Molecular and genetic determinants of the Listeria monocytogenes infectious process

From the beginning to the present state of molecular microbial pathogenesis-A tribute to Pascale Cossart

The universe of Molecular Microbial Pathogenesis is filled with many female and male stars. But there are two particularly bright shining supernovae-like stars: the late Stanley Falkow and the very lively and creative Pascale Cossart. These two outstanding luminaries, surrounded by numerous planets, do not only belong to different scientific generations but their splendor also comes from very different scientific concepts. Stanley Falkow, often referred to as the 'Father of Molecular Microbial Pathogenesis', made many groundbreaking contributions to this field by addressing almost all important bacterial pathogens. Pascale Cossart, who could be called in analogy the 'Queen of Modern Molecular Microbial Pathogenesis' by combining the Microbiology and Cell Biology, concentrates in her similarly impressive scientific work essentially on a single bacterial species which she studied and still studies in great depth: the facultative intracellular bacterial pathogen Listeria monocytogenes-and the vast majority of her most prominent publications deals with this pathogen in almost all facets. It is certainly not an exaggeration to say that she together with her co-workers and collaborators developed this model bacterium into a paradigm among the intracellular bacterial pathogens.

Exploring specific primers targeted against different genes for a multiplex PCR for detection of Listeria monocytogenes

The efficacy of six different sets of primers targeted against 16S rRNA and virulence genes such as ‘iap’, ‘hly’ and ‘prf’ was evaluated in separate PCR assays. The primer pairs targeted against 16S rRNA resulted into amplification of 1.2 kb PCR product. However, sets of primers targeted against different regions of ‘iap’ produced 371 and 660 bp PCR products, respectively. The primer pair targeted against ‘prf’ gene could produce 508 bp product. Three primer pairs targeted against different regions of ‘hly’, i.e., ‘hly’, ‘hly A’ and ‘hly K9’ were able to amplify 713, 276 and 384 bp products, respectively. The PCR conditions were also optimized in respect of two internal sets of primers falling within ‘iap’ and ‘hly’ genes that amplified 119 and 188 bp products to verify the PCR results obtained with respective external sets of primers. Three different combinations involving four sets of primers based on 16S rRNA, ‘iap’, ‘hly’ and ‘prf’ were explored in respective multiplex PCR assays in order to select a suitable combination. Combination 1 and 3 worked successfully as revealed by amplification of all the four bands of expected sizes on agarose gel. However, while optimizing the different parameters for developing a functional multiplex PCR, it was observed that in both these combinations, only two of the amplified products, i.e., 1.2 kb and 713 bp could be invariably detected. Hence, these two primers were combined in the multiplex PCR and the conditions were optimized for application in dairy foods for detection of Listeria monocytogenes.

Involvement of Reactive Oxygen Intermediate in the Enhanced Expression of Virulence-Associated Genes ofListeria monocytogenesinside Activated Macrophages

Listeriolysin O encoded by 1,587 bp hly is the essential virulence factor of Listeria monocytogenes that replicates in the cytosolic space after escaping from phagosome of macrophages. By using murine macrophage-like J774.1 cells with or without activation by IFN-gamma plus LPS, the expression of both hly and its positive regulator prfA was monitored by means of RT-PCR. In activated J774.1 cells, the level of hly expression was enhanced although the multiplication of bacteria was significantly suppressed. The elevated expression of hly inside activated macrophage was abolished by addition of SOD and catalase, suggesting that reactive oxygen intermediates contribute to the upregulation of prfA and hly transcriptions. Moreover, we found that exposure of L. monocytogenes to H2O2 dramatically enhanced the expression of both prfA and hly mRNAs. Spontaneous ONOO- generator, SIN-1, also promoted the transcription to a certain level. These results suggested that oxygen radicals generated in activated macrophages provide a positive signal for up-regulation of virulence genes in L. monocytogenes.

Role of a GntR-family response regulator LbrA in Listeria monocytogenes biofilm formation

The formation of Listeria monocytogenes biofilms contributes to persistent contamination in food processing facilities. A microarray comparison of L. monocytogenes between the transcriptome of the strong biofilm forming strain (Bfm^s) Scott A and the weak biofilm forming (Bfm^w) strain F2365 was conducted to identify genes potentially involved in biofilm formation. Among 951 genes with significant difference in expression between the two strains, a GntR-family response regulator encoding gene (LMOf2365_0414), designated lbrA, was found to be highly expressed in Scott A relative to F2365. A Scott A lbrA-deletion mutant, designated AW3, formed biofilm to a much lesser extent as compared to the parent strain by a rapid attachment assay and scanning electron microscopy. Complementation with lbrA from Scott A restored the Bfm^s phenotype in the AW3 derivative. A second microarray assessment using the lbrA deletion mutant AW3 and the wild type Scott A revealed a total of 304 genes with expression significantly different between the two strains, indicating the potential regulatory role of LbrA in L. monocytogenes. A cloned copy of Scott A lbrA was unable to confer enhanced biofilm forming potential in F2365, suggesting that additional factors contributed to weak biofilm formation by F2365.

Proteoglycans in host-pathogen interactions: molecular mechanisms and therapeutic implications

Many microbial pathogens subvert proteoglycans for their adhesion to host tissues, invasion of host cells, infection of neighbouring cells, dissemination into the systemic circulation, and evasion of host defence mechanisms. Where studied, specific virulence factors mediate these proteoglycan-pathogen interactions, which are thus thought to affect the onset, progression and outcome of infection. Proteoglycans are composites of glycosaminoglycan (GAG) chains attached covalently to specific core proteins. Proteoglycans are expressed ubiquitously on the cell surface, in intracellular compartments, and in the extracellular matrix. GAGs mediate the majority of ligand-binding activities of proteoglycans, and many microbial pathogens elaborate cell-surface and secreted factors that interact with GAGs. Some pathogens also modulate the expression and function of proteoglycans through known virulence factors. Several GAG-binding pathogens can no longer attach to and invade host cells whose GAG expression has been reduced by mutagenesis or enzymatic treatment. Furthermore, GAG antagonists have been shown to inhibit microbial attachment and host cell entry in vitro and reduce virulence in vivo. Together, these observations underscore the biological significance of proteoglycan-pathogen interactions in infectious diseases.

A novel prfA mutation that promotes Listeria monocytogenes cytosol entry but reduces bacterial spread and cytotoxicity

Listeria monocytogenes is an environmental bacterium that becomes a pathogen following ingestion by a mammalian host. The transition from environmental organism to pathogen requires significant changes in gene expression, including the increased expression of gene products that contribute to bacterial growth within host cells. PrfA is an L. monocytogenes transcriptional regulator that becomes activated upon bacterial entry into mammalian cells and induces the expression of gene products required for virulence. How PrfA activation occurs is not known, however several mutations have been identified that increase PrfA activity in strains grown in vitro (prfA mutations). Here we describe a novel prfA mutation that enhances extracellular PrfA-dependent gene expression but in contrast to prfA mutants inhibits the cytosol-mediated induction of virulence genes. prfA Y154C strains entered cells and escaped from phagosomes with an efficiency similar to wild type bacteria, however the mutation prevented efficient L. monocytogenes actin polymerization and reduced spread of bacteria to adjacent cells. The prfA Y154C mutation severely attenuated bacterial virulence in mice but the mutant strains did generate target antigen specific CD8(+) effector cells. Interestingly, the prfA Y154C mutant was significantly less cytotoxic for host cells than wild type L. monocytogenes. The prfA Y154C mutant strain may therefore represent a novel attenuated strain of L. monocytogenes for antigen delivery with reduced host cell toxicity.

