genes were found in L. welshimeri, however, no correlation between the occurrence of these genes and larval survival was confirmed. This research highlights the importance of the potential impact that Listeria spp. strains isolated from fish and shrimps may have on consumers.

Two Prevalent Listeria ivanovii subsp. ivanovii Clonal Strains With Different Virulence Exist in Wild Rodents and Pikas of China

Listeria ivanovii subsp. ivanovii is an intracellular bacterium distributed widely in nature, causing the listeriosis in ruminants and humans. Previous researches had isolated 116 strains of L. ivanovii subsp. ivanovii from wild rodents and pikas of different regions in China, and the predominant sequence types were ST1 and ST2. In this study, we first investigated the biological characteristics and virulence of these two clonal strains including motility, metabolism and virulence in cells and mouse model. The results demonstrated the ST1 strains exhibited motility, wide metabolic activity and hypervirulence, whereas the ST2 strains showed non-motility, relative lower metabolic activity and virulence. Considering the transmissible ability from wild rodents and pikas to ecological environment, the L. ivanovii subsp. ivanovii with potential pathogenicity to humans and ruminants should be monitored.

High resolution melting analysis for the characterization of lineage II Listeria monocytogenes serovars 1/2a and 1/2c based on single nucleotide polymorphisms identification within the Listeria Pathogenicity Island-1 and inlAB operon: a novel approach for epidemiological surveillance

AIMS

A high resolution melting (HRM) assay was developed for characterizing lineage II Listeria monocytogenes based on the amplification and the melting profiles analysis of 81 fragments targeting the region from the prs to ldh loci, including the Listeria Pathogenicity Island-1 (LIPI-1) genes and the inlAB operon.

METHODS AND RESULTS

Real-time PCR and HRM protocols were standardized using 10 replicate assays from L. monocytogenes EGD-e reference strain (serovar 1/2a). Twenty wild-type isolates of serovar 1/2a and two of serovar 1/2c were tested, and differences between EGD-e strain and the wild-type isolates were defined if the melting temperature (Tm ) of an amplicon was not within the lower and the upper limits calculated from replicate testing on EGD-e. The analysis revealed 17 and 19 HRM profiles with respect to prs/LIPI-1/ldh and inlAB target regions (Simpson's Index of Diversity 0·979 and 0·983) respectively. The 1/2c cultures showed 98·1% similarity to melting characteristics with EGD-e, whilst 1/2a isolates had the greatest heterogeneity that was related to inlA, inlB and actA genes. Sequencing of amplicons generating different Tm values from EGD-e confirmed the presence of point mutations.

CONCLUSIONS

This method was useful for L. monocytogenes subtyping based on single nucleotide polymorphisms detection through the melting behaviour analysis of main virulence genes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study underlines the effectiveness of HRM in differentiating L. monocytogenes strains with high discriminatory power, thus rendering it useful for epidemiological surveillance.

Virulence Gene Sequencing Highlights Similarities and Differences in Sequences in Listeria monocytogenes Serotype 1/2a and 4b Strains of Clinical and Food Origin From 3 Different Geographic Locations

The prfA-virulence gene cluster (pVGC) is the main pathogenicity island in Listeria monocytogenes, comprising the prfA, plcA, hly, mpl, actA, and plcB genes. In this study, the pVGC of 36 L. monocytogenes isolates with respect to different serotypes (1/2a or 4b), geographical origin (Australia, Greece or Ireland) and isolation source (food-associated or clinical) was characterized. The most conserved genes were prfA and hly, with the lowest nucleotide diversity (π) among all genes (P < 0.05), and the lowest number of alleles, substitutions and non-synonymous substitutions for prfA. Conversely, the most diverse gene was actA, which presented the highest number of alleles (n = 20) and showed the highest nucleotide diversity. Grouping by serotype had a significantly lower π value (P < 0.0001) compared to isolation source or geographical origin, suggesting a distinct and well-defined unit compared to other groupings. Among all tested genes, only hly and mpl were those with lower nucleotide diversity in 1/2a serotype than 4b serotype, reflecting a high within-1/2a serotype divergence compared to 4b serotype. Geographical divergence was noted with respect to the hly gene, where serotype 4b Irish strains were distinct from Greek and Australian strains. Australian strains showed less diversity in plcB and mpl relative to Irish or Greek strains. Notable differences regarding sequence mutations were identified between food-associated and clinical isolates in prfA, actA, and plcB sequences. Overall, these results indicate that virulence genes follow different evolutionary pathways, which are affected by a strain's origin and serotype and may influence virulence and/or epidemiological dominance of certain subgroups.

