Penetration of Salmonella through a polarized Madin-Darby canine kidney epithelial cell monolayer

Destruction of the brush border by Salmonella enterica sv. Typhimurium subverts resorption by polarized epithelial cells

Salmonella enterica serovar Typhimurium is an invasive, facultative intracellular gastrointestinal pathogen that destroys the brush border of polarized epithelial cells (PEC). The brush border is critical for the functions of PEC because it resorbs nutrients from the intestinal lumen and builds a physical barrier to infecting pathogens. The manipuation of PEC during infection by Salmonella was investigated by live-cell imaging and ultrastructural analysed of the brush border. We demonstrate that the destruction of the brush border by Salmonella significantly reduces the resorption surface of PEC along with the abrogation of endocytosis at the apical side of PEC. Both these changes in the physiology of PEC were associated with the translocation of type III secretion system effector protein SopE. Additionally, the F-actin polymerization rate at the apical side of PEC was highly altered by SopE, indicating that reduced endocytosis observed in infected PEC is related to the manipulation of F-actin polymerization mediated by SopE and, to a lesser extent, by effectors SopE2 or SipA. We further observed that in the absence of SopE, Salmonella effaced microvilli and induced reticular F-actin by bacterial accumulation during prolonged infection periods. In contrast to strains translocating SopE, strains lacking SopE did not alter resorption by PEC. Finally, we observed that after engulfment of Salmonella, ezrin was lost from the apical side of PEC and found later in early endosomes containing Salmonella. Our observations suggest that the destruction of the brush border by Salmonella may contribute to the pathogenesis of diarrhea.

Manipulation of microvillar proteins during Salmonella enterica invasion results in brush border effacement and actin remodeling

Enterocyte invasion by the gastrointestinal pathogen Salmonella enterica is accompanied by loss of brush border and massive remodeling of the actin cytoskeleton, leading to microvilli effacement and formation of membrane ruffles. These manipulations are mediated by effector proteins translocated by the Salmonella Pathogenicity Island 1-encoded type III secretion system (SPI1-T3SS). To unravel the mechanisms of microvilli effacement and contribution of SPI1-T3SS effector proteins, the dynamics of host-pathogen interactions was analyzed using live cell imaging (LCI) of polarized epithelial cells (PEC) expressing LifeAct-GFP. PEC were infected with S. enterica wild-type and mutant strains with defined defects in SPI1-T3SS effector proteins, and pharmacological inhibition of actin assembly were applied. We identified that microvilli effacement involves two distinct mechanisms: i) F-actin depolymerization mediated by villin and ii), the consumption of cytoplasmic G-actin by formation of membrane ruffles. By analyzing the contribution of individual SPI1-T3SS effector proteins, we demonstrate that SopE dominantly triggers microvilli effacement and formation of membrane ruffles. Furthermore, SopE via Rac1 indirectly manipulates villin, which culminates in F-actin depolymerization. Collectively, these results indicate that SopE has dual functions during F-actin remodeling in PEC. While SopE-Rac1 triggers F-actin polymerization and ruffle formation, activation of PLCγ and villin by SopE depolymerizes F-actin in PEC. These results demonstrate the key role of SopE in destruction of the intestinal barrier during intestinal infection by Salmonella.

Modulation of Intestinal Paracellular Transport by Bacterial Pathogens

The passive and regulated movement of ions, solutes, and water via spaces between cells of the epithelial monolayer plays a critical role in the normal intestinal functioning. This paracellular pathway displays a high level of structural and functional specialization, with the membrane-spanning complexes of the tight junctions, adherens junctions, and desmosomes ensuring its integrity. Tight junction proteins, like occludin, tricellulin, and the claudin family isoforms, play prominent roles as barriers to unrestricted paracellular transport. The past decade has witnessed major advances in our understanding of the architecture and function of epithelial tight junctions. While it has been long appreciated that microbes, notably bacterial and viral pathogens, target and disrupt junctional complexes and alter paracellular permeability, the precise mechanisms remain to be defined. Notably, renewed efforts will be required to interpret the available data on pathogen-mediated barrier disruption in the context of the most recent findings on tight junction structure and function. While much of the focus has been on pathogen-induced dysregulation of junctional complexes, commensal microbiota and their products may influence paracellular permeability and contribute to the normal physiology of the gut. Finally, microbes and their products have become important tools in exploring host systems, including the junctional properties of epithelial cells. © 2018 American Physiological Society. Compr Physiol 8:823-842, 2018.