From hot dogs to host cells: how the bacterial pathogen Listeria monocytogenes regulates virulence gene expression

Environmental pathogens are organisms that normally spend a substantial part of their lifecycle outside of human hosts, but when introduced into humans are capable of causing disease. Such organisms are often able to transition between disparate environments ranging from the soil to the cytosol of host cells. The food-borne bacterial pathogen Listeria monocytogenes serves as a model system for understanding how an environmental organism makes the transition into mammalian hosts. A transcriptional regulatory protein known as PrfA appears to serve as a critical switch, enabling L. monocytogenes to transition from the outside environment to life within the host cell cytosol. PrfA is required for the expression of many L. monocytogenes gene products associated with virulence, and multiple mechanisms serve to regulate the expression and activity of PrfA. Increasing evidence suggests that specific environmental stresses help prime L. monocytogenes for life within the host, and cross-talk between the stress response regulator sigma-B and PrfA may mediate the transition from outside environment to cytosol. Once within the host cytosol, multiple changes in bacterial metabolism and gene expression help to complete the transformation of L. monocytogenes from soil dweller to intracellular pathogen.

A novel mutation within the central Listeria monocytogenes regulator PrfA that results in constitutive expression of virulence gene products

The PrfA protein of Listeria monocytogenes functions as a key regulatory factor for the coordinated expression of many virulence genes during bacterial infection of host cells. PrfA activity is controlled by multiple regulatory mechanisms, including an apparent requirement for either the presence of a cofactor or some form of posttranslational modification that regulates the activation of PrfA. In this study, we describe the identification and characterization of a novel PrfA mutation that results in constitutive activation of the PrfA protein. The PrfA L140F mutation was found to confer high-level expression of PrfA-regulated genes and to be functionally dominant over the wild-type allele. The presence of the PrfA L140F mutation resulted in the aggregation of L. monocytogenes in broth culture and, unlike previously described prfA mutations, appeared to be slightly toxic to the bacteria. High-level PrfA-dependent gene expression showed no additional increase in L. monocytogenes strains containing an additional copy of prfA L140F despite a >4-fold increase in PrfA protein levels. In contrast, the introduction of multiple copies of the wild-type prfA allele to L. monocytogenes resulted in a corresponding increase in PrfA-dependent gene expression, although overall expression levels remained far below those observed for PrfA L140F strains. These results suggest a hierarchy of PrfA regulation, such that the relative levels of PrfA protein present within the cell correlate with the levels of PrfA-dependent gene expression when the protein is not in its fully activated state; however, saturating levels of the protein are then quickly reached when PrfA is converted to its active form. Regulation of the PrfA activation status must be an important facet of L. monocytogenes survival, as mutations that result in constitutive PrfA activation may have deleterious consequences for bacterial physiology.

Modulation of the immune system by Listeria monocytogenes-mediated gene transfer into mammalian cells

In this study, we established a method for Listeria monocytogenes(Lm)-mediated gene transfer into mammalian cells to manipulate the immune response of the host during infection by pathogens. We used the Lm-mediated gene transfer method in an in vivo study to manipulate host immune responses against Leishmania major(L. major )-infection. The injection of Lm modulated the susceptible host into a resistant state against L. major-infection. A more efficient protective effect was obtained with the injection of IL-12-cDNA containing Lm, and the protective effect was stronger than that of the resistant strain. The protective mechanism of Lm-injection against L. major-infection observed here appeared to be a result of the activation of the local immune system by the Lm-mediated gene transfer method. The present study is the first demonstration that a gene introduced into a host by Lm works to modulate the murine host immune response against infections in vivo. Since this system strongly induces Th1 responses and suppresses Th2 responses in infected hosts, the system can be used for controlling infectious diseases and for protection against allergic responses in the future.

Genome diversification in phylogenetic lineages I and II of Listeria monocytogenes: identification of segments unique to lineage II populations

Thirteen different serotypes of Listeria monocytogenes can be distinguished on the basis of variation in somatic and flagellar antigens. Although the known virulence genes are present in all serotypes, greater than 90% of human cases of listeriosis are caused by serotypes 1/2a, 1/2b, and 4b and nearly all outbreaks of food-borne listeriosis have been caused by serotype 4b strains. Phylogenetic analysis of these three common clinical serotypes places them into two different lineages, with serotypes 1/2b and 4b belonging to lineage I and 1/2a belonging to lineage II. To begin examining evolution of the genome in these serotypes, DNA microarray analysis was used to identify lineage-specific and serotype-specific differences in genome content. A set of 44 strains representing serotypes 1/2a, 1/2b, and 4b was probed with a shotgun DNA microarray constructed from the serotype 1/2a strain 10403s. Clones spanning 47 different genes in 16 different contiguous segments relative to the lineage II 1/2a genome were found to be absent in all lineage I strains tested (serotype 4b and 1/2b) and an additional nine were altered exclusively in 4b strains. Southern hybridization confirmed that conserved alterations were, in all but two loci, due to absence of the segments from the genome. Genes within these contiguous segments comprise five functional categories, including genes involved in synthesis of cell surface molecules and regulation of virulence gene expression. Phylogenetic reconstruction and examination of compositional bias in the regions of difference are consistent with a model in which the ancestor of the two lineages had the 1/2 somatic serotype and the regions absent in the lineage I genome arose by loss of ancestral sequences.

In vivo depletion of CD11c+ dendritic cells abrogates priming of CD8+ T cells by exogenous cell-associated antigens

Cytotoxic T lymphocytes (CTL) respond to antigenic peptides presented on MHC class I molecules. On most cells, these peptides are exclusively of endogenous, cytosolic origin. Bone marrow-derived antigen-presenting cells, however, harbor a unique pathway for MHC I presentation of exogenous antigens. This mechanism permits cross-presentation of pathogen-infected cells and the priming of CTL responses against intracellular microbial infections. Here, we report a novel diphtheria toxin-based system that allows the inducible, short-term ablation of dendritic cells (DC) in vivo. We show that in vivo DC are required to cross-prime CTL precursors. Our results thus define a unique in vivo role of DC, i.e., the sensitization of the immune system for cell-associated antigens. DC-depleted mice fail to mount CTL responses to infection with the intracellular bacterium Listeria monocytogenes and the rodent malaria parasite Plasmodium yoelii.