Comparison of listeriolysin O and phospholipases PlcA and PlcB activities, and initial intracellular growth capability among food and clinical strains of Listeria monocytogenes

AIMS

We investigated whether Listeria monocytogenes strains differ in their ability to escape from the primary phagosome after internalization into human intestinal epithelial cells.

METHODS AND RESULTS

Food and clinical strains were used to study specific alleles; the activities of listeriolysin O (LLO) and phospholipases PlcA and PlcB, which promote rupture of the phagocytic vacuole; and initial intracellular bacterial growth in Caco-2 cells. Results showed no difference in LLO activities between food and clinical strains or among serotypes. In contrast, the LLO truncation mutant lacked detectable haemolytic activity and intracellular growth. PlcA and PlcB produced by the strains of serotypes 4b/4e and 1/2b exhibited significantly lower activities than those of serotypes 1/2a and 1/2c. In contrast, the strains of serotype 1/2b grew significantly faster than those of serotypes 4b/4e and 1/2a. Moreover, the PrfA truncation mutants lacked LLO and phospholipases activities and did not show intracellular growth.

CONCLUSIONS

We determined that LLO and PrfA mutants exert a significant effect on intracellular growth, although it was unclear from this study whether PlcA and PlcB alleles affect escape from vacuoles.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study estimates that low-virulence L. monocytogenes strains associated with escape ability from the primary vacuoles are not widely distributed among food strains.

Foodborne Listeria monocytogenes: A Real Challenge in Quality Control

Listeria monocytogenes is a foodborne pathogen, and the detection and differentiation of this bacterium from the nonpathogenic Listeria species are of great importance to the food industry. Differentiation of Listeria species is very difficult, even with the sophisticated MALDI-TOF MS technique because of the close genetic relationship of the species and the usual gene transfer. The present paper emphasizes the difficulties of the differentiation through the standardized detection and confirmation according to ISO 11290-1:1996 and basic available L. monocytogenes detection methods and tests (such as API Listeria test, MALDI-TOF MS analysis, and hly gene PCR). With the increase of reports on the pathogenesis of atypical Listeria strains in humans, the significance of species level determination has become questionable, especially in food quality control, and the detection of pathogenic characteristics seems to be more relevant.

Listeria monocytogenes isolated from food samples from a Romanian black market show distinct virulence profiles

