A spontaneous genomic deletion inListeria ivanoviiidentifies LIPI-2, a species-specific pathogenicity island encoding sphingomyelinase and numerous internalins

Reducing the degree of crosslinking of peptidoglycan in Listeria monocytogenes promoted the secretion of membrane vesicles

Listeria monocytogenes (LM) is a Gram-positive (G+) bacterium that secretes nanoscale membrane vesicles (MVs). LM MVs comprise various bacterial components and may have potential as an antigen or drug-delivery vehicle; however, the low yield of the LM MVs limits related research. G+-bacterial MVs germinate from the bacterial plasma membrane and must pass through a thick crosslinked peptidoglycan layer for release. Herein, we aimed to increase the release of MVs by reducing the degree of crosslinking of peptidoglycan. We knocked out two genes related to the longitudinal crosslinking of peptidoglycan, dal and dat, and supplemented the knocked-out dal gene through plasmid expression to obtain a stably inherited recombinant strain LMΔdd::pCW633. The structure, particle size, and main protein components of MVs secreted by this recombinant strain were consistent with those secreted from the wild strain, but the yield of MVs was considerably increased (p < 0.05). Furthermore, Listeria ivanovii (LI) was found to secrete MVs that differed in the composition of the main proteins compared with those of LM MVs. The abovementioned method was also feasible for promoting the secretion of MVs from the attenuated LM strain and LI wild-type and attenuated strains. Our study provides a new method to increase the secretion of MVs derived from Listeria that could be extended to other G+ bacteria.

High density genomic surveillance and risk profiling of clinical Listeria monocytogenes subtypes in Germany

BACKGROUND

Foodborne infections such as listeriosis caused by the bacterium Listeria monocytogenes represent a significant public health concern, particularly when outbreaks affect many individuals over prolonged time. Systematic collection of pathogen isolates from infected patients, whole genome sequencing (WGS) and phylogenetic analyses allow recognition and termination of outbreaks after source identification and risk profiling of abundant lineages.

METHODS

We here present a multi-dimensional analysis of > 1800 genome sequences from clinical L. monocytogenes isolates collected in Germany between 2018 and 2021. Different WGS-based subtyping methods were used to determine the population structure with its main phylogenetic sublineages as well as for identification of disease clusters. Clinical frequencies of materno-foetal and brain infections and in vitro infection experiments were used for risk profiling of the most abundant sublineages. These sublineages and large disease clusters were further characterised in terms of their genetic and epidemiological properties.

RESULTS

The collected isolates covered 62% of all notified cases and belonged to 188 infection clusters. Forty-two percent of these clusters were active for > 12 months, 60% generated cases cross-regionally, including 11 multinational clusters. Thirty-seven percent of the clusters were caused by sequence type (ST) ST6, ST8 and ST1 clones. ST1 was identified as hyper- and ST8, ST14, ST29 as well as ST155 as hypovirulent, while ST6 had average virulence potential. Inactivating mutations were found in several virulence and house-keeping genes, particularly in hypovirulent STs.

CONCLUSIONS

Our work presents an in-depth analysis of the genomic characteristics of L. monocytogenes isolates that cause disease in Germany. It supports prioritisation of disease clusters for epidemiological investigations and reinforces the need to analyse the mechanisms underlying hyper- and hypovirulence.

Prevalence, Genotypic Characteristics, and Antibiotic Resistance of Listeria monocytogenes From Retail Foods in Huzhou, China

Listeria monocytogenes are considered to be the major foodborne pathogen worldwide. To understand the prevalence and potential risk of L. monocytogenes in retail foods, a total of 1243 retail foods in 12 food categories were sampled and screened for L. monocytogenes from 2020 to 2022 in Huzhou, China. A total of 46 out of 1234 samples were confirmed to be L. monocytogenes positive with a total rate of 3.7%. The contamination rate of seasoned raw meat (15.2%) was the highest, followed by raw poultry meat and raw livestock meat (9.9%) and salmon sashimi (9.5%). The L. monocytogenes isolates belonged to four serotypes, 1/2a,1/2b, 1/2c, and 4b, with the most prevalent serotype being 1/2a (47.9%). All isolates were grouped into 15 sequence types (STs) belonging to 14 clonal complexes (CCs) via multilocus sequence typing (MLST). The most prevalent ST was ST9/CC9 (23.9%), followed by ST3/CC3 (19.6%) and ST121/CC121 (17.4%). Notably, 11 STs were detected from ready-to-eat (RTE) foods, some of them have been verified to be strongly associated with clinical origin listeriosis cases, such as ST3, ST2, ST5, ST8, and ST87. Listeria pathogenicity islands 1 (LIPI-1) and LIPI-2 were detected in approximately all L. monocytogenes isolates, whereas the distribution of both LIPI-3 genes and LIPI-4 genes exhibited association with specific ST, with LIPI-3 in ST3 and ST288, and LIPI-4 in ST87. The strains carrying LIPI-3 and LIPI-4 virulence genes in this study were all isolated from RTE foods. Antimicrobial susceptibility tests showed that >90% of isolates were susceptible to PEN, AMP, ERY, CIP, SXT, VAN, CHL, and GEN, indicating the antibiotic treatment might be still efficient for most of the L. monocytogenes strains. However, for the three clinical first-line antibiotics (PEN, AMP, and GEN), we also observed three and four strains showing MIC values greater than the susceptibility standards for PEN and AMP, respectively, and one strain showing resistance to GEN.

Human Listeriosis

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen that can cause severe invasive infections upon ingestion with contaminated food. Clinically, listerial disease, or listeriosis, most often presents as bacteremia, meningitis or meningoencephalitis, and pregnancy-associated infections manifesting as miscarriage or neonatal sepsis. Invasive listeriosis is life-threatening and a main cause of foodborne illness leading to hospital admissions in Western countries. Sources of contamination can be identified through international surveillance systems for foodborne bacteria and strains' genetic data sharing. Large-scale whole genome studies have increased our knowledge on the diversity and evolution of L. monocytogenes, while recent pathophysiological investigations have improved our mechanistic understanding of listeriosis. In this article, we present an overview of human listeriosis with particular focus on relevant features of the causative bacterium, epidemiology, risk groups, pathogenesis, clinical manifestations, and treatment and prevention.