Intracellular induction of Listeria monocytogenes actA expression

Following entry into the host cytosol, the bacterial pathogen Listeria monocytogenes dramatically increases the expression of several key virulence factors. The expression of actA, whose protein product is required for L. monocytogenes actin-based intracellular motility, is increased by more than 200-fold in cytosolic bacteria in comparison to broth-grown cultures. Two distinct promoter elements have been reported to regulate actA expression. One promoter is located immediately upstream of actA coding sequences, while the second promoter is contributed by the upstream mpl gene via the generation of an mpl-actA-plcB transcript. A series of L. monocytogenes mutants were constructed to define the contributions of individual promoter elements to actA expression. The intracellular induction of actA expression was found to be dependent upon the actA proximal promoter; the mpl promoter appeared to contribute to the extracellular induction of actA but did not affect intracellular levels of expression. The actA promoter is dependent upon a regulatory factor known as PrfA for transcriptional activation; however, no increase in actA expression was detected following the introduction of a high-affinity PrfA binding site within the actA promoter. The presence of a mutationally activated form of PrfA, known as PrfA*, increased overall actA expression in broth-grown cultures of both wild-type and actA promoter mutant strains, but the levels of induction observed were still approximately 50-fold lower than those observed for intracellularly grown L. monocytogenes. Collectively, these results indicate that the dramatic induction of actA expression that occurs in the host cell cytosol is mediated through a single promoter element. Furthermore, intracellular induction of actA appears to require additional steps or factors beyond those necessary for the activation and binding of PrfA to the actA promoter.

RAPD- and actA gene-typing of Listeria monocytogenes isolates of human listeriosis, the intestinal contents of cows and beef

Seventy-five L. monocytogenes isolates of human listeriosis, the intestinal contents of cows and beef were divided into 5 major clusters, 17 sub-clusters and 28 minor clusters by typing using random amplification of polymorphic DNA (RAPD). According to their major RAPD category, L. monocytogenes isolates serotyped as 1/2b and 4b were distinguished from L. monocytogenes isolates of serovars 1/2a and 1/2c. Moreover serovar 4b was distinguished from serovar 1/2b by a difference in the RAPD sub-cluster category. All L. monocytogenes were found to possess either actA gene Type I or II, and only one actA gene type was detected in each RAPD minor cluster. actA gene Type II was observed in 32.0%, 38.5% and 18.9% of isolates from humans, cows and beef, respectively, and was detected more frequently in serovar 4b (46.9%) than in serovars 1/2a (22.2%), 1/2b (7.7%) and 1/2c (0.0%). Twenty (80%) of 25 human isolates fell within three minor RAPD types (II-d (16%), V-p-1 (36%), V-p-2 (28%)). Two isolates from humans and beef were found to have the same RAPD type (Type IV-k-1), actA gene type (Type I) and serovar (1/2b). Our results suggest that only a few genotypes of L. monocytogenes are predominant in human listeriosis in Japan, although the human isolates were collected over a broad span of time and a wide geographical range. Our results also suggest that RAPD-, actA gene- and sero-typing can be useful for epidemiological analysis.

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

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

Dendritic cells are early cellular targets of Listeria monocytogenes after intestinal delivery and are involved in bacterial spread in the host

We studied the sequence of cellular events leading to the dissemination of Listeria monocytogenes from the gut to draining mesenteric lymph nodes (MLNs) by confocal microscopy of immunostained tissue sections from a rat ligated ileal loop system. OX-62-positive cells beneath the epithelial lining of Peyer's patches (PPs) were the first Listeria targets identified after intestinal inoculation. These cells had other features typical of dendritic cells (DCs): they were large, pleiomorphic and major histocompatibility complex class II(hi). Listeria were detected by microscopy in draining MLNs as early as 6 h after inoculation. Some 80-90% of bacteria were located in the deep paracortical regions, and 100% of the bacteria were present in OX-62-positive cells. Most infected cells contained more than five bacteria each, suggesting that they had arrived already loaded with bacteria. At later stages, the bacteria in these areas were mostly present in ED1-positive mononuclear phagocytes. These cells were also infected by an actA mutant defective in cell-to-cell spreading. This suggests that Listeria are transported by DCs from PPs to the deep paracortical regions of draining MLNs and are then transmitted to other cell populations by mechanisms independent of ActA. Another pathway of dissemination to MLNs was identified, probably involving free Listeria and leading to the infection of ED3-positive mononuclear phagocytes in the subcapsular sinus and adjacent paracortical areas. This study provides evidence that DCs are major cellular targets of L. monocytogenes in PPs and that DCs may be involved in the early dissemination of this pathogen. DCs were not sites of active bacterial replication, making these cells ideal vectors of infection.

Antibodies against listerial protein 60 act as an opsonin for phagocytosis of Listeria monocytogenes by human dendritic cells

Human-monocyte-derived dendritic cells (MoDC) are very efficient in the uptake of Listeria monocytogenes, a gram-positive bacterium which is an important pathogen in humans and animals causing systemic infections with symptoms such as septicemia and meningitis. In this work, we analyzed the influence of blood plasma on the internalization of L. monocytogenes into human MoDC and compared the uptake of L. monocytogenes with that of Salmonella enterica serovar Typhimurium and Yersinia enterocolitica. While human plasma did not significantly influence the uptake of serovar Typhimurium and Y. enterocolitica by human MoDC, the efficiency of the uptake of L. monocytogenes by these phagocytes was strongly enhanced by human plasma. In plasma-free medium the internalization of L. monocytogenes was very low, whereas the addition of pooled human immunoglobulins resulted in the internalization of these bacteria to a degree comparable to the highly efficient uptake observed with human plasma. All human plasma tested contained antibodies against the 60-kDa extracellular protein of L. monocytogenes (p60), and anti-p60 antibodies were also found in the commercially available pooled immunoglobulins. Strikingly, in contrast to L. monocytogenes wild type, an iap deletion mutant (totally deficient in p60) showed only a minor difference in the uptake by human MoDC in the presence or the absence of human plasma. These results support the assumption that antibodies against the listerial p60 protein may play an important role in Fc-receptor-mediated uptake of L. monocytogenes by human MoDC via opsonization of the bacteria. This process may have a major impact in preventing systemic infection in L. monocytogenes in immunocompetent humans.

Identification of new genes involved in the virulence of Listeria monocytogenes by signature-tagged transposon mutagenesis

Listeria monocytogenes is a gram-positive, facultative intracellular pathogen that can cause severe food-born infections in humans and animals. We have adapted signature-tagged transposon mutagenesis to L. monocytogenes to identify new genes involved in virulence in the murine model of infection. We used transposon Tn1545 carried on the integrative vector pAT113. Forty-eight tagged transposons were constructed and used to generate banks of L. monocytogenes mutants. Pools of 48 mutants were assembled, taking one mutant from each bank, injected into mice, and screened for those affected in their multiplication in the brains of infected animals. From 2,000 mutants tested, 18 were attenuated in vivo. The insertions harbored by these mutants led to the identification of 10 distinct loci, 7 of which corresponded to previously unknown genes. The properties of four loci involving putative cell wall components were further studied in vitro and in vivo. The data suggested that these components are involved in bacterial invasion and multiplication in the brain.

