A nonvirulent mutant of Listeria monocytogenes does not move intracellularly but still induces polymerization of actin

Carboxyl-Terminal Residues N478 and V479 Required for the Cytolytic Activity of Listeriolysin O Play a Critical Role in Listeria monocytogenes Pathogenicity

Listeria monocytogenes is a facultative intracellular pathogen that secretes the cytolysin listeriolysin O (LLO), which enables the bacteria to cross the phagosomal membrane. L. monocytogenes regulates LLO activity in the phagosome and minimizes its activity in the host cytosol. Mutants that fail to compartmentalize LLO activity are cytotoxic and have attenuated virulence. Here, we showed that residues N478 and V479 of LLO are required for LLO hemolytic activity and bacterial virulence. A single N478A mutation (LLO_N478A) significantly increased the hemolytic activity of LLO at a neutral pH, while no difference was observed at the optimum acidic pH, compared with wild-type LLO. Conversely, the mutant LLO_V479A exhibited lower hemolytic activity at the acidic pH, but not at the neutral pH. The double mutant LLO_N478AV479A showed a greater decrease in hemolytic activity at both the acidic and neutral pHs. Interestingly, strains producing LLO_N478A or LLO_V479A lysed erythrocytes similarly to the wild-type strain. Surprisingly, bacteria-secreting LLO_N478AV479A had barely detectable hemolytic activity, but exhibited host cell cytotoxicity, escaped from the phagosome, grew intracellularly, and spread cell-to-cell with the same efficiency as the wild-type strain, but were highly attenuated in virulence in mice. These data demonstrate that these two residues are required for LLO hemolytic activity and pathogenicity in mice, but not for escape from the phagosome and cell-to-cell spreading. The finding that the nearly non-hemolytic LLO_N478AV479A mutant grew intracellularly indicates that mutagenesis of a virulence determinant is a novel approach for the development of live vaccine strains.

Use of the plaque assay for testing the antibiotic susceptibility of intracellular bacteria

The plaque assay was first described for titration of bacterial inoculums and clonal isolation, and was later adapted for testing antibiotics susceptibility and to study virulence factors and motility of bacteria. Over time, the sensitivity and reproducibility of the technique has been improved. Usually, the number of plaques is counted; however, the recent development of informatics tools has stimulated interest in the quantification of plaque size. Owing to this new approach, the plaque assay has been used to characterize the host cell response when infected cells are treated with antimicrobial agents. It was found that statins prevented cell lesions following rickettsial infection; in other studies, some antibiotics were found to cause apoptosis of host cells, suggesting a toxic activity. Here, we present an overview of the plaque assay as it has been used to investigate intracellular bacteria.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Comparison of widely used Listeria monocytogenes strains EGD, 10403S, and EGD-e highlights genomic variations underlying differences in pathogenicity

For nearly 3 decades, listeriologists and immunologists have used mainly three strains of the same serovar (1/2a) to analyze the virulence of the bacterial pathogen Listeria monocytogenes. The genomes of two of these strains, EGD-e and 10403S, were released in 2001 and 2008, respectively. Here we report the genome sequence of the third reference strain, EGD, and extensive genomic and phenotypic comparisons of the three strains. Strikingly, EGD-e is genetically highly distinct from EGD (29,016 single nucleotide polymorphisms [SNPs]) and 10403S (30,296 SNPs), and is more related to serovar 1/2c than 1/2a strains. We also found that while EGD and 10403S strains are genetically very close (317 SNPs), EGD has a point mutation in the transcriptional regulator PrfA (PrfA*), leading to constitutive expression of several major virulence genes. We generated an EGD-e PrfA* mutant and showed that EGD behaves like this strain in vitro, with slower growth in broth and higher invasiveness in human cells than those of EGD-e and 10403S. In contrast, bacterial counts in blood, liver, and spleen during infection in mice revealed that EGD and 10403S are less virulent than EGD-e, which is itself less virulent than EGD-e PrfA*. Thus, constitutive expression of PrfA-regulated virulence genes does not appear to provide a significant advantage to the EGD strain during infection in vivo, highlighting the fact that in vitro invasion assays are not sufficient for evaluating the pathogenic potential of L. monocytogenes strains. Together, our results pave the way for deciphering unexplained differences or discrepancies in experiments using different L. monocytogenes strains.

Over the past 3 decades, Listeria has become a model organism for host-pathogen interactions, leading to critical discoveries in a broad range of fields, including bacterial gene regulation, cell biology, and bacterial pathophysiology. Scientists studying Listeria use primarily three pathogenic strains: EGD, EGD-e, and 10403S. Despite many studies on EGD, it is the only one of the three strains whose genome has not been sequenced. Here we report the sequence of its genome and a series of important genomic and phenotypic differences between the three strains, in particular, a critical mutation in EGD’s PrfA, the main regulator of Listeria virulence. Our results show that the three strains display differences which may play an important role in the virulence differences observed between the strains. Our findings will be of critical relevance to listeriologists and immunologists who have used or may use Listeria as a tool to study the pathophysiology of listeriosis and immune responses.

Investigation of specific substitutions in virulence genes characterizing phenotypic groups of low-virulence field strains of Listeria monocytogenes

Several models have shown that virulence varies from one strain of Listeria monocytogenes to another, but little is known about the cause of low virulence. Twenty-six field L. monocytogenes strains were shown to be of low virulence in a plaque-forming assay and in a subcutaneous inoculation test in mice. Using the results of cell infection assays and phospholipase activities, the low-virulence strains were assigned to one of four groups by cluster analysis and then virulence-related genes were sequenced. Group I included 11 strains that did not enter cells and had no phospholipase activity. These strains exhibited a mutated PrfA; eight strains had a single amino acid substitution, PrfAK220T, and the other three had a truncated PrfA, PrfADelta174-237. These genetic modifications could explain the low virulence of group I strains, since mutated PrfA proteins were inactive. Group II and III strains entered cells but did not form plaques. Group II strains had low phosphatidylcholine phospholipase C activity, whereas group III strains had low phosphatidylinositol phospholipase C activity. Several substitutions were observed for five out of six group III strains in the plcA gene and for one out of three group II strains in the plcB gene. Group IV strains poorly colonized spleens of mice and were practically indistinguishable from fully virulent strains on the basis of the above-mentioned in vitro criteria. These results demonstrate a relationship between the phenotypic classification and the genotypic modifications for at least group I and III strains and suggest a common evolution of these strains within a group.