Hyperinvasiveness of Salmonella enterica serovar Choleraesuis linked to hyperexpression of type III secretion systems in vitro

Salmonella enterica serovars Choleraesuis and Typhimurium are among the non-typhoid Salmonella serovars that are important zoonotic pathogens. In clinical observation, S. Typhimurium typically causes diarrheal diseases; however, S. Choleraesuis shows high predilection to cause bacteremia. The mechanism why S. Choleraesuis is more invasive to humans remains unknown. In this study, we compared the S. Typhimurium LT2 and S. Choleraesuis SC-B67 proteomes through stable isotope labeling of amino acid in cell culture (SILAC). In SILAC, the expression of many virulence proteins in two type III secretion systems (T3SSs) were significantly higher in S. Choleraesuis than in S. Typhimurium. Similar differences were also found at the transcriptional level. Compared to S. Typhimurium, S. Choleraesuis showed a higher penetration level to Caco-2 (>100-fold) and MDCK (>10-fold) monolayers. In mice after oral challenge, the invasion of spleen and liver was also higher in S. Choleraesuis than in S. Typhimurium. The transcription of hilD in S. Choleraesuis was increased in physiological (1 mM) or high (10 mM) concentrations of Mg2+, but not in low (8 μM) concentration. We conclude that S. Choleraesuis showed hyperinvasiveness in cellular as well as mouse models due to hyperexpression of T3SS genes.

Pharmacokinetic profile that reduces nephrotoxicity of gentamicin in a perfused kidney-on-a-chip

Nephrotoxicity is often underestimated because renal clearance in animals is higher compared to in humans. This paper aims to illustrate the potential to fill in such pharmacokinetic gaps between animals and humans using a microfluidic kidney model. As an initial demonstration, we compare nephrotoxicity of a drug, administered at the same total dosage, but using different pharmacokinetic regimens. Kidney epithelial cell, cultured under physiological shear stress conditions, are exposed to gentamicin using regimens that mimic the pharmacokinetics of bolus injection or continuous infusion in humans. The perfusion culture utilized is important both for controlling drug exposure and for providing cells with physiological shear stress (1.0 dyn cm(-2)). Compared to static cultures, perfusion culture improves epithelial barrier function. We tested two drug treatment regimens that give the same gentamycin dose over a 24 h period. In one regimen, we mimicked drug clearance profiles for human bolus injection by starting cell exposure at 19.2 mM of gentamicin and reducing the dosage level by half every 2 h over a 24 h period. In the other regimen, we continuously infused gentamicin (3 mM for 24 h). Although junctional protein immunoreactivity was decreased with both regimens, ZO-1 and occludin fluorescence decreased less with the bolus injection mimicking regimen. The bolus injection mimicking regimen also led to less cytotoxicity and allowed the epithelium to maintain low permeability, while continuous infusion led to an increase in cytotoxicity and permeability. These data show that gentamicin disrupts cell-cell junctions, increases membrane permeability, and decreases cell viability particularly with prolonged low-level exposure. Importantly a bolus injection mimicking regimen alleviates much of the nephrotoxicity compared to the continuous infused regimen. In addition to potential relevance to clinical gentamicin administration regimens, the results are important in demonstrating the general potential of using microfluidic cell culture models for pharmacokinetics and toxicity studies.

Combined optical and electronic sensing of epithelial cells using planar organic transistors

A planar, conducting-polymer-based transistor for combined optical and electronic monitoring of live cells provides a unique platform for monitoring the health of cells in vitro. Monitoring of MDCK-I epithelial cells over several days is shown, along with a demonstration of the device for toxicology studies, of use in future drug discovery or diagnostics applications.

Dynamic monitoring of Salmonella typhimurium infection of polarized epithelia using organic transistors

Ion flow across polarized epithelia is a tightly regulated process. Measurement of the transepithelial resistance is a highly relevant parameter for assessing the function or health of the tissue. Dynamic, electrical measurements of transepithelial ion flow are preferred as they provide the most accurate snapshot of effects of external stimuli. Enteric pathogens such as Salmonella typhimurium are known to disrupt ion flow in gastrointestinal epithelia. Here, for the first time, the use of organic transistors as a powerful potential alternative for front-line, disposable, high-throughput diagnostics of enteric pathogens is demonstrated. The transistors' ability to detect early and subtle changes in transepithelial ion flow is capitalized upon to develop a highly sensitive detector of epithelial integrity. Stable operation of the organic devices under physiological conditions is shown, followed by dynamic, pathogen-specific diagnosis of infection of epithelia. Further, operation of the device is possible in complex matrices, showing particular promise for food and safety applications.

In vitro Intestinal Mucosal Epithelial Responses to Wild-Type Salmonella Typhi and Attenuated Typhoid Vaccines

Typhoid fever, caused by S. Typhi, is responsible for approximately 200,000 deaths per year worldwide. Little information is available regarding epithelium-bacterial interactions in S. Typhi infection. We have evaluated in vitro the effects of wild-type S. Typhi, the licensed Ty21a typhoid vaccine and the leading strains CVD 908-htrA and CVD 909 vaccine candidates on intestinal barrier function and immune response. Caco2 monolayers infected with wild-type S. Typhi exhibited alterations in the organization of tight junctions, increased paracellular permeability, and a rapid decrease in Trans-Epithelial Electrical Resistance as early as 4 h post-exposure. S. Typhi triggered the secretion of interleukin (IL)-8 and IL-6. Caco2 cells infected with the attenuated strains exhibited a milder pro-inflammatory response with minimal disruption of the barrier integrity. We conclude that wild-type S. Typhi causes marked transient alterations of the intestinal mucosa that are more pronounced than those observed with Ty21a or new generation attenuated typhoid vaccine candidates.