Listeria monocytogenes is a facultative intracellular foodborne pathogen responsible for listeriosis. In a recent study, in which we investigated neglected exogenous routes of transmission of foodborne pathogens into the European Union, we have isolated 15 L. monocytogenes strains in food products, which were imported from the Republic of Moldavia to Romania and illegally sold at a local market. The aim of this study was to characterize the subtype and virulence potential of these 15 L. monocytogenes strains. Multilocus sequence typing revealed that these L. monocytogenes strains belong to six different sequence types (ST2, ST8, ST9, ST20, ST121 and ST155). In addition, in vitro virulence assays using human intestinal epithelial Caco2 and macrophage-like THP1 cells showed a high strain variability regarding the invasion efficiency in Caco2 cells (0.98-2.78%) and the intracellular growth rate in both cell types. Both ST121 strains and the ST9 isolate were unable to invade Caco2 cells, and all ST155 strains showed no proliferation inside macrophages and revealed low cytotoxicity. Furthermore we performed sequence analysis of three main virulence factors: PrfA, internalin A (InlA) and listeriolysin O (LLO). The Romanian food isolates showed a high diversity in the InlA and LLO amino acid sequences, whereas the amino acid sequence of PrfA of all strains was identical. Overall, the amino acid sequences of PrfA, InlA and LLO were identical for strains belonging to the same ST. We detected in total 30 different amino acid substitutions, resulting in seven different InlA variants, two of which have not yet been described. The three strains, which were unable to invade Caco2 cells, harboured a premature stop codon resulting in truncated InlA. Furthermore, we detected four different amino acid substitutions in the LLO sequence, which are present in four variants. The number of LLO mutations correlates negatively with intracellular growth in Caco2 and THP1 cells and subsequently with cytotoxicity. In conclusion, we show that L. monocytogenes isolated from food samples from a Romanian black market show distinct virulence profiles, due to a high diversity in the amino acid sequence of main virulence factors.

Is the exoproteome important for bacterial pathogenesis? Lessons learned from interstrain exoprotein diversity in Listeria monocytogenes grown at different temperatures

Bacterial exoproteomes vary in composition and quantity among species and within each species, depending on the environmental conditions to which the cells are exposed. This article critically reviews the literature available on exoproteins synthesized by the foodborne pathogenic bacterium Listeria monocytogenes grown at different temperatures. The main challenges posed for exoproteome analyses and the strategies that are being used to overcome these constraints are discussed. Over thirty exoproteins from L. monocytogenes are considered, and the multifunctionality of some of them is discussed. Thus, at the host temperature of 37°C, good examples are provided by Lmo0443, a potential marker for low virulence, and by the virulence factors internalin C (InlC) and listeriolysin O (LLO). Based on the reported LLO-induced mucin exocytosis, a model is proposed for the involvement of extracellular LLO in optimizing the conditions for InlC intervention in the invasion of intestinal epithelial cells. At lower growth temperatures, exoproteins such as flagellin (FlaA) and oligopeptide permease (OppA) may explain the persistence of particular strains in the food industry environment, eventually allowing the development of new tools to eradicate L. monocytogenes, a major concern for public health.

A novel autoantibody against moesin in the serum of patients with MPO-ANCA-associated vasculitis

BACKGROUND

Antineutrophil cytoplasmic autoantibody (ANCA) directed against myeloperoxidase (MPO), a diagnostic criterion in MPO-ANCA-associated vasculitis (MPO-AAV), does not always correlate with disease activity. Here, we detected autoantibodies against moesin, which was located on the surface of stimulated endothelial cells, in the serum of patients.

METHODS

The anti-moesin autoantibody titer was evaluated by ELISA. Seventeen kinds of cytokines/chemokines were measured by a Bio-Plex system.

RESULTS

Serum creatinine in the anti-moesin autoantibody-positive group was higher than that in the negative group. Additionally, interferon (IFN)-γ, macrophage chemotactic peptide-1 (MCP-1), interleukin (IL)-2, IL-7, IL-12p70, IL-13, granulocyte/macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor were significantly higher in the positive group. Furthermore, IL-7 and IL-12p70 levels correlated with the anti-moesin autoantibody titer. Based on these findings and the binding of anti-moesin IgG to neutrophils and monocytes, we detected the secretion of cytokines/chemokines such as IFN-γ, MCP-1 and GM-CSF from these cells.

CONCLUSIONS

The anti-moesin autoantibody existed in the serum of patients with MPO-AAV and was associated with the production of inflammatory cytokines/chemokines targeting neutrophils with a cytoplasmic profile, which suggests that the anti-moesin autoantibody has the possibility to be a novel autoantibody developing vasculitis via neutrophil and endothelial cell activation.