ClpC ATPase is required for cell adhesion and invasion of Listeria monocytogenes

We studied the role of two members of the 100-kDa heat shock protein family, the ClpC and ClpE ATPases, in cell adhesion and invasion of the intracellular pathogen Listeria monocytogenes. During the early phase of infection, a clpC mutant failed to disseminate to hepatocytes in the livers of infected mice whereas the invasive capacity of a clpE mutant remained unchanged. This was confirmed by a confocal microscopy study on infected cultured hepatocyte and epithelial cell lines, showing a strong reduction of cell invasion only by the clpC mutant. Western blot analysis with specific antisera showed that the absence of ClpC, but not that of ClpE, reduced expression of the virulence factors InlA, InlB, and ActA. ClpC-dependent modulation of these factors occurs at the transcriptional level with a reduction in the transcription of inlA, inlB, and actA in the clpC mutant, in contrast to the clpE mutant. This work provides the first evidence that, in addition to promoting escape from the phagosomes, ClpC is required for adhesion and invasion and modulates the expression of InlA, InlB, and ActA, further supporting the major role of the Clp chaperones in the virulence of intracellular pathogens.

Activation of host phospholipases C and D in macrophages after infection with Listeria monocytogenes

Infection of the J774 murine macrophage-derived cell line with Listeria monocytogenes results in several elevations of intracellular calcium during the first 15 min of infection. These appear to result from the actions of secreted bacterial proteins, including phosphatidylinositol-specific phospholipase C (PI-PLC), a broad-range phospholipase C, and listeriolysin O (LLO) (S. J. Wadsworth and H. Goldfine, Infect. Immun. 67:1770-1778, 1999). We have measured hydrolysis of host PI and the activation of host polyphosphoinositide-specific PLC and host phospholipase D (PLD) during infection with wild-type and mutant L. monocytogenes. Elevated hydrolysis of host PI occurred within the first 10 min of infection and was dependent on both bacterial PI-PLC and LLO, both of which were required for the earliest elevations of intracellular calcium in the host cell. A more rapid hydrolysis of host PI was observed at 30 min after infection, at the time when wild-type bacteria have been internalized. Activation of host PLC, also occurred in the first 10 min of infection but was not dependent on the presence of bacterial PI-PLC. Similar observations were made in murine bone marrow-derived macrophages. In J774 cells, activation of host PLD was observed after 20 min of infection and was dependent on bacterial LLO. Mutants in the bacterial phospholipases produced levels of PLD activation similar to those produced by the wild type. Phorbol myristate acetate (PMA) also activated host PLD, while long-term treatment with PMA resulted in loss of the ability of L. monocytogenes to activate host PLD, suggesting an involvement of protein kinase C (PKC) in the activation of PLD. Rottlerin, an inhibitor of PKC delta in J774 cells, also inhibited the activation of PLD, but hispidin, an inhibitor of PKC betaI and betaII, did not. Pretreatment of J774 cells with the PLD inhibitor, 2, 3-diphosphoglycerate partially inhibited escape of the bacteria from the primary phagocytic vacuole.

The contribution of both oxygen and nitrogen intermediates to the intracellular killing mechanisms of C1q-opsonized Listeria monocytogenes by the macrophage-like IC-21 cell line

Listeria monocytogenes is a facultative intracellular pathogen which is internalized by host mammalian cells upon binding to their surface. Further listerial growth occurs in the cytosol after escape from the phagosomal-endosomal compartment. We have previously reported that C1q is able to potentiate L. monocytogenes phagocytosis upon bacterial opsonization by ingestion through C1q-binding structures. In this report, we analysed the post-phagocytic events upon internalization of C1q-opsonized L. monocytogenes and found an induction of macrophage (Mphi)-like IC-21 cell bactericidal mechanisms displayed by the production of oxygen and nitrogen metabolites. Both types of molecules are effective in L. monocytogenes killing. Further analysis of the cellular responses promoted by interaction of C1q with its surface binding structures, leads us to consider C1q as a collaborative molecule involved in Mphi activation. Upon interaction with surface binding structures, C1q was able to trigger and/or amplify the production of reactive oxygen and nitrogen intermediates induced by stimuli such as interferon-gamma and L. monocytogenes phagocytosis.

Listeria monocytogenes-infected human dendritic cells: uptake and host cell response

Dendritic cells (DCs) are potent antigen-presenting cells and play a crucial role in initiation and modulation of specific immune responses. Various pathogens are able to persist inside DCs. However, internalization of the gram-positive bacterium Listeria monocytogenes into human DCs has not yet been shown. In the present study, we demonstrate that human monocyte-derived immature DCs can efficiently phagocytose L. monocytogenes. This uptake is independent of listerial adhesion factors internalin A and internalin B but requires cytoskeletal motion and factors present in human plasma. A major portion of internalized bacteria is found in membrane-bound phagosomes and is rarely free in the cytosol, as shown by transmission electron microscopy and by using an L. monocytogenes strain expressing green fluorescent protein when in the host cell cytosol. The infection caused maturation of the immature DCs into mature DCs displaying high levels of CD83, CD25, major histocompatibility complex class II, and the CD86 costimulator molecule. This effect appeared to be largely mediated by listerial lipoteichoic acid. Although L. monocytogenes infection is known to induce death in other cell types, infection of human DCs was found to induce necrotic but not apoptotic death in fewer than 20% of DCs. Therefore, the ability of DCs to act as effective antigen-presenting cells for listerial immunity is probably enhanced by their resistance to cell death, as well as their ability to rapidly differentiate into mature, immunostimulatory DCs upon encountering bacteria.

Listeriolysin O as a reporter to identify constitutive and in vivo-inducible promoters in the pathogen Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular gram-positive bacterium capable of growing in the cytoplasm of infected host cells. Bacterial escape from the phagosomal vacuole of infected cells is mainly mediated by the pore-forming hemolysin listeriolysin O (LLO) encoded by hly. LLO-negative mutants of L. monocytogenes are avirulent in the mouse model. We have developed a genetic system with hly as a reporter gene allowing the identification of both constitutive and in vivo-inducible promoters of this pathogen. Genomic libraries were created by randomly inserting L. monocytogenes chromosomal fragments upstream of the promoterless hly gene cloned into gram-positive and gram-negative shuttle vectors and expressed in an LLO-negative mutant strain. With this hly-based promoter trap system, combined with access to the L. monocytogenes genome database, we identified 20 in vitro-transcribed genes, including genes encoding (i) p60, a previously known virulence gene, (ii) a putative new hemolysin, and (iii) two proteins of the general protein secretion pathway. By using the hly-based system as an in vivo expression technology tool, nine in vivo-induced loci of L. monocytogenes were identified, including genes encoding (i) the previously known in vivo-inducible phosphatidylinositol phospholipase C and (ii) a putative N-acetylglucosamine epimerase, possibly involved in teichoic acid biosynthesis. The use of hly as a reporter is a simple and powerful alternative to classical methods for transcriptional analysis to monitor promoter activity in L. monocytogenes.