Impact of Salmonella enterica Type III Secretion System Effectors on the Eukaryotic Host Cell

Type III secretion systems are molecular machines used by many Gram-negative bacterial pathogens to inject proteins, known as effectors, directly into eukaryotic host cells. These proteins manipulate host signal transduction pathways and cellular processes to the pathogen’s advantage. Salmonella enterica possesses two virulence-related type III secretion systems that deliver more than forty effectors. This paper reviews our current knowledge about the functions, biochemical activities, host targets, and impact on host cells of these effectors. First, the concerted action of effectors at the cellular level in relevant aspects of the interaction between Salmonella and its hosts is analyzed. Then, particular issues that will drive research in the field in the near future are discussed. Finally, detailed information about each individual effector is provided.

Near surface swimming of Salmonella Typhimurium explains target-site selection and cooperative invasion

Targeting of permissive entry sites is crucial for bacterial infection. The targeting mechanisms are incompletely understood. We have analyzed target-site selection by S. Typhimurium. This enteropathogenic bacterium employs adhesins (e.g. fim) and the type III secretion system 1 (TTSS-1) for host cell binding, the triggering of ruffles and invasion. Typically, S. Typhimurium invasion is focused on a subset of cells and multiple bacteria invade via the same ruffle. It has remained unclear how this is achieved. We have studied target-site selection in tissue culture by time lapse microscopy, movement pattern analysis and modeling. Flagellar motility (but not chemotaxis) was required for reaching the host cell surface in vitro. Subsequently, physical forces trapped the pathogen for ∼1.5–3 s in “near surface swimming”. This increased the local pathogen density and facilitated “scanning” of the host surface topology. We observed transient TTSS-1 and fim-independent “stopping” and irreversible TTSS-1-mediated docking, in particular at sites of prominent topology, i.e. the base of rounded-up cells and membrane ruffles. Our data indicate that target site selection and the cooperative infection of membrane ruffles are attributable to near surface swimming. This mechanism might be of general importance for understanding infection by flagellated bacteria.

The animal body is protected by physical, chemical and immunological barriers. Identification of “promising” target sites is therefore of importance for any pathogen. This crucial step of the infection is still poorly understood. Here, we have studied target site selection by the flagellated Gram-negative gut pathogen Salmonella Typhimurium. Using a well-established tissue culture model system, we found that flagella-driven motility forces the bacterium into a “near surface swimming” mode which facilitates “scanning” of the host cell surface. The near surface swimming was found to target the pathogen towards sites with particular topological features, i.e., rounded cells and membrane ruffles. This explains how S. Typhimurium “identifies” particular target sites and infects membrane ruffles in a cooperative manner. Interestingly, the near surface swimming is attributable to generic physical principles acting on moving particles. Therefore, our findings might be of general importance for the infection by motile pathogens.

Near surface swimming of Salmonella Typhimurium explains target-site selection and cooperative invasion

Targeting of permissive entry sites is crucial for bacterial infection. The targeting mechanisms are incompletely understood. We have analyzed target-site selection by S. Typhimurium. This enteropathogenic bacterium employs adhesins (e.g. fim) and the type III secretion system 1 (TTSS-1) for host cell binding, the triggering of ruffles and invasion. Typically, S. Typhimurium invasion is focused on a subset of cells and multiple bacteria invade via the same ruffle. It has remained unclear how this is achieved. We have studied target-site selection in tissue culture by time lapse microscopy, movement pattern analysis and modeling. Flagellar motility (but not chemotaxis) was required for reaching the host cell surface in vitro. Subsequently, physical forces trapped the pathogen for ∼1.5-3 s in "near surface swimming". This increased the local pathogen density and facilitated "scanning" of the host surface topology. We observed transient TTSS-1 and fim-independent "stopping" and irreversible TTSS-1-mediated docking, in particular at sites of prominent topology, i.e. the base of rounded-up cells and membrane ruffles. Our data indicate that target site selection and the cooperative infection of membrane ruffles are attributable to near surface swimming. This mechanism might be of general importance for understanding infection by flagellated bacteria.

Simvastatin represses translocation of Pseudomonas aeruginosa across Madin-Darby canine kidney cell monolayers

Pseudomonas aeruginosa causes both invasive (bacteremic) and chronic noninvasive infections. An increase in intestinal epithelial permeability is a characteristic of severe sepsis. Alterations in the normal barrier function of the gut mucosa may result in the translocation of microbial cells and products. On the otherhand, it has been demonstrated that statin use is associated with a lower risk of mortality from bloodstream infections. Therefore, we investigated the ability of P. aeruginosa PAO1 to translocate across the Madin-Darby canine kidney (MDCK) cell monolayers in the presence and absence of simvastatin. The bacteria readily translocated across MDCK cell monolayers after 3 h of infection irrespective of the presence or absence of the drug in the medium. However, the bacteria were less able to penetrate the MDCK monolayers in the presence of simvastatin than in its absence. A gentamicin survival assay demonstrated that simvastatin did not affect the bacteria's invasive behavior in the MDCK cells.

