iactA of Listeria ivanovii, although distantly related to Listeria monocytogenes actA, restores actin tail formation in an L. monocytogenes actA mutant

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.

Entropy-driven translocation of disordered proteins through the Gram-positive bacterial cell wall

In Gram-positive bacteria, a thick cross-linked cell wall separates the membrane from the extracellular space. Some surface-exposed proteins, such as the Listeria monocytogenes actin nucleation-promoting factor ActA, remain associated with the bacterial membrane but somehow thread through tens of nanometres of cell wall to expose their amino terminus to the exterior. Here, we report that entropy enables the translocation of disordered transmembrane proteins through the Gram-positive cell wall. We build a physical model, which predicts that the entropic constraint imposed by a thin periplasm is sufficient to drive the translocation of an intrinsically disordered protein such as ActA across a porous barrier similar to a peptidoglycan cell wall. We experimentally validate our model and show that ActA translocation depends on the cell-envelope dimensions and disordered-protein length, and that translocation is reversible. We also show that disordered regions of eukaryotic proteins can translocate Gram-positive cell walls via entropy. We propose that entropic forces are sufficient to drive the translocation of specific proteins to the outer surface.

Multiplex PCR for the Identification of Pathogenic Listeria in Flammulina velutipes Plant Based on Novel Specific Targets Revealed by Pan-Genome Analysis

Listeria spp. is an important foodborne disease agent, often found in the fresh mushroom (Flammulina velutipes) and its production environment. The aim of this study was to develop multiplex PCR for rapid identification of Listeria monocytogenes and Listeria ivanovii, and nonpathogenic Listeria in F. velutipes plants. Pan-genome analysis was first used to identify five novel Listeria-specific targets: one for the Listeria genus, one for L. monocytogenes, and three for L. ivanovii. Primers for the novel targets were highly specific in individual reactions. The detection limits were 10³-10⁴ CFU/mL, meeting the requirements of molecular detection. A mPCR assay for the identification of pathogenic Listeria, with primers targeting the novel genes specific for Listeria genus (LMOSLCC2755_0944), L. monocytogenes (LMOSLCC2755_0090), and L. ivanovii (queT_1) was then designed. The assay specificity was robustly verified by analyzing nonpathogenic Listeria and non-Listeria spp. strains. The determined detection limits were 2.0 × 10³ CFU/mL for L. monocytogenes and 3.4 × 10³ CFU/mL for L. ivanovii, for pure culture analysis. Further, the assay detected 7.6 × 10⁴ to 7.6 × 10⁰ CFU/10 g of pathogenic Listeria spiked into F. velutipes samples following 4-12 h enrichment. The assay feasibility was evaluated by comparing with a traditional culture-based method, by analyzing 129 samples collected from different F. velutipes plants. The prevalence of Listeria spp. and L. monocytogenes was 58.1% and 41.1%, respectively. The calculated κ factors for Listeria spp., L. monocytogenes, and L. ivanovii were 0.97, 0.97, and 1, respectively. The results of the novel mPCR assay were highly consistent with those of the culture-based method. The new assay thus will allow rapid, specific, and accurate detection and monitoring of pathogenic Listeria in food and its production environment.

Spontaneous Loss of Virulence in Natural Populations of Listeria monocytogenes

The pathogenesis of Listeria monocytogenes depends on the ability of this bacterium to escape from the phagosome of the host cells via the action of the pore-forming toxin listeriolysin O (LLO). Expression of the LLO-encoding gene (hly) requires the transcriptional activator PrfA, and both hly and prfA genes are essential for L. monocytogenes virulence. Here, we used the hemolytic activity of LLO as a phenotypic marker to screen for spontaneous virulence-attenuating mutations in L. monocytogenes. Sixty nonhemolytic isolates were identified among a collection of 57,820 confirmed L. monocytogenes strains isolated from a variety of sources (0.1%). In most cases (56/60; 93.3%), the nonhemolytic phenotype resulted from nonsense, missense, or frameshift mutations in prfA. Five strains carried hly mutations leading to a single amino acid substitution (G299V) or a premature stop codon causing strong virulence attenuation in mice. In one strain, both hly and gshF (encoding a glutathione synthase required for full PrfA activity) were missing due to genomic rearrangements likely caused by a transposable element. The PrfA/LLO loss-of-function (PrfA⁻/LLO⁻) mutants belonged to phylogenetically diverse clades of L. monocytogenes, and most were identified among nonclinical strains (57/60). Consistent with the rare occurrence of loss-of-virulence mutations, we show that prfA and hly are under purifying selection. Although occurring at a low frequency, PrfA⁻/LLO⁻ mutational events in L. monocytogenes lead to niche restriction and open an evolutionary path for obligate saprophytism in this facultative intracellular pathogen.

Actin-based motility of bacterial pathogens: mechanistic diversity and its impact on virulence

A diverse spectrum of intracellular bacterial pathogens that inhabit the cytosol have evolved the ability to polymerize actin on their surface to power intracellular actin-based motility (ABM). These include species of Listeria, Burkholderia and Rickettsia, as well as Shigella and Mycobacteria Here, we provide an overview of the roles of bacterial ABM in survival and virulence. Moreover, we survey the molecular mechanisms of actin polymerization in host cells and describe how bacterial pathogens mimic or harness the full diversity of these mechanisms for ABM. Finally, we present ABM through a new lens by comparing motility mechanisms between related species of Listeria, Burkholderia, and Rickettsia Through these comparisons, we hope to illuminate how exploitation of different actin polymerization mechanisms influences ABM as well as pathogenicity and virulence in humans and other animals.

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.