PrfA mediates specific binding of RNA polymerase of Listeria monocytogenes to PrfA-dependent virulence gene promoters resulting in a transcriptionally active complex

There is accumulating evidence that the coordinate transcription of the virulence genes in Listeria monocytogenes constitutes a very complex regulation mechanism which might require other factors in addition to PrfA. We previously described an unknown proteinaceous component from crude bacterial cell extracts, which, together with PrfA, formed a specific complex (CI) in electrophoretic mobility shift assays (EMSA) with an hly promoter probe. Here we identify the RNA polymerase (RNAP) of L. monocytogenes as an essential component of the CI complex. Addition of purified RNAP plus PrfA to the hly promoter probe allowed reconstitution of a complex migrating at the same height as CI. By using EMSA and DNaseI footprint experiments it could be shown that PrfA leads to an enhanced and specific binding of RNAP. Transcriptional activity of RNAP in vitro, using the actA promoter, was strictly dependent on PrfA.

Frequency of Escherichia coli strains producing the cytotoxic necrotizing factor (CNF1) in nosocomial urinary tract infections

The presence of cytotoxic necrotizing factor 1 (CNF1), together with various associated virulence factors (alpha-haemolysin, P-, S- and A-fimbriae), was screened in 175 uropathogenic Escherichia coli strains isolated from hospitalized adult patients. The cnf1 gene was detected in 30% of the selected strains independently of the severity of the clinical urinary infection. A significant association between CNF1, haemolytic activity and the products of the pap/sfa genes was found. However, CNF1 appeared not to play a major role in nosocomial E. coli urinary tract infections.

Sequence variations within PrfA DNA binding sites and effects on Listeria monocytogenes virulence gene expression

Reporter gene fusions were used to investigate the contributions of PrfA DNA binding sites to Listeria monocytogenes virulence gene expression. Our results suggest that the DNA sequence of PrfA binding sites determines the levels of expression of certain virulence genes, such as hly and mpl. Other virulence genes, such as actA and plcB, may depend upon additional factors for full regulation of gene expression.

CtsR controls class III heat shock gene expression in the human pathogen Listeria monocytogenes

Stress proteins play an important role in virulence, yet little is known about the regulation of stress response in pathogens. In the facultative intracellular pathogen Listeria monocytogenes, the Clp ATPases, including ClpC, ClpP and ClpE, are required for stress survival and intracellular growth. The first gene of the clpC operon of L. monocytogenes encodes a homologue of the Bacillus subtilis CtsR repressor of stress response genes. An L. monocytogenes ctsR-deleted mutant displayed enhanced survival under stress conditions (growth in the presence of 2% NaCl or at 42 degrees C), but its level of virulence in the mouse was not affected. The virulence of a wild-type strain constitutively expressing CtsR is significantly attenuated, presumably because of repression of the stress response. Regulation of the L. monocytogenes clpC, clpP and clpE genes was investigated using transcriptional fusions in B. subtilis as a host. The L. monocytogenes ctsR gene was placed under the control of an inducible promoter, and regulation by CtsR and heat shock was demonstrated in vivo in B. subtilis. The purified CtsR protein of L. monocytogenes binds specifically to the clpC, clpP and clpE regulatory regions, and the extent of the CtsR binding sites was defined by DNase I footprinting. Our results demonstrate that this human pathogen possesses a CtsR regulon controlling class III heat shock genes, strikingly similar to that of the saprophyte B. subtilis. This is the first description of a stress response regulatory gene in a pathogen.

PrfA, the transcriptional activator of virulence genes, is upregulated during interaction of Listeria monocytogenes with mammalian cells and in eukaryotic cell extracts

Most virulence genes of Listeria monocytogenes are activated by the transcriptional regulator PrfA. Previous studies have shown that environmental parameters, such as temperature, pH, stress conditions and medium composition, affect the expression of PrfA and PrfA-dependent proteins. In this report, we demonstrate a threefold increase in PrfA protein synthesis during infection of mammalian cells, which correlates with the increased activity of the plcA promoter, the major prfA promoter. Increased PrfA synthesis begins when L. monocytogenes adheres to host cells. In addition, we show that the observed induction of PrfA during the interaction of L. monocytogenes with mammalian cells can be reproduced in vitro using total cell extracts. Our data suggest a role for host proteinase K-sensitive protein(s) in PrfA upregulation.

Cutting Edge: Paradigm Revisited: Antibody Provides Resistance to Listeria Infection

Listeriolysin O (LLO) is a secreted pore-forming toxin of the facultative intracellular bacterium Listeria monocytogenes. We assessed the ability of a murine anti-LLO mAb to affect the course of infection in mice challenged with Listeria. This mAb was previously shown to be capable of neutralizing LLO-mediated pore formation in vitro, and here we show that the passive administration of this Ab to mice before infection provides increased resistance. Mice treated with the mAb were protected from a lethal challenge with virulent Listeria and showed a significant reduction in Listeria burden during the first hours to days postinfection. These effects of the Ab were independent of host B or T cells, since treatment with the mAb provided enhanced resistance to SCID mice. The titer of anti-LLO Abs during the regular infection of mice with Listeria was found to be low to negative.

The smcL gene of Listeria ivanovii encodes a sphingomyelinase C that mediates bacterial escape from the phagocytic vacuole

The ruminant pathogen Listeria ivanovii differs from Listeria monocytogenes in that it causes strong, bizonal haemolysis and a characteristic shovel-shaped co-operative haemolytic ('CAMP-like') reaction with Rhodococcus equi. We cloned the gene responsible for the differential haemolytic properties of L. ivanovii, smcL. It encodes a sphingomyelinase C (SMase) highly similar (> 50% identity) to the SMases from Staphylococcus aureus (beta-toxin), Bacillus cereus and Leptospira interrogans. smcL was transcribed monocistronically and was expressed independently of PrfA. Low-stringency Southern blots demonstrated that, within the genus Listeria, smcL was present only in L. ivanovii. We constructed an smcL knock-out mutant. Its phenotype on blood agar was identical to that of L. monocytogenes (i.e. weak haemolysis and no shovel-shaped CAMP-like reaction with R. equi ). This mutant was less virulent for mice, and its intracellular proliferation was impaired in the bovine epithelial-like cell line MDBK. The role of SmcL in intracellular survival was investigated using an L. monocytogenes mutant lacking the membrane-damaging determinants hly, plcA and plcB, being thus unable to grow intracellularly. Complementation of this mutant with smcL on a plasmid was sufficient to promote bacterial intracellular proliferation in MDBK cells. Transmission electron microscopy showed that SmcL mediates the disruption of the phagocytic vacuole and the release of bacteria into the cytosol. Therefore, L. ivanovii possesses a third phospholipase with membrane-damaging activity that, together with PlcA and PlcB, may act in concert with the pore-forming toxin Hly to mediate efficient escape from the vacuolar compartment. The 5' end of smcL is contiguous with the internalin locus i-inlFE, which is also specific to L. ivanovii and is required for full virulence in mice. Thus, smcL forms part of a novel virulence gene cluster in Listeria that is species specific.