Salmonella gut invasion involves TTSS-2-dependent epithelial traversal, basolateral exit, and uptake by epithelium-sampling lamina propria phagocytes

Salmonella Typhimurium causes diarrhea by infecting the epithelium and lamina propria of the intestinal mucosa and by secreting various effector proteins through type III secretion systems (TTSSs). However, the mechanisms by which Salmonella transverses the epithelium and is subsequently released into the lamina propria are poorly understood. Using a murine Salmonella-diarrhea model and in vivo microscopy, we show that epithelial traversal requires TTSS-1-mediated invasion and TTSS-2-dependent trafficking to the basolateral side. After being released into the lamina propria, the bacterium is transiently extracellular before being taken up by phagocytes, including CD11c(+)CX(3)CR1(high) monocytic phagocytes (MPs), which were found to constitutively sample cellular material shed from the basolateral side of the epithelium. Thus, Salmonella infects the cecal mucsa through a step-wise process wherein the bacterium transverses the epithelium through TTSS-2-dependent trafficking and then likely exploits lamina propria MPs, which are sampling the epithelium, to enter and replicate within the host.

Bactericidal effects of antimicrobial agents on epithelial cell-associated Pseudomonas aeruginosa

It is not clear whether antipseudomonal agents can kill cell-associated bacteria within a short time. Madin-Darby canine kidney (MDCK) and A549 cells were infected with Pseudomonas aeruginosa ATCC 27853 and PAO1 and the bactericidal activity of ceftazidime, imipenem, meropenem, gentamicin, and ciprofloxacin against the organisms was investigated. In both MDCK and A549 cells, β-lactams could not kill epithelial cell-associated bacteria within 2 h. Gentamicin at concentrations ≤32 μg/ml killed more than 99% of epithelial cell-associated bacteria. Ciprofloxacin at 0.5 μg/ml killed more than 99.9% of MDCK cell-associated bacteria. Ciprofloxacin has the strongest and most rapid bactericidal activity against epithelial cell-associated bacteria, which may be explained by the combination of potent in-vitro bactericidal activity and high penetration ability into epithelial cells.

Phenotypic pattern-based assay for dynamically monitoring host cellular responses to Salmonella infections

The interaction between mammalian host cells and bacteria is a dynamic process, and the underlying pathologic mechanisms are poorly characterized. Limited information describing the host-bacterial interaction is based mainly on studies using label-based endpoint assays that detect changes in cell behavior at a given time point, yielding incomplete information. In this paper, a novel, label-free, real-time cell-detection system based on electronic impedance sensor technology was adapted to dynamically monitor the entire process of intestinal epithelial cells response to Salmonella infection. Changes in cell morphology and attachment were quantitatively and continuously recorded following infection. The resulting impedance-based time-dependent cell response profiles (TCRPs) were compared to standard assays and showed good correlation and sensitivity. Biochemical assays further suggested that TCRPs were correlated with cytoskeleton-associated morphological dynamics, which can be largely attenuated by inhibitions of actin and microtubule polymerization. Collectively, our data indicate that cell-electrode impedance measurements not only provide a novel, real-time, label-free method for investigating bacterial infection but also help advance our understanding of host responses in a more physiological and continuous manner that is beyond the scope of current endpoint assays.

Exit of intracellular Porphyromonas gingivalis from gingival epithelial cells is mediated by endocytic recycling pathway

Gingival epithelial cells function as an innate host defence system to prevent intrusion by periodontal bacteria. Nevertheless, Porphyromonas gingivalis, the most well-known periodontal pathogen, can enter gingival epithelial cells and pass through the epithelial barrier into deeper tissues. However, it is poorly understood how this pathogen exits from infected cells for further transcellular spreading. The present study was performed to elucidate the cellular machinery exploited by P. gingivalis to exit from immortalized human gingival epithelial cells. P. gingivalis was shown to be internalized with early endosomes positive for the FYVE domain of EEA1 and transferrin receptor, and about half of the intracellular bacteria were then sorted to lytic compartments, including autolysosomes and late endosomes/lysosomes, while a considerable number of the remaining organisms were sorted to Rab11- and RalA-positive recycling endosomes. Inhibition experiments revealed that bacterial exit was dependent on actin polymerization, lipid rafts and microtubule assembly. Dominant negative forms and RNAi knockdown of Rab11, RalA and exocyst complex subunits (Sec5, Sec6 and Exo84) significantly disturbed the exit of P. gingivalis. These results strongly suggest that the recycling pathway is exploited by intracellular P. gingivalis to exit from infected cells to neighbouring cells as a mechanism of cell-to-cell spreading.