Listeria monocytogenes ActA: a new function for a 'classic' virulence factor

Listeria monocytogenes (Lm) is ubiquitous and widespread in the environment. It is responsible for one of the most severe human foodborne infection. Lm is a facultative intracellular bacterium that can cross the intestinal barrier, disseminate via the bloodstream and reach the liver, spleen, central nervous system and fetus. The bacterial surface protein ActA is one of the most critical and best characterized virulence factors of Lm. It fulfills many essential functions within host cells, allowing Lm escape from autophagy and recruiting an actin polymerization complex that promotes Lm actin-based motility, cell-to-cell spread and dissemination within host tissues. We have recently shown that ActA also acts extracellularly. It mediates Lm aggregation and biofilm formation in vitro and in vivo, and long-term colonization of the gut lumen. This new property of ActA favors Lm transmission and may participate in the selective pressure on Lm to maintain ActA.

ActA promotes Listeria monocytogenes aggregation, intestinal colonization and carriage

Listeria monocytogenes (Lm) is a ubiquitous bacterium able to survive and thrive within the environment and readily colonizes a wide range of substrates, often as a biofilm. It is also a facultative intracellular pathogen, which actively invades diverse hosts and induces listeriosis. So far, these two complementary facets of Lm biology have been studied independently. Here we demonstrate that the major Lm virulence determinant ActA, a PrfA-regulated gene product enabling actin polymerization and thereby promoting its intracellular motility and cell-to-cell spread, is critical for bacterial aggregation and biofilm formation. We show that ActA mediates Lm aggregation via direct ActA-ActA interactions and that the ActA C-terminal region, which is not involved in actin polymerization, is essential for aggregation in vitro. In mice permissive to orally-acquired listeriosis, ActA-mediated Lm aggregation is not observed in infected tissues but occurs in the gut lumen. Strikingly, ActA-dependent aggregating bacteria exhibit an increased ability to persist within the cecum and colon lumen of mice, and are shed in the feces three order of magnitude more efficiently and for twice as long than bacteria unable to aggregate. In conclusion, this study identifies a novel function for ActA and illustrates that in addition to contributing to its dissemination within the host, ActA plays a key role in Lm persistence within the host and in transmission from the host back to the environment.

Listeria monocytogenes (Lm) is a ubiquitous bacterium that survives and thrives within the environment, and a facultative intracellular pathogen that induces listeriosis. So far, these two complementary facets of Lm biology have been studied independently. Here we identify ActA, which is a major Lm virulence determinant mediating actin-based motility, as critical for bacterial aggregation and biofilm formation. ActA promotes Lm aggregation via direct ActA-ActA interaction and ActA C-terminal region, which is not involved in actin polymerization, is essential for aggregation. Whereas ActA-mediated Lm aggregation is not observed in infected tissues, it occurs in the gut lumen. Strikingly, ActA-dependent aggregating bacteria exhibit an increased ability to persist within the gut lumen, and are shed in the feces three order of magnitude more and for twice as long than bacteria unable to aggregate. This study identifies a novel function for ActA, which plays a key role in Lm persistence within the host and transmission.

Role for telomerase in Listeria monocytogenes infection

Human telomerase reverse transcriptase (hTERT) is the catalytic subunit of the human telomerase complex. Growing evidence suggests that hTERT also contributes to the cell physiology independently of telomere elongation. However, its role in bacterial infection is unknown. Here we show that hTERT is critical for Listeria monocytogenes infection, as the depletion of hTERT impaired bacterial intracellular replication. In addition, we observed that L. monocytogenes caused a decrease in hTERT levels at early time points of the infectious process. This effect was mediated by the pore-forming toxin listeriolysin O (LLO) and did not require bacterial entry into host cells. Calcium influx through the LLO pores contributed to a proteasome-independent decrease in hTERT protein levels. Together, our data provide evidence that these bacteria trigger hTERT degradation, an event that is detrimental to bacterial replication.

Critical role for the host GTPase-activating protein ARAP2 in InlB-mediated entry of Listeria monocytogenes

The bacterial pathogen Listeria monocytogenes causes food-borne illnesses culminating in gastroenteritis, meningitis, or abortion. Listeria induces its internalization into some mammalian cells through binding of the bacterial surface protein InlB to the host receptor tyrosine kinase Met. Interaction of InlB with the Met receptor elicits host downstream signaling pathways that promote F-actin cytoskeletal changes responsible for pathogen engulfment. Here we show that the mammalian signaling protein ARAP2 plays a critical role in cytoskeletal remodeling and internalization of Listeria. Depletion of ARAP2 through RNA interference (RNAi) caused a marked inhibition of InlB-mediated F-actin rearrangements and bacterial entry. ARAP2 contains multiple functional domains, including a GTPase-activating protein (GAP) domain that antagonizes the GTPase Arf6 and a domain capable of binding the GTPase RhoA. Genetic data indicated roles for both the Arf GAP and RhoA binding domains in Listeria entry. Experiments involving Arf6 RNAi or a constitutively activated allele of Arf6 demonstrated that one of the ways in which ARAP2 promotes bacterial uptake is by restraining the activity of Arf6. Conversely, Rho activity was dispensable for Listeria internalization, suggesting that the RhoA binding domain in ARAP2 acts by engaging a host ligand other than Rho proteins. Collectively, our findings indicate that ARAP2 promotes InlB-mediated entry of Listeria, in part, by antagonizing the host GTPase Arf6.

Branched-chain fatty acids promote Listeria monocytogenes intracellular infection and virulence

Anteiso-branched-chain fatty acids (BCFA) represent the dominant group of membrane fatty acids and have been established as crucial determinants in resistance against environmental stresses in Listeria monocytogenes, a facultative intracellular pathogen. Here, we investigate the role of anteiso-BCFA in L. monocytogenes virulence by using mutants deficient in branched-chain alpha-keto acid dehydrogenase (BKD), an enzyme complex involved in the synthesis of BCFA. In tissue culture models of infection, anteiso-BCFA contributed to intracellular growth and survival in macrophages and significantly enhanced plaque formation upon prolonged infection in L2 fibroblasts. The intracellular defects observed could be attributed partially to insufficient listeriolysin O (LLO) production, indicating a requirement for anteiso-BCFA in regulating virulence factor production. In a murine model of infection, the BKD-deficient mutant was highly attenuated, further emphasizing the importance of BKD-mediated metabolism in L. monocytogenes virulence. This study demonstrates an underappreciated role for BCFA in bacterial pathogenesis, which may provide insight into the development and application of antimicrobial agents.

