Entry of Shigella flexneri into HeLa cells: evidence for directed phagocytosis involving actin polymerization and myosin accumulation

Infection of cultured rat enterocytes by Ileal symbiont intracellularis depends on host cell function and actin polymerisation

The mechanisms of entry of Ileal symbiont intracellularis into IEC-18 rat enterocyte cells and subsequent bacterial proliferation were examined in centrifuge-assisted and static infections. Live, oxygen or neomycin damaged, and formalin killed bacteria, each rapidly entered viable cells. Live or damaged bacteria did not enter cells nor proliferate within cells after static infection of cells cooled to 5 degrees C. Infection of cells was greatly reduced at 20 degrees or 32 degrees compared to infection at 37 degrees C. Centrifuge-assisted infection was also reduced by chilling the cells. Cytochalasin D but not B inhibited the entry process indicating an actin-dependent infection, although other pathways may also be involved in centrifuge-assisted infections. Drugs capable of modifying cell membrane charge, heparin receptors or trypsin-labile proteins were all inactive in preventing or enhancing infection. We therefore conclude that infection of enterocytes by IS intracellularis is dependent on host cell activity and actin polymerization, but is independent of bacterial viability.

The evolution of invasion by enteric bacteria

Despite differences in disease pathologies and host range, many enteric pathogens, including Salmonella and Shigella spp., utilize a remarkably similar machinery to secrete proteins that promote their entry into host cells. Analogous structures are required for the export of virulence proteins in other animal and plant pathogens. While the structure and organization of the gene complexes specifying these secretory pathways are broadly conserved, their phylogenetic distribution and genomic locations suggest that these sequences arose independently in divergent pathogens.

Invasion of epithelial cells by Shigella flexneri induces tyrosine phosphorylation of cortactin by a pp60c-src-mediated signalling pathway

Shigella flexneri causes bacillary dysentery in humans by invading epithelial cells of the colon. Cell invasion occurs via bacterium-directed phagocytosis, a process requiring polymerization of actin at the site of bacterial entry. We show that invasion of HeLa cells by S.flexneri induces tyrosine phosphorylation of cortactin, a host cell protein previously identified as a cytoskeleton-associated protein tyrosine kinase (PTK) substrate for the proto-oncoprotein pp60c-src. Immunolocalization experiments indicate that cortactin is recruited to submembranous actin filaments formed during bacterial entry. In particular, cortactin is highly enriched in membrane ruffles of the entry structure, which engulf entering bacteria, and also in the periphery of the phagosome early after bacterial internalization. The proto-oncoprotein pp60c-src appears to mediate tyrosine phosphorylation of cortactin, since overexpression of this PTK in HeLa cells specifically increases the level of cortactin tyrosine phosphorylation induced during bacterial entry. Immunolocalization studies in pp60c-src-overexpressing HeLa cells indicate that pp60c-src is recruited to the entry structure and to the periphery of the phagosome, where pp60c-src appears to accumulate in association with the membrane. Our results suggest that epithelial cell invasion by S.flexneri involves recruitment and kinase activation of pp60c-src. Signalling by the proto-oncoprotein pp60c-src may play a role in cytoskeletal changes that facilitate S.flexneri uptake into epithelial cells, since transient overexpression of pp60c-src in HeLa cells can provoke membrane ruffling and appears also to stimulate bacterial uptake of a non-invasive S.flexneri strain.

Human microvascular endothelial tissue culture cell model for studying pathogenesis of Brazilian purpuric fever

Brazilian purpuric fever (BPF) is a fulminant pediatric disease characterized by fever, with rapid progression to purpura, hypotensive shock, and death. All known BPF cases have been caused by three clones of Haemophilus influenzae biogroup aegyptius and have occurred in either Brazil or Australia. Using an immortalized line of human vascular endothelial cells, we developed an in vitro assay that identifies all known BPF-causing H. influenzae biogroup aegyptius strains (R. S. Weyant, F. D. Quinn, E. A. Utt, M. Worley, V. G. George, F. J. Candal, and E. W. Ades, J. Infect. Dis. 169:430-433, 1994). With multiplicities of infection (MOIs) as low as one bacterium per 1,000 tissue culture cells, BPF-associated strains produce a unique cytotoxic effect in which the tissue culture cells detach and aggregate in large floating masses after 48 h of incubation. In this study, using a BPF-associated strain and a non-BPF-associated control, we demonstrated that strains which produce the cytotoxic phenotype were able to replicate intracellularly whereas non-BPF-associated strains, with MOIs of > or = 1,000 did not replicate and did not produce the phenotype. We also showed that this phenotype is not caused by the activity of an endotoxin or the release of some other compound from the bacterial cell, since neither gamma irradiation-killed whole BPF clone bacteria nor bacterial cell fractions at MOIs of > 1,000 produced the cytotoxic effect. Furthermore, bacteria in numbers equal to MOIs of > 1,000 treated with chloramphenicol did not produce the cytotoxic phenotype, suggesting a requirement for bacterial protein synthesis. In addition, viable bacteria separated from the tissue culture monolayer by a 0.2-micron-pore-size membrane also failed to produce the phenotype. The ability of the bacterium to invade, replicate, and produce the phenotype appears to be primarily parasite directed since phagocytosis, pinocytosis, and eukaryotic protein synthesis inhibitors, including cycloheximide, cytochalasin D, and methylamine, had no effect on the ability of the bacterium to invade and cause a cytotoxic response. Understanding the basic mechanisms involved in this tissue-destructive process should enhance our knowledge of the general pathogenesis of BPF.