Salmonella enterica serovar Typhimurium binds to HeLa cells via Fim-mediated reversible adhesion and irreversible type three secretion system 1-mediated docking

The food-borne pathogen Salmonella enterica serovar Typhimurium invades mammalian epithelial cells. This multistep process comprises bacterial binding to the host cell, activation of the Salmonella type three secretion system 1 (T1), injection of effector proteins, triggering of host cell actin rearrangements, and S. Typhimurium entry. While the latter steps are well understood, much less is known about the initial binding step. Earlier work had implicated adhesins (but not T1) or T1 (but not other adhesins). We have studied here the Salmonella virulence factors mediating S. Typhimurium binding to HeLa cells. Using an automated microscopy assay and isogenic S. Typhimurium mutants, we analyzed the role of T1 and of several known adhesins (Fim, Pef, Lpf, Agf, and Shd) in host cell binding. In wild-type S. Typhimurium, host cell binding was mostly attributable to T1. However, in the absence of T1, Fim (but not Pef, Lpf, Agf, and Shd) also mediated HeLa cell binding. Furthermore, in the absence of T1 and type I fimbriae (Fim), we still observed residual binding, pointing toward at least one additional, unidentified binding mechanism. Dissociation experiments established that T1-mediated binding was irreversible ("docking"), while Fim-mediated binding was reversible ("reversible adhesion"). Finally, we show that noninvasive bacteria docking via T1 or adhering via Fim can efficiently invade HeLa cells, if actin rearrangements are triggered in trans by a wild-type S. Typhimurium helper strain. Our data show that binding to HeLa cells is mediated by at least two different mechanisms and that both can lead to invasion if actin rearrangements are triggered.

Transcellular passage of Neisseria meningitidis across a polarized respiratory epithelium

Neisseria meningitidis is a major cause of sepsis and meningitis but is also a common commensal, present in the nasopharynx of between 8 and 20% of healthy individuals. During carriage, the bacterium is found on the surface of the nasopharyngeal epithelium and in deeper tissues, while to develop disease the meningococcus must spread across the respiratory epithelium and enter the systemic circulation. Therefore, investigating the pathways by which N. meningitidis crosses the epithelial barrier is relevant for understanding carriage and disease but has been hindered by the lack of appropriate models. Here, we have established a physiologically relevant model of the upper respiratory epithelial cell barrier to investigate the mechanisms responsible for traversal of N. meningitidis. Calu-3 human respiratory epithelial cells were grown on permeable cell culture membranes to form polarized monolayers of cells joined by tight junctions. We show that the meningococcus crosses the epithelial cell barrier by a transcellular route; traversal of the layer did not disrupt its integrity, and bacteria were detected within the cells of the monolayer. We demonstrate that successful traversal of the epithelial cell barrier by N. meningitidis requires expression of its type 4 pili (Tfp) and capsule and is dependent on the host cell microtubule network. The Calu-3 model should be suitable for dissecting the pathogenesis of infections caused by other respiratory pathogens, as well as the meningococcus.

Hypoxia-inducible factor-dependent regulation of platelet-activating factor receptor as a route for gram-positive bacterial translocation across epithelia

Results from these studies reveal that some strains of Gram-positive bacteria exploit hypoxia-inducible factor-regulated platelet-activating factor receptor as a means for translocation through intestinal epithelial cells.

Mucosal surfaces, such as the lung and intestine, are lined by a monolayer of epithelia that provides tissue barrier and transport function. It is recently appreciated that a common feature of inflammatory processes within the mucosa is hypoxia (so-called inflammatory hypoxia). Given the strong association between bacterial translocation and mucosal inflammatory disease, we hypothesized that intestinal epithelial hypoxia influences bacterial translocation. Initial studies revealed that exposure of cultured intestinal epithelia to hypoxia (pO₂, 20 torr; 24–48 h) resulted in a increase of up to 40-fold in the translocation of some strains of Gram-positive bacteria, independently of epithelial barrier function. A screen of relevant pathway inhibitors identified a prominent role for the platelet-activating factor receptor (PAFr) in hypoxia-associated bacterial translocation, wherein pharmacologic antagonists of PAFr blocked bacterial translocation by as much as 80 ± 6%. Extensions of these studies revealed that hypoxia prominently induces PAFr through a hypoxia-inducible factor (HIF)-dependent mechanism. Indeed, HIF and PAFr loss of function studies (short hairpin RNA) revealed that apically expressed PAFr is central to the induction of translocation for the Gram-positive bacteria Enterococcus faecalis. Together, these findings reveal that some strains of Gram-positive bacteria exploit HIF-regulated PAFr as a means for translocation through intestinal epithelial cells.