Combination of growth conditions and InlB-specific dot-immunoassay for rapid detection of Listeria monocytogenes in raw milk

The gram-positive bacterium Listeria monocytogenes is an important foodborne pathogen contaminating dairy products. Closely related to L. monocytogenes saprophytic Listeria spp. are also frequent contaminators of food and, particularly, dairy products. To distinguish L. monocytogenes from nonpathogenic Listeria spp. and other bacteria, a dot-immunoassay was developed. The immunoassay is based on the polyclonal antibody to the secreted form of the surface virulence-associated L. monocytogenes-specific InlB protein. To increase InlB production, bacteria were grown on the brain-heart infusion agar supplemented with 0.2% activated charcoal (BHIC agar). Direct plating of artificially contaminated raw milk samples on the BHIC agar followed by the dot-immunoassay allowed a rapid identification of L. monocytogenes in concentrations as little as 10 cfu/mL. Using the developed approach, preliminary results were obtained within 14 h, and the final results were obtained after 26 h. The dot-immunoassay was tested on L. monocytogenes strains belonging to different clonal complexes and phylogenetic lineages, Listeria spp., and other bacterial species. Results showed the exceptional specificity of the developed dot-immunoassay for the rapid identification of L. monocytogenes.

A Comprehensive Virulence and Resistance Characteristics of Listeria monocytogenes Isolated from Fish and the Fish Industry Environment

Listeria monocytogenes is an important pathogen, often associated with fish, that can adapt and survive in products and food processing plants, where it can persist for many years. It is a species characterized by diverse genotypic and phenotypic characteristics. Therefore, in this study, a total of 17 L. monocytogenes strains from fish and fish-processing environments in Poland were characterized for their relatedness, virulence profiles, and resistance genes. The Core Genome Multilocus Sequence Typing (cgMLST) analysis revealed that the most frequent serogroups were IIa and IIb; sequence types (ST) were ST6 and ST121; and clonal complexes (CC) were CC6 and CC121. Core genome multilocus sequence typing (cgMLST) analysis was applied to compare the present isolates with the publicly available genomes of L. monocytogenes strains recovered in Europe from humans with listeriosis. Despite differential genotypic subtypes, most strains had similar antimicrobial resistance profiles; however, some of genes were located on mobile genetic elements that could be transferred to commensal or pathogenic bacteria. The results of this study showed that molecular clones of tested strains were characteristic for L. monocytogenes isolated from similar sources. Nevertheless, it is worth emphasizing that they could present a major public health risk due to their close relation with strains isolated from human listeriosis.

Wholegenome sequencing as the gold standard approach for control of Listeria monocytogenes in the food chain

Listeria monocytogenes has been implicated in numerous outbreaks and related deaths of listeriosis. In food production, L. monocytogenes occurs in raw food material and above all, through postprocessing contamination. The use of next-generation sequencing technologies such as whole-genome sequencing (WGS) facilitates foodborne outbreak investigations, pathogen source tracking and tracing geographic distributions of different clonal complexes, routine microbiological/epidemiological surveillance of listeriosis, and quantitative microbial risk assessment. WGS can also be used to predict various genetic traits related to virulence, stress, or antimicrobial resistance, which can be of great benefit for improving food safety management as well as public health.

A Listeria ivanovii balanced-lethal system may be a promising antigen carrier for vaccine construction

Expressing heterologous antigens by plasmids may cause antibiotic resistance. Additionally, antigen expression via plasmids is unstable due to the loss of the plasmid. Here, we developed a balanced‐lethal system. The Listeria monocytogenes (LM) balanced‐lethal system has been previously used as an antigen carrier to induce cellular immune response. However, thus far, there has been no reports on Listeria ivanovii (LI) balanced‐lethal systems. The dal and dat genes from the LI‐attenuated LIΔatcAplcB (LIΔ) were deleted consecutively, resulting in a nutrient‐deficient LIΔdd strain. Subsequently, an antibiotic resistance‐free plasmid carrying the LM dal gene was transformed into the nutrient‐deficient strain to generate the LI balanced‐lethal system LIΔdd:dal. The resultant bacterial strain retains the ability to proliferate in phagocytic cells, as well as the ability to adhere and invade hepatocytes. Its genetic composition was stable, and compared to the parent strain, the balanced‐lethal system was substantially attenuated. In addition, LIΔdd:dal induced specific CD4⁺/CD8⁺ T‐cell responses and protected mice against LIΔ challenge. Similarly, we constructed an LM balanced‐lethal system LMΔdd:dal. Sequential immunization with different recombinant Listeria strains will significantly enhance the immunotherapeutic effect. Thus, LIΔdd:dal combined with LMΔdd:dal, or with other balanced‐lethal systems will be more promising alternative for vaccine development.

Virulence Characteristics and Distribution of the Pathogen Listeria ivanovii in the Environment and in Food

Listeria ivanovii and L. monocytogenes, are the only pathogenic species of the genus Listeria and share many virulence factors and mechanisms of pathogenicity. L. ivanovii shows host tropism towards small ruminants and rodents and much lower virulence for humans compared to L. monocytogenes. However, severe infections caused by L. ivanovii, resulting in bacteremia, abortion and stillbirth, occasionally occurred in immunocompromised persons and in pregnant women, while in immunocompetent hosts L. ivanovii can cause gastroenteritis. In this review, the updated knowledge on virulence aspects and distribution of L. ivanovii in the environment and in food is summarized. Recent research on its virulence characters at genome level gave indications on how pathogenicity evolved in this bacterial species. As for L. monocytogenes, L. ivanovii infections occurred after the ingestion of contaminated food, so an overview of reports regarding its distribution in food products was carried out to obtain indications on the categories of foods exposed to contamination by L. ivanovii. It was found that a wide variety of food products can be a source of this microorganism and that, like L. monocytogenes, L. ivanovii is able to persist in the food production environment. Studies on its ability to grow in enrichment and isolation media suggested that its occurrence in nature might be underestimated. Moreover, virulence varies among strains for differences in virulence character regulation, presence/absence of genetic regions and the possible instability of a Listeria pathogenicity genomic island, LIPI-2, which is unique to L. ivanovii. We can conclude that L. ivanovii, as a possible pathogen for animals and humans, requires more focused investigations regarding its occurrence in the environment and in food and on intra-species variability of pathogenic potential.

Recombinant Listeria ivanovii strain expressing listeriolysin O in place of ivanolysin O might be a potential antigen carrier for vaccine construction

Listeria monocytogenes (LM) induces efficient and specific T-cell immune responses in the host. Listeriolysin O (LLO) is the main virulence protein of LM. LLO helps LM escape from the lysosome. However, the pronounced pathogenicity of LM limits its practical application as a live bacterial vector. Listeria ivanovii (LI) also displays intracellular parasitic abilities, cell to cell transfer, and other LM properties, with an elevated biosafety relative to LM. We have confirmed that LI can be used as a viable bacterial vaccine vector. However, we have also observed in vivo that LI vector vaccine candidates survive in the immune organ (spleen) for a shorter time compared with the survival time of LM and elicit weaker immune responses compared with LM. Studies have confirmed that hemolysin correlates with some important biological properties of Listeria, including cell invasion, intracellular proliferation, and the ability to induce immune responses. We speculated that the weaker immunogenicity of LI compared to LM may be related to the function of ivanolysin O (ILO). Here, we established a hemolysin gene deletion strain, LIΔilo, and a modified strain, LIΔilo:hly, whose ilo was replaced by hly. The hemolysin-modified strain was attenuated; however, it led to significantly improved invasive and proliferative activities of antigen-presenting cells, including those of RAW 264.7 macrophages, compared with the effects of LI. Mice immunized twice with LIΔilo:hly showed higher cytokine levels and better challenge protection rates than LI-immunized mice. This is the first description in Listeria carrier vaccine research of the modification of LI hemolysin to obtain a better vaccine carrier than LI. The recombinant strain LIΔilo:hly showed good biosafety and immunogenicity, and thus appears to be a good vector strain for vaccine development.