Cytoskeletal rearrangements and the functional role of T-plastin during entry of Shigella flexneri into HeLa cells

Shigella flexneri is an enteroinvasive bacterium which causes bacillary dysentery in humans. A major feature of its pathogenic potential is the capacity to invade epithelial cells. Shigella entry into epithelial cells is considered a parasite-induced internalization process requiring polymerization of actin. Here we describe the cytoskeletal rearrangements during S. flexneri invasion of HeLa cells. After an initial contact of the bacterium with the cell surface, distinct nucleation zones of heavy chain actin polymerization appear in close proximity to the contact site underneath the parasite with long filaments being polymerized. These structures then push cellular protrusions that rise beside the entering bacterium, being sustained by tightly bundled long actin filaments organized in parallel orientation with their positive ends pointing to the cytoplasmic membrane. Finally, the cellular projections coalesce above the bacterial body, leading to its internalization. In addition, we found the actin-bundling protein plastin to be concentrated in these protrusions. Since plastin is known to bundle actin filaments in parallel orientation, colocalization of parallel actin filaments and plastin in the cellular protrusions strongly suggested a functional role of this protein in the architecture of parasite-induced cellular projections. Using transfection experiments, we show the differential recruitment of the two plastin isoforms (T- and L-) into Shigella entry zones. By transient expression of a truncated T-plastin which is deprived of one of its actin-binding sites, we also demonstrate the functional role of T-plastin in Shigella entry into HeLa cells.

Penetration and damage of endothelial cells by Candida albicans

The mechanisms of phagocytosis of Candida albicans by human vascular endothelial cells and subsequent endothelial cell injury were examined in vitro. Both live and killed C. albicans cells were phagocytized by endothelial cells. This organism specifically induced endothelial cell phagocytosis because neither Candida tropicalis nor Torulopsis glabrata was ingested. Endothelial cell microfilaments polymerized around C. albicans as the organisms were phagocytized. Cytochalasin D inhibited this polymerization of microfilaments around C. albicans and blocked phagocytosis. The blocking of actin depolymerization with phalloidin had no effect on microfilament condensation around the organism, indicating that the microfilaments surrounding C. albicans are formed from a pool of G-actin. Intact microtubules were also necessary for the phagocytosis of C. albicans, since the depolymerizing of endothelial cell microtubules with nocodazole prevented the condensation of actin filaments around the organisms and inhibited phagocytosis. In contrast, microtubule depolymerization was not required for microfilament function because the blocking of microtubule depolymerization with taxol had no effect on microfilament condensation around C. albicans. The phagocytosis of C. albicans was pivotal in the induction of endothelial cell damage, since the blocking of candidal internalization significantly reduced endothelial cell injury. Endothelial cells were not damaged by phagocytosis of dead organisms, indicating that injury was caused by a factor associated with viable organisms. Therefore, C. albicans is uniquely able to induce endothelial cell phagocytosis by comparison with non-albicans species of Candida. Furthermore, at least two components of the endothelial cytoskeleton, microfilaments and microtubules, are necessary for the phagocytosis of C. albicans.

Rickettsia conorii entry into Vero cells

The entry of rickettsiae into eukaryotic cells is mediated by an induced phagocytosis, but rickettsiae have never been observed in a closed phagocytic vacuole. In this study, Rickettsia conorii entry into Vero cells was observed by transmission electron microscopy during a period of 3 to 20 min after bacterium-cell contact. The entry occurred within 3 min after bacterium-cell contact, and R. conorii was observed in the process of engulfment, within a phagocytic vacuole, or free in the cytosol. Escape from the phagosome is a very rapid step since phagosome lysis was only occasionally observed. By 12 min, 90% of bacteria were internalized and half were free in the cytosol. This report confirms that rickettsiae penetrate nonphagocytic cells by induced phagocytosis and is the first demonstration of rickettsiae within a complete phagocytic vacuole.

Avirulence of rough mutants of Shigella flexneri: requirement of O antigen for correct unipolar localization of IcsA in the bacterial outer membrane

Mutations in the lipopolysaccharide (LPS) of Shigella spp. result in attenuation of the bacteria in both in vitro and in vivo models of virulence, although the precise block in pathogenesis is not known. We isolated defined mutations in two genes, galU and rfe, which directly affect synthesis of the LPS of S. flexneri 2a, in order to determine more precisely the step in virulence at which LPS mutants are blocked. The galU and rfe mutants invaded HeLa cells but failed to generate the membrane protrusions (fireworks) characteristic of intracellular motility displayed by wild-type shigellae. Furthermore, the galU mutant was unable to form plaques on a confluent monolayer of eucaryotic cells and the rfe mutant generated only tiny plaques. These observations indicated that the mutants were blocked in their ability to spread from cell to cell. Western immunoblot analysis of expression of IcsA, the protein essential for intracellular motility and intercellular spread, demonstrated that both mutants synthesized IcsA, although they secreted less of the protein to the extracellular medium than did the wild-type parent. More strikingly, the LPS mutants showed aberrant surface localization of IcsA. Unlike the unipolar localization of IcsA seen in the wild-type parent, the galU mutant expressed the protein in a circumferential fashion. The rfe mutant had an intermediate phenotype in that it displayed some localization of IcsA at one pole while also showing diffuse localization around the bacterium. Given the known structures of the LPS of wild-type S. flexneri 2a, the rfe mutant, and the galU mutant, we hypothesize that the core and O-antigen components of LPS are critical elements in the correct unipolar localization of IcsA. These observations indicate a more precise role for LPS in Shigella pathogenesis.