Salmonella enterica serovar Typhimurium mutants unable to convert malate to pyruvate and oxaloacetate are avirulent and immunogenic in BALB/c mice

Previously, we showed that the Salmonella enterica serovar Typhimurium SR-11 tricarboxylic acid (TCA) cycle must operate as a complete cycle for full virulence after oral infection of BALB/c mice (M. Tchawa Yimga, M. P. Leatham, J. H. Allen, D. C. Laux, T. Conway, and P. S. Cohen, Infect. Immun. 74:1130-1140, 2006). In the same study, we showed that for full virulence, malate must be converted to both oxaloacetate and pyruvate. Moreover, it was recently demonstrated that blocking conversion of succinyl-coenzyme A to succinate attenuates serovar Typhimurium SR-11 but does not make it avirulent; however, blocking conversion of succinate to fumarate renders it completely avirulent and protective against subsequent oral infection with the virulent serovar Typhimurium SR-11 wild-type strain (R. Mercado-Lubo, E. J. Gauger, M. P. Leatham, T. Conway, and P. S. Cohen, Infect. Immun. 76:1128-1134, 2008). Furthermore, the ability to convert succinate to fumarate appeared to be required only after serovar Typhimurium SR-11 became systemic. In the present study, evidence is presented that serovar Typhimurium SR-11 mutants that cannot convert fumarate to malate or that cannot convert malate to both oxaloacetate and pyruvate are also avirulent and protective in BALB/c mice. These results suggest that in BALB/c mice, the malate that is removed from the TCA cycle in serovar Typhimurium SR-11 for conversion to pyruvate must be replenished by succinate or one of its precursors, e.g., arginine or ornithine, which might be available in mouse phagocytes.

Tight junctions as targets of infectious agents

The epithelial barrier is a critical border that segregates luminal material from entering tissues. Essential components of this epithelial fence are physical intercellular structures termed tight junctions. These junctions use a variety of transmembrane proteins coupled with cytoplasmic adaptors, and the actin cytoskeleton, to attach adjacent cells together thereby forming intercellular seals. Breaching of this barrier has profound effects on human health and disease, as barrier deficiencies have been linked with the onset of inflammation, diarrhea generation and pathogenic effects. Although tight junctions efficiently restrict most microbes from penetrating into deeper tissues and contain the microbiota, some pathogens have developed specific strategies to alter or disrupt these structures as part of their pathogenesis, resulting in either pathogen penetration, or other consequences such as diarrhea. Understanding the strategies that microorganisms use to commandeer the functions of tight junctions is an active area of research in microbial pathogenesis. In this review we highlight and overview the tactics bacteria and viruses use to alter tight junctions during disease. Additionally, these studies have identified novel tight junction protein functions by using pathogens and their virulence factors as tools to study the cell biology of junctional structures.

The Fortunate Professor

My professional life can be summarized by a quote from the Talmud.

Much have I learned from my teachers, More from my colleagues, But most from my students.

It is the fortunate professor who learns from the student.

Enhanced microscopic definition of Campylobacter jejuni 81-176 adherence to, invasion of, translocation across, and exocytosis from polarized human intestinal Caco-2 cells

Campylobacter jejuni-mediated pathogenesis involves gut adherence and translocation across intestinal cells. The current study was undertaken to examine the C. jejuni interaction with and translocation across differentiated Caco-2 cells to better understand Campylobacter's pathogenesis. The efficiency of C. jejuni 81-176 invasion of Caco-2 cells was two- to threefold less than the efficiency of invasion of INT407 cells. Adherence-invasion analyses indicated that C. jejuni 81-176 adhered to most INT407 cells but invaded only about two-thirds of the host cells over 2 h (two bacteria/cell). In contrast, only 11 to 17% of differentiated Caco-2 cells were observed to bind and internalize either C. jejuni strain 81-176 or NCTC 11168, and a small percentage of infected Caco-2 cells contained 5 to 20 internalized bacteria per cell after 2 h. Electron microscopy revealed that individual C. jejuni cells adhered to the tips of host cell microvilli via intimate flagellar contacts and by lateral bacterial binding to the sides of microvilli. Next, bacteria were observed to bind at the apical host membrane surface via presumed interactions at one pole of the bacterium and with host membrane protrusions located near intercellular junctions. The latter contacts apparently resulted in coordinated, localized plasma membrane invagination, causing simultaneous internalization of bacteria into an endosome. Passage of this Campylobacter endosome intracellularly from the apical surface to the basolateral surface occurred over time, and bacterial release apparently resulted from endosome-basolateral membrane fusion (i.e., exocytosis). Bacteria were found intercellularly below tight junctions at 60 min postinfection, but not at earlier times. This study revealed unique host cell adherence contacts, early endocytosis-specific structures, and a presumptive exocytosis component of the transcellular transcytosis route.