A proteomic approach to study the acid response in Listeria monocytogenes

The responses of Listeria monocytogenes to acidic conditions were studied at the level of protein synthesis at a lethal acidic pH (acid stress) and an intermediary nonlethal acidic pH (acid adaptation). The radiolabeled acid-induced proteins were separated by two-dimensional (2-D) electrophoresis and analyzed by a computer-aided 2-D gel analysis system. The two acidic conditions upgraded a number of constitutive proteins and induced synthesis of a number of novel proteins. The majority of these induced proteins were common to the two pHs and the lethal acidic pH induced more proteins than the mildly acidic pH, suggesting that the responses to the two acidic conditions involve many common proteins and that additional proteins are required when the bacteria have to face more severe acidic conditions. In waiting for identification of more proteins involved in order to have a wholesome mechanistic picture of the acid response in L. monocytogenes, we present here the first results obtained from identification of the most abundant of these acid-induced proteins using peptide mass fingerprinting and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) technique.

Examination of Listeria monocytogenes intracellular gene expression by using the green fluorescent protein of Aequorea victoria

The ActA protein of Listeria monocytogenes is an essential virulence factor and is required for intracellular bacterial motility and cell-to-cell spread. plcB, cotranscribed with actA, encodes a broad-specificity phospholipase C that contributes to lysis of host cell vacuoles and cell-to-cell spread. Construction of a transcriptional fusion between actA-plcB and the green fluorescent protein gene of Aequorea victoria has facilitated the detailed examination of patterns of actA/plcB expression within infected tissue culture cells. actA/plcB expression began approximately 30 min postinfection and was dependent upon entry of L. monocytogenes into the host cytosol. L. monocytogenes Deltahly mutants, which are unable to escape from host cell vacuoles, did not express actA/plcB at detectable levels within infected tissue culture cells; however, complementation of the hly defect allowed entry of the bacteria into the host cytoplasm and subsequent actA/plcB expression. These results emphasize the ability of L. monocytogenes to sense the different host cell compartment environments encountered during the course of infection and to regulate virulence gene expression in response.

Listeriolysin O-dependent activation of endothelial cells during infection with Listeria monocytogenes: activation of NF-kappa B and upregulation of adhesion molecules and chemokines

The facultative intracellular bacterium Listeria monocytogenes is an invasive pathogen that crosses the vascular endothelium and disseminates to the placenta and the central nervous system. Its interaction with endothelial cells is crucial for the pathogenesis of listeriosis. By infecting in vitro human umbilical vein endothelial cells (HUVEC) with L. monocytogenes, we found that wild-type bacteria induced the expression of the adhesion molecules (ICAM-1 and E-selectin), chemokine secretion (IL-8 and monocyte chemotactic protein-1) and NF-kappa B nuclear translocation. The activation of HUVEC required viable bacteria and was abolished in prfA-deficient mutants of L. monocytogenes, suggesting that virulence genes are associated with endothelial cell activation. Using a genetic approach with mutants of virulence genes, we found that listeriolysin O (LLO)-deficient mutants inactivated in the hly gene did not induce HUVEC activation, as opposed to mutants inactivated in the other virulence genes. Adhesion molecule expression, chemokine secretion and NF-kappa B activation were fully restored by a strain of Listeria innocua transformed with the hly gene encoding LLO. The relevance in vivo of endothelial cell activation for listerial pathogenesis was investigated in transgenic mice carrying an NF-kappa B-responsive lacZ reporter gene. NF-kappa B activation was visualized by a strong lacZ expression in endothelial cells of capillaries of mice infected with a virulent haemolytic strain, but was not seen in those infected with a non-haemolytic isogenic mutant. Direct evidence that LLO is involved in NF-kappa B activation in transgenic mice was provided by injecting intravenously purified LLO, thus inducing stimulation of NF-kappa B in endothelial cells of blood capillaries. Our results demonstrate that functional listeriolysin O secreted by bacteria contributes as a potent inflammatory stimulus to inducing endothelial cell activation during the infectious process.

Intracellular delivery and antibacterial activity of gentamicin encapsulated in pH-sensitive liposomes

Cell membranes are relatively impermeable to the antibiotic gentamicin, a factor that, along with the toxicity of gentamicin, precludes its use against many important intracellular bacterial infections. Liposomal encapsulation of this drug was used in order to achieve intracellular antibiotic delivery and therefore increase the drug's therapeutic activity against intracellular pathogens. Gentamicin encapsulation in several dipalmitoylphosphatidylcholine (DPPC) and pH-sensitive dioleoylphosphatidylethanolamine (DOPE)-based carrier systems was characterized. To systematically test the antibacterial efficacies of these formulations, a tissue culture assay system was developed wherein murine macrophage-like J774A.1 cells were infected with bacteria and were then treated with encapsulated drug. Of these formulations, DOPE-N-succinyl-DOPE and DOPE-N-glutaryl-DOPE (70:30;mol:mol) containing small amounts of polyethyleneglycol-ceramide showed appreciable antibacterial activities, killing greater than 75% of intracellular vacuole-resident wild-type Salmonella typhimurium compared to the level of killing of the control formulations. These formulations also efficiently eliminated intracellular infections caused by a recombinant hemolysin-expressing S. typhimurium strain and a Listeria monocytogenes strain, both of which escape the vacuole and reside in the cytoplasm. Control non-pH-sensitive liposomal formulations of gentamicin had poor antibacterial activities. A fluorescence resonance energy transfer assay indicated that the efficacious formulations undergo a pH-dependent lipid mixing and fusion event. Intracellular delivery of the fluorescent molecules encapsulated in these formulations was confirmed by confocal fluorescence microscopy and was shown to be dependent on endosomal acidification. This work shows that encapsulation of membrane-impermeative antibiotics in appropriately designed lipid-based delivery systems can enable their use in treating intracellular infections and details the development of a general assay for testing the intracellular delivery of encapsulated drug formulations.