Shigella flexneri-HeLa cell interactions: a putative role for host cell protein kinases

Epithelial cell invasion has been shown to be a prerequisite for Shigella flexneri virulence. Recently, we have documented the induction of transcription factor DNA binding activities as a result of S. flexneri challenge of HeLa cells. In this report, we show that HeLa cells challenged with S. flexneri display differences in phosphotyrosine-containing proteins. These changes are detected as early as 5 min post-challenge. Challenge with a noninvasive ipaB mutant strain resulted in the induction of a similar, but less intense, profile of phosphotyrosine-containing host cell proteins. Phosphotyrosine-containing proteins could be detected in S. flexneri, but were unique from those detected following HeLa cell challenge. S. flexneri invasion of HeLa cell monolayers was reduced by treatment with protein kinase inhibitors. These data suggest a role for protein kinases in the initial response of host cells to S. flexneri.

Group A streptococci efficiently invade human respiratory epithelial cells

Although infection by group A streptococci is a model of extracellular mucosal pathogenesis, these organisms can be associated with highly invasive infections resulting in sepsis and shock. Over the last 6 yr this species has renewed its reputation as a significant cause of sepsis and has piqued interest in the mechanism by which some strains are better able to breach mucosal barriers to gain access to the bloodstream than are others. An internalization assay was developed on the basis of resistance of intracellular streptococci to penicillin and gentamicin. Experiments showed that stationary-phase, as opposed to logarithmic-phase, bacteria are efficiently internalized and can persist in cultured human cells. Electron microscopy confirmed that streptococci were contained within intracellular vacuoles. Various strains of streptococci revealed significant differences in their capacity to be internalized. Two type M1 streptococci isolated from blood infections were internalized at frequencies equal to those reported for Salmonella and Listeria monocytogenes and greater than the frequency of a clonal variant from a case of pharyngitis.

Invasion and intracellular survival of Bordetella bronchiseptica in mouse dendritic cells

We have studied the interaction between the respiratory pathogen Bordetella bronchiseptica and murine spleen dendritic cells, important antigen-presenting cells that are found in the airway epithelium. Wild-type B. bronchiseptica 5376 attached very efficiently to dendritic cells, whereas the bvg mutant ATCC 10580, wild-type strain BB7865, and its spontaneous delta bvgS mutant BB7866 bound less efficiently. However, all tested B. bronchiseptica strains were able to invade dendritic cells and survive intracellularly for at least 72 h. These results suggest that bvg-independent or bvg-downregulated products are involved in the uptake and intracellular survival. Transmission electron microscopic analysis revealed that bacteria grew and replicated intracellularly and were present in typical phagosomes, which fused with lysosomes during the initial infection period. However, in later infection stages some bacteria seemed to escape into an unfused endocytic compartment, where individual bacteria were tightly surrounded by a membrane. The in vitro interaction of B. bronchiseptica with dendritic cells reported here may be relevant to natural infections caused by this organism that lead to chronicity or an altered immune response.

Pseudomonas aeruginosa selective adherence to and entry into human endothelial cells

The pathogenesis of Pseudomonas aeruginosa disseminated infections depends on bacterial interaction with blood vessels. We have hypothesized that in order to traverse the endothelial barrier, bacteria would have to adhere to and damage endothelial cells. To test this hypothesis, we studied the adherence to human endothelial cells in primary culture of the piliated P. aeruginosa strain PAK and of two isogenic nonpiliated strains: PAK/p-, which carries a mutation in the pilin structural gene, and PAK-N1, a mutant defective in the regulatory rpoN gene. PAK adhered significantly more than did the pilus-lacking strains. P. aeruginosa was also taken up by endothelial cells, as determined by quantitative bacteriologic assays and by transmission electron microscopy. This internalization of P. aeruginosa seems to be a selective process, since the piliated strain was taken up significantly more than the nonpiliated bacteria and the avirulent Escherichia coli DH5 alpha, even following bacterial centrifugation onto the cell monolayers. A significant fraction of the internalized P. aeruginosa PAK was recovered in a viable form after 6 h of residence within endothelial cells. Progressive endothelial cell damage resulted from PAK intracellular harboring, as indicated by the release of lactate dehydrogenase. An increasing concentration of PAK cells was recovered from the extracellular medium with time, suggesting that ingested bacteria were released from endothelial cells and multiplied freely. We speculate that in vivo the ability of some P. aeruginosa strains to resist intracellular residence would afford protection from host defenses and antibiotics and that the release of viable bacteria into bloodstream may represent a central feature of the pathogenesis of bacteremia in compromised patients.

Extracellular association and cytoplasmic partitioning of the IpaB and IpaC invasins of S. flexneri

Shigella species cause bacillary dysentery in humans by invading colonic epithelial cells. IpaB and IpaC, two major invasins of these pathogens, are secreted into the extracellular milieu. We show here that IpaB and IpaC form a complex in the extracellular medium and that each binds independently to a 17 kDa polypeptide, IpgC, in the bacterial cytoplasm. The IpgC polypeptide was found to be necessary for bacterial entry into epithelial cells, to stabilize the otherwise unstable IpaB protein, and to prevent the proteolytic degradation of IpaC that occurs through its association with unprotected IpaB. We propose that IpgC, which is not secreted and thus acts as a molecular chaperone, serves as a receptor that prevents premature oligomerization of IpaB and IpaC within the cytoplasm of Shigella cells.