Foodborne Salmonella ecology in the avian gastrointestinal tract

Foodborne Salmonella continues to be a major cause of salmonellosis with Salmonella Enteritidis and S. Typhimurium considered to be responsible for most of the infections. Investigation of outbreaks and sporadic cases has indicated that food vehicles such as poultry and poultry by-products including raw and uncooked eggs are among the most common sources of Salmonella infections. The dissemination and infection of the avian intestinal tract remain somewhat unclear. In vitro incubation of Salmonella with mammalian tissue culture cells has shown that invasion into epithelial cells is complex and involves several genetic loci and host factors. Several genes are required for the intestinal phase of Salmonella invasion and are located on Salmonella pathogenicity island 1 (SPI 1). Salmonella pathogenesis in the gastrointestinal (GI) tract and the effects of environmental stimuli on gene expression influence bacterial colonization and invasion. Furthermore, significant parameters of Salmonella including growth physiology, nutrient availability, pH, and energy status are considered contributing factors in the GI tract ecology. Approaches for limiting Salmonella colonization have been primarily based on the microbial ecology of the intestinal tract. In vitro studies have shown that the toxic effects of short chain fatty acids (SCFA) to some Enterobacteriaceae, including Salmonella, have resulted in a reduction in population. In addition, it has been established that native intestinal microorganisms such as Lactobacilli provide protective mechanisms against Salmonella in the ceca. A clear understanding of the key factors involved in Salmonella colonization in the avian GI tract has the potential to lead to better approach for more effective control of this foodborne pathogen.

Growth and genetic responses of Salmonella Typhimurium to pH-shifts in an anaerobic continuous culture

Salmonella infection of chickens that leads to potential human foodborne salmonellosis continues to be a concern. Changes in the pH of poultry gastrointestinal tract could influence Salmonella growth and virulence response. In the current study, growth responses of a chicken isolate Salmonella enterica serovar Typhimurium (ST) to three incremental pH-shifts (6.17-7.35) in continuous cultures (CC) were evaluated. The expression of rpoS and hilA was determined by real time-polymerase chain reaction (RT-PCR) as well. Increases in pH resulted in higher cell protein concentrations, glucose disappearance, and glucose and ATP yields. Although with some inconsistency between the two trials, the data indicated that the ammonia release into media was favored by low pH. The pH shifts did not significantly affect acetate biosynthesis. No consistent trends of pH influence on propionate and butyrate production could be detected. In all three pH shifts, relative expression of hilA was dominant at 0h which represented CC steady state. In pH shift 7.35-6.86 (Trial 1), the relative expression of rpoS at time 0 and 1h were over five-fold higher than after 3 and 6h of growth. Overall, the results suggest that ST physiology is altered by changes in pH, which could be determinant factors for ST survival in the poultry gastrointestinal ecosystems.

Salmonella Pathogenicity Island 4 encodes a giant non-fimbrial adhesin and the cognate type 1 secretion system

Pathogenicity Islands play a major role in the pathogenesis of infections by Salmonella enterica. The molecular function of Salmonella Pathogenicity Island 4 (SPI4) is largely unknown, but recent work indicated a role of SPI4 for Salmonella pathogenesis in certain animal models. We analysed the virulence functions of SPI4 and observed that SPI4 is contributing to intestinal inflammation in a mouse model. On a cellular level, SPI4 mediates adhesion to epithelial cells. We demonstrate the function of SPI4-encoded proteins as a type I secretion system (T1SS) and identify SiiE as the substrate protein of the T1SS. SiiE is secreted into the culture medium but mediates contact-dependent adhesion to epithelial cell surfaces. SiiE is a very large non-fimbrial adhesin of 600 kDa and consists of 53 repeats of Ig domains. Our study describes the first T1SS-secreted protein that functions as a non-fimbrial adhesin in binding to eukaryotic cells. The SPI4-encoded T1SS and SiiE might functionally resemble the type I fimbrial adhesins.

Applications of cell imaging in Salmonella research

Salmonella enterica is a Gram-negative enteropathogen that can cause localized infections, typically resulting in gastroenteritis, or systemic infection, e.g., typhoid fever, in both humans and warm-blooded animals. Understanding the mechanisms by which Salmonella induce disease has been the focus of intensive research. This has revealed that Salmonella invasion requires dynamic cross-talk between the microbe and host cells, in which bacterial adherence rapidly leads to a complex sequence of cellular responses initiated by proteins translocated into the host cell by a type III secretion system (T3SS). Once these Salmonella-induced responses have resulted in bacterial invasion, proteins translocated by a second T3SS initiate further modulation of cellular activities to enable survival and replication of the invading pathogen. These processes contribute to Salmonella entry into the host and the clinical symptoms of gastrointestinal and systemic infection. Elucidation of the complex and highly dynamic pathogen-host interactions ultimately requires analysis at the level of single cells and single infection events. To achieve this goal, researchers have applied a diverse range of microscopical methods to examine Salmonella infection in models ranging from whole animal to isolated cells and simple eukaryotic organisms. For example, electron microscopy and confocal microscopy can reveal the juxtaposition of Salmonella, its products, and cellular components at high resolution. Simple light microscopy (LM) can also be used to investigate the interaction of bacteria with host cells and has advantages for live cell imaging, which enables detailed analysis of the dynamics of infection and cellular responses. Here we review the use of imaging techniques in Salmonella research and compare the capabilities of different classes of microscope to address specific types of research question. We also provide protocols and notes on several LM techniques routinely used in our own research.