Bactofection of mammalian cells by Listeria monocytogenes: improvement and mechanism of DNA delivery

Bacteria-mediated transfer of plasmid DNA into mammalian cells (bactofection) is a potent approach to express plasmid-encoded heterologous proteins (protein antigens, toxins or enzymes) in a large set of different cell types including phagocytic and nonphagocytic mammalian cells. Previously, we have described a Listeria monocytogenes-mediated DNA delivery system, which releases plasmid DNA directly into the cytosol of mammalian cells by partial self-destruction of the carrier bacteria. Here we report on a second generation of this phage lysin supported bactofection system, which is greatly improved with respect to plasmid stability, transfer efficacy and biosafety. In this case, DNA release is initiated by spontaneous bacterial lysis in the infected cells cytosol which is subsequently enhanced by the simultaneously released phage lysin produced by the intracellular carrier bacteria. Bacteria that are capable of cell-to-cell spread are found to be much more efficient in bactofection than their non spreading counterparts.

Deletion of the gene encoding p60 in Listeria monocytogenes leads to abnormal cell division and loss of actin-based motility

Protein p60 encoded by the iap gene is regarded as an essential gene product of Listeria monocytogenes. Here we report, however, the successful construction of a viable iap deletion mutant of L. monocytogenes EGD. The mutant, which produces no p60, shows abnormal septum formation and tends to form short filaments and hooked forms during logarithmic growth. These abnormal bacterial cells break into almost normal sized single bacteria in the late-stationary-growth phase. The iap mutant is strongly attenuated in a mouse model after intravenous injection, demonstrating the importance of p60 during infection, and the invasiveness of the Deltaiap mutant for 3T6 fibroblasts and Caco-2 epithelial cells is slightly reduced. Upon uptake by epithelial cells and macrophages, the iap mutant escapes from the phagosome into the cytosol with the same efficiency as the wild-type strain, and the mutant bacteria also grow intracellularly at a rate similar to that of the wild-type strain. Intracellular movement and cell-to-cell spread are drastically reduced in various cell lines, since the iap-negative bacteria fail to induce the formation of actin tails. However, the bacteria are covered with actin filaments. Most intracellular bacteria show a nonpolar and uneven distribution of ActA around the cell, in contrast to that for the wild-type strain, where ActA is concentrated at the old pole. In an iap(+) revertant strain that produces wild-type levels of p60, intracellular movement, cell-to-cell spread, and polar distribution of ActA are fully restored. In vitro analysis of ActA distribution on the filaments of the Deltaiap strain shows that the loss of bacterial septum formation leads to ActA accumulation at the presumed division sites. In the light of data presented here and elswhere, we propose to rename iap (invasion-associated protein) cwhA (cell wall hydrolase A).

Pivotal role of VASP in Arp2/3 complex-mediated actin nucleation, actin branch-formation, and Listeria monocytogenes motility

The Listeria monocytogenes ActA protein mediates actin-based motility by recruiting and stimulating the Arp2/3 complex. In vitro, the actin monomer-binding region of ActA is critical for stimulating Arp2/3-dependent actin nucleation; however, this region is dispensable for actin-based motility in cells. Here, we provide genetic and biochemical evidence that vasodilator-stimulated phosphoprotein (VASP) recruitment by ActA can bypass defects in actin monomer-binding. Furthermore, purified VASP enhances the actin-nucleating activity of wild-type ActA and the Arp2/3 complex while also reducing the frequency of actin branch formation. These data suggest that ActA stimulates the Arp2/3 complex by both VASP-dependent and -independent mechanisms that generate distinct populations of actin filaments in the comet tails of L. monocytogenes. The ability of VASP to contribute to actin filament nucleation and to regulate actin filament architecture highlights the central role of VASP in actin-based motility.

LaXp180, a mammalian ActA-binding protein, identified with the yeast two-hybrid system, co-localizes with intracellular Listeria monocytogenes

The Listeria monocytogenes surface protein ActA is an important virulence factor required for listerial intracellular movement by inducing actin polymerization. The only host cell protein known that directly interacts with ActA is the phosphoprotein VASP, which binds to the central proline-rich repeat region of ActA. To identify additional ActA-binding proteins, we applied the yeast two-hybrid system to search for mouse proteins that interact with ActA. A mouse cDNA library was screened for ActA-interacting proteins (AIPs) using ActA from strain L. monocytogenes EGD as bait. Three different AIPs were identified, one of which was identical to the human protein LaXp180 (also called CC1). Binding of LaXp180 to ActA was also demonstrated in vitro using recombinant histidine-tagged LaXp180 and recombinant ActA. Using an anti-LaXp180 antibody and fluorescence microscopy, we showed that LaXp180 co-localizes with a subset of intracellular, ActA-expressing L. monocytogenes but was never detected on intracellularly growing but ActA-deficient mutants. Furthermore, LaXp180 binding to intracellular L. monocytogenes was asymmetrical and mutually exclusive with F-actin polymerization on the bacterial surface. LaXp180 is a putative binding partner of stathmin, a protein involved in signal transduction pathways and in the regulation of microtubule dynamics. Using immunofluorescence, we showed that stathmin co-localizes with intracellular ActA-expressing L. monocytogenes.

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.

Interaction of Listeria monocytogenes with human brain microvascular endothelial cells: InlB-dependent invasion, long-term intracellular growth, and spread from macrophages to endothelial cells

Invasion of endothelial tissues may be crucial in a Listeria monocytogenes infection leading to meningitis and/or encephalitis. Internalization of L. monocytogenes into endothelial cells has been previously demonstrated by using human umbilical vein endothelial cells as a model system. However, during the crossing of the blood-brain barrier, L. monocytogenes most likely encounters brain microvascular endothelial cells which are strikingly different from macrovascular or umbilical vein endothelial cells. In the present study human brain microvascular endothelial cells (HBMEC) were used to study the interaction of L. monocytogenes with endothelial cells, which closely resemble native microvascular endothelial cells of the brain. We show that L. monocytogenes invades HBMEC in an InlB-dependent and wortmannin-insensitive manner. Once within the HBMEC, L. monocytogenes replicates efficiently over a period of at least 18 h, moves intracellularly by inducing actin tail formation, and spreads from cell to cell. Using a green fluorescent protein-expressing L. monocytogenes strain, we present direct evidence that HBMEC are highly resistant to damage by intracellularly growing L. monocytogenes. Infection of HBMEC with L. monocytogenes results in foci of heavily infected, but largely undamaged endothelial cells. Heterologous plaque assays with L. monocytogenes-infected P388D1 macrophages as vectors demonstrate efficient spreading of L. monocytogenes into HBMEC, fibroblasts, hepatocytes, and epithelial cells, and this phenomenon is independent of the inlC gene product.