Fluorescent choleretic and cholestatic bile salts take different paths across the hepatocyte: transcytosis of glycolithocholate leads to an extensive redistribution of annexin II

We have used fluorescent derivatives of the choleretic bile salts cholate and chenodeoxycholate, the cholestatic salt lithocholate, and the therapeutic agent ursodeoxycholate to visualize distinct routes of transport across the hepatocyte and delivery to the canalicular vacuole of isolated hepatocyte couplets. The cholate and chenodeoxycholate derivatives produced homogeneous intracellular fluorescence and were rapidly transported to the vacuole, while the lithocholate analogue accumulated more slowly in the canalicular vacuole and gave rise to punctate fluorescence within the cell. Fluorescent ursodeoxycholate showed punctate intracellular fluorescence against a high uniform background indicating use of both pathways. Inhibition of vesicular transport by treatment with colchicine and Brefeldin A had no effect on the uptake of any of the compounds used, but it dramatically impaired delivery of both the lithocholate and the ursodeoxycholate derivatives to the canalicular vacuole. We conclude that while the chenodeoxycholate and cholate analogues traverse the hepatocyte by a cytoplasmic route, lithocholate and ursodeoxycholate analogues are transported by vesicle-mediated transcytosis. Treatment of couplets with glycine derivatives of lithocholate and ursodeoxycholate, but not cholate or chenodeoxycholate, led to a marked relocalization of annexin II, which initially became concentrated at the basolateral membrane, then moved to a perinuclear distribution and finally to the apical membrane as the incubation progressed. This suggests that lithocholate and ursodeoxycholate treatment leads to a rapid induction of transcytosis and that annexin II exchange occurs upon membrane fusion at all stages of the hepatocyte transcytotic pathway. These results indicate that isolated hepatocyte couplets may provide an inducible model system for the study of vesicle-mediated transcytosis.

Acute inflammation causes epithelial invasion and mucosal destruction in experimental shigellosis

The gram-negative pathogen Shigella flexneri causes bacillary dysentery, an invasive disease of the human colonic mucosa. A major characteristic of the infectious process is the occurrence of an acute inflammatory reaction of mucosal tissues which is generally consequence of primary invasion and destruction of colonic epithelial cells by the pathogen. Confirming in vitro demonstration that S. flexneri is unable to invade the apical pole of colonic cells and that polymorphonuclear (PMN) cells may assist them in reaching the basal side of epithelial cells where they can invade, we have provided here in vivo evidence that S. flexneri enters the epithelial barrier essentially through the dome of lymphoid follicles at the early stage of infection and that subsequent invasion and destruction of the epithelium is primarily due to immigration of leukocytes, particularly PMN that destroy cohesion of the epithelial barrier. These conclusions are based on experiments carried out in infected rabbit ligated intestinal loops, with some animals treated by an anti-CD18 monoclonal antibody that blocked immigration of leukocytes into infected tissues.

Cell association and invasion of Caco-2 cells by Campylobacter jejuni

Adherence and invasion studies were conducted in monolayers of Caco-2 cells. Three-day-old monolayers were inoculated with Campylobacter jejuni 81-176 at a bacterium/cell ratio of 1,000:1. Saturation studies demonstrated time- and dose-dependent saturation curves for C. jejuni cell association and invasion into Caco-2 cells. Electron microscopy revealed intracellular C. jejuni located within membrane-bound vacuoles. Cell association and invasion were inhibited by 0.3 and 0.5 M concentrations of various sugars, including D-glucose, D-mannose, and D-fucose. However, there was no inhibition with the corresponding L-sugars, indicating physiological specificity. The inhibition of cell association with phloridzin was less pronounced. There was no inhibition of bacterial entry with monodansylcadaverine or g-strophanthin, indicating that it was unlikely that coated-pit formation is important in the invasion of C. jejuni into Caco-2 cells. Furthermore, there was no inhibition with cytochalasin D, vincristine, or vinblastine. Inhibition of cell association was demonstrated at 4 degrees C. Significantly decreased cell association and invasion were seen in potassium-depleted cells. Treatment of cells with bromelain also caused reduction in the number of C. jejuni binding to cells. A nonmotile aflagellate variant of C. jejuni also showed reduced invasion. The results of this study are consistent with energy-dependent invasion mechanisms. The results do not support an endocytic method of invasion for C. jejuni into Caco-2 cells.

Infection of murine fetal brain cell cultures with Listeria monocytogenes

The uptake of Listeria monocytogenes by different brain cells was studied in primary dissociated brain cell cultures derived from murine fetuses. In respect to the supposed intraaxonal migration of Listeria monocytogenes in the pathogenesis of listeric focal brain stem encephalitis, it was examined whether the bacterium was internalized by neurons. Infection rates of distinct cell types were determined by double immunofluorescence with antibodies against cell type-specific markers and the bacterial pathogen. Because of the changing composition of the cultures and time-dependent expression of the oligodendrocyte marker galactocerebroside (GC), infections were carried out on day 4, 6, 8, and 15 in vitro. Listeria monocytogenes was detected predominantly within macrophages. Astrocytes, oligodendrocytes, and fibronectin-expressing cells were infected to a lesser extent. The lowest rates of infection were observed in neurons. A tropism of Listeria monocytogenes for neurons was not detected in vitro.

Salmonella typhimurium induces selective aggregation and internalization of host cell surface proteins during invasion of epithelial cells

Salmonella interact with eucaryotic membranes to trigger internalization into non-phagocytic cells. In this study we examined the distribution of host plasma membrane proteins during S. typhimurium invasion of epithelial cells. Entry of S. typhimurium into HeLa epithelial cells produced extensive aggregation of cell surface class I MHC heavy chain, beta 2-microglobulin, fibronectin-receptor (alpha 5 beta 1 integrin), and hyaluronate receptor (CD-44). Other cell surface proteins such as transferrin-receptor or Thy-1 were aggregated by S. typhimurium to a much lesser extent. Capping of these plasma membrane proteins was observed in membrane ruffles localized to invading S. typhimurium and in the area surrounding these structures. In contrast, membrane ruffling induced by epidermal growth factor only produced minor aggregations of surface proteins, localized exclusively in the membrane ruffle. This result suggests that extensive redistribution of these proteins requires a signal related to bacterial invasion. This bacteria-induced process was associated with rearrangement of polymerized actin but not microtubules, since preincubation of epithelial cells with cytochalasin D blocked aggregation of these proteins while nocodazole treatment did not. Of the host surface proteins aggregated by S. typhimurium, only class I MHC heavy chain was predominantly present in the bacteria-containing vacuoles. No extensive aggregation of host plasma membrane proteins was detected when HeLa epithelial cells were infected with invasive bacteria that do not induce membrane ruffling, including Yersinia enterocolitica, a bacterium that triggers internalization via binding to beta 1 integrin, and a S. typhimurium invasion mutant that utilizes the Yersinia-internalization route. In contrast to the situation with S. typhimurium, class I MHC heavy chain was not selectively internalized into vacuoles containing these other bacteria. Extensive aggregation of host plasma membrane proteins was also not observed when other S. typhimurium mutants that are defective for invasion were used. The amount of internalized host plasma membrane proteins in the bacteria-containing vacuoles decreased over time with all invasive bacteria examined, indicating that modification of the composition of these vacuoles occurs. Therefore, our data show that S. typhimurium induces selective aggregation and internalization of host plasma membrane proteins, processes associated with the specific invasion strategy used by this bacterium to enter into epithelial cells.