Identification of the gene encoding the alternative sigma factor sigmaB from Listeria monocytogenes and its role in osmotolerance

Listeria monocytogenes is well known for its robust physiology, which permits growth at low temperatures under conditions of high osmolarity and low pH. Although studies have provided insight into the mechanisms used by L. monocytogenes to allay the physiological consequences of these adverse environments, little is known about how these responses are coordinated. In the studies presented here, we have cloned the sigB gene and several rsb genes from L. monocytogenes, encoding homologs of the alternative sigma factor sigmaB and the RsbUVWX proteins, which govern transcription of a general stress regulon in the related bacterium Bacillus subtilis. The L. monocytogenes and B. subtilis sigB and rsb genes are similar in sequence and physical organization; however, we observed that the activity of sigmaB in L. monocytogenes was uniquely responsive to osmotic upshifting, temperature downshifting, and the presence of EDTA in the growth medium. The magnitude of the response was greatest after an osmotic upshift, suggesting a role for sigmaB in coordinating osmotic responses in L. monocytogenes. A null mutation in the sigB gene led to substantial defects in the ability of L. monocytogenes to use betaine and carnitine as osmoprotectants. Subsequent measurements of betaine transport confirmed that the absence of sigmaB reduced the ability of the cells to accumulate betaine. Thus, sigmaB coordinates responses to a variety of physical and chemical signals, and its function facilitates the growth of L. monocytogenes under conditions of high osmotic strength.

Entry of Listeria monocytogenes into neurons occurs by cell-to-cell spread: an in vitro study

Listeria monocytogenes is an intracellular pathogen that causes severe central nervous system infection in humans and animals. The ability of this bacterium to penetrate nerve cells was investigated by using rat spinal cell cultures. Entry into distinct cell types, i. e., glial cells and neurons, was monitored by a differential immunofluorescence technique with antibodies against cell type-specific markers and the bacterial pathogen. L. monocytogenes was detected predominantly within macrophages constituting the microglia. Astrocytes and oligodendrocytes, the major components of macroglia, were infected to a lesser extent. Surprisingly, Listeria innocua, a noninvasive and nonpathogenic species, also has the capacity to enter into these three types of glial cells. Entry into neurons was a very rare event. In contrast, we found that L. monocytogenes could efficiently invade neurons when these latter cells were cocultivated with Listeria-infected mouse macrophages. In this case, infection of neurons occurs by cell-to-cell spread via an actA-dependent mechanism. These data support the notion that infected phagocytes can be vectors by which L. monocytogenes gains access to privileged niches such as the central nervous system.

Sequence analysis of the actA gene of Listeria monocytogenes isolated from human

The region encoding proline-rich units of actA genes was amplified from 24 strains of Listeria monocytogenes using polymerase chain reaction (PCR). PCR products of 13 strains showed the expected size of 623 bp, whereas those of 11 strains showed a short size of 518 bp. The shortening of these PCR products resulted from the deletion of one proline-rich unit. These results indicate that ActA proteins are divided into at least two different types which are unrelated to bacterial serotypes.

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.

Differential interaction of the transcription factor PrfA and the PrfA-activating factor (Paf) of Listeria monocytogenes with target sequences

The interaction of the purified PrfA transcription factor with the regulatory sequences located upstream of the PrfA-dependent listeriolysin (hly) and internalin (inlA) genes was studied in the presence and in the absence of Paf (PrfA-activating factor)-containing extracts. It is shown that PrfA protein is able to bind, independently of additional factors, to a 109bp DNA fragment including the entire hly promoter sequence with the anticipated PrfA binding site ('PrfA-box'). PrfA alone, but not in combination with Paf, can also bind to a shorter target sequence of 28 bp comprising essentially the PrfA-box of the hly promoter. The addition of a Paf-containing extract does not lead to significant protein binding to these two hly target sequences in the absence of PrfA but converts the complex (CIII) consisting of PrfA and the 109 bp hly DNA fragment to a slower migrating PrfA-Paf-DNA complex (CI). Incubation of cell-free extracts of wild-type Listeria monocytogenes with the 109 bp DNA fragment leads to the formation of CI. The addition of polyclonal PrfA antibodies causes a supershift of CIII. Purified PrfA and PrfA-Paf also bind to a DNA fragment containing the PrfA-dependent promoter P2 of inlA, albeit at a lower rate when compared with the corresponding hly sequence. In contrast to the hly target DNA, the inlA promoter sequence efficiently binds Paf alone, and this Paf binding reduces that of PrfA and PrfA-Paf to the inlA target DNA. DNase I footprint experiments show that purified PrfA protects sequences of dyad symmetry previously proposed as PrfA binding sites in the hly and in the inlA promoter regions.

The host response to Listeria monocytogenes mutants defective in genes encoding phospholipases C (plcA, plcB) and actin assembly (actA)

Several genes involved in the determination of Listeria monocytogenes pathogenesis have been identified. Among them, plcA gene encodes phosphatidylinositol-specific phospholipase C (PI-PLC), plcB gene encodes a broad-range phospholipase C (PC-PLC), and actA encodes a protein contributing to actin assembly in infected cells. The interaction of L. monocytogenes wild type (LO 28) strain and two derivative mutants, plcA- (BUG 206) and actA-/plcB- (LUT 12), with macrophages and T lymphocytes was investigated in a mouse model of listeriosis. Both mutants showed evidence of attenuation. The plcA- mutant, but not the plcB- mutant, expressed an increase in susceptibility to the anti-listerial activity of macrophages. Both mutants showed a decreased ability to induce IL-12 production by bone marrow macrophages when co-stimulated with E. coli LPS or IFN-gamma. In vivo, L. monocytogenes plcA- mutant was found to be a more effective stimulator of T cells than the wild LO 28 strain.

Glucose-1-phosphate utilization by Listeria monocytogenes is PrfA dependent and coordinately expressed with virulence factors

Virulence genes of the facultative intracellular pathogen Listeria monocytogenes are coordinately regulated by the activator protein PrfA, encoded by prfA, a member of the cyclic AMP receptor protein family of bacterial transcription factors. We found that prfA* mutants that constitutively overexpress the virulence regulon due to a Gly145Ser substitution in PrfA (M.-T. Ripio, G. Domínguez-Bernal, M. Lara, M. Suárez, and J.-A. Vázquez-Boland, J. Bacteriol. 179:1533-1540, 1997) rapidly utilized glucose-1-phosphate (G-1-P) as a carbon source for growth, in contrast to wild-type strains, which characteristically do not. Wild-type strains acquired the capacity for readily metabolizing G-1-P upon exposure to environmental conditions that activate the expression of prfA and PrfA-dependent virulence genes (i.e., culture at 37 degrees C in charcoal-treated medium). In these strains, G-1-P utilization followed an expressional pattern identical to that of virulence genes controlled by PrfA, with repression at 20 degrees C. Tn917 insertions in L. monocytogenes mutants selected for G-1-P utilization deficiency mapped to the plcA-prfA operon, a deltaprfA strain was totally unable to utilize G-1-P, and trans complementation with prfA constructs restored the ability to efficiently metabolize and grow on G-1-P to these mutants. Thus, G-1-P utilization by L. monocytogenes is under the tight positive control of the central virulence regulator, PrfA, and is coexpressed with PrfA-dependent pathogenicity determinants. It was recently reported that readily utilized carbohydrates, such as glucose or cellobiose, repress virulence genes in L. monocytogenes. We confirmed this but, interestingly, found that G-1-P does not inhibit expression of the PrfA regulon, indicating that this sugar follows a catabolic pathway that bypasses the repressor mechanism triggered by other readily metabolized carbon sources. PrfA dependence and coexpression with virulence genes suggest that utilization of exogenous G-1-P may be relevant to Listeria pathogenesis. G-1-P is the precursor metabolite and primary degradation product of glycogen and is therefore available within the mammalian cell. Based on our results, we hypothesize that G-1-P could play an important role as a growth substrate for intracellular Listeria.