Landscape of Stress Response and Virulence Genes Among Listeria monocytogenes Strains

The pathogenic microorganism Listeria monocytogenes is ubiquitous and responsible for listeriosis, a disease with a high mortality rate in susceptible people. It can persist in different habitats, including the farm environment, the food production environments, and in foods. This pathogen can grow under challenging conditions, such as low pH, low temperatures, and high salt concentrations. However, L. monocytogenes has a high degree of strain divergence regarding virulence potential, environmental adaption, and stress response. This review seeks to provide the reader with an up-to-date overview of clonal and serotype-specific differences among L. monocytogenes strains. Emphasis on the genes and genomic islands responsible for virulence and resistance to environmental stresses is given to explain the complex adaptation among L. monocytogenes strains. Moreover, we highlight the use of advanced diagnostic technologies, such as whole-genome sequencing, to fine-tune quantitative microbiological risk assessment for better control of listeriosis.

Application of Whole Genome Sequencing to Aid in Deciphering the Persistence Potential of Listeria monocytogenes in Food Production Environments

Listeria monocytogenes is the etiological agent of listeriosis, a foodborne illness associated with high hospitalizations and mortality rates. This bacterium can persist in food associated environments for years with isolates being increasingly linked to outbreaks. This review presents a discussion of genomes of Listeria monocytogenes which are commonly regarded as persisters within food production environments, as well as genes which are involved in mechanisms aiding this phenotype. Although criteria for the detection of persistence remain undefined, the advent of whole genome sequencing (WGS) and the development of bioinformatic tools have revolutionized the ability to find closely related strains. These advancements will facilitate the identification of mechanisms responsible for persistence among indistinguishable genomes. In turn, this will lead to improved assessments of the importance of biofilm formation, adaptation to stressful conditions and tolerance to sterilizers in relation to the persistence of this bacterium, all of which have been previously associated with this phenotype. Despite much research being published around the topic of persistence, more insights are required to further elucidate the nature of true persistence and its implications for public health.

Combination immunotherapy with two attenuated Listeria strains carrying shuffled HPV-16 E6E7 protein causes tumor regression in a mouse tumor model

Cervical cancer continues to impose a heavy burden worldwide, and human papilloma virus (HPV) infection, especially persistent infection with type 16 (HPV-16), is known to be the primary etiological factor. Therapeutic vaccines are urgently needed because prophylactic vaccines are ineffective at clearing pre-existing HPV infection. Here, two recombinant Listeria strains (LMΔ-E6E7 & LIΔ-E6E7) with deletions of the actA and plcB genes, expressing the shuffled HPV-16 E6E7 protein were constructed. The strains were delivered into the spleen and liver by intravenous inoculation, induced antigen-specific cellular immunity and were eliminated completely from the internal organs several days later. Intravenously treating with single strain for three times, or with both strains alternately for three times significantly reduced the tumor size and prolonged the survival time of model mice. Combination immunotherapy with two strains seemed more effective than immunotherapy with single strain in that it enhanced the survival of the mice, and the LMΔ-E6E7-prime-LIΔ-E6E7-boost strategy showed significant stronger efficacy than single treatment with the LIΔ-E6E7 strain. The antitumor effect of this treatment might due to its ability to increase the proportion of CD8⁺ T cells and reduce the proportion of T regulatory cells (Tregs) in the intratumoral milieu. This is the first report regarding Listeria ivanovii-based therapeutic vaccine candidate against cervical cancer. Most importantly we are the first to confirm that combination therapy with two different recombinant Listeria strains has a more satisfactory antitumor effect than administration of a single strain. Thus, we propose a novel prime-boost treatment strategy.

Retrospective Use of Whole-Genome Sequencing Expands the Multicountry Outbreak Cluster of Listeria monocytogenes ST1247

Listeria monocytogenes sequence type 1247 clonal complex 8 caused a prolonged multicountry outbreak in five EU countries: Denmark, Estonia, Finland, France, and Sweden. A total of 22 disease cases were identified with onset of symptoms between July 2014 and February 2019. Five patients died due to, or with, the disease. The retrospective analysis of L. monocytogenes isolate VLTRLM2013 revealed the presence of an outbreak-related strain (cgMLST type L2-SL8-ST1247-CT4158) in ready-to-eat fish product more than a year prior to the first outbreak-related cases. Reference outbreak strain and VLTRLM2013 strain were compared using core genome and whole-genome multilocus sequence typing analyses. Genomic level differences of the persistent L. monocytogenes strains associated with a prolonged multicountry foodborne listeriosis outbreak are described. It was concluded that the persistent nature of the multicountry outbreak-related L. monocytogenes strain VLTRLM2013 together with stress island, virulence, and antibiotic resistance genes could potentially be the determining factors for the extensive and prolonged outbreak affecting five European Union countries. Our results support the systematic application of whole-genome sequencing in food and public health surveillance and further encourages its wide adoption.

Making Sense of the Biodiversity and Virulence of Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen responsible for listeriosis, an infection that can manifest in humans as bacteremia, meningoencephalitis in immunocompromised patients and the elderly, and fetal-placental infection in pregnant women. Reference strains from this facultative intracellular bacterium have been instrumental in the investigation of basic mechanisms in microbiology, immunology, and cell biology. The integration of bacterial population genomics with environmental, epidemiological, and clinical data allowed the uncovering of new factors involved in the virulence of L. monocytogenes and its adaptation to different environments. This review illustrates how these investigations have led to a better understanding of the bacterium's virulence and the driving forces that shaped it.