Models of invasion of enteric and periodontal pathogens into epithelial cells: a comparative analysis

Bacterial invasion of epithelial cells is associated with the initiation of infection by many bacteria. To carry out this action, bacteria have developed remarkable processes and mechanisms that co-opt host cell function and stimulate their own uptake and adaptation to the environment of the host cell. Two general types of invasion processes have been observed. In one type, the pathogens (e.g., Salmonella and Yersinia spp.) remain in the vacuole in which they are internalized and replicate within the vacuole. In the other type, the organism (e.g., Actinobacillus actinomycetemcomitans, Shigella flexneri, and Listeria monocytogenes) is able to escape from the vacuole, replicate in the host cell cytoplasm, and spread to adjacent host cells. The much-studied enteropathogenic bacteria usurp primarily host cell microfilaments for entry. Those organisms which can escape from the vacuole do so by means of hemolytic factors and C type phospholipases. The cell-to-cell spread of these organisms is mediated by microfilaments. The investigation of invasion by periodontopathogens is in its infancy in comparison with that of the enteric pathogens. However, studies to date on two invasive periodontopathogens. A actinomycetemcomitans and Porphyromonas (Bacteroides) gingivalis, reveal that these bacteria have developed invasion strategies and mechanisms similar to those of the enteropathogens. Entry of A. actinomycetemcomitans is mediated by microfilaments, whereas entry of P. gingivalis is mediated by both microfilaments and microtubules. A. actinomycetemcomitans, like Shigella and Listeria, can escape from the vacuole and spread to adjacent cells. However, the spread of A. actinomycetemcomitans is linked to host cell microtubules, not microfilaments. The paradigms presented establish that bacteria which cause chronic infections, such as periodontitis, and bacteria which cause acute diseases, such as dysentery, have developed similar invasion strategies.

High concentrations of phosphatidylinositol-4,5-bisphosphate may promote actin filament growth by three potential mechanisms: inhibiting capping by neutrophil lysates, severing actin filaments and removing capping protein-beta2 from barbed ends

Cell locomotion requires rapid growth of cortical actin filaments whose barbed ends are capped in the resting cell. Phosphatidylinositol-4,5-bisphosphate (PIP2) may play a critical role as an intracellular messenger in cytoskeletal rearrangement after stimulation. We have examined the effects of PIP2 micelles on the Ca2+-independent actin filament capping activity in high speed supernatants of neutrophil lysates which we had previously demonstrated to be almost entirely due to capping protein-beta2, a homologue of cap Z. High concentrations of PIP2 totally prevented the capping of exogenous spectrin-F-actin seeds by dilute supernatants of neutrophil extracts. Capping could also be inhibited, albeit less effectively, by PIP and PI, but not by other phospholipids. When incubated with filaments in the absence of supernatant, PIP2 increased the number of growing ends. PIP2 also uncapped previously capped actin filaments, as demonstrated by incubating supernatant-capped and uncapped seeds with and without PIP2 and then comparing the initial elongation rates after addition of pyrenyl-G-actin. Incubation of capped seeds with high concentrations of PIP2 increased the number of free barbed ends to a level comparable to that of the uncapped seeds exposed to PIP2. PIP2 caused uncapping to occur too quickly to be explained simply by the off-rate of capping protein-beta2, implying that PIP2 interacted directly with capping protein on the filament ends. In fact, PIP2 transiently uncapped capped seeds in the presence of excess free capping protein. From our data, we estimate that millimolar concentrations of PIP2 (almost 100-fold higher than the amount predicted from the effective concentration in purified systems) would be required to inhibit all the capping protein-beta2 in the cytosol. This discrepancy probably results, in large part, from sequestration of PIP2 by other PIP2-binding proteins in the cytoplasm. If PIP2 mediates differential cytoskeletal growth after chemoattractant stimulation in vivo, very high concentrations may be required subjacent to the plasma membrane for regional severing and uncapping of actin filaments to occur quickly near the perturbed membrane.

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.

How the Listeria monocytogenes ActA protein converts actin polymerization into a motile force

The ActA protein is an essential determinant of pathogenicity that is responsible for the actin-based motility of Listeria monocytogenes in mammalian cells and cell-free extracts. ActA appears to control at least four functions that collectively lead to actin-based motility: (1) initiation of actin polymerization, (2) polarization of ActA function, (3) transformation of actin polymerization into a motile force and (4) acceleration of movement mediated by the host protein profilin.

The tandem repeat domain in the Listeria monocytogenes ActA protein controls the rate of actin-based motility, the percentage of moving bacteria, and the localization of vasodilator-stimulated phosphoprotein and profilin

The ActA protein is responsible for the actin-based movement of Listeria monocytogenes in the cytosol of eukaryotic cells. Analysis of mutants in which we varied the number of proline-rich repeats (PRR; consensus sequence DFPPPPTDEEL) revealed a linear relationship between the number of PRRs and the rate of movement, with each repeat contributing approximately 2-3 microns/min. Mutants lacking all functional PRRs (generated by deletion or point mutation) moved at rates 30% of wild-type. Indirect immunofluorescence indicated that the PRRs were directly responsible for binding of vasodilator-stimulated phosphoprotein (VASP) and for the localization of profilin at the bacterial surface. The long repeats, which are interdigitated between the PRRs, increased the frequency with which actin-based motility occurred by a mechanism independent of the PRRs, VASP, and profilin. Lastly, a mutant which expressed low levels of ActA exhibited a phenotype indicative of a threshold; there was a very low percentage of moving bacteria, but when movement did occur, it was at wild-type rates. These results indicate that the ActA protein directs at least three separable events: (1) initiation of actin polymerization that is independent of the repeat region; (2) initiation of movement dependent on the long repeats and the amount of ActA; and (3) movement rate dependent on the PRRs.

Lack of cleavage of IcsA in Shigella flexneri causes aberrant movement and allows demonstration of a cross-reactive eukaryotic protein

Once in the cytoplasm of mammalian cells, Shigella flexneri expresses a motile phenotype caused by polar directional assembly of actin. This process depends on accumulation of IcsA (VirG), a 120-kDa protein with ATPase activity, at the pole of the bacterium opposite to that at which ongoing septation occurs. IcsA is also secreted into the bacterial supernatant as a 95-kDa species, after cleavage at an SSRRASS sequence which, when mutagenized, blocks processing. MAbF15, an anti-IcsA monoclonal antibody, recognizes an epitope located within repeated Gly-rich boxes in the N-terminal half of the protein. We used this monoclonal antibody to visualize the location of a noncleavable 120-kDa IcsA mutant protein expressed in S. flexneri. We found that this noncleavable IcsA protein no longer localized exclusively to the pole of the bacterium but also could be detected circumferentially. Whereas the monoclonal antibody detected the wild-type cleavable form of IcsA in only 40% of the cells expressing this protein, the noncleavable was easily detectable in all the cells carrying the icsA mutant allele. Similar aberrant localization of the IcsA mutant protein on bacteria growing within the cytoplasm of HeLa cells was observed. The strains expressing the noncleavable IcsA protein expressed abnormal intracellular movement and were often observed moving in a direction perpendicular to their longitudinal axis. The putative protease which processes IcsA may therefore play a role in achieving polar expression of this protein and providing maximum asymmetry essential to directional movement. In addition, MAbF15 allowed us to identify a 70-kDa eukaryotic protein cross-reacting with IcsA. This protein accumulated in the actin tails of motile bacteria and in membrane ruffles of the cells.