Crystal structure of the phosphatidylinositol-specific phospholipase C from the human pathogen Listeria monocytogenes

The X-ray crystal structure of the phosphatidylinositol-specific phospholipase C (PI-PLC) from the human pathogen Listeria monocytogenes has been determined both in free form at 2.0 A resolution, and in complex with the competitive inhibitor myo-inositol at 2.6 A resolution. The structure was solved by a combination of molecular replacement using the structure of Bacillus cereus PI-PLC and single isomorphous replacement. The enzyme consists of a single (beta alpha)8-barrel domain with the active site located at the C-terminal side of the beta-barrel. Unlike other (beta alpha)8-barrels, the barrel in PI-PLC is open because it lacks hydrogen bonding interactions between beta-strands V and VI. myo-Inositol binds to the active site pocket by making specific hydrogen bonding interactions with a number of charged amino acid side-chains as well as a coplanar stacking interaction with a tyrosine residue. Despite a relatively low sequence identity of approximately 24%, the structure is highly homologous to that of B.cereus PI-PLC with an r.m.s. deviation for 228 common C alpha positions of 1.46 A. Larger differences are found for loop regions that accommodate most of the numerous amino acid insertions and deletions. The active site pocket is also well conserved with only two amino acid replacements directly implicated in inositol binding.

Construction of the temperature-sensitive vectors pLUCH80 and pLUCH88 for delivery of Tn917::NotI/SmaI and use of these vectors to derive a circular map of Listeria monocytogenes Scott A, a serotype 4b isolate

A physical map of Listeria monocytogenes Scott A was generated by the pulsed-field technique of contour-clamped-homogeneous-electric-field (CHEF) electrophoresis. The circular genome of this serotype 4b strain contains 12 AscI fragments (38 to 790 kb), 5 NotI fragments (55 to 1,400 kb), 3 SrfI fragments (110, 1,110, and 2,000 kb), and 2 SfiI fragments (1,320 and 1,920 kb). Summation of individually sized fragments derived by digestion of Scott A genomic DNA with each of these four enzymes provided an average estimated genome length of 3,210 +/- 60 kb. Efforts to assemble the macrorestriction map benefited greatly from the construction and use of pLUCH80 and pLUCH88, temperature-sensitive vectors for delivering transposon Tn917::NotI/SmaI to the chromosome of Scott A. As another component of this study, the positions of four known virulence genes (inlA, mpl, hly, and prf) and three L. monocytogenes-specific sequences (lisM44, lisM51, and lisM52) were localized on the physical map of Scott A by hybridization. Probes prepared from lisM44, lisM51, and the four virulence genes hybridized within a cluster on a 150-kb fragment of the Scott A genome that overlaps part of the NotI-B and AscI-D fragments. The lisM52 probe hybridized with the AscI-F2 (120-kb) fragment of Scott A, which is separated from the NotI-B-AscI-D region by about 300 kb. These results established the first physical and genetic map of a serotype 4b strain of L. monocytogenes and provided further insight on this important food-borne pathogen at the genome level.

Listeria monocytogenes infection of P388D1 macrophages results in a biphasic NF-kappaB (RelA/p50) activation induced by lipoteichoic acid and bacterial phospholipases and mediated by IkappaBalpha and IkappaBbeta degradation

As previously reported, Listeria monocytogenes infection of P388D1 macrophages results in a rapid induction of NF-kappaB DNA-binding activity. Here we show that this induction of NF-kappaB activity occurs in a biphasic mode: first, a transient, IkappaBalpha degradation-dependent phase of activity, also induced by the nonvirulent species Listeria innocua, which is mediated by binding of the bacteria to the macrophage, or by adding Listeria-derived lipoteichoic acid to the macrophage; the second persistent phase of activation is only markedly induced when the bacteria enter the cytoplasm of the host cell and express the virulence genes plcA and plcB, encoding two phospholipases. We suggest that products of the enzymatic activity of phospholipases directly interfere with host cell signal transduction pathways, thus leading to persistent NF-kappaB activation via persistent IkappaBbeta degradation.

Responses by murine macrophages infected with Listeria monocytogenes crucial for the development of immunity to this pathogen

Macrophages and other mammalian cells respond to infection with Listeria monocytogenes (L. monocytogenes) by the transient or persistent activation of host cell signal transduction pathways. In addition, L. monocytogenes infection influences expression of various host cell genes, such as stress genes, genes from the MHC I and II complex, cytokine genes, and cytokine receptor genes. The possible influences of the different host cell responses on the outcome of an L. monocytogenes infection in vitro as well as for the development of immunity are discussed.

Listeria monocytogenes in laboratory mice: a model of short-term infectious and pathogenic processes controllable by regulated protective immune responses

The mammalian immune system is an integrated network of tissue, leukocytes and effector and regulatory molecules. All these components operate i) to maintain the proper structure of and processes expressed by each tissue. and ii) to protect the hosts from those microorganisms that generally invade them as part of their life cycle. Among the invading microorganisms. Listeria monocytogenes (L. monocytogenes) can persist as a live organism independent of the host, and is, thus, able to drive short-term infectious and pathogenic processes that are controlled by integrated innate and adaptive protective immune responses driven by CD8 and CD4 type 1 T lymphocytes acting on non-T non-B leukocytes. Although the effector functions and the fine specificity of T lymphocytes have been more and more characterized, an understanding of the precise regulation of both leukocyte traffic and T-lymphocyte migration depends on knowledge of the early tissue distribution of L. monocytogenes, points that are addressed in this review.

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

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

Listeria monocytogenes: a potent vaccine vector for neoplastic and infectious disease

Listeria monocytogenes (L. monocytogenes) is a promising candidate vaccine vector that naturally infects antigen-presenting cells, and targets antigen delivery to both the class I MHC pathway of endogenous antigen presentation and the class II pathway of exogenous antigen presentation. At the same time, L. monocytogenes stimulates the innate immune response to produce cytokines that enhance antigen-presenting function and induce a Th1-type cytokine profile associated with cell-mediated immune responses. Immune responses with these features are considered to be particularly important for clearance of viruses, tumors, and intracellular infections. In this review, we describe the development of methods to transform L. monocytogenes to express and secrete foreign antigens and the studies that have demonstrated that genetically engineered L. monocytogenes mutants are highly effective vectors for the induction of potent immune responses against viral antigens and tumor cells. In addition, we discuss the strengths and weaknesses of L. monocytogenes as a vaccine vector.