σ(B) plays a limited role in the ability of Listeria monocytogenes strain F2365 to survive oxidative and acid stress and in its virulence characteristics

Listeria monocytogenes strain F2365 was the first strain representative of serotype 4b (lineage I) to be sequenced in 2004, suggesting it could become the model organism for this serotype, which is associated with most human outbreaks of listeriosis worldwide to date. F2365 itself is an outbreak strain that was involved in the listeriosis outbreak associated with Mexican-style soft cheese in California in 1985. In this study, we show through phenotypic and transcriptomic analysis that L. monocytogenes strain F2365 has reduced ability to respond to acid and oxidative stress. F2365 has neither the σ(B)-dependent ability to survive acid or oxidative stress nor the σ(B)-dependent ability to infect Caco-2 epithelial cells in vitro or guinea pigs in vivo. More studies are needed to determine whether the atypical σ(B)-independent response to stress observed in F2365 is strain specific, serotype specific, or even lineage specific.

Protective immunity induced by a LLO-deficient Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen able to cause serious disease in human and animals. Listeriolysin O (LLO), a major virulence factor secreted by this bacterium, is a vacuole-specific lysin that facilitates bacterial entrance into the host cytosol. Thus, LLO plays a key role in the translocation and intracellular spread of L. monocytogenes. To study the effect of LLO on virulence and immunopotency, a LLO-deficient L. monocytogenes mutant was constructed using a shuttle vector followed by homologous recombination. The mutant strain had lost hemolytic activity, which resulted in an extremely reduced virulence, 5 logs lower than that of the parent strain, yzuLM4, in BALB/c mice. The number of bacteria detected in the spleens and livers of mice infected with the mutant was greatly reduced, and the bacteria were rapidly eliminated by the host. Kinetics studies in this murine model of infection showed that the invasion ability of the mutant strain was much lower than that of the parent strain. Moreover, immunization with the mutant strain conferred protective immunity against listerial infection. In particular, stimulation with Ag85B240-259, strong specific Th1 type cellular immunity was elicited by vaccination C57BL/6 mice with hly deficient strain delivering Mycobacterium tuberculosis fusion antigen Ag85B-ESAT-6 via intravenous inoculation. These results clearly show that highly attenuated LLO-deficient L. monocytogenes is an attractive vaccine carrier for delivering heterologous antigens.

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.

Development and implementation of a multiplex single-nucleotide polymorphism genotyping assay for detection of virulence-attenuating mutations in the Listeria monocytogenes virulence-associated gene inlA

The virulence factor internalin A (InlA) facilitates the uptake of Listeria monocytogenes by epithelial cells that express the human isoform of E-cadherin. Previous studies identified naturally occurring premature stop codon (PMSC) mutations in inlA and demonstrated that these mutations are responsible for virulence attenuation. We assembled >1,700 L. monocytogenes isolates from diverse sources representing 90 EcoRI ribotypes. A subset of this isolate collection was selected based on ribotype frequency and characterized by a Caco-2 cell invasion assay. The sequencing of inlA genes from isolates with attenuated invasion capacities revealed three novel inlA PMSCs which had not been identified previously among U.S. isolates. Since ribotypes include isolates with and without inlA PMSCs, we developed a multiplex single-nucleotide polymorphism (SNP) genotyping assay to detect isolates with virulence-attenuating PMSC mutations in inlA. The SNP genotyping assay detects all inlA PMSC mutations that have been reported worldwide and verified in this study to date by the extension of unlabeled primers with fluorescently labeled dideoxynucleoside triphosphates. We implemented the SNP genotyping assay to characterize human clinical and food isolates representing common ribotypes associated with novel inlA PMSC mutations. PMSCs in inlA were significantly (ribotypes DUP-1039C and DUP-1045B; P < 0.001) or marginally (ribotype DUP-1062D; P = 0.11) more common among food isolates than human clinical isolates. SNP genotyping revealed a fourth novel PMSC mutation among U.S. L. monocytogenes isolates, which was observed previously among isolates from France and Portugal. This SNP genotyping assay may be implemented by regulatory agencies and the food industry to differentiate L. monocytogenes isolates carrying virulence-attenuating PMSC mutations in inlA from strains representing the most significant health risk.