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.

Cadherin expression is required for the spread of Shigella flexneri between epithelial cells

Shigella flexneri, a gram-negative pathogen, invades the human colonic epithelium. After entering epithelial cells, bacteria escape into the cytoplasm, move intracellularly, and pass from cell to cell. The bacterium diverts actin and associated actin-binding proteins to generate a cytoskeleton-based motor that pushes forward the bacterium. As the moving bacterium reaches the inner face of the host-cell cytoplasmic membrane, a protrusion forms that allows passage of this bacterium into a neighboring cell. We show here that components of the intermediate junction are used by the bacterium to allow this passage. Using S180, a mouse fibroblastic sarcoma cell line that does not produce cell adhesion molecules (CAM), and S180L and S180cadN, the same cell line transfected with L-CAM and N-cadherin cDNA, respectively, we demonstrate that expression of a cadherin is required for cell-to-cell spread to occur.

Polymorphonuclear leukocyte transmigration promotes invasion of colonic epithelial monolayer by Shigella flexneri

In vivo and in vitro, Shigella flexneri, an invasive pathogen of the human colon, cannot invade epithelial cells through their apical pole. To identify ways by which it may reach the cellular basolateral domain in order to invade, we have established an assay using the human colonic T-84 cell line grown on permeable filters. Human PMN were added to the basal pole of the cells, and invasive shigellae to their apical pole. Apical addition of bacteria induced strong transmigration of PMN, reaching a maximum after 1 h of incubation. Transmigration depended on a receptor-specific interaction since it was inhibited by an anti-CD18 monoclonal antibody that antagonizes binding of MAC1 on its putative epithelial cell receptor. After 1 h of PMN transmigration, shigellae started to invade the monolayer in areas of intense PMN infiltration. Invasion was clearly dependant on PMN transmigration since it was also inhibited by addition of an anti-CD18 monoclonal antibody. This in vitro assay is consistent with in vivo observations showing early PMN efflux within colonic crypts in the course of shigellosis. PMN transmigration may therefore allow invasion in the colon by opening the paracellular pathway to invasive microorganisms.

Infection and apparent invasion of Vero cells by Paracoccidioides brasiliensis

Paracoccidioides brasiliensis probably uses many different mechanisms to establish itself in the host and cause disease. In this work, we assess an in vitro model system which uses cultured mammalian cells to investigate the virulence factors of P. brasiliensis. We were able to demonstrate an invasion process of the yeast form of this fungus in Vero cell cultures. We deduced that the overall invasive process involved three steps: adhesion, followed by invasion of individual epithelial cells and spread to adjacent cells.

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.

Shigella subversion of the cellular cytoskeleton: a strategy for epithelial colonization

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Shigella flexneri invasion of HeLa cells induces NF-kappa B DNA-binding activity

Although information about the genetic basis and mechanisms of Shigella flexneri cellular invasion is accumulating, little is known about changes in cell signaling and their consequences following bacterium-host cell interactions. A general result of signal transduction is alterations in the levels and/or activities of transcription factors. Alterations in transcription factor binding activities were observed following challenge with S. flexneri. Changes in the DNA-binding activities of cellular transcription factors to AP1, AP2, cyclic AMP response element, CTF1/NF1, NF-kappa B/Rel, OCT1, and SP1 DNA-binding sites were investigated by electrophoretic mobility shift assays. NF-kappa B/Rel DNA-binding activity was enhanced more than 11-fold by cellular invasion; noninvasive S. flexneri strains induced low levels of kappa B DNA binding. Both subunits of the NF-kappa B transcription factor, p50 and p65, but not c-Rel (p85), are components of the kappa B DNA-binding activity. These data suggest that changes in cellular transcription factor binding activity are a consequence of S. flexneri invasion, and these changes could play a role in the initial host response or in the pathogenesis of the disease.

Electron microscopic study of the attachment and penetration of rabbit intestinal epithelium by Providencia alcalifaciens

Ilea of adult rabbits with removable ileal ties (RITARD model) that developed diarrhoea and ileal loops of adult rabbits after inoculation with Providencia alcalifaciens, isolated from two patients with diarrhoea, were studied by transmission electron microscopy for enterocyte-bacterial interactions. Two modes of entry of bacteria into epithelial cells were seen: one directly by endocytosis associated with polymerization of cytoskeletal components and the other by disruption of tight junctions with entry into and proliferation in intercellular spaces.