Prevalence and sequence diversity of a factor required for actin-based motility in natural populations of Burkholderia species

Actin-based motility of the melioidosis pathogen Burkholderia pseudomallei requires BimA. We report a high degree of conservation of bimA in 99 B. pseudomallei isolates from the area of endemicity. A geographically restricted subset of B. pseudomallei isolates harbored a B. mallei-like bimA allele (12.1%), confounding a differential diagnostic test based on amplification of species-specific bimA regions.

The carboxyl-terminal SH3 domain of the mammalian adaptor CrkII promotes internalization of Listeria monocytogenes through activation of host phosphoinositide 3-kinase

The intracellular bacterial pathogen Listeria monocytogenes causes food-borne illnesses leading to gastroenteritis, meningitis or abortion. Listeria induces its internalization into some mammalian cells through binding of the bacterial surface protein InlB to its host receptor, the Met Receptor Tyrosine Kinase. InlB-induced activation of Met stimulates host signal transduction pathways that culminate in cell surface changes driving pathogen engulfment. One mammalian protein with the potential to couple Met to downstream signalling is the adaptor CrkII. CrkII contains an unusual carboxyl-terminal SH3 domain (SH3C) that promotes entry of Listeria. However, binding partners or downstream effectors of SH3C remain unknown. Here, we use RNA interference and overexpression studies to demonstrate that SH3C affects bacterial uptake, at least in part, through stimulation of host phosphatidylinositide (PI) 3-kinase. Experiments with latex beads coated with InlB protein indicated that one potential role of SH3C and PI 3 kinase is to promote changes in the F-actin cytoskeleton necessary for particle engulfment. Taken together, our results indicate that the CrkII SH3C domain engages a cellular ligand that regulates PI 3 kinase activity and host cell surface rearrangements.

The Listeria monocytogenes virulence factor InlJ is specifically expressed in vivo and behaves as an adhesin

The food-borne pathogen Listeria monocytogenes is adapted to a diversity of environments, such as soil, food, body fluids, and the cytosol of eukaryotic cells. The transition between saprophytic and pathogenic life is mediated through complex regulatory pathways that modulate the expression of virulence factors. Here we examined the expression of inlJ, a recently identified gene encoding a protein of the LPXTG-internalin family and involved in pathogenesis. We show that inlJ expression is controlled neither by the major listerial regulator of virulence genes, PrfA, nor by AxyR, a putative AraC regulator encoded by a gene adjacent to inlJ and divergently transcribed. The InlJ protein is not produced by bacteria grown in vitro in brain heart infusion medium or replicating in the cytosol of tissue-cultured cells. In contrast, it is efficiently produced and localized at the surface of bacteria present in the liver and blood of infected animals. Strikingly, the expression of inlJ by a heterologous promoter in L. monocytogenes or L. innocua promotes bacterial adherence to human cells in vitro. Taken together, these results strongly suggest that InlJ acts as a novel L. monocytogenes sortase-anchored adhesin specifically expressed during infection in vivo.

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.

Control ofListeriaSuperoxide Dismutase by Phosphorylation

Superoxide dismutases (SODs) are enzymes that protect organisms against superoxides and reactive oxygen species (ROS) produced during their active metabolism. ROS are major mediators of phagocytes microbicidal activity. Here we show that the cytoplasmic Listeria monocytogenes MnSOD is phosphorylated on serine and threonine residues and less active when bacteria reach the stationary phase. We also provide evidence that the most active nonphosphorylated form of MnSOD can be secreted via the SecA2 pathway in culture supernatants and in infected cells, where it becomes phosphorylated. A Deltasod deletion mutant is impaired in survival within macrophages and is dramatically attenuated in mice. Together, our results demonstrate that the capacity to counteract ROS is an essential component of L. monocytogenes virulence. This is the first example of a bacterial SOD post-translationally controlled by phosphorylation, suggesting a possible new host innate mechanism to counteract a virulence factor.

Actin-dependent movement of bacterial pathogens

Listeria, Rickettsia, Burkholderia, Shigella and Mycobacterium species subvert cellular actin dynamics to facilitate their movement within the host cytosol and to infect neighbouring cells while evading host immune surveillance and promoting their intracellular survival. 'Attaching and effacing' Escherichia coli do not enter host cells but attach intimately to the cell surface, inducing motile actin-rich pedestals, the function of which is currently unclear. The molecular basis of actin-based motility of these bacterial pathogens reveals novel insights about bacterial pathogenesis and fundamental host-cell pathways.

Host adaptor proteins Gab1 and CrkII promote InlB-dependent entry of Listeria monocytogenes

The bacterial surface protein InlB mediates internalization of Listeria monocytogenes into mammalian cells through interaction with the host receptor tyrosine kinase, Met. InlB/Met interaction results in activation of the host phosphoinositide (PI) 3-kinase p85-p110, an event required for bacterial entry. p85-p110 activation coincides with tyrosine phosphorylation of the host adaptor Gab1, and formation of complexes between Gab1 and the p85 regulatory subunit of PI 3-kinase. When phosphorylated in response to agonists, Gab1 is known to recruit several Src-homology 2 (SH2) domain-containing proteins including p85, the tyrosine phosphatase Shp2 and the adaptor CrkII. Here, we demonstrate that Gab1.p85 and Gab1.CrkII complexes promote entry of Listeria. Overexpression of wild-type Gab1 stimulated entry, whereas Gab1 alleles unable to recruit all SH2 proteins known to bind wild-type Gab1 inhibited internalization. Further analysis with Gab1 alleles defective in binding individual effectors suggested that recruitment of p85 and CrkII are critical for entry. Consistent with this data, overexpression of wild-type CrkII stimulated bacterial uptake. Experiments with mutant CrkII alleles indicated that both the first and second SH3 domains of this adaptor participate in entry, with the second domain playing the most critical role. Taken together, these findings demonstrate novel roles for Gab1 and CrkII in Listeria internalization.