Unraveling the emergence and population diversity of Listeria monocytogenes in a newly built meat facility through whole genome sequencing

The food processing environments of a newly opened meat processing facility were sampled in ten visits carried out during its first 1.5 years of activity and analyzed for the presence of Listeria monocytogenes. A total of 18 L. monocytogenes isolates were obtained from 229 samples, and their genomes were sequenced to perform comparative genomic analyses. An increase in the frequency of isolation of L. monocytogenes and in the diversity of sequence types (STs) detected was observed along time. Although the strains isolated belonged to six different STs (ST8, ST9, ST14, ST37, ST121 and ST155), ST9 was the most abundant (8 out of 18 strains). Low (0 and 2) single nucleotide polymorphism (SNP) distances were found between two pairs of ST9 strains isolated in both cases 3 months apart from the same processing room (Lm-1267 and Lm-1705, with a 2 SNPs distance in the core genome; Lm-1265 and Lm-1706, with a 0 SNPs distance), which suggests that these strains may be persistent L. monocytogenes strains in the food processing environment. Most strains showed an in silico attenuated virulence potential either through the truncation of InlA (in 67% of the isolates) or the absence of other virulence factors involved in cell adhesion or invasion. Twelve of the eighteen L. monocytogenes isolates contained a plasmid, which ranged in size from 4 to 87 Kb and harbored stress survival, in addition to heavy metals and biocides resistance determinants. Identical or highly similar plasmids were identified for various sets of L. monocytogenes ST9 isolates, which suggests the clonal expansion and persistence of plasmid-containing ST9 strains in the processing environments of the meat facility. Finally, the analysis of the L. monocytogenes genomes available in the NCBI database, and their associated metadata, evidenced that strains from ST9 are more frequently reported in Europe, linked to foods, particularly to meat and pork products, and less represented among clinical isolates than other L. monocytogenes STs. It also showed that the ST9 strains here isolated were more closely related to the European isolates, which clustered together and separated from ST9 North American isolates.

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.

A hybrid sub-lineage of Listeria monocytogenes comprising hypervirulent isolates

The foodborne pathogen Listeria monocytogenes (Lm) is a highly heterogeneous species and currently comprises of 4 evolutionarily distinct lineages. Here, we characterize isolates from severe ovine listeriosis outbreaks that represent a hybrid sub-lineage of the major lineage II (HSL-II) and serotype 4h. HSL-II isolates are highly virulent and exhibit higher organ colonization capacities than well-characterized hypervirulent strains of Lm in an orogastric mouse infection model. The isolates harbour both the Lm Pathogenicity Island (LIPI)-1 and a truncated LIPI-2 locus, encoding sphingomyelinase (SmcL), a virulence factor required for invasion and bacterial translocation from the gut, and other non-contiguous chromosomal segments from another pathogenic species, L. ivanovii. HSL-II isolates exhibit a unique wall teichoic acid (WTA) structure essential for resistance to antimicrobial peptides, bacterial invasion and virulence. The discovery of isolates harbouring pan-species virulence genes of the genus Listeria warrants global efforts to identify further hypervirulent lineages of Lm.

Listeria monocytogenes contamination of ready-to-eat foods and the risk for human health in the EU

Food safety criteria for Listeria monocytogenes in ready-to-eat (RTE) foods have been applied from 2006 onwards (Commission Regulation (EC) 2073/2005). Still, human invasive listeriosis was reported to increase over the period 2009-2013 in the European Union and European Economic Area (EU/EEA). Time series analysis for the 2008-2015 period in the EU/EEA indicated an increasing trend of the monthly notified incidence rate of confirmed human invasive listeriosis of the over 75 age groups and female age group between 25 and 44 years old (probably related to pregnancies). A conceptual model was used to identify factors in the food chain as potential drivers for L. monocytogenes contamination of RTE foods and listeriosis. Factors were related to the host (i. population size of the elderly and/or susceptible people; ii. underlying condition rate), the food (iii. L. monocytogenes prevalence in RTE food at retail; iv. L. monocytogenes concentration in RTE food at retail; v. storage conditions after retail; vi. consumption), the national surveillance systems (vii. improved surveillance), and/or the bacterium (viii. virulence). Factors considered likely to be responsible for the increasing trend in cases are the increased population size of the elderly and susceptible population except for the 25-44 female age group. For the increased incidence rates and cases, the likely factor is the increased proportion of susceptible persons in the age groups over 45 years old for both genders. Quantitative modelling suggests that more than 90% of invasive listeriosis is caused by ingestion of RTE food containing > 2,000 colony forming units (CFU)/g, and that one-third of cases are due to growth in the consumer phase. Awareness should be increased among stakeholders, especially in relation to susceptible risk groups. Innovative methodologies including whole genome sequencing (WGS) for strain identification and monitoring of trends are recommended.

Whole genome sequencing analyses of Listeria monocytogenes that persisted in a milkshake machine for a year and caused illnesses in Washington State

Background

In 2015, in addition to a United States multistate outbreak linked to contaminated ice cream, another outbreak linked to ice cream was reported in the Pacific Northwest of the United States. It was a hospital-acquired outbreak linked to milkshakes, made from contaminated ice cream mixes and milkshake maker, served to patients. Here we performed multiple analyses on isolates associated with this outbreak: pulsed-field gel electrophoresis (PFGE), whole genome single nucleotide polymorphism (SNP) analysis, species-specific core genome multilocus sequence typing (cgMLST), lineage-specific cgMLST and whole genome-specific MLST (wgsMLST)/outbreak-specific cgMLST. We also analyzed the prophages and virulence genes.

Results

The outbreak isolates belonged to sequence type 1038, clonal complex 101, genetic lineage II. There were no pre-mature stop codons in inlA. Isolates contained Listeria Pathogenicity Island 1 and multiple internalins. PFGE and multiple whole genome sequencing (WGS) analyses all clustered together food, environmental and clinical isolates when compared to outgroup from the same clonal complex, which supported the finding that L. monocytogenes likely persisted in the soft serve ice cream/milkshake maker from November 2014 to November 2015 and caused 3 illnesses, and that the outbreak strain was transmitted between two ice cream production facilities. The whole genome SNP analysis, one of the two species-specific cgMLST, the lineage II-specific cgMLST and the wgsMLST/outbreak-specific cgMLST showed that L. monocytogenes cells persistent in the milkshake maker for a year formed a unique clade inside the outbreak cluster. This clustering was consistent with the cleaning practice after the outbreak was initially recognized in late 2014 and early 2015. Putative prophages were conserved among prophage-containing isolates. The loss of a putative prophage in two isolates resulted in the loss of the AscI restriction site in the prophage, which contributed to their AscI-PFGE banding pattern differences from other isolates.

Conclusions

The high resolution of WGS analyses allowed the differentiation of epidemiologically unrelated isolates, as well as the elucidation of the microevolution and persistence of isolates within the scope of one outbreak. We applied a wgsMLST scheme which is essentially the outbreak-specific cgMLST. This scheme can be combined with lineage-specific cgMLST and species-specific cgMLST to maximize the resolution of WGS.