Characterization of Ehrlichia risticii binding, internalization, and proliferation in host cells by flow cytometry

The binding, internalization, and proliferation of Ehrlichia risticii in P388D1 cells and equine polymorphonuclear (PMN) leukocytes were studied by immunofluorescent staining and flow cytometric analysis. The binding of ehrlichiae to P388D1 cells at 4 degrees C was dose dependent, and the antigens of bound organisms were susceptible to pronase treatment. Additionally, the binding of ehrlichiae to P388D1 cells was diminished when either P388D1 cells or ehrlichiae were treated with 1% paraformaldehyde for 30 min or 0.25% trypsin for 15 min. These results indicate that the ehrlichial ligand and host cell receptor are likely surface proteins. Following incubation at 37 degrees C, bound E. risticii and/or its antigens were removed with pronase and indirect immunofluorescent staining in the presence of saponin was used to examine intracellular ehrlichiae. Our results indicate that E. risticii was internalized into P388D1 cells within 3 h and proliferated by 48 h of incubation. The microfilament-disrupting agent cytochalasin D and the transglutaminase inhibitor monodansylcadaverine were used to differentiate between phagocytosis (sensitive to cytochalasin) and receptor-mediated endocytosis (sensitive to monodansylcadaverine) of E. risticii by P388D1 cells. In concentrations that produced distinctive morphological changes and inhibited phagocytosis of polystyrene latex beads, cytochalasin D did not suppress the infectivity of E. risticii. Binding, internalization, or proliferation of E. risticii was not affected by cytochalasin D. However, monodansylcadaverine inhibited infection of E. risticii in a dose-dependent manner. The agent did not affect the attachment of ehrlichiae to host cells, but it did suppress internalization and proliferation. These results suggest that E. risticii is internalized by receptor-mediated endocytosis and that productive infection by E. risticii does not depend on phagocytosis by the P388D1 cells. Although E. risticii did not bind to the surface of equine PMN leukocytes at 4 degrees C, organisms were taken up by this cell at 37 degrees C. E. risticii, however, failed to survive in equine PMN leukocytes.

Acute infectious diarrhea

Diarrhea continues to be a major cause of mortality and morbidity in third world countries as well as a major symptomatic complaint in the primary care setting in the United States. The etiologic pathogen depends on an exposure history to include recent travel to foreign countries, consuming fecally contaminated water or food, prior use of antibiotics, or homosexual behavior. A careful history from patients directed at attempting to identify particular risk factors may help in making a diagnosis. Not all patients require a diagnostic workup. A large number of patients may only require oral rehydration, careful observation over time with or without use of antimotility agents. In toxic appearing patients or patients with fever, however, bloody stools, abdominal pain or tenesmus, a selective diagnostic workup is indicated. Antimicrobial treatments are not always required, some pathogens clearly call for treatment while some have less clear indications and other pathogens are not responsive to antimicrobial agents at all. Finally, one needs to remember that the differential diagnosis of acute diarrhea includes many noninfectious origins.

OmpC is involved in invasion of epithelial cells by Shigella flexneri

Osmoregulation of the Shigella flexneri ompC gene and the role of OmpC in Shigella virulence have been investigated. OmpC was highly expressed when bacteria were grown in medium of either low or high osmolarity. This constitutive expression is in contrast with the regulation observed in Escherichia coli, in which the expression of OmpC is repressed at low osmolarity and induced at high osmolarity. In addition, the Shigella ompC gene was barely expressed by a delta ompB (delta ompR and delta envZ) mutant. We described in a previous report that such a mutant was severely impaired in virulence both in vitro and in vivo. Starting from this observation, and in order to assess which gene(s) regulated by ompR and envZ are involved in virulence, we constructed an S. flexneri delta ompC mutant. Three S. flexneri mutants, ompF'-lacZ, delta ompC, and delta ompB, were compared for virulence. The ompF'lacZ mutant behaved like the S. flexneri serotype 5 wild-type strain M90T in all in vitro and in vivo virulence tests. On the contrary, the delta ompB and delta ompC strains were considerably impaired in their virulence phenotypes. The ability of these two mutants to spread from cell to cell and to kill epithelial cells was severely affected. Consequently delta ompC, as previously described for delta ompB, was unable to elicit a positive Sereny test. The delta ompB mutant was restored to virulence by introducing a recombinant multicopy plasmid carrying the cloned E. coli ompC gene, indicating that a functional OmpC protein was necessary and sufficient to restore virulence to this mutant of S. flexneri.

Nonpolar mutagenesis of the ipa genes defines IpaB, IpaC, and IpaD as effectors of Shigella flexneri entry into epithelial cells

A 31-kb fragment of the large virulence plasmid of Shigella flexneri is necessary for bacterial entry into epithelial cells in vitro. One locus of this fragment encodes the IpaA, -B, -C, and -D proteins, which are the dominant antigens of the humoral immune response during shigellosis. To address the role of the ipa genes, which are clustered in an operon, we constructed a selectable cassette that does not affect transcription of downstream genes and used this cassette to inactivate the ipaB, ipaC, and ipaD genes. Each of these nonpolar mutants was defective in entry and lysis of the phagocytic vacuole but was not impaired in adhesion to the cells. We showed that, like IpaB and IpaC, IpaD is secreted into the culture supernatant and that none of these proteins is necessary for secretion of the other two. This result differentiates the Ipa proteins, which direct the entry process, from the Mxi and Spa proteins, which direct secretion of the Ipa proteins. Moreover, lack of either IpaB or IpaD resulted in the release of larger amounts of the other Ipa polypeptides into the culture medium, which indicates that, in addition to their role in invasion, IpaB and IpaD are each involved in the maintenance of the association of the Ipa proteins with the bacterium.

Newborn piglet model for campylobacteriosis

An in vivo model system for human campylobacteriosis has been developed in which colostrum-deprived newborn piglets are orally challenged with an invasive strain of Campylobacter jejuni. Piglets developed clinical symptoms and histopathological lesions similar to those observed in humans infected with C. jejuni. Gross lesion examination at autopsy revealed the presence of edema, hyperemia, and mucus. Histopathologic examinations by light and transmission electron microscopy demonstrated damage to surface epithelial cells with the presence of intracellular bacteria, mainly in the large intestine. Similar lesions were not demonstrated in control piglets.