inlA premature stop codons are common among Listeria monocytogenes isolates from foods and yield virulence-attenuated strains that confer protection against fully virulent strains

Previous studies showed that a considerable proportion of Listeria monocytogenes isolates obtained from foods carry a premature stop codon (PMSC) mutation in inlA that leads to production of a truncated and secreted InlA. To further elucidate the role these mutations play in virulence of L. monocytogenes, we created isogenic mutants, including (i) natural isolates where an inlA PMSC was reverted to a wild-type inlA allele (without a PMSC) and (ii) natural isolates where a PMSC mutation was introduced into a wild-type inlA allele; isogenic mutant sets were constructed to represent two distinct inlA PMSC mutations. Phenotypical and transcriptional analysis data showed that inlA PMSC mutations do not have a polar effect on the downstream inlB. Isogenic and natural strains carrying an inlA PMSC showed significantly reduced invasion efficiencies in Caco-2 and HepG2 cell lines as well as reduced virulence in oral guinea pig infections. Guinea pigs were also orally infected with a natural strain carrying the most common inlA PMSC mutation (vaccinated group), followed by challenge with a fully virulent L. monocytogenes strain 15 days postvaccination to probe potentially immunizing effects of exposure to L. monocytogenes with inlA PMSC mutations. Vaccinated guinea pigs showed reduced bacterial loads in internal organs and improved weight gain postchallenge, indicating reduced severity of infections in guinea pigs exposed to natural strains with inlA PMSC mutations. Our data support that (i) inlA PMSC mutations are causally associated with attenuated virulence in mammalian hosts and (ii) naturally occurring virulence-attenuated L. monocytogenes strains commonly found in food confer protective immunity.

Virulence of Listeria monocytogenes isolated from the cheese dairy environment, other foods and clinical cases

The virulence potential of 51 Listeria monocytogenes isolates, including strains from cheese, cheese production environments and from human cases of listeriosis, was evaluated in this study. The isolates were used to infect HT-29 cell monolayers in an in vitro test of virulence, based on a plaque-forming assay (PFA). Fifteen selected isolates were used for subcutaneous footpad inoculation in mice and subsequent recovery of the bacterium from the spleen 3 days after inoculation. In the PFA, two isolates from milk (serovar 1/2a) were not significantly different (P<0.05) from the low-virulence strain (442) used as reference. Thirty-three isolates were not significantly different (P<0.05) from the virulent strain (EGDe) used as reference. Nine isolates were significantly more virulent (highly virulent) than the EGDe strain and seven isolates were significantly less virulent. The nine highly virulent isolates were either from humans (four), from cheese dairy environments (two isolates of a strain were found persistently in two dairies), from cheese (one), from milk (one) and the reference strain for serovar 1/2b (CECT 936). The two milk isolates with low virulence in the PFA were found to be virulent in mice. In conclusion, all the isolates from food and food-related environments were potentially virulent or highly virulent. These results stress the risk of listeriosis associated with the consumption of cheese contaminated with L. monocytogenes, and once more emphasize the importance of good manufacturing practices (GMPs) together with sanitation standard operating procedures (SSOPs) throughout the food chain.