Comparative experimental infection of Listeria monocytogenes and Listeria ivanovii in bovine trophoblasts

Listeria monocytogenes is a Gram-positive, facultative intracellular and invasive bacterium that has tropism to the placenta, and causes fetal morbidity and mortality in several mammalian species. While infection with L. monocytogenes and L. ivanovii are known as important causes of abortion and reproductive failure in cattle, the pathogenesis of maternal-fetal listeriosis in this species is poorly known. This study used the bovine chorioallantoic membrane explant model to investigate the kinetics of L. monocytogenes, L. ivanovii, and L. innocua infections in bovine trophoblastic cells for up to 8 h post infection. L. monocytogenes and L. ivanovii were able to invade and multiply in trophoblastic cells without causing cell death or inducing expression of pro-inflammatory genes. Although L. innocua was unable to multiply in bovine trophoblastic cells, it induced transcription of the pro-inflammatory mediator CXCL6. This study demonstrated for the first time the susceptibility of bovine trophoblastic cells to L. monocytogenes and L. ivanovii infection.

Listeria ivanovii Infection in Mice: Restricted to the Liver and Lung with Limited Replication in the Spleen

Listeria monocytogenes (LM) vectors have shown much promise in delivery of viral and tumor antigens for the development of vaccines. L. ivanovii (LI) is a closely related bacterium with a similar intracellular life cycle that may offer advantages over LM because it is not a human pathogen, but can infect other animal species. Recent studies show that recombinant LI expressing Mycobacterium tuberculosis antigens is effective in inducing protective immunity in mouse models, demonstrating the potential of LI as a live vaccine vector. However, a key barrier in the development of LI into a live vaccine vector is that its pathogenic and immunogenic characteristics have yet to be fully understood. Therefore, in this research, C57BL/6J mice were inoculated with LM or LI intravenously or intranasally, and bacterial loads, histopathologic changes, and cytokine production were determined at indicated days post inoculation. Results showed that after intravenous infection with LM or LI, bacteria were found proliferating in the liver, spleen, and lung. However, LI could only reach a heavy burden in the liver and its ability to multiply and to resist host immunity seemed limited in the spleen and lung. After intranasal inoculation with LI, bacteria were mainly localized in the lung and failed to infect liver or spleen, while LM could. In organs with heavy LI burden, lesions were isolated, localized and densely packed, compared to lesions caused by LM, which were invasive. In the liver of intravenously inoculated mice and lung of intranasally inoculate mice, LI was able to elicit comparable cytokine production with LM and cause less severe histopathologic damages, and thus could be considered as a vector for treating or preventing hepatic or pulmonary diseases.

Comparative Genomics of Listeria Sensu Lato: Genus-Wide Differences in Evolutionary Dynamics and the Progressive Gain of Complex, Potentially Pathogenicity-Related Traits through Lateral Gene Transfer

Historically, genome-wide and molecular characterization of the genus Listeria has concentrated on the important human pathogen Listeria monocytogenes and a small number of closely related species, together termed Listeria sensu strictu. More recently, a number of genome sequences for more basal, and nonpathogenic, members of the Listeria genus have become available, facilitating a wider perspective on the evolution of pathogenicity and genome level evolutionary dynamics within the entire genus (termed Listeria sensu lato). Here, we have sequenced the genomes of additional Listeria fleischmannii and Listeria newyorkensis isolates and explored the dynamics of genome evolution in Listeria sensu lato. Our analyses suggest that acquisition of genetic material through gene duplication and divergence as well as through lateral gene transfer (mostly from outside Listeria) is widespread throughout the genus. Novel genetic material is apparently subject to rapid turnover. Multiple lines of evidence point to significant differences in evolutionary dynamics between the most basal Listeria subclade and all other congeners, including both sensu strictu and other sensu lato isolates. Strikingly, these differences are likely attributable to stochastic, population-level processes and contribute to observed variation in genome size across the genus. Notably, our analyses indicate that the common ancestor of Listeria sensu lato lacked flagella, which were acquired by lateral gene transfer by a common ancestor of Listeria grayi and Listeria sensu strictu, whereas a recently functionally characterized pathogenicity island, responsible for the capacity to produce cobalamin and utilize ethanolamine/propane-2-diol, was acquired in an ancestor of Listeria sensu strictu.

Rapid and sensitive detection of Listeria ivanovii by loop-mediated isothermal amplification of the smcL gene

A loop-mediated isothermal amplification (LAMP) assay for rapid and sensitive detection of the L. ivanovii strains had been developed and evaluated in this study. Oligonucleotide primers specific for L. ivanovii species were designed corresponding to smcL gene sequences. The primers set comprise six primers targeting eight regions on the species-specific gene smcL. The LAMP assay could be completed within 1 h at 64°C in a water bath. Amplification products were directly observed by the Loopamp Fluorescent Detection Reagent (FD) or detected by agarose gel electrophoresis. Moreover, the LAMP reactions were also detected by real-time measurement of turbidity. The exclusivity of 77 non-L. ivanovii and the inclusivity of 17 L. ivanovii were both 100% in the assay. Sensitivity of the LAMP assay was 250 fg DNA and 16 CFU per reaction for detection of L. ivanovii in pure cultures and simulated human stool. The LAMP assay was 10 and 100-fold more sensitive than quantitative PCR (qPCR) and conventional PCR assays,respectively. When applied to human stool samples spiked with low level (8 CFU/0.5 g) of L. ivanovii strains, the new LAMP assay described here achieved positive detection after 6 hours enrichment. In conclusion, the new LAMP assay in this study can be used as a valuable, rapid and sensitive detection tool for the detection of L. ivanovii in field, medical and veterinary laboratories.

Listeria floridensis sp. nov., Listeria aquatica sp. nov., Listeria cornellensis sp. nov., Listeria riparia sp. nov. and Listeria grandensis sp. nov., from agricultural and natural environments

Sampling of agricultural and natural environments in two US states (Colorado and Florida) yielded 18 Listeria-like isolates that could not be assigned to previously described species using traditional methods. Using whole-genome sequencing and traditional phenotypic methods, we identified five novel species, each with a genome-wide average BLAST nucleotide identity (ANIb) of less than 85% to currently described species. Phylogenetic analysis based on 16S rRNA gene sequences and amino acid sequences of 31 conserved loci showed the existence of four well-supported clades within the genus Listeria; (i) a clade representing Listeria monocytogenes, L. marthii, L. innocua, L. welshimeri, L. seeligeri and L. ivanovii, which we refer to as Listeria sensu stricto, (ii) a clade consisting of Listeria fleischmannii and two newly described species, Listeria aquatica sp. nov. (type strain FSL S10-1188(T) = DSM 26686(T) = LMG 28120(T) = BEI NR-42633(T)) and Listeria floridensis sp. nov. (type strain FSL S10-1187(T) = DSM 26687(T) = LMG 28121(T) = BEI NR-42632(T)), (iii) a clade consisting of Listeria rocourtiae, L. weihenstephanensis and three novel species, Listeria cornellensis sp. nov. (type strain TTU A1-0210(T) = FSL F6-0969(T) = DSM 26689(T) = LMG 28123(T) = BEI NR-42630(T)), Listeria grandensis sp. nov. (type strain TTU A1-0212(T) = FSL F6-0971(T) = DSM 26688(T) = LMG 28122(T) = BEI NR-42631(T)) and Listeria riparia sp. nov. (type strain FSL S10-1204(T) = DSM 26685(T) = LMG 28119(T) = BEI NR- 42634(T)) and (iv) a clade containing Listeria grayi. Genomic and phenotypic data suggest that the novel species are non-pathogenic.