Ultrastructural observations on caecal epithelial cells invaded by first-generation merozoites of Eimeria tenella in vivo

Transmission electron microscopy was used to study the effects of invasion of caecal epithelial cells by first-generation merozoites of Eimeria tenella. At 53 h post-infection, merozoites were found within mature meronts and in enterocytes of the crypt epithelium. Following merozoite invasion, the terminal web of microfilaments of the invaded cell was disrupted and cell extensions were present on the enterocyte surface. There was a marked morphological alteration of the host cell resulting in loss of microvilli and extensive bulging of cytoplasm into the crypt lumen. Multiple invasion of enterocytes and invasion of goblet cells were observed. Large numbers of mast cells infiltrated the mucosa, identified by the metachromatic staining of their granules, and were found within the lamina propria, epithelium and lumen. Merozoites were found within mast cells and lymphocytes within the lumen.

Unusual microtubule-dependent endocytosis mechanisms triggered by Campylobacter jejuni and Citrobacter freundii

Bacterial invasion of six different human epithelial cell lines showed that some strains of the intestinal pathogen Campylobacter jejuni invaded intestinal cell lines at a level 10(2)-10(4) times higher than reported previously for other Campylobacter strains. Separately, urinary tract isolates of Citrobacter freundii triggered a high-efficiency invasion of bladder cells. Use of multiple inhibitors with known effects on eukaryotic cell structures/processes allowed us to define in these genetically distinct bacterial genera unusual bacterial invasion mechanisms that uniquely require microtubules but not microfilaments. Campylobacter jejuni strain 81-176 uptake into 407 intestinal cells and Citrobacter entry into T24 bladder cells was blocked by microtubule depolymerization and inhibitors of coated-pit formation but not by microfilament depolymerization. Inhibitors of endosome acidification had no significant impact on intracellular survival of Campylobacter jejuni or Citrobacter freundii, but monensin markedly reduced Citrobacter uptake. Epithelial cell invasion by both of these bacterial genera was dependent upon de novo bacterial protein synthesis but not upon de novo eukaryotic cell protein synthesis. In contrast to the T24 cell line-specific, strict microtubule-dependent uptake, Citrobacter entry into other cell lines was inhibited by both microtubule- and microfilament-depolymerization, suggesting that these bacteria encode two separate pathways for uptake (i, microtubule-dependent; ii, microfilament-dependent) that are cell line-specific and are recognized perhaps depending on the presence and abundance of appropriate eukaryotic receptors.

Characterization of the Shigella flexneri ipgD and ipgF genes, which are located in the proximal part of the mxi locus

The Shigella flexneri invasion process requires the synthesis of the Ipa proteins and their secretion by specific factors encoded by the mxi and spa genes, which are clustered upstream from the ipa operon. We report here the characterization of the ipgD, ipgE, and ipgF genes, which are located in the 5' end of the mxi locus. Analysis of IpgF-PhoA fusions endowed with high levels of alkaline phosphatase activity confirmed the functionality of a classical signal sequence detected in the sequence of IpgF. The ipgD and ipgF genes were each inactivated on the large virulence plasmid by insertion of a nonpolar cassette; each of the ipgD and ipgF mutants thus constructed showed the same invasive phenotype as the wild-type strain and was able to provoke keratoconjunctivitis in guinea pigs. It thus appears that two genes located at the ipa-proximal part of the mxi locus are not directly involved in invasion. Analysis of concentrated culture supernatants of the wild-type and ipgD strains indicated that secretion of one polypeptide, whose size was consistent with that predicted for the IpgD protein (60 kDa), was abolished in the ipgD mutant.

Clonal structure and pathogenicity of Shiga-like toxin-producing, sorbitol-fermenting Escherichia coli O157:H-

We compared a collection of sorbitol-fermenting (SF) Escherichia coli O157:H- strains with SF E. coli O157:H45 and non-SF E. coli O157:H7 and E. coli O157:H- strains by pulsed-field gel electrophoresis. The SF E. coli O157:H- strains had identical or closely related XbaI patterns that differed markedly from those for the other E. coli O157 strains. Plasmid content and the presence of Shiga-like toxin-converting phages were determined for the SF E. coli O157:H- strains, indicating that these strains harbor a single 90-kb plasmid. They are lysogenized by toxin-converting phages and harbor the eae gene. Nonmotile E. coli O157 strains were observed to adhere more efficiently to HEp-2 cells than the motile strains. From their phenotypic and genotypic features, the SF E. coli O157:H- strains may well represent a new clone with non-SF E. coli O157:H7 pathogenic characteristics.

Unipolar localization and ATPase activity of IcsA, a Shigella flexneri protein involved in intracellular movement

Shigella flexneri uses elements of the host cell cytoskeleton to move within cells and from cell to cell. IcsA, an S. flexneri protein involved in this movement, was purified and studied in vitro. IcsA bound the radiolabelled ATP analog 3'(2')-O-(4-benzoyl)benzoyl-ATP and hydrolyzed ATP. In addition, the surface localization of IcsA on both extracellular and intracellular shigellae was unipolar. Further, in HeLa cells infected with shigellae, IcsA antiserum labelled the actin tail throughout its length, thereby suggesting that IcsA interacts with elements within the tail. Localization of IcsA within the tail at a distance from the bacterium would require its secretion; we demonstrate here that in vitro IcsA is secreted into the culture supernatant in a cleaved form.