Toward an improved laboratory definition of Listeria monocytogenes virulence

Listeria monocytogenes is an opportunistic foodborne pathogen that encompasses a diversity of strains with varied virulence. The ability to rapidly determine the pathogenic potential of L. monocytogenes strains is integral to the control and prevention campaign against listeriosis. Early methods for assessing L. monocytogenes virulence include in vivo bioassays and in vitro cell assays. While in vivo bioassays provide a measurement of all virulence determinants of L. monocytogenes, they are not applied routinely due to their reliance on experimental animals whose costs have become increasingly prohibitive. As a low cost alternative, in vitro cell assays are useful for estimating the virulence of L. monocytogenes strains. However, these assays are often slow, and at times variable. Prior attempts to ascertain L. monocytogenes virulence by targeting virulence-associated proteins and genes have been largely unsuccessful, since many of the assay targets are present in both virulent and avirulent strains. Recent identification of novel virulence-specific genes (particularly internalin gene inlJ) has opened a new avenue for rapid, sensitive, and precise differentiation of virulent L. monocytogenes strains from avirulent strains. The application of DNA sequencing technique also offers an additional tool for assessing L. monocytogenes virulence potential. By providing an update on the laboratory methods that have been reported for the determination of L. monocytogenes pathogenicity, this review discusses future research needs that may help achieve an improved laboratory definition of L. monocytogenes virulence.

Nonsense-mutated inlA and prfA not widely distributed in Listeria monocytogenes isolates from ready-to-eat seafood products in Japan

InlA is a surface protein participating in the entry of Listeria monocytogenes into mammalian non-phagocytic cells. PrfA is a positive regulatory factor that regulates the expression of a set of virulence genes. Recent studies revealed that some L. monocytogenes strains have a truncated form of these proteins because of nonsense mutations in their sequences, and these truncations contribute to the significant reduction in virulence of this pathogen. In this study, sequence analyses of inlA and prfA among L. monocytogenes isolated from ready-to-eat seafood revealed that only one out of 59 isolates had a nonsense-mutated inlA and all had non-mutated prfA. This indicated that these strains could be fully virulent based on the sizes of these proteins.

Listeria monocytogenes F2365 carries several authentic mutations potentially leading to truncated gene products, including inlB, and demonstrates atypical phenotypic characteristics

Searches of the genome annotation of Listeria monocytogenes F2365, an isolate from the 1985 listeriosis epidemic in California, showed that this strain carries 20 authentic mutations resulting in premature stop codons, including a nonsense mutation in inlB. Here we showed that L. monocytogenes F2365 demonstrates atypical virulence-associated characteristics, including significantly (P < 0.05) reduced invasion efficiency in Caco-2 cells as compared with a closely related lineage I serotype 4b strain as well as significantly (P < 0.05) greater variation in invasiveness when grown under different conditions compared with standard laboratory control and other lineage I serotype 4b strains. In addition, L. monocytogenes F2365 demonstrated distinct growth characteristics, including a significantly (P < 0.05) reduced exponential growth rate when compared with laboratory control and other lineage I serotype 4b outbreak-associated strains as well as a significantly (P < 0.05) longer lag phase duration time compared with another lineage I serotype 4b strain. Our results support that L. monocytogenes F2365 is characterized by genotypic and phenotypic properties that are atypical of other L. monocytogenes strains.

SigmaB activation under environmental and energy stress conditions in Listeria monocytogenes

To measure sigmaB activation in Listeria monocytogenes under environmental or energy stress conditions, quantitative reverse transcriptase PCR (TaqMan) was used to determine the levels of transcripts for the sigmaB -dependent opuCA and clpC genes in strains having null mutations in genes encoding regulator of sigma B proteins (rsbT and rsbV) and sigma B (sigB) and in the L. monocytogenes wild-type 10403S strain under different stress conditions. The DeltasigB, DeltarsbT, and DeltarsbV strains previously exhibited increased hemolytic activities compared to the hemolytic activity of the wild-type strain; therefore, transcript levels for hly were also determined. RsbT, RsbV, and sigmaB were all required for opuCA expression during growth under carbon-limiting conditions or following exposure to pH 4.5, salt, ethanol, or the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP). Expression of clpC was RsbT, RsbV, and sigmaB dependent in the presence of CCCP but not under the other conditions. hly expression was not RsbT, RsbV, or sigmaB dependent in the presence of either CCCP or salt. opuCA transcript levels did not increase in the presence of rapidly lethal stresses (i.e., pH 2.5 or 13 mM cumene hydroperoxide) despite the enhanced survival of the wild type compared with the survival of the mutant strains under these conditions. These findings highlight the importance of complementing phenotypic characterizations with gene expression studies to identify direct and indirect effects of null mutations in regulatory genes, such as sigB. Overall, our data show that while sigmaB activation occurs through a single pathway under both environmental and energy stress conditions, regulation of expression of some stress response and virulence genes in the sigmaB regulon (e.g., clpC) appears to require networks involving multiple transcriptional regulators.