An iron-dependent mutant of Listeria monocytogenes of attenuated virulence

A bank of Tn917-insertional mutants from the facultative intracellular pathogen Listeria monocytogenes was screened by an original method based on bacterial growth on synthetic medium under iron-limiting conditions. One mutant, whose in vitro growth in synthetic medium was specifically dependent upon the availability of iron in its environment, was isolated and characterized. The insertional event occurred in a non-coding region, upstream of a rrn operon and located within a 1100-kb NotI fragment of the physical map, where the virulence genes already identified in L. monocytogenes were also present. Protein analysis by SDS-PAGE revealed a pleiotropic effect of the insertional event on cell-associated proteins, suggesting a polar effect of the transposon on adjacent unknown gene(s). The virulence in the mouse of this mutant was strongly impaired, although it was capable in vitro of growing intracellularly and of spreading from cell to cell, as shown by the production of lytic plaques on cell culture.

Intracellular killing of Listeria monocytogenes in the J774.1 macrophage-like cell line and the lipopolysaccharide (LPS)-resistant mutant LPS1916 cell line defective in the generation of reactive oxygen intermediates after LPS treatment

Listeria monocytogenes is a facultative intracellular pathogen and survives within phagocytic cells by escaping from phagosomes into the cytoplasm. It has been reported that, in vivo, L. monocytogenes is effectively eliminated through cell-mediated immunity, especially by macrophages which have been immunologically activated by cytokines such as gamma interferon (IFN-gamma). However, this killing mechanism for L. monocytogenes and the role of macrophage activation in this bacterial killing are unclear. We demonstrated the listericidal effect of oxidative radicals induced by lipopolysaccharide (LPS) and IFN-gamma, using a macrophage-like cell line, J774.1, and a mutant cell line, LPS1916. LPS1916 cells do not exhibit normal generation of O2- and H2O2 after treatment with 0.1 microgram of LPS per ml, although J774.1 cells generate 100 times the normal level of oxidative radicals with the same LPS treatment. The growth of L. monocytogenes was strongly inhibited in J774.1 cells pretreated with 0.1 microgram of LPS per ml or the combination of 0.1 microgram of LPS per ml and 10 U of IFN-gamma per ml. On the other hand, in LPS1916 cells, the growth of L. monocytogenes was not inhibited by treatment with LPS only, although LPS1916 cells pretreated with the combination of LPS and IFN-gamma showed moderate inhibition of listerial growth. This killing was not influenced by treatment with NG-monomethyl-L-arginine, which is a strong inhibitor of nitrite oxide generation. Interestingly, J774.1 cells treated with LPS did not show enhanced intraphagosomal killing of a nonhemolytic strain of avirulent L. monocytogenes that lacks the ability to escape from phagosomes, and this killing was not influenced by treatment with NG-monomethyl-L-arginine either. These results suggest that the reactive oxygen radicals are more important than nitric oxide in the mechanism underlying the intracellular killing of virulent L. monocytogenes and that there seem to be different killing mechanisms for virulent and avirulent strains of L. monocytogenes in activated-macrophage cell lines.

Organization and structure of actin filament bundles in Listeria-infected cells

During its motion inside host cells, Listeria monocytogenes promotes the formation of a column of actin filaments that extends outward from the distal end of the moving bacterium. The column is constructed of short actin filaments that polymerize at the bacteria-column interface. To get a measure of filament organization in the column, Listeria grown in cultured PtK2 cells were studied with steady state fluorescence polarization, confocal microscopy, and whole cell intermediate voltage electron microscopy. Although actin filament ordering was higher in nearby stress fibers than in the Listeria-associated actin, four distinct areas of ordering could be observed in fluorescence polarization ratio images of bacteria: 1) the surface of the bacteria, 2) the cytoplasm next to the bacteria, 3) the outer shell of the actin column, and 4) the core of the column. Filaments were preferentially oriented parallel to the long axis of the column with highest ordering along the long axis of the bacterial surface and in the shell of the tail. The lowest ordering was in the core (where filaments are possibly also shorter with respect to the cup and the shell), whereas in the adjacent cytoplasm, filaments were oriented perpendicular to the column. A mutant of Listeria that can polymerize actin around itself but cannot move intracellularly does not have its actin organized along the bacterial surface. Thus the alignment of the actin filaments along the bacterial surfaces may be important for the intracellular movement. These conclusions are also supported by confocal microscopy and whole mount electron microscopic data that also reveal that actin filaments can be deposited asymmetrically around the long axis of the bacteria, a distribution that may affect the direction of motility of Listeria monocytogenes inside infected cells.

The actin-based motility of the facultative intracellular pathogen Listeria monocytogenes

The Gram-positive bacterium Listeria monocytogenes is a facultative intracellular parasite that invades and multiplies within diverse eukaryotic cell types. An essential pathogenicity determinant is its ability to move in the host cell cytoplasm and to spread within tissues by directly passing from one cell to another. The propulsive force for intracellular movement is thought to be generated by continuous actin assembly at the rear end of the bacterium. Moving bacteria that reach the plasma membrane induce the formation of long membranous protrusions that are internalized by neighbouring cells, thus mediating the spread of infection. The unrelated pathogens Shigella and Rickettsia use a similar process of actin-based motility to disseminate in infected tissues. This review focuses on the bacterial and cellular factors involved in the actin-based motility of L. monocytogenes.