Listeria ivanovii ATCC 19119 strain behaviour is modulated by iron and acid stress

It has been suggested that the rarity of human listeriosis due to Listeria ivanovii reflects not only host tropism factors but also the rare occurrence of this species in the environment, compared with Listeria monocytogenes. In the present study we evaluate the effects on the reference strain L. ivanovii ATCC 19119 behaviour of two combined stresses, low iron availability and acid environment, that bacteria can encounter in the passage from saprophytic life to the host. In these conditions, L. ivanovii evidenced a different behaviour compared to L. monocytogenes exposed to similar conditions. L. ivanovii was not able to mount an acid tolerance response (ATR) even if, upon entry into the stationary phase in iron-loaded medium, growth phase-dependent acid resistance (AR) was evidenced. Moreover, bacteria grown in iron excess and acidic pH showed the higher invasion value in Caco-2 cells, even though it was not able to efficiently multiply. On the contrary, low iron and acidic conditions improved invasion ability in amniotic WISH cells.

Horizontally Transferred Genetic Elements and Their Role in Pathogenesis of Bacterial Disease

This article reviews the roles that laterally transferred genes (LTG) play in the virulence of bacterial pathogens. The features of LTG that allow them to be recognized in bacterial genomes are described, and the mechanisms by which LTG are transferred between and within bacteria are reviewed. Genes on plasmids, integrative and conjugative elements, prophages, and pathogenicity islands are highlighted. Virulence genes that are frequently laterally transferred include genes for bacterial adherence to host cells, type 3 secretion systems, toxins, iron acquisition, and antimicrobial resistance. The specific roles of LTG in pathogenesis are illustrated by specific reference to Escherichia coli, Salmonella, pyogenic streptococci, and Clostridium perfringens.

Reassessment of the Listeria monocytogenes pan-genome reveals dynamic integration hotspots and mobile genetic elements as major components of the accessory genome

Background

Listeria monocytogenes is an important food-borne pathogen and model organism for host-pathogen interaction, thus representing an invaluable target considering research on the forces governing the evolution of such microbes. The diversity of this species has not been exhaustively explored yet, as previous efforts have focused on analyses of serotypes primarily implicated in human listeriosis. We conducted complete genome sequencing of 11 strains employing 454 GS FLX technology, thereby achieving full coverage of all serotypes including the first complete strains of serotypes 1/2b, 3c, 3b, 4c, 4d, and 4e. These were comparatively analyzed in conjunction with publicly available data and assessed for pathogenicity in the Galleria mellonella insect model.

Results

The species pan-genome of L. monocytogenes is highly stable but open, suggesting an ability to adapt to new niches by generating or including new genetic information. The majority of gene-scale differences represented by the accessory genome resulted from nine hyper variable hotspots, a similar number of different prophages, three transposons (Tn916, Tn554, IS3-like), and two mobilizable islands. Only a subset of strains showed CRISPR/Cas bacteriophage resistance systems of different subtypes, suggesting a supplementary function in maintenance of chromosomal stability. Multiple phylogenetic branches of the genus Listeria imply long common histories of strains of each lineage as revealed by a SNP-based core genome tree highlighting the impact of small mutations for the evolution of species L. monocytogenes. Frequent loss or truncation of genes described to be vital for virulence or pathogenicity was confirmed as a recurring pattern, especially for strains belonging to lineages III and II. New candidate genes implicated in virulence function were predicted based on functional domains and phylogenetic distribution. A comparative analysis of small regulatory RNA candidates supports observations of a differential distribution of trans-encoded RNA, hinting at a diverse range of adaptations and regulatory impact.

Conclusions

This study determined commonly occurring hyper variable hotspots and mobile elements as primary effectors of quantitative gene-scale evolution of species L. monocytogenes, while gene decay and SNPs seem to represent major factors influencing long-term evolution. The discovery of common and disparately distributed genes considering lineages, serogroups, serotypes and strains of species L. monocytogenes will assist in diagnostic, phylogenetic and functional research, supported by the comparative genomic GECO-LisDB analysis server (http://bioinfo.mikrobio.med.uni-giessen.de/geco2lisdb).

Comparative genomics and transcriptomics of lineages I, II, and III strains of Listeria monocytogenes

Background

Listeria monocytogenes is a food-borne pathogen that causes infections with a high-mortality rate and has served as an invaluable model for intracellular parasitism. Here, we report complete genome sequences for two L. monocytogenes strains belonging to serotype 4a (L99) and 4b (CLIP80459), and transcriptomes of representative strains from lineages I, II, and III, thereby permitting in-depth comparison of genome- and transcriptome -based data from three lineages of L. monocytogenes. Lineage III, represented by the 4a L99 genome is known to contain strains less virulent for humans.

Results

The genome analysis of the weakly pathogenic L99 serotype 4a provides extensive evidence of virulence gene decay, including loss of several important surface proteins. The 4b CLIP80459 genome, unlike the previously sequenced 4b F2365 genome harbours an intact inlB invasion gene. These lineage I strains are characterized by the lack of prophage genes, as they share only a single prophage locus with other L. monocytogenes genomes 1/2a EGD-e and 4a L99. Comparative transcriptome analysis during intracellular growth uncovered adaptive expression level differences in lineages I, II and III of Listeria, notable amongst which was a strong intracellular induction of flagellar genes in strain 4a L99 compared to the other lineages. Furthermore, extensive differences between strains are manifest at levels of metabolic flux control and phosphorylated sugar uptake. Intriguingly, prophage gene expression was found to be a hallmark of intracellular gene expression. Deletion mutants in the single shared prophage locus of lineage II strain EGD-e 1/2a, the lma operon, revealed severe attenuation of virulence in a murine infection model.

Conclusion

Comparative genomics and transcriptome analysis of L. monocytogenes strains from three lineages implicate prophage genes in intracellular adaptation and indicate that gene loss and decay may have led to the emergence of attenuated lineages.

Complete genome sequence of the animal pathogen Listeria ivanovii, which provides insights into host specificities and evolution of the genus Listeria

We report the complete and annotated genome sequence of the animal pathogen Listeria ivanovii subsp. ivanovii strain PAM 55 (serotype 5), isolated in 1997 in Spain from an outbreak of abortion in sheep. The sequence and its analysis are available at an interactive genome browser at the Institut Pasteur (http://genolist.pasteur.fr/LivaList/).