Actin-based motility of intracellular pathogens

The actin cytoskeleton is harnessed by several pathogenic bacteria that are capable of entering into non-phagocytic cells, the so-called 'invasive bacteria'. Among them, a few also exploit the host actin cytoskeleton to move intra- and inter-cellularly. Our knowledge of the basic mechanisms underlying actin-based motility has dramatically increased and the list of bacteria that are able to move in this way is also increasing including not only Listeria, Shigella and Rickettsia species but also Mycobacterium marinum and Burkholderia pseudomallei. In all cases the central player is the Arp2/3 complex. Vaccinia virus moves intracellularly on microtubules and just after budding, triggers actin polymerization and the formation of protrusions similar to that of adherent enteropathogenic Escherichia coli, that involve the Arp2/3 complex and facilitate its inter-cellular spread.

Redundant roles for Met docking site tyrosines and the Gab1 pleckstrin homology domain in InlB-mediated entry of Listeria monocytogenes

The bacterial pathogen Listeria monocytogenes causes food-borne illnesses leading to gastroenteritis, meningitis, or abortion. Listeria induces its internalization into some mammalian cells through interaction of the bacterial surface protein InlB with host Met receptor tyrosine kinase. Binding of InlB leads to phosphorylation of Met and the adapter Gab1 and to activation of host phosphoinositide (PI) 3-kinase. The mammalian ligand of Met, hepatocyte growth factor, promotes cell motility and morphogenesis in a manner dependent on phosphorylation of two docking site tyrosines at positions 1349 and 1356 in the receptor's cytoplasmic tail. Here we determined if these tyrosines were essential for Listeria entry. A derivative of the human cell line T47D stably expressing a truncated Met lacking most of its cytoplasmic domain was unable to support InlB-mediated signaling or entry. Surprisingly, cells expressing mutant Met containing phenylalanine substitutions in both tyrosines 1349 and 1356 (MetYF) allowed entry and InlB-induced Gab1 phosphorylation. However, in contrast to the situation in cells expressing wild-type Met, Gab1 phosphorylation in MetYF cells required PI 3-kinase activity. The Gab1 pleckstrin homology (PH) domain was constitutively associated with the plasma membrane of cells in a PI 3-kinase-dependent manner. Overexpression of the PH domain blocked entry of Listeria into cells expressing MetYF but not into cells expressing wild-type Met. Taken together, these results indicate that the docking site tyrosines are dispensable for internalization when membrane localization of Gab1 is constitutive. Distinct pathways of recruitment by phosphorylated tyrosines in Met and PH domain ligands in the membrane are redundant for bacterial entry.

WASP-related proteins, Abi1 and Ena/VASP are required forListeriainvasion induced by the Met receptor

Internalisation of the pathogenic bacterium Listeria monocytogenes involves interactions between the invasion protein InlB and the hepatocyte growth factor receptor, Met. Using colocalisation studies, dominant-negative constructs and small interfering RNA (siRNA), we demonstrate a cell-type-dependent requirement for various WASP-related proteins in Listeria entry and InlB-induced membrane ruffling. The WAVE2 isoform is essential for InlB-induced cytoskeletal rearrangements in Vero cells. In HeLa cells, WAVE1, WAVE2 and N-WASP cooperate to promote these processes. Abi1, a key component of WAVE complexes, is recruited at the entry site in both cell types and its inactivation by RNA interference impairs InlB-mediated processes. Ena/VASP proteins also play a role in Listeria internalization, and their deregulation by sequestration or overexpression, modifies actin cups beneath entering particles. Taken together, these results identify the WAVE complex, N-WASP and Ena/VASP as key effectors of the Met signalling pathway and of Listeria entry and highlight the existence of redundant and/or cooperative functions among WASP-family members.

Actin-based bacterial motility: towards a definition of the minimal requirements

At the border line between microbiology and cell biology, the spectacular capacity o f some intracellular bacterial pathogens, including Listeria monocytogenes, Shigella flexneri and several Rickettsias, to use actin polymerization as a driving force for intracellular movement, cell-to-cell spreading and dissemination within the infected tissue is being increasingly studied. Now that it is possible to manipulate the bacterial surface proteins involved in this process - ActA o f L. monocytogenes and IcsA of S. flexneri - these bacterial systems are providing experimental models in which to investigate the role o f actin filament dynamics in cell motility.

Evolutionary history of the genus Listeria and its virulence genes

The genus Listeria contains the two pathogenic species Listeria monocytogenes and Listeria ivanovii and the four apparently apathogenic species Listeria innocua, Listeria seeligeri, Listeria welshimeri, and Listeria grayi. Pathogenicity of the former two species is enabled by an approximately 9 kb virulence gene cluster which is also present in a modified form in L. seeligeri. For all Listeria species, the sequence of the virulence gene cluster locus and its flanking regions was either determined in this study or assembled from public databases. Furthermore, some virulence-associated internalin loci were compared among the six species. Phylogenetic analyses were performed on a data set containing the sequences of prs, ldh, vclA, and vclB (all directly flanking the virulence gene cluster), as well as the iap gene and the 16S and 23S-rRNA coding genes which are located at different sites in the listerial chromosomes. L. grayi represents the deepest branch within the genus. The remaining five species form two groupings which have a high bootstrap support and which are consistently found by using different treeing methods. One lineage represents L. monocytogenes and L. innocua, while the other contains L. welshimeri, L. ivanovii and L. seeligeri, with L. welshimeri forming the deepest branch. Based on this perception, we tried to reconstruct the evolution of the virulence gene cluster. Since no traces of lateral gene transfer events could be detected the most parsimonious scenario is that the virulence gene cluster was present in the common ancestor of L. monocytogenes, L. innocua, L. ivanovii, L. seeligeri and L. welshimeri and that the pathogenic capability has been lost in two separate events represented by L. innocua and L. welshimeri. This hypothesis is also supported by the location of the putative deletion breakpoints of the virulence gene cluster within L. innocua and L. welshimeri.