Quorum sensing system lactones do not increase invasiveness of a MexAB-OprM efflux mutant but do play a partial role in Pseudomonas aeruginosa invasion

We studied the quorum sensing (QS) system and the related homoserine lactones (HSLs) observing Pseudomonas aeruginosa invasion using the epithelial cell monolayer penetration assay model. Compared to the PAO1 wild-type, the QS mutants, DeltalasI and DeltarhlI, were compromised in their capacity to invade. The decreased invasiveness of DeltarhlI was restored by adding 100 microM exogenous C(4)-HSL. However, the decreased invasiveness of an efflux mutant, DeltamexAB-oprM, was not restored in the presence of exogenous HSLs. The QS system partially plays a role in P. aeruginosa invasion; however, C(4)-HSL and 3-O-C(12)-HSL are not the essential determinants for invasiveness for P. aeruginosa.

Invasion of host cells by Salmonella typhimurium requires focal adhesion kinase and p130Cas

Salmonella typhimurium colonizes the intestinal epithelium by injecting an array of effector proteins into host cells that induces phagocytic uptake of attached bacteria. However, the host molecules targeted by these effectors remain poorly defined. Here, we demonstrate that S. typhimurium induces formation of focal adhesion-like complexes at sites of bacterial attachment and that both focal adhesion kinase (FAK) and the scaffolding protein p130Cas are required for Salmonella uptake. Entry of Salmonella into FAK(-/-) cells is dramatically impaired and can be restored to control levels by expression of wild-type FAK. Surprisingly, reconstitution of bacterial internalization requires neither the kinase domain of FAK nor activation of c-Src, but does require a C-terminal PXXP motif through which FAK interacts with Cas. Infection of Cas(-/-) cells is also impaired, and reconstitution of invasiveness requires the central Cas YXXP repeat domain. The invasion defect in Cas(-/-) cells can be suppressed by overexpression of FAK, suggesting a functional link between FAK and Cas in the regulation of Salmonella invasion. Together, these findings reveal a novel role for focal adhesion proteins in the invasion of host cells by Salmonella.

Three-dimensional organotypic models of human colonic epithelium to study the early stages of enteric salmonellosis

In vitro cell culture models used to study how Salmonella initiates disease at the intestinal epithelium would benefit from the recognition that organs and tissues function in a three-dimensional (3-D) environment and that this spatial context is necessary for development of cultures that more realistically resemble in vivo tissues/organs. Our aim was to establish and characterize biologically meaningful 3-D models of human colonic epithelium and apply them to study the early stages of enteric salmonellosis. The human colonic cell line HT-29 was cultured in 3-D and characterized by immunohistochemistry, histology, and scanning electron microscopy. Wild-type Salmonella typhimurium and an isogenic SPI-1 type three secretion system (TTSS) mutant derivative (invA) were used to compare the interactions with 3-D cells and monolayers in adherence/invasion, tissue pathology, and cytokine expression studies. The results showed that 3-D culture enhanced many characteristics normally associated with fully differentiated, functional intestinal epithelia in vivo, including better organization of junctional, extracellular matrix, and brush-border proteins, and highly localized mucin production. Wild-type Salmonella demonstrated increased adherence, but significantly lower invasion for 3-D cells. Interestingly, the SPI-I TTSS mutant showed wild-type ability to invade into the 3-D cells but did not cause significant structural changes to these cells. Moreover, 3-D cells produced less interleukin-8 before and after Salmonella infection. These results suggest that 3-D cultures of human colonic epithelium provide valuable alternative models to study human enteric salmonellosis with potential for novel insight into Salmonella pathogenesis.

Differential effects of clathrin and actin inhibitors on internalization of Escherichia coli and Salmonella choleraesuis in porcine jejunal Peyer's patches

Peyer's patches constitute both an inductive immune site and an enteropathogen invasion route. Peyer's patch mucosae from porcine jejunum were mounted in Ussing chambers, and either Salmonella choleraesuis vaccine strain SC-54 or non-pathogenic rodent and porcine Escherichia coli strains contacted the Peyer's patch mucosa for 90 min. Internalized bacteria were quantified by a gentamicin resistance assay. Monodansylcadaverine (300 microM, luminal addition), an inhibitor of clathrin-mediated endocytosis, significantly inhibited internalization of both E. coli strains relative to tissues untreated with the inhibitor; internalization of SC-54 was unaffected. The actin-disrupting agent cytochalasin D (10 microM, luminal addition), inhibited internalization of pig-adapted E. coli but not that of rodent-adapted E. coli or SC-54. Internalization of SC-54 and non-pathogenic E. coli in Peyer's patches appears to occur through different cellular routes.

The role of host cell death in Salmonella infections