Streptolysin S-like virulence factors: the continuing sagA

Streptolysin S (SLS) is a potent cytolytic toxin and virulence factor that is produced by nearly all Streptococcus pyogenes strains. Despite a 100-year history of research on this toxin, it has only recently been established that SLS is just one of an extended family of post-translationally modified virulence factors (the SLS-like peptides) that are produced by some streptococci and other Gram-positive pathogens, such as Listeria monocytogenes and Clostridium botulinum. In this Review, we describe the identification, genetics, biochemistry and various functions of SLS. We also discuss the shared features of the virulence-associated SLS-like peptides, as well as their place within the rapidly expanding family of thiazole/oxazole-modified microcins (TOMMs).

A population genetics-based and phylogenetic approach to understanding the evolution of virulence in the genus Listeria

The genus Listeria includes (i) the opportunistic pathogens L. monocytogenes and L. ivanovii, (ii) the saprotrophs L. innocua, L. marthii, and L. welshimeri, and (iii) L. seeligeri, an apparent saprotroph that nevertheless typically contains the prfA virulence gene cluster. A novel 10-loci multilocus sequence typing scheme was developed and used to characterize 67 isolates representing six Listeria spp. (excluding L. grayi) in order to (i) provide an improved understanding of the phylogeny and evolution of the genus Listeria and (ii) use Listeria as a model to study the evolution of pathogenicity in opportunistic environmental pathogens. Phylogenetic analyses identified six well-supported Listeria species that group into two main subdivisions, with each subdivision containing strains with and without the prfA virulence gene cluster. Stochastic character mapping and phylogenetic analysis of hly, a gene in the prfA cluster, suggest that the common ancestor of the genus Listeria contained the prfA virulence gene cluster and that this cluster was lost at least five times during the evolution of Listeria, yielding multiple distinct saprotrophic clades. L. welshimeri, which appears to represent the most ancient clade that arose from an ancestor with a prfA cluster deletion, shows a considerably lower average sequence divergence than other Listeria species, suggesting a population bottleneck and a putatively different ecology than other saprotrophic Listeria species. Overall, our data suggest that, for some pathogens, loss of virulence genes may represent a selective advantage, possibly by facilitating adaptation to a specific ecological niche.

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.

Human listeriosis caused by Listeria ivanovii

Two species of Listeria are pathogenic; L. monocytogenes infects humans and animals, and L. ivanovii has been considered to infect ruminants only. We report L. ivanovii–associated gastroenteritis and bacteremia in a man. This isolate was indistinguishable from prototypic ruminant strains. L. ivanovii is thus an enteric opportunistic human pathogen.

Sphingomyelin metabolism at the plasma membrane: implications for bioactive sphingolipids

The plasma membrane (PM) is a major resource for production of bioactive lipids and contains a large proportion of the cellular sphingomyelin (SM) content. Consequently, the regulation of SM levels at the PM by enzymes such as sphingomyelinase (SMase) and SM synthase 2 (SMS2) can have profound effects - both on biophysical properties of the membrane, but also on cellular signaling. Over the past 20 years, there has been considerable research into the physiological and cellular functions associated with regulation of SM levels, notably with regards to the production of ceramide. In this review, we will summarize this research with particular focus on the SMases and SMS2. We will outline what biological functions are associated with SM metabolism/production at the PM, and discuss what we believe are major challenges that need to be addressed in future studies.

Birth-and-death evolution of the internalin multigene family in Listeria

The birth-and-death model of multigene family evolution describes patterns of gene origination, diversification and loss within multigene families. Since it was first developed in the 1990s, the model has been found to characterize a large number of eukaryotic multigene families. In this paper, we report for the first time a bacterial multigene family that undergoes birth-and-death evolution. By analyzing the evolutionary relationships among internalins, a relatively large and diverse family of genes associated with key virulence functions in Listeria, we demonstrate the importance of birth-and-death evolution in the diversification of this important bacterial pathogen. We also detected two instances of lateral gene transfer within the internalins, but the estimated frequency would have been much higher had it not been analyzed within the context of birth-and-death evolutionary dynamics and a phenomenon that we term "paralog-sorting", which involves the unequal transmittal of gene duplicates during or subsequent to the speciation process. As such, in addition to providing the first demonstration of birth-and-death evolution within a bacterial multigene family, our results indicate that the extent of lateral transfer in bacterial multigene families should be re-examined in the light of birth-and-death evolution.

Listeriolysin S, a novel peptide haemolysin associated with a subset of lineage I Listeria monocytogenes

Streptolysin S (SLS) is a bacteriocin-like haemolytic and cytotoxic virulence factor that plays a key role in the virulence of Group A Streptococcus (GAS), the causative agent of pharyngitis, impetigo, necrotizing fasciitis and streptococcal toxic shock syndrome. Although it has long been thought that SLS and related peptides are produced by GAS and related streptococci only, there is evidence to suggest that a number of the most notorious Gram-positive pathogenic bacteria, including Listeria monocytogenes, Clostridium botulinum and Staphylococcus aureus, produce related peptides. The distribution of the L. monocytogenes cluster is particularly noteworthy in that it is found exclusively among a subset of lineage I strains; i.e., those responsible for the majority of outbreaks of listeriosis. Expression of these genes results in the production of a haemolytic and cytotoxic factor, designated Listeriolysin S, which contributes to virulence of the pathogen as assessed by murine- and human polymorphonuclear neutrophil-based studies. Thus, in the process of establishing the existence of an extended family of SLS-like modified virulence peptides (MVPs), the genetic basis for the enhanced virulence of a proportion of lineage I L. monocytogenes may have been revealed.

Invasive pathway of Listeria ivanovii in human amnion-derived WISH cells

Among Listeria genus, only two species, Listeria ivanovii and Listeria monocytogenes, are pathogenic. L. ivanovii is almost only associated with infections in animals, mainly sheep and cattle, and has rarely been associated with human infections, whereas L. monocytogenes causes severe illnesses in both humans and animals. To further investigate the pathogenetic features of L. ivanovii in humans, we undertook a study in which the intracellular behaviour of this pathogen was analysed in WISH cells, a cell line derived from human amniotic tissue, and compared to that of L. monocytogenes. Using microbiological, biochemical, and ultrastructural approaches, we demonstrate that L. ivanovii can adhere to and invade human amniotic cells, lyse the phagosomal membrane, polymerize host cell actin, and spread from cell to cell more efficiently than L. monocytogenes. However, although L. ivanovii is capable of specifically infecting and replicating in human amnion cells, its survival in cytoplasm is limited compared to that of L. monocytogenes.