Interferon-gamma Mediates Neuronal Killing of Intracellular Bacteria

Neurons can be targets for microbes, which could kill the neurons. Just in reverse, we, in this study, report that bacteria can be killed when entering a neuron. Primary cultures of foetal mouse hippocampal neurons and a neuronal cell line derived from mouse hypothalamus were infected by Listeria monocytogenes. Treatment with interferon-gamma (IFN-gamma) did not affect bacterial uptake, but resulted in increased killing of intracellular bacteria, whereas the neuronal cell remained intact. The IFN-gamma-mediated bacterial killing was mapped to the neuronal cytosol, before listerial actin tail formation. Treatment with IFN-gamma induced phosphorylation of the transcription factor STAT-1 in neurons and IFN-gamma-mediated listerial killing was not observed in STAT-1(-/-) neurons or neurons treated with IFN regulatory factor-1 antisense oligonucleotides. IFN-gamma-treated neuronal cells showed increased levels of inducible nitric oxide synthase (iNOS) mRNA, and antisense iNOS oligonucleotides hampered the bacterial killing by neurons upon IFN-gamma treatment. This novel neuronal function - i.e., that of a microbe killer - could play a crucial role in the control of infections in the immuno-privileged nervous system.

Auto, a surface associated autolysin of Listeria monocytogenes required for entry into eukaryotic cells and virulence

Listeria monocytogenes is an opportunistic food-borne human and animal pathogen. Several surface proteins expressed by this intracellular pathogen are critical for the infectious process. By in silico analysis we compared the surface protein repertories of L. monocytogenes and of the non-pathogenic species Listeria innocua and identified a gene encoding a surface protein of L. monocytogenes absent in L. innocua. This gene that we named aut encodes a protein (Auto) of 572 amino acids containing a signal sequence, a N-terminal autolysin domain and a C-terminal cell wall-anchoring domain made up of four GW modules. We show here that the aut gene is expressed independently of the virulence gene regulator PrfA and encodes a surface protein with an autolytic activity. We provide evidence that Auto is required for entry of L. monocytogenes into cultured non-phagocytic eukaryotic cells. The low invasiveness of an aut deletion mutant correlates with its reduced virulence following intravenous inoculation of mice and oral infection of guinea pigs. During infection, the autolytic activity of Auto may also be critical. Auto appears thus as a novel type of L. monocytogenes virulence factor.

CD44-regulated intracellular proliferation of Listeria monocytogenes

CD44 has been implicated in immune and inflammatory processes. We have analyzed the role of CD44 in the outcome of Listeria monocytogenes infection in murine bone marrow-derived macrophages (BMM). Surprisingly, a dramatically decreased intracellular survival of L. monocytogenes was observed in CD44(-/-) BMM. CD44(-/-) heart or lung fibroblast cultures also showed reduced bacterial levels. Moreover, livers from CD44(-/-)-infected mice showed diminished levels of L. monocytogenes. In contrast, intracellular growth of Salmonella enterica serovar Typhimurium was the same in CD44(-/-) and control BMM. The CD44-mediated increased bacterial proliferation was not linked to altered BMM differentiation or to secretion of soluble factors. CD44 did not mediate listerial uptake, and it played no role in bacterial escape from the primary phagosome or formation of actin tails. Furthermore, CD44-enhanced listerial proliferation occurred in the absence of intracellular bacterial spreading. Interestingly, coincubation of BMM with hyaluronidase or anti-CD44 antibodies that selectively inhibit hyaluronan binding increased intracellular listerial proliferation. Treatment of cells with hyaluronan, in contrast, diminished listerial growth and induced proinflammatory transcript levels. We suggest that L. monocytogenes takes advantage of the CD44-mediated signaling to proliferate intracellularly, although binding of CD44 to certain ligands will inhibit such response.

Enteral immunization with attenuated recombinant Listeria monocytogenes as a live vaccine vector: organ-dependent dynamics of CD4 T lymphocytes reactive to a Leishmania major tracer epitope

Listeria monocytogenes is considered as a potential live bacterial vector, particularly for the induction of CD8 T cells. The CD4 T-cell immune response triggered after enteral immunization of mice has not yet been thoroughly characterized. The dynamics of gamma interferon (IFN-gamma)- and interleukin-4 (IL-4)-secreting CD4 T cells were analyzed after priming through intragastric delivery of an attenuated delta actA recombinant L. monocytogenes strain expressing the Leishmania major LACK protein; a peptide of this protein, LACK(158-173) peptide (pLACK), is a well-characterized CD4 T-cell target in BALB/c mice. Five compartments were monitored: Peyer's patches, mesenteric lymph nodes (MLN), spleen, liver, and blood. A single intragastric inoculation of delta actA-LACK-LM in BALB/c mice led to colonization of the MLN and spleen at a significant level for at least 3 days. Efficient priming of IFN-gamma-secreting pLACK-reactive CD4 T cells was observed in all tested compartments. Interestingly, IL-4-secreting pLACK-reactive CD4 T cells were detectable at day 6 or 7 only in blood and liver. The absence of translocation of viable bacteria through the intestinal epithelium after further delta actA-LACK-LM inoculations was concomitant with the absence of an increase in the level of IFN-gamma secreted by the MLN, blood, and splenic pLACK-reactive Th1 T cells, although the levels remained significantly above the basal level. No change in this population size was detected in the spleen. However, an increase in the number of intragastric inoculations had a clinical beneficial effect in L. major-infected BALB/c mice. L. monocytogenes thus presents the potential of an efficient vector for induction of CD4 T cells when administered by the enteral route.