Modification of a virulence-associated phenotype after growth of Listeria monocytogenes on food

AIM

To assess the effect of different foods, which have been implicated or not in cases of listeriosis, on the in vitro virulence-associated phenotype level of different Listeria monocytogenes strains.

METHODS AND RESULTS

The virulence-associated phenotype level of L. monocytogenes was studied with the in vitro cell test based on a plaque-forming assay with a human adenocarcinoma cell line (HT-29) monolayer. Three strains of L. monocytogenes were grown in preparations (homogenate, 1-mum filtrate or 0.2-mum filtrate) of different food extracts ['rillettes' (potted minced pork), milk, raw salmon and cold-smoked salmon] or in a control medium, brain heart infusion (BHI). The bacterial suspensions grown in food extracts or in BHI at 37 degrees C were diluted with their growth medium (food extract or BHI) or with minimum essential medium before seeding on confluent HT-29 cell monolayers. Filtration of food extracts had no significant effect on the plaque numbers formed by the bacteria. A significant decrease in the plaque numbers was noted for the three strains when they grew in the rillettes extracts, compared with the other food extracts and BHI. The levels of in vitro virulence-associated phenotype of the strains after growth in the rillettes extract were similar to or lower than that of the hypovirulent internal reference strain L. monocytogenes 442. After growth in milk and cold-smoked salmon, the impact on virulence-associated phenotype depended on the strain. In contrast, plaque-forming assay indicated increased virulence-associated phenotype when the strains were switched from a nutrient-rich medium (food extract or BHI) to a minimum essential medium.

CONCLUSIONS

In vitro virulence-associated phenotype level of the studied strains grown in BHI or cold-smoked salmon was the same as the control virulent strain EGD. In contrast, the nutrients present in rillettes may therefore substantially reduce the number of plaques but not the growth of L. monocytogenes. The utilization of minimum essential medium as diluent attenuates changes the effect of the food extract on virulence-associated phenotype in vitro.

SIGNIFICANCE AND IMPACT OF THE STUDY

In the experimental design of this study, we showed that the nature of the food could affect the in vitro virulence-associated phenotype level of L. monocytogenes.

Identification, subtyping and virulence determination of Listeria monocytogenes, an important foodborne pathogen

Listeria monocytogenes is an opportunistic intracellular pathogen that has become an important cause of human foodborne infections worldwide. Given its close relationship to other Listeria species and its tendency to produce non-specific clinical symptoms, the availability of rapid, sensitive and specific diagnostic tests for the differentiation of L. monocytogenes from other Listeria species is helpful for selecting appropriate treatment regimens. In addition, with L. monocytogenes comprising a diversity of strains of varying pathogenicity, the ability to precisely track the strains involved in listeriosis outbreaks and speedily determine their pathogenic potential is critical for the control and prevention of further occurrences of this deadly disease. Extensive research in recent decades has revealed significant insights regarding the molecular mechanisms of L. monocytogenes infection. This in turn has facilitated the development of laboratory procedures for enhanced detection and identification of L. monocytogenes, and has also contributed to the implementation of improved control and prevention strategies against listeriosis. The purpose of this review is to summarize recent progress in the species-specific identification, subtyping and virulence determination of L. monocytogenes strains, and to discuss future research needs pertaining to these important areas of listeriosis.