A monoclonal antibody to OspA inhibits association of Borrelia burgdorferi with human endothelial cells

Previously, it has been shown that polyclonal antibodies to Borrelia burgdorferi and some monoclonal antibodies (MAbs) to borrelia major surface proteins caused inhibition of adherence of the bacteria to cultured human umbilical vein endothelial (HUVE) cells. In this study, fragment antigen binding (Fab) molecules generated from the immunoglobulin G fraction of rabbit anti-recombinant OspA serum were found to inhibit the adherence of B. burgdorferi to HUVE cells by 73%. Subsequently, MAbs were generated for use in determining whether or how B. burgdorferi outer surface proteins (Osps) A and/or B are involved in mediating attachment to, and/or invasion of, HUVE cells by B. burgdorferi. Twenty-two MAbs were generated to borrelial proteins with apparent molecular masses (determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) of 19, 31 (OspA), 34 (OspB), and 35 kDa. Fab molecules from one anti-OspA MAb, 9B3D, demonstrated an inhibitory effect on bacterial association with HUVE cells. None of the other MAbs, including the other anti-OspA MAbs, showed an inhibitory effect on cell association of greater than 5%. This effect of Fab 9B3D was concentration dependent and plateaued at approximately 6 micrograms of Fab per ml (nearly 80% inhibition of the bacterial association with the monolayer). Penetration assays and cell association experiments performed by using immunofluorescence also suggested that the inhibitory action of 9B3D occurs at the level of adherence. MAb 9B3D recognized the OspA of every North American strain tested (n = 19) but only 3 [corrected] of 20 strains from western Europe, Russia, and Japan, suggesting that the North American strains and strains from other parts of the world may use different molecules and/or different OspA epitopes to interact with endothelial cells. Immunoblots of Escherichia coli expressing different OspA fusion peptides suggested that the 9B3D epitope resides in the carboxy-terminal half of OspA. MAb 9B3D promises to be a valuable tool for elucidating the domain or domains of OspA involved in the endothelial cell cytadherence of North American strains of B. burgdorferi.

Strategies for development of potential candidate Shigella vaccines

Bacillary dysentery, caused by Shigella bacteria, is a major enteric disease responsible for over 200 million infections annually with 650,000 fatal cases. Due to its high communicability, improvement of hygienic standards alone should reduce the spread of dysentery. However, such measures are expensive, and in the communities (e.g. penitentiaries and asylums) or in the areas of the world where bacillary dysentery is most frequently encountered (e.g. in the developing countries) they are not likely to take effect in the reasonably near future. Therefore the possibility of other preventive means such as anti-dysentery vaccines have been explored over the past 40 years. Recently, increased understanding of the molecular biology of bacillary dysentery and the possibility of designing well characterized vaccine strains have increased interest in the field. Several promising vaccine candidates are at various levels of investigations, but to date no Shigella vaccines are available for public health purposes. In this review, beyond the relevant basic information about the pathology, pathomechanism and molecular biology of bacillary dysentery, the various approaches and strategies to construct a safe and immunogenic anti-dysentery vaccine are critically discussed.

Effects of cytochalasins B and D on Staphylococcus aureus adherence to and ingestion by mouse renal cells from primary culture

Cytochalasin B (CB) and cytochalasin D (CD), inhibitors of microfilament function of host cell, were examined for their effects on Staphylococcus aureus Cowan I adherence to and ingestion by several types of the hyperosmolarity-tolerant (HOT) cells obtained from primary culture of mouse kidney. Staphylococcal adherence to the HOT cells with epithelial appearance was extraordinarily enhanced by the treatment of those cells with both 5 micrograms/ml of CB and CD. In particular, staphylococci adhered to the periphery rather than the center of each cytochalasin-treated cell. Staphylococcal ingestion by all types of the HOT cells was markedly inhibited by CD in spite of the enhanced adherence. Contrary to our expectation, inhibition by CB was incomplete, and the enhanced adherence of staphylococci to CB-treated cells resulted in the enhanced ingestion.

MxiD, an outer membrane protein necessary for the secretion of the Shigella flexneri lpa invasins

The invasive phenotype of Shigella flexneri is conferred by a 220 kb virulence plasmid, pWR100, that encodes both the lpa proteins, which are involved in the entry process, and factors which are required for the export and correct localization of the lpa proteins. We have characterized the mxiD gene, whose expression, like that of the ipa operon, is regulated by temperature. After inactivation of mxiD, the mutant strain was unable to invade HeLa cells and to provoke keratoconjunctivitis in guinea-pigs. Analysis of culture supernatants indicated that wild-type S. flexneri secretes about nine polypeptides and that secretion of several of these, including lpaA, lpaB, and lpaC, is abolished in the mxiD mutant. Examination of the membrane proteins of the wild-type and mxiD strains suggested that MxiD is an outer membrane protein. Amino acid sequence comparison revealed that MxiD is homologous to the YscC protein of Yersinia enterocolitica and to the C-terminal region of the PulD protein of Klebsiella pneumoniae. Both YscC and PulD are involved in extracellular protein secretion. These results indicate that MxiD is an essential component of the lpa secretion apparatus.

Lysosome recruitment and fusion are early events required for trypanosome invasion of mammalian cells

Trypanosoma cruzi invades most nucleated cells by a mechanism distinct from classical phagocytosis. Although parasites enter at the lysosome-poor peripheral cell margins, lysosomal markers are immediately incorporated into the parasitophorous vacuole. No accumulation of polymerized actin was detected around recently internalized parasites, and disruption of microfilaments significantly facilitated invasion. Lysosomes were observed to aggregate at the sites of trypanosome attachment and to fuse with the vacuole at early stages of its formation. Experimentally induced, microtubule-dependent movement of lysosomes from the perinuclear area to the cell periphery enhanced entry. Conditions that deplete cells of peripheral lysosomes or interfere with lysosomal fusion capacity inhibited invasion. These observations reveal a novel mechanism for cell invasion:recruitment of lysosomes for fusion at the site of parasite internalization.