Effect of intragastric and intraperitoneal immunisation with attenuated and wild-type LACK-expressing Listeria monocytogenes on control of murine Leishmania major infection

Stable chromosomal constructs of attenuated DeltaactA and wild-type Listeria monocytogenes expressing the Leishmania major protein LACK were tested as live vaccine vectors in the Th2-orientated chronic L. major murine infection model. These vectors, either by intraperitoneal (i.p.) or intragastric (i.g.) route, were able to induce a strong CD4 Th1 immune response that was correlated with slower parasite growth in the infected footpad. Significant protection against L. major infection was observed in BALB/c mice, ranging from delay in the lesion onset to full protection in 80% of the challenged animals, depending on the size of the parasite inoculum challenge. The i.g. route gave clinically higher protection level than the i.p. route. Both bacterial vectors were as efficient, suggesting that the extent of in vivo bacterial dissemination and multiplication did not seem to be a key parameter for induction of an efficient protective immune response.

Actin-based motility of intracellular microbial pathogens

A diverse group of intracellular microorganisms, including Listeria monocytogenes, Shigella spp., Rickettsia spp., and vaccinia virus, utilize actin-based motility to move within and spread between mammalian host cells. These organisms have in common a pathogenic life cycle that involves a stage within the cytoplasm of mammalian host cells. Within the cytoplasm of host cells, these organisms activate components of the cellular actin assembly machinery to induce the formation of actin tails on the microbial surface. The assembly of these actin tails provides force that propels the organisms through the cell cytoplasm to the cell periphery or into adjacent cells. Each of these organisms utilizes preexisting mammalian pathways of actin rearrangement to induce its own actin-based motility. Particularly remarkable is that while all of these microbes use the same or overlapping pathways, each intercepts the pathway at a different step. In addition, the microbial molecules involved are each distinctly different from the others. Taken together, these observations suggest that each of these microbes separately and convergently evolved a mechanism to utilize the cellular actin assembly machinery. The current understanding of the molecular mechanisms of microbial actin-based motility is the subject of this review.

A role for cofilin and LIM kinase in Listeria-induced phagocytosis

The pathogenic bacterium Listeria monocytogenes is able to invade nonphagocytic cells, an essential feature for its pathogenicity. This induced phagocytosis process requires tightly regulated steps of actin polymerization and depolymerization. Here, we investigated how interactions of the invasion protein InlB with mammalian cells control the cytoskeleton during Listeria internalization. By fluorescence microscopy and transfection experiments, we show that the actin-nucleating Arp2/3 complex, the GTPase Rac, LIM kinase (LIMK), and cofilin are key proteins in InlB-induced phagocytosis. Overexpression of LIMK1, which has been shown to phosphorylate and inactivate cofilin, induces accumulation of F-actin beneath entering particles and inhibits internalization. Conversely, inhibition of LIMK's activity by expressing a dominant negative construct, LIMK1(-), or expression of the constitutively active S3A cofilin mutant induces loss of actin filaments at the phagocytic cup and also inhibits phagocytosis. Interestingly, those constructs similarly affect other actin-based phenomenons, such as InlB-induced membrane ruffling or Listeria comet tail formations. Thus, our data provide evidence for a control of phagocytosis by both activation and deactivation of cofilin. We propose a model in which cofilin is involved in the formation and disruption of the phagocytic cup as a result of its local progressive enrichment.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Pathogenicity islands and virulence evolution in Listeria

As in other bacterial pathogens, the virulence determinants of Listeria species are clustered in genomic islands scattered along the chromosome. This review summarizes current knowledge about the structure, distribution and role in pathogenesis of Listeria virulence loci. Hypotheses about the mode of acquisition and evolution of these loci in this group of Gram-positive bacteria are presented and discussed.

Actin-based motility of pathogens: the Arp2/3 complex is a central player

Bacterial actin-based motility has provided cell biologists with tools that led to the recent discovery that, in many forms of actin-based motilities, a key player is a protein complex named the Arp2/3 complex. The Arp2/3 complex is evolutionally conserved and made up of seven polypeptides involved in both actin filament nucleation and organization. Interestingly, this complex is inactive by itself and recent work has highlighted the fact that its activation is achieved differently in the different types of actin-based motilities, including the well-known examples of Listeria and Shigella motilities. Proteins of the WASP family and small G-proteins are involved in most cases. It is interesting that bacteria bypass or mimic some of the events occurring in eukaryotic systems. The Shigella protein IcsA recruits N-WASP and activates it in a Cdc42-like fashion. This activation leads to Arp2/3 complex recruitment, activation of the complex and ultimately actin polymerization and movement. The Listeria ActA protein activates Arp2/3 directly and, thus, seems to mimic proteins of the WASP family. A breakthrough in the field is the recent reconstitution of the actin-based motilities of Listeria and N-WASP-coated E. coli (IcsA) using a restricted number of purified cellular proteins including F-actin, the Arp2/3 complex, actin depolymerizing factor (ADF or cofilin) and capping protein. The movement was more effective upon addition of profilin, alpha-actinin and VASP (for Listeria). Bacterial actin-based motility is now one of the best-documented examples of the exploitation of mammalian cell machineries by bacterial pathogens.

Poisons, ruffles and rockets: bacterial pathogens and the host cell cytoskeleton

The cytoskeleton of eukaryotic cells is affected by a number of bacterial and viral pathogens. In this review we consider three recurring themes of cytoskeletal involvement in bacterial pathogenesis: 1) the effect of bacterial toxins on actin-regulating small GTP-binding proteins; 2) the invasion of non-phagocytic cells by the bacterial induction of ruffles at the plasma membrane; 3) the formation of actin tails and pedestals by intracellular and extracellular bacteria, respectively. Considerable progress has been made recently in the characterization of these processes. It is becoming clear that bacterial pathogens have developed a variety of sophisticated mechanisms for utilizing the complex cytoskeletal system of host cells. These bacterially-induced processes are now providing unique insights into the regulation of fundamental eukaryotic mechanisms.