Arrest of Listeria movement in host cells by a bacterial ActA analogue: implications for actin-based motility

Upon entering the host cell's cytoplasm, the pathogen Listeria monocytogenes can subvert the normal contractile system of the host cell; subsequent assembly of polar actin-filament structures is likely to provide the force for rapid intracellular bacterial movement and its cell-to-cell spread. We have now investigated the functional consequences of microinjecting Listeria-infected PtK2 cells with a synthetic peptide, CFEFPPPPTDE. This peptide represents one of four related oligoproline stretches in ActA, a bacterial surface protein necessary for Listeria-induced actin assembly. Over an estimated intracellular concentration range of 80 nM to 0.8 microM, this analogue rapidly blocks the formation of the actin-filament tails and arrests intracellular bacterial motility. Over the same time scale and concentration range, introduction of the ActA analogue also causes host cell membrane retraction. Bodipyphallacidin staining reveals that microinjection of the ActA analogue results in massive retraction of the actin cytoskeleton. Microinjection of 1-20 microM poly(L-proline) (intracellular concentration) fails to block Listeria intracellular movement or polar actin-filament assembly. As observed with ActA, however, poly(L-proline) does cause membrane retraction. Our findings demonstrate the efficacy of low molecular weight peptides in efforts to distinguish mechanistic features in Listeria motility and PtK2 host cell membrane reorganization. These observations also suggest that a cytoskeletal component sensitive to specific oligoproline peptides may participate in protein-protein interactions essential for both of these actin-associated processes.

Nucleotide sequence of the rhamnose biosynthetic operon of Shigella flexneri 2a and role of lipopolysaccharide in virulence

N1308, a chromosomal Tn5 mutant of Shigella flexneri 2a, was described previously as a lipopolysaccharide (LPS) mutant with a short O side chain. N1308 formed foci, but not plaques, in LLC-MK2 cell monolayers and was negative in the Serény test. In this study, the wild-type locus inactivated in N1308 was cloned and further defined by means of complementation analysis. A 4.3-kb BstEII-XhoI fragment of S. flexneri 2a YSH6200 DNA was sufficient to restore both normal LPS and virulence phenotype to the mutant. DNA sequencing of this region revealed four genes, rfbA, rfbB, rfbC, and rfbD, encoding the enzymes required for the biosynthesis of activated rhamnose. The four genes were expressed in Escherichia coli, and the expected protein products were visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. N1308 was shown to have normal levels of surface IpaC and IpaD, while a Western blot (immunoblot) of whole-cell lysates or outer membrane fractions indicated an elevated level of appropriately localized VirG. An in vitro invasion assay revealed that N1308 had normal primary invasive capacity and was able to multiply and move normally within the initial infected cell. However, it exhibited a significant reduction in its ability to spread from cell to cell in the monolayer. A double immunofluorescence assay revealed differences between LLC-MK2 cells infected with the wild-type YSH6000 and those infected with N1308. The wild-type bacteria elicited the formation of the characteristic F-actin tails, whereas N1308 failed to do so. However, N1308 was capable of inducing deposition of F-actin, which accumulated in a peribacterial fashion with only slight, if any, unipolar accumulation of the cytoskeletal protein.

Morphologic evaluation of the pathogenesis of bacterial enteric infections

Current advances in the understanding of the pathogenicity of the agents of diarrheal infections, Vibrio cholerae, diarrheagenic E. coli, Shigella, Salmonella, and enteropathogenic Yersinia, have, to a great extent, become possible due to morphological studies of host-pathogen interactions in natural and experimental infections. Despite a multigenic nature and a diversity of pathogenic features in the bacterial species and even in serogroups of the same species, it is now possible to delineate four major patterns of interaction of enteric pathogens with their cellular targets, the enterocytes, and with the immune apparatus of the gut. These patterns, epicellular cytotonic, epicellular restructuring cytotonic, invasive intraepithelial cytotonic and cytotoxic, and invasive transcellular cytotonic and cytotoxic bacteremic, underlie early pathogenesis and clinical manifestations in the respective diarrheal diseases. In this review, the results of the morphological analyses of these patterns over the last 3 decades as well as some methodological problems encountered in the interpretation of morphological observations are discussed.

Intact alpha-actinin molecules are needed for both the assembly of actin into the tails and the locomotion of Listeria monocytogenes inside infected cells

After the infectious bacterium, Listeria monocytogenes, is phagocytosed by a host cell, it leaves the lysosome and recruits the host cell's cytoskeletal proteins to assemble a stationary tail composed primarily of actin filaments cross-linked with alpha-actinin. The continual recruitment of contractile proteins to the interface between the bacterium and the tail accompanies the propulsion of the bacterium ahead of the elongating tail. When a bacterium contacts the host cell membrane, it pushes out the membrane into an undulating tubular structure or filopodium that envelops the bacterium at the tip with the tail of cytoskeletal proteins behind it. Previous work has demonstrated that alpha-actinin can be cleaved into two proteolytic fragments whose microinjection into cells interferes with stress fiber integrity. Microinjection of the 53 kD alpha-actinin fragment into cells infected with Listeria monocytogenes, induces the loss of tails from bacteria and causes the bacteria to become stationary. Infected cells that possess filopodia when injected with the 53 kD fragment lose their filopodia. These results indicate that intact alpha-actinin molecules play an important role in the intracellular motility of Listeria, presumably by stabilizing the actin fibers in the stationary tails that are required for the bacteria to move forward. Fluorescently labeled vinculin associated with the tails when it was injected into infected cells. Talin antibody staining indicated that this protein, also, is present in the tails. These observations suggest that the tails share properties of attachment plaques normally present in the host cells. This model would explain the ability of the bacterium (1) to move within the cytoplasm and (2) to push out the surface of the cell to form a filopodium. The attachment plaque proteins, alpha-actinin, talin, and vinculin, may bind and stabilize the actin filaments as they polymerize behind the bacteria and additionally could also enable the tails to bind to the cell membrane in the filopodia.

Immunogold labelling of Listeria monocytogenes virulence-related factors within Caco-2 cells

We demonstrated by immunoelectron microscopy that listeriolysin O (LLO), phospholipases and other putative virulence-related proteins produced by Listeria monocytogenes were primarily cell-wall-associated when the bacterium infected Caco-2 tissue culture cell monolayers. Antibodies made to LLO, serogroup 1/2a reacted poorly with serogroup 4b cells and vice-versa, indicating fundamental structural differences in the two proteins. Finally, comet-tail pseudopod structures shown to be involved in cell-to-cell passage of Listeria in Caco-2 cells did not possess detectable Listeria antigens on their anterior surface or within their structure, suggesting that the phagocytic process is primarily host-cell-dependent once it is initiated by the bacterial cell.

Polarized distribution of Listeria monocytogenes surface protein ActA at the site of directional actin assembly

The facultative intracellular pathogen Listeria monocytogenes can infect host tissues by using directional actin assembly to propel itself from one cell into another. The movement is generated by continuous actin assembly from one end of the bacterium into a tail, which is left behind in the cytoplasm. Bacterial actin assembly requires expression of the bacterial gene actA. We have used immunocytochemistry to show that the actA gene product, ActA, is distributed asymmetrically on the bacterial surface: it is not expressed at one pole and is increasingly concentrated towards the other. This polarized distribution of ActA was linked to bacterial division: ActA protein was not, or only faintly, expressed at the pole that had been formed during the previous division. On intracellular bacteria ActA was expressed at the site of actin assembly, suggesting that ActA may be involved in actin filament nucleation off the bacterial surface. We predict that the asymmetrical distribution of this protein is required for the ability of intracellular Listeria to move in the direction of the non-ActA expressing pole.