Induction of suicidal erythrocyte death by listeriolysin from Listeria monocytogenes

Listeriolysin, the secreted cytolysin of the facultative intracellular bacterium Listeria monocytogenes, is its major virulence factor. Previously, non-lytic concentrations of listeriolysin were shown to induce Ca2+-permeable nonselective cation channels in human embryonic kidney cells. In erythrocytes, Ca2+ entry is followed by activation of K+ channels resulting in K+-exit as well as by membrane scrambling resulting in phosphatidylserine exposure at the cell surface. Phosphatidylserine-exposing erythrocytes are recognized by macrophages, engulfed, degraded and thus cleared from circulating blood. Phosphatidylserine exposure is a key event of eryptosis, the suicidal death of erythrocytes. The present study utilized patch-clamp technique, Fluo3-fluorescence, and annexin V-binding in FACS analysis to determine the effect of listeriolysin on cell membrane conductance, cytosolic free Ca2+ concentration, and phosphatidylserine exposure, respectively. Within 30 minutes, exposure of human peripheral blood erythrocytes to low concentrations of listeriolysin (which were non-hemolytic for the majority of cells) induced a Ca2+-permeable cation conductance in the erythrocyte cell membrane, increased cytosolic Ca2+ concentration, and triggered annexin V-binding. Increase of extracellular K+ concentration blunted, but did not prevent, listeriolysin-induced annexin V-binding. In conclusion, listeriolysin triggers suicidal death of erythrocytes, an effect at least partially due to depletion of intracellular K+. Listeriolysin induced suicidal erythrocyte death could well contribute to the pathophysiology of L. monocytogenes infection.

Internalins: a complex family of leucine-rich repeat-containing proteins in Listeria monocytogenes

The Listeria monocytogenes genome includes a large family of proteins harbouring leucine-rich repeats known as internalins (Inl). The generation of novel mutants and comparative analysis of Inl variability among Listeria and other bacterial genomes suggest that beyond the extensively-studied invasins, InlA and InlB, additional internalins also play important functions in the infectious process.

Biodiversity of the species Listeria monocytogenes and the genus Listeria

This review describes the Listeria monocytogenes genome sequences available today and their comparison with that of Listeria innocua and Listeria welshimeri by highlighting their characteristic features and common traits. The diversity present among them is analysed with emphasis on putative virulence and host-pathogen interaction related functions. Then large-scale studies comparing gene content of Listeria and how these studies contributed to typing applications will be discussed. Finally, evolutionary conclusions and future perspectives in Listeria genomics are presented.

Temperature-dependent expression of Listeria monocytogenes internalin and internalin-like genes suggests functional diversity of these proteins among the listeriae

The Listeria monocytogenes genome contains genes encoding several internalins and internalin-like proteins. As L. monocytogenes is present in many environments and can infect numerous, diverse host species, the environmental temperature was hypothesized to be a signal that might affect internalin gene transcription. A subgenomic microarray was used to investigate temperature-dependent transcription of 24 members of the internalin gene family in L. monocytogenes 10403S. The levels of internalin gene transcripts for cells grown at 37 degrees C were compared to the levels of transcripts for cells grown at 16, 30, and 42 degrees C using competitive microarray hybridization, and the results were confirmed by performing quantitative reverse transcriptase PCR for 14 internalin genes. Based on these studies, the internalin genes can be grouped into the following five temperature-dependent categories: (i) four sigma(B)-dependent internalin genes (inlC2, inlD, lmo0331, and lmo0610) with the highest levels of transcripts at 16 degrees C and generally the lowest levels of transcripts at 37 degrees C; (ii) three partially PrfA-dependent internalin genes (inlA, inlB, and inlC) with the lowest levels of transcripts at 16 degrees C and the highest levels of transcripts at 37 and 42 degrees C; (iii) four genes (inlG, inlJ, lmo0514, and lmo1290) with the lowest levels of transcripts at 16 degrees C and the highest levels of transcripts at 30 and/or 37 degrees C; (iv) one gene (lmo0327) with the highest levels of transcripts at 16 degrees C and low levels of transcripts at higher temperatures; and (v) 12 internalin genes with no differences in the levels of transcripts at the temperatures used in this study. The temperature-dependent transcription patterns suggest that the relative importance of different internalins varies by environment, which may provide insight into the specific functions of these proteins.

Comparative and functional genomics of Listeria spp

The genus Listeria comprises a group of non-sporulating, Gram-positive, soil bacteria belonging to the low G+C group of microorganisms. The genus consists of only six species, L. monocytogenes, L. ivanovii, L. seeligeri, L. innocua, L. welshimeri, and L. grayi.L. monocytogenes and L. ivanovii are the only known pathogens of this group. Comparative whole-genome sequencing of representative strains comprising the entire genus is currently being performed and nearing completion. In the genus Listeria, genome reduction has led to the generation of non-pathogenic species from pathogenic progenitor strains. Indeed, many of the regions absent in the non-pathogenic species represent commonly deleted genes. Speciation and diversity of strains has been achieved by horizontal gene transfer of DNA encoding novel genes probably required for niche specific survival. The sequencing of several listerial genomes has also been accompanied by studies using global strategies involving whole-genome transcriptional profiling and proteomics to examine the adaptative changes of L. monocytogenes to growth in different environments and to catalogue the genes mediating these responses. We review this data and present information on the expression profile of L. monocytogenes EGD-e inside the vacuolar and the cytosolic environments of the host cell using whole-genome microarray analysis. Of the 484 genes regulated during intracellular growth 41 genes are species-specific, being absent from the genome of the non-pathogenic L. innocua CLIP 11262 strain. There were 25 genes that are strain-specific i.e. absent from the genome of the L. monocytogenes F2365 serotype 4b strain suggesting heterogeneity in the gene pool required for intracellular survival of L. monocytogenes in host cells.

Whole-genome sequence of Listeria welshimeri reveals common steps in genome reduction with Listeria innocua as compared to Listeria monocytogenes

We present the complete genome sequence of Listeria welshimeri, a nonpathogenic member of the genus Listeria. Listeria welshimeri harbors a circular chromosome of 2,814,130 bp with 2,780 open reading frames. Comparative genomic analysis of chromosomal regions between L. welshimeri, Listeria innocua, and Listeria monocytogenes shows strong overall conservation of synteny, with the exception of the translocation of an F(o)F(1) ATP synthase. The smaller size of the L. welshimeri genome is the result of deletions in all of the genes involved in virulence and of "fitness" genes required for intracellular survival, transcription factors, and LPXTG- and LRR-containing proteins as well as 55 genes involved in carbohydrate transport and metabolism. In total, 482 genes are absent from L. welshimeri relative to L. monocytogenes. Of these, 249 deletions are commonly absent in both L. welshimeri and L. innocua, suggesting similar genome evolutionary paths from an ancestor. We also identified 311 genes specific to L. welshimeri that are absent in the other two species, indicating gene expansion in L. welshimeri, including horizontal gene transfer. The species L. welshimeri appears to have been derived from early evolutionary events and an ancestor more compact than L. monocytogenes that led to the emergence of nonpathogenic Listeria spp.