Actin and phosphoinositide binding by the ActA protein of the bacterial pathogen Listeria monocytogenes

The surface protein ActA of the pathogenic bacterium Listeria monocytogenes induces actin-driven movement of bacteria in the cytoplasm of infected host cells and serves as a model for actin-based motility in general. We generated and purified soluble recombinant fragments of ActA and assessed their ability to interact with the acidic phospholipids phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate, both implicated in the regulation of actin polymerization. Purified ActA consisted of biologically active, elongated molecules with an alpha-helix and beta-sheet content of 11 and 32%, respectively. In the presence of either phosphatidylinositol 4,5-bisphosphate or phosphatidylinositol 3,4,5-trisphosphate, but not phosphatidylcholine, ActA molecules underwent a structural change that raised the alpha-helix content to 19% and lowered the beta-sheet content to 27%. Co-sedimentation experiments with phosphatidylcholine vesicles containing different acidic phospholipids demonstrated that ActA binds preferentially to D-3 phosphoinositides. The D-3 phosphoinositide binding activity was mapped to a small subregion in the N-terminal domain of ActA. This subregion comprised 19 amino acids and showed homology to cecropins. In addition, we found that amino acids 33 to 74 of ActA mediated actin binding by the whole, folded ActA molecule. These findings shed new light on ActA function.

Intracellular pathogens and the actin cytoskeleton

Many pathogens actively exploit the actin cytoskeleton during infection. This exploitation may take place during entry into mammalian cells after engagement of a receptor and/or as series of signaling events culminating in the engulfment of the microorganism. Although actin rearrangements are a common feature of most internalization events (e.g. entry of Listeria, Salmonella, Shigella, Yersinia, Neisseria, and Bartonella), bacterial and other cellular factors involved in entry are specific to each bacterium. Another step during which pathogens harness the actin cytoskeleton takes place in the cytosol, within which some bacteria (Listeria, Shigella, Rickettsia) or viruses (vaccinia virus) are able to move. Movement is coupled to a polarized actin polymerization process, with the formation of characteristic actin tails. Increasing attention has focused on this phenomenon due to its striking similarity to cellular events occurring at the leading edge of locomoting cells. Thus pathogens are convenient systems in which to study actin cytoskeleton rearrangements in response to stimuli at the plasma membrane or inside cells.

Identification of cofilin, coronin, Rac and capZ in actin tails using a Listeria affinity approach

Actin assembly is involved in cell motility and intracellular movement of Listeria monocytogenes. Induction of Listeria actin tails is mediated by the surface protein ActA. The N-terminal domain of ActA is sufficient for this function. Cell components known to play a role in the actin-based motility of Listeria are VASP (vasodilatator-stimulated phosphoprotein), the multiprotein Arp2/3 complex and cofilin. VASP interacts with the central domain of ActA. Proteins interacting with the N-terminal domain of ActA have not been identified. To identify novel host cell components of ActA-induced actin tails, we used bovine brain extracts and an affinity approach with Listeria as matrix. Several known components of Listeria tails were isolated including VASP, Arp3 and cofilin. Cofilin was identified by peptide sequencing, and cofilin recruitment and Listeria tail length were found to be pH-dependent, in agreement with its recently reported role in enhancing actin filament turnover. In addition, three proteins not previously known to be associated with Listeria tails, coronin, Rac and capZ, were identified in our affinity approach. In infected cells, the localization of the identified proteins was studied by immunofluorescence. Our findings suggest that these latter proteins, which are known to play critical roles in cellular actin rearrangements, may also be involved in the dynamics of Listeria-induced actin assembly.

The InIB protein of Listeria monocytogenes is sufficient to promote entry into mammalian cells

InIB is one of the two Listeria monocytogenes invasion proteins required for bacterial entry into mammalian cells. Entry into human epithelial cells such as Caco-2 requires InIA, whereas InIB is needed for entry into cultured hepatocytes and some epithelial or fibroblast cell lines such as Vero, HEp-2 and HeLa cells. InIB-mediated entry requires tyrosine phosphorylation, cytoskeletal rearrangements and activation of the host protein phosphoinositide (PI) 3-kinase, probably in response to engagement of a receptor. In this study, we demonstrate for the first time that InIB is sufficient to promote internalization. Indeed, coating of normally non-invasive bacteria or inert latex beads with InIB leads to internalization into mammalian cells. In addition, a soluble form of InIB also appears to promote uptake of non-invasive bacteria, albeit at a very low level. Similar to entry of L. monocytogenes, uptake of InIB-coated beads required tyrosine phosphorylation in the host cell, PI 3-kinase activity and cytoskeletal reorganization. Taken together, these data indicate that InIB is sufficient for entry of L. monocytogenes into host cells and suggest that this protein is an effector of host cell signalling pathways.

Host-pathogen interactions during entry and actin-based movement of Listeria monocytogenes

Listeria monocytogenes is a pathogenic bacterium that induces its own uptake into mammalian cells, and spreads from one cell to another by an actin-based motility process. Entry into host cells involves the bacterial surface proteins InlA (internalin) and InlB. The receptor for InlA is the cell adhesion molecule E-cadherin. InlB-mediated entry requires activation of the host protein phosphoinositide (PI) 3-kinase, probably in response to engagement of a receptor. Actin-based movement of L. monocytogenes is mediated by the bacterial surface protein ActA. The N-terminal region of this protein is necessary and sufficient for polymerization of host cell actin. Other host proteins involved in bacterial motility include profilin, Vasodilator-Stimulated Phosphoprotein (VASP), the Arp2/Arp3 complex, and cofilin. Studies of entry and intracellular movement of L. monocytogenes could lead to a better understanding of receptor-ligand signaling and dynamics of actin polymerization in mammalian cells.