Localization of the ActA polypeptide of Listeria monocytogenes in infected tissue culture cell lines: ActA is not associated with actin "comets"

The ActA protein of the gram-positive pathogen Listeria monocytogenes is a 90-kDa polypeptide required for interaction of the bacteria with components of the host cell microfilament system to generate intra- and intercellular movement. To study the localization, distribution, and expression of the ActA polypeptide in L. monocytogenes grown either in broth culture or in infected tissue culture cells, we first isolated ActA by monoclonal antibody-based immunoaffinity chromatography. Polyclonal rabbit antisera raised against purified ActA revealed that ActA was associated with the cell wall and exposed on the surface of the bacteria, readily accessible to ActA antibodies. In contrast, a C-terminally truncated ActA1 polypeptide expressed by the isogenic actA1 mutant was detected only in the supernatant fluids. Immunofluorescence microscopy and electron microscopic studies using immunogold labeling showed that ActA was present on the surface of the bacteria infecting PtK2 and J774 cells at all stages of the infection cycle and was not found to be associated with the actin "tail" of individual bacteria. For the isogenic actA1 mutant strain, which grew as microcolonies within infected cells, only diffuse staining of the secreted ActA1 polypeptide in the host cytoplasm was observed. The ActA polypeptide therefore appears to be required in the initiation of actin accumulation by the bacterium and is apparently not directly involved in the generation of the actin tail. Analysis of strains of several L. monocytogenes serotypes indicated microheterogeneity in the molecular weights of the ActA polypeptides of individual strains and led to the detection of a serotype 3a strain that does not produce ActA.

Role of actin polymerization in cell locomotion: molecules and models

Actin filaments forming at the anterior margin of a migrating cell are essential for the formation of filopodia, lamellipodia, and pseudopodia, the "feet" that the cell extends before it. These structures in turn are required for cell locomotion. Yet the molecular nature of the "nucleator" that seeds the polymerization of actin at the leading edge is unknown. Recent advances, including video microscopy of actin dynamics, discovery of proteins unique to the leading edge such as ponticulin, the Mab 2E4 antigen, and ABP 120, and novel experimental models of actin polymerization such as the actin-based movements of intracellular parasites, promise to shed light on this problem in the near future.

Directional actin polymerization associated with spotted fever group Rickettsia infection of Vero cells

Members of the spotted fever group (SFG) of rickettsiae spread rapidly from cell to cell by an unknown mechanism(s). Staining of Rickettsia rickettsii-infected Vero cells with rhodamine phalloidin demonstrated unique actin filaments associated with one pole of intracellular rickettsiae. F-actin tails greater than 70 microns in length were seen extending from rickettsiae. Treatment of infected cells with chloramphenicol eliminated rickettsia-associated F-actin tails, suggesting that de novo protein synthesis of one or more rickettsial proteins is required for tail formation. Rickettsiae were coated with F-actin as early as 15 min postinfection, and tail formation was detected by 30 min. A survey of virulent and avirulent species within the SFG rickettsiae demonstrated that all formed actin tails. Typhus group rickettsiae, which do not spread directly from cell to cell, lacked F-actin tails entirely or exhibited only very short tails. Transmission electron microscopy demonstrated fibrillar material in close association with R. rickettsii but not Rickettsia prowazekii. Biochemical evidence that actin polymerization plays a role in movement was provided by showing that transit of R. rickettsii from infected cells into the cell culture medium was inhibited by treatment of host cells with cytochalasin D. These data suggest that the cell-to-cell transmission of SFG rickettsiae may be aided by induction of actin polymerization in a fashion similar to that described for Shigella flexneri and Listeria monocytogenes.

Listeria monocytogenes--a model system for studying the pathomechanisms of an intracellular microorganism

Virulence of Listeria monocytogenes is determined by a cluster of five genes in the order plcA, hly, mpl, actA and plcB, which are coordinately regulated by a transcriptional activator, termed PrfA. The gene for PrfA is located in front of plcA. Mutations within each of these genes reduce the virulence considerably and render the mutants unable to properly multiply and/or spread within the infected host cells. Under growth-limiting conditions PrfA-dependent proteins are preferentially synthesised. These studies indicate the existence of additional PrfA-regulated proteins in L. monocytogenes. The synthesis of catalase, superoxide dismutase, LmaA and p60 is not under the control of PrfA. These proteins seem to be also associated with virulence of L. monocytogenes. P60-related proteins are found as major extracellular proteins in all Listeria species but only p60 of L. monocytogenes is able to restore the failure of R-mutants (exhibiting a drastically reduced synthesis of p60) to adhere to 3T6 mouse fibroblasts. Adherence of L. monocytogenes to the epithelial Caco-2 cells seem to be independent of p60. The p60 protein of L. monocytogenes differs characteristically from the p60-related proteins of the nonvirulent Listeria species.

Listeria ivanovii is capable of cell-to-cell spread involving actin polymerization

Listeria ivanovii has been considered to be pathogenic to animals but has rarely been found associated with human infections. It has been claimed that L. ivanovii lacks the actA gene, which in L. monocytogenes encodes a protein required for interaction with host cell actin. Using fluorescence microscopy and electron microscopy, we demonstrate that L. ivanovii can invade mammalian cells, lyse the phagosomal membrane, polymerize host cell actin, reorganize actin to form tails, and spread from cell to cell. However, no DNA homologous to the actA gene could be detected by polymerase chain reaction. Further, L. ivanovii lacks the 90-kDa surface protein which in L. monocytogenes is encoded by actA. Despite the ability to spread from cell to cell, L. ivanovii differed significantly from L. monocytogenes in being unable to form plaques on monolayers of 3T3 fibroblast cells.

Unipolar reorganization of F-actin layer at bacterial division and bundling of actin filaments by plastin correlate with movement of Shigella flexneri within HeLa cells

Shigella flexneri causes bacillary dysentery, an invasive disease of colonic epithelial cells in humans. The capacity of bacteria, once they have entered into a cell and escaped the phagocytic vacuole, to spread intracellularly and directly to adjacent cells without further extracellular passage is a key factor in invasion of the epithelial layer. Movement of intracellular bacteria is dependent upon the polymerization of actin; concentration of the formed filaments to one end of the bacterium is associated with initiation of movement. This movement may lead to the formation of a protrusion at the cell surface through which the bacterium passes to an adjacent cell. Development of these protrusions in infected HeLa cells is described, with emphasis on two critical observations. First, initiation of movement is coupled with bacterial division since elongation of the bacterial body is associated with relocalization of the previously uniformly distributed layer of actin to one pole of the bacterium. Second, the actin-bundling protein plastin appears to bundle the actin filaments just posterior to the bacterium, producing an ongoing contraction of the cylindrical actin tail that may be associated with forward movement of the bacterium within the protrusion.