Discovery of natural atypical nonhemolytic Listeria seeligeri isolates

We found seven Listeria isolates, initially identified as isolates with the Xyl(+) Rha(-) biotype of Listeria welshimeri by phenotypic tests, which exhibited discrepant genotypic properties in a well-validated Listeria species identification oligonucleotide microarray. The microarray gives results of these seven isolates being atypical hly-negative L. seeligeri isolates, not L. welshimeri isolates. The aberrant L. seeligeri isolates were d-xylose fermentation positive, l-rhamnose fermentation negative (Xyl(+) Rha(-)), and nonhemolytic on blood agar and in the CAMP test with both Staphylococcus aureus (S(-) reaction) and Rhodococcus equi (R(-) reaction). All genes of the prfA cluster of L. seeligeri, located in the prs-ldh region, including the orfA2, orfD, prfA, orfE, plcA, hly, orfK, mpl, actA, dplcB, plcB, orfH, orfX, orfI, orfP, orfB, and orfA genes, were checked by PCR and direct sequencing for evidence of their presence in the atypical isolates. The prs-prfA cluster-ldh region of the L. seeligeri isolates was approximately threefold shorter due to the loss of orfD, prfA, orfE, plcA, hly, orfK, mpl, actA, dplcB, plcB, orfH, orfX, and orfI. The genetic map order of the cluster genes of all the atypical L. seeligeri isolates was prs-orfA2-orfP-orfB-orfA-ldh, which was comparable to the similar region in L. welshimeri, with the exception of the presence of orfA2. DNA sequencing and phylogenetic analysis of 17 housekeeping genes indicated an L. seeligeri genomic background in all seven of the atypical hly-negative L. seeligeri isolates. Thus, the novel biotype of Xyl(+) Rha(-) Hly(-) L. seeligeri strains can only be distinguished from Xyl(+) Rha(-) L. welshimeri strains genotypically, not phenotypically. In contrast, the Rha(+) Xyl(+) biotype of L. welshimeri would not present an identification issue.

Invasiveness is a variable and heterogeneous phenotype in Listeria monocytogenes serotype strains

The ability of Listeria monocytogenes to breach mucosal and endothelial barriers of the host during infection is a hallmark property mediated by the internalins (Inl) A and B. We examined the invasive property of several L. monocytogenes strains representing 13 serotypes. We found that invasiveness is a heterogeneous phenotype amongst L. monocytogenes serotype strains. Despite this, many of the poorly invasive and non-invasive strains of L. monocytogenes express internalins at levels comparable to those of invasive isolates. Introduction of the inlAB locus from EGD-e into several poorly invasive strains had no effect on their invasive properties. A strain from serotype 4b that exhibits highly invasive properties was further examined. Deletion of the inlAB locus abrogated invasion of this strain while reintroduction of the inlAB locus into this strain restored invasiveness. An analysis of regions flanking the inlAB locus revealed considerable differences in the strains studied. Our results suggest that efficacious entry of L. monocytogenes into eukaryotic cells is complex and requires additional factors apart from internalins. Data presented here also suggest that the inlAB locus was introduced into L. monocytogenes by horizontal gene transfer with subsequent deletion and rearrangements occurring during evolution of this species.

Allelic exchange and site-directed mutagenesis probe the contribution of ActA amino-acid variability to phosphorylation and virulence-associated phenotypes among Listeria monocytogenes strains

To test the hypothesis that actA allelic variation contributes to virulence differences among Listeria monocytogenes strains, cell-to-cell spread and intracellular ActA phosphorylation patterns were characterized for 14 wild-type isolates and selected isogenic mutants. Our data show that (i) while actA allelic variation is not responsible for enhanced cell-to-cell spread observed in epidemic clone I strains, actA allelic variation may contribute to reduced plaque size observed in some isolates, (ii) actA sequence alone determines phosphorylation-dependent ActA banding patterns, and (iii) sequence variation at the positively selected ActA residue 498 does not contribute to ActA phosphorylation patterns or to differences in cell-to-cell spread.