Interaction of vaccinia virus with the actin cytoskeleton

Vaccinia virus infection results in large rearrangements of the host actin cytoskeleton including the formation of actin tails that are strikingly similar to those seen in Listeria, Shigella and Rickettsia infections. Using actin polymerization as the driving force the intracellular enveloped form of the vaccinia virus (IEV) is propelled on the tip of actin tails at a speed of 2.8 microns/min, both intra- and intercellularly. The similarities between the actin-based motility of the vaccinia virus, Listeria, Shigella and Rickettsia suggest that intracellular pathogens have developed a common strategy to exploit the actin cytoskeleton of the host to facilitate their intercellular spread. This review focuses on our current understanding of the interactions between the vaccinia virus and the actin cytoskeleton.

The role of actin binding proteins in epithelial morphogenesis: models based upon Listeria movement

We summarize recent findings on the organization of the protein actin in eucaryotic cells. In particular we focus on how actin can be used to generate a vectorial force that is required for cell movement. These forces arise from protein molecules that recruit actin to the plasma membrane in such a manner that actin filaments extend outward from the cell body. This type of actin dependent force generation has been described in a nucleation-release model, which is one of several models currently being tested to explain actin dependent cell movement. Data in support of this model has arisen unexpectedly from studies of an intracellular bacteria, Listeria monocytogenes. This bacteria uses actin to propel itself during infection of eucaryotic cells. By studying Listeria movement, the roles of several eucaryotic actin interacting proteins have been identified. One of these is zyxin, a human protein that shares important structural and possibly functional properties with ActA, an actin dependent force generating protein of Listeria. We intend to test the function of these and other actin interacting proteins in a simplified system that should facilitate precise measurement of their properties of force generation in vitro.

A novel proline-rich motif present in ActA of Listeria monocytogenes and cytoskeletal proteins is the ligand for the EVH1 domain, a protein module present in the Ena/VASP family

The ActA protein of the intracellular pathogen Listeria monocytogenes induces a dramatic reorganization of the actin-based cytoskeleton. Two profilin binding proteins, VASP and Mena, are the only cellular proteins known so far to bind directly to ActA. This interaction is mediated by a conserved module, the EVH1 domain. We identify E/DFPPPPXD/E, a motif repeated 4-fold within the primary sequence of ActA, as the core of the consensus ligand for EVH1 domains. This motif is also present and functional in at least two cellular proteins, zyxin and vinculin, which are in this respect major eukaryotic analogs of ActA. The functional importance of the novel protein-protein interaction was examined in the Listeria system. Removal of EVH1 binding sites on ActA reduces bacterial motility and strongly attenuates Listeria virulence. Taken together we demonstrate that ActA-EVH1 binding is a paradigm for a novel class of eukaryotic protein-protein interactions involving a proline-rich ligand that is clearly different from those described for SH3 and WW/WWP domains. This class of interactions appears to be of general importance for processes dependent on rapid actin remodeling.

Identification of two regions in the N-terminal domain of ActA involved in the actin comet tail formation by Listeria monocytogenes

The ActA protein of Listeria monocytogenes induces actin nucleation on the bacterial surface. The continuous process of actin filament elongation provides the driving force for bacterial propulsion in infected cells or cytoplasmic extracts. Here, by fusing the N-terminus of ActA (residues 1-234) to the omega fragment of beta-galactosidase, we present the first evidence that this domain contains all the necessary elements for actin tail formation. A detailed analysis of ActA variants, in which small fragments of the N-terminal region were deleted, allowed the identification of two critical regions. Both are required to initiate the actin polymerization process, but each has in addition a specific role to maintain the dynamics of the process. The first region (region T, amino acids 117-121) is critical for filament elongation, as shown by the absence of actin tail in a 117-121 deletion mutant or when motility assays are performed in the presence of anti-region T antibodies. The second region (region C, amino acids 21-97), is more specifically involved in maintenance of the continuity of the process, probably by F-actin binding or prevention of barbed end capping, as strongly suggested by both a deletion (21-97) leading to 'discontinuous' actin tail formation and in vitro experiments showing that a synthetic peptide covering residues 33-74 can interact with F-actin. Our results provide the first insights in the molecular dissection of the actin polymerization process induced by the N-terminal domain of ActA.

Host cell heparan sulfate proteoglycans mediate attachment and entry of Listeria monocytogenes, and the listerial surface protein ActA is involved in heparan sulfate receptor recognition

The mechanisms by which the intracellular pathogen Listeria monocytogenes interacts with the host cell surface remain largely unknown. In this study, we investigated the role of heparan sulfate proteoglycans (HSPG) in listerial infection. Pretreatment of bacteria with heparin or heparan sulfate (HS), but not with other glycosaminoglycans, inhibited attachment and subsequent uptake by IC-21 murine macrophages and CHO epithelial-like cells. Specific removal of HS from target cells with heparinase III significantly impaired listerial adhesion and invasion. Mutant CHO cells deficient in HS synthesis bound and internalized significantly fewer bacteria than wild-type cells did. Pretreatment of target cells with the HS-binding proteins fibronectin and platelet factor 4, or with heparinase III, impaired listerial infectivity only in those cells expressing HS. Moreover, a synthetic peptide corresponding to the HS-binding ligand in Plasmodium falciparum circumsporozoite protein (pepPf1) inhibited listerial attachment to IC-21 and CHO cells. A motif very similar to the HS-binding site of pepPf1 was found in the N-terminal region of ActA, the L. monocytogenes surface protein responsible for actin-based bacterial motility and cell-to-cell spread. In the same region of ActA, several clusters of positively charged amino acids which could function as HS-binding domains were identified. An ActA-deficient mutant was significantly impaired in attachment and entry due to altered HS recognition functions. This work shows that specific interaction with an HSPG receptor present on the surface of both professional and nonprofessional phagocytes is involved in L. monocytogenes cytoadhesion and invasion and strongly suggests that the bacterial surface protein ActA may be a ligand mediating HSPG receptor recognition.