Host cell actin assembly is necessary and likely to provide the propulsive force for intracellular movement of Listeria monocytogenes

Listeria monocytogenes is able to escape from the phagolysosome and grow within the host cell cytoplasm. By 3 h after initiation of infection, actin filaments begin to concentrate at one end of the bacterium. Polarization of F-actin is associated with intracellular bacterial movement, long projections of actin filaments forming directly behind the moving bacteria. New actin monomers are added to the region of the projection in proximity to the bacterium. The rate of new actin filament growth correlates closely with the speed of bacterial migration. This actin structure is anchored within the cytoplasm, serving as a fixed platform for directional expansion of the actin filament network. The actin projection progressively lengthens as the bacterium migrates. Cytochalasin blocks both elongation of the projection and bacterial movement but does not result in complete depolymerization of the bacterially induced actin structure, residual actin and alpha-actinin persisting in proximity to one end of the bacterium. Bacteria initially migrate within the cortical cytoplasm but later move to the peripheral membrane, where they form filopodiumlike structures which pivot and undulate in the extracellular medium. In the filopodia, bacteria are occasionally seen to abruptly change direction, turn 180 degrees, and move back into the medullary region of the host cell. All filopodium movement ceases once the bacterium containing the F-actin projection returns to the cortical cytoplasm. These results indicate that host cell actin polymerization is necessary for intracellular migration of listeriae and suggest that directional actin assembly may in fact generate the propulsive force for bacterial and filopodial movement.

Expression of receptors for enterotoxigenic Escherichia coli during enterocytic differentiation of human polarized intestinal epithelial cells in culture

To study the expression of human intestinal receptors for enterotoxigenic Escherichia coli (ETEC), the human polarized intestinal epithelial cell line Caco-2 in culture and several subpopulations of HT-29 cells in culture--parental (mainly undifferentiated) HT-29 cells (HT-29 Std), an enterocytelike subpopulation obtained by selection through glucose deprivation (HT-29 Glc-), and an enterocytelike subpopulation obtained by selection through glucose deprivation which maintains its differentiation characteristics when switched back to standard glucose-containing medium (HT-29 Glc-/+)--were used. Since Caco-2 spontaneously differentiated in culture under standard culture conditions (in the presence of glucose) and HT-29 cells were undifferentiated when cultured under standard conditions (HT-29 Std) and differentiated when grown in a glucose-free medium (HT-29 Glc-), we studied the expression of the receptors for colonization factor antigens (CFA) I, II, and III and the 2230 antigen of ETEC in relation to enterocytic differentiation. We provide evidence that expression of ETEC CFA receptors develops in parallel with other differentiation functions of the cultured cells. The expression of ETEC-specific brush border receptors was studied by indirect immunofluorescence using antibodies raised against purified ETEC CFA. No ETEC receptors were detected in HT-29 Std or short-term-cultured Caco-2 cells. However, among the population of HT-29 Std cells, 2 to 4% of the cells were found to bind ETEC, and these cells expressed positive carcinoembryonic antigen immunoreactivity. This indicated that among the population of undifferentiated HT-29 cells, clusters of differentiated cells were present. ETEC CFA receptors were expressed in the apical and basolateral domains of differentiated HT-29 cells, whereas in differentiated Caco-2 cells only apical expression was observed. Both in HT-29 cells (HT-29 Glc-/+) and in Caco-2 cells cultured under standard conditions, ETEC CFA receptors develop as a function of day in culture. This indicated that the expression of the ETEC CFA receptors was a growth-related event. Indeed, ETEC CFA receptors developed in step with the apical expression of differentiation-associated proteins.

Immunoaffinity purification, stabilization and comparative characterization of listeriolysin O from Listeria monocytogenes serotypes 1/2a and 4b

We developed a simple and highly effective procedure for stabilizing the haemolytic activity of listeriolysin O (LLO) from Listeria monocytogenes after immunoaffinity purification. The haemolytic activity of LLO was stabilized by eluting it directly into tubes containing an alkaline buffer (5 mM lysine, 140 mM KCl, 50% ethylene glycol, pH 11.5). The purified LLO retained 100% of its haemolytic activity after 6 weeks of storage at -20 degrees C. LLO purified from a strain of L. monocytogenes serotype 1/2a (ATCC 43249) and LLO purified from a strain of L. monocytogenes serotype 4b (F 2365) isolated from a Mexican-style cheese, showed no significant differences in pH and temperature stability. When incubated in buffers at pH values from 4 to 12 at 4 degrees C and 25 degrees C, LLO from serotypes 1/2a and 4b retained maximal haemolytic activity at pH 8 after 4 h of incubation. LLO from both serotypes lost their haemolytic activity after incubation at 50 degrees C for 25 min.

Movement along actin filaments of the perijunctional area and de novo polymerization of cellular actin are required for Shigella flexneri colonization of epithelial Caco-2 cell monolayers

Shigella flexneri invades eucaryotic cells and grows in the cytoplasm. Lysis of the phagosomal membrane is a prerequisite for both intracellular multiplication and movement of the bacteria that gain direct access to the host cell actin. In HeLa cells, bacteria generate their own movement essentially by inducing actin polymerization. Polymerization of actin enables them to move rapidly and randomly in the cytoplasm and to spread from one cell to another through protrusions of the host cell membrane. This movement was designated the Ics phenotype. In contrast, in chicken embryo fibroblasts, bacteria move along actin filaments in a very organized manner, following the cytoskeletal architecture; this movement was designated the Olm phenotype. Bacterial movement is a major virulence factor in that it is necessary for efficient colonization of the intestinal epithelium of infected macaque monkeys. Further characterization of the cellular events that lead to colonization of the colonic intestinal epithelium was needed. In order to characterize the movement in vitro in a cell assay system more closely related to the intestinal epithelium, we used human colonic epithelial Caco-2 cells. The movement of bacteria as observed by using immunofluorescence and confocal microscopy appeared to result from the expression of both the Olm and Ics phenotypes. The former allowed colonization of cells along the actin filament ring of the perijunctional area. The latter promoted passage from one cell to adjacent cells. This in vitro pattern of movement and multiplication gives S. flexneri, once it has entered an epithelial cell, the unique capacity to spread through the entire epithelial layer without having further contact with the extracellular compartment.

L. monocytogenes-induced actin assembly requires the actA gene product, a surface protein

The intracellular pathogenic bacterium L. monocytogenes can spread directly from cell to cell without leaving the cytoplasm. The mechanism of this movement, generated through bacterially induced actin polymerization, is not understood. By analyzing an avirulent Tn917-lac mutant defective for actin polymerization, we have identified a bacterial component involved in this process. The transposon had inserted in actA, the second gene of an operon. Gene disruption of downstream genes and transformation of the mutant strain with actA showed that the actA gene encodes a surface protein necessary for bacterially induced actin assembly. Our results indicate that it is a 610 amino acid protein with an apparent molecular weight of 90 kd.

Nucleotide sequence of the lecithinase operon of Listeria monocytogenes and possible role of lecithinase in cell-to-cell spread

The lecithinase gene of the intracellular pathogen Listeria monocytogenes, plcB, was identified in a 5,648-bp DNA fragment which expressed lecithinase activity when cloned into Escherichia coli. This fragment is located immediately downstream of the previously identified gene mpl (prtA). It contains five open reading frames, named actA, plcB, and ORFX, -Y, and -Z, which, together with mpl, form an operon, since a 5.7-kb-long transcript originates from a promoter located upstream of mpl (J. Mengaud, C. Geoffroy, and P. Cossart, Infect. Immun. 59:1043-1049, 1991). A second promoter was detected in front of actA which encodes a putative membrane protein containing a region of internal repeats. plcB encodes the lecithinase, a predicted 289-amino-acid protein homologous to the phosphatidylcholine-specific phospholipases C of Bacillus cereus and Clostridium perfringens (alpha-toxin). plcB mutants produce only small plaques on fibroblast monolayers, and an electron microscopic analysis of infected macrophages suggests that lecithinase is involved in the lysis of the two-membrane vacuoles that surround the bacteria after cell-to-cell spread. On the opposite DNA strand, downstream of the operon, three more open reading frames, ldh, ORFA, and ORFB, were found. The deduced amino acid sequence of the first one is homologous to lactate dehydrogenases. Low-stringency Southern hybridization experiments suggest that these three open reading frames lie outside of the L. monocytogenes virulence region: mpl and actA were specific for L. monocytogenes, sequences hybridizing to plcB were detected in L. ivanovii and L. seeligeri, and sequences hybridizing to ORFX, -Y, and -Z were found in L. innocua. In contrast to this, sequences hybridizing to ldh or ORFB were detected in all Listeria species (including the nonpathogenic ones).

Shigella flexneri enters human colonic Caco-2 epithelial cells through the basolateral pole

The commonly accepted view that enteroinvasive bacteria enter cells of the intestinal epithelial lining through the apical surface can be challenged in the case of shigellosis. This study is based on in vitro experiments that showed that the invasion of human colonic Caco-2 cells by Shigella flexneri occurred through the basolateral pole of these cells. In these experiments, the few bacteria that interacted with the apical surface either bound to microvilli of the cell dome without causing detectable alteration or bound at the level of intercellular junctions at which they demonstrated a limited capacity for paracellular invasion, which permitted subsequent entry through the lateral domain of the cells. Treatment of Caco-2 cell monolayers with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), which disrupts intercellular junctions, greatly enhanced the rate of cell infection. These observations suggest a physiopathological paradox that may have important consequences for the understanding of the process of colonic invasion in vivo during shigellosis.

Coordinate regulation of virulence genes in Listeria monocytogenes requires the product of the prfA gene

The prfA gene of Listeria monocytogenes encodes a protein that activates transcription of the listeriolysin gene (lisA). In order to explore the role of the prfA gene product in the pathogenesis of listerial infection, we constructed a site-directed insertion mutation in prfA by the chromosomal integration of a novel suicide vector containing a portion of the prfA coding region. This mutation not only transcriptionally silenced the listeriolysin (lisA) gene but also abrogated production of specific RNA transcripts corresponding to the phosphatidylinositol-specific phospholipase C (pic) and metalloprotease (mpl) genes, two further virulence gene products expressed only by pathogenic Listeria strains. The strain was also found to be avirulent when tested in a mouse model of listerial infection. The concomitant loss of multiple characteristics such as production of LisA, Pic, Mpl, and loss of virulence in a mouse infection model is the result of a mutation in a single gene and demonstrates that the prfA gene product is a positive regulator of multiple virulence determinants in L. monocytogenes.

Intercellular spread of Shigella flexneri through a monolayer mediated by membranous protrusions and associated with reorganization of the cytoskeletal protein vinculin

The spread of Shigella flexneri in a monolayer of infected Henle and HeLa cells was studied by using immunofluorescence and electron microscopy. Infected cells produced numerous bacterium-containing membranous protrusions up to 18 microns in length that penetrated adjacent cells and were subsequently phagocytosed. Fluorescence staining of actin and vinculin in infected cells with phalloidin and monoclonal antibody to vinculin, respectively, demonstrated that the protrusions containing the bacteria consisted of these cytoskeletal proteins. Actin accumulated predominantly at the poles of bacteria distal to the tip of protrusions and appeared as trails extending back towards the host cell cytoplasm. Vinculin, however, was distributed uniformly around the bacteria and throughout the protrusion. A profound rearrangement of vinculin occurred in Henle and HeLa cells following infection with shigellae: whereas in uninfected cells it was distributed mainly around the cell periphery, in infected cells it concentrated mainly around clusters of bacteria in the cytoplasm. This suggests a possible involvement of the vinculin cytoskeletal protein in the intercellular spread of shigellae during an infection.

Listeria monocytogenes, a food-borne pathogen

The gram-positive bacterium Listeria monocytogenes is an ubiquitous, intracellular pathogen which has been implicated within the past decade as the causative organism in several outbreaks of foodborne disease. Listeriosis, with a mortality rate of about 24%, is found mainly among pregnant women, their fetuses, and immunocompromised persons, with symptoms of abortion, neonatal death, septicemia, and meningitis. Epidemiological investigations can make use of strain-typing procedures such as DNA restriction enzyme analysis or electrophoretic enzyme typing. The organism has a multifactorial virulence system, with the thiol-activated hemolysin, listeriolysin O, being identified as playing a crucial role in the organism's ability to multiply within host phagocytic cells and to spread from cell to cell. The organism occurs widely in food, with the highest incidences being found in meat, poultry, and seafood products. Improved methods for detecting and enumerating the organism in foodstuffs are now available, including those based on the use of monoclonal antibodies, DNA probes, or the polymerase chain reaction. As knowledge of the molecular and applied biology of L. monocytogenes increases, progress can be made in the prevention and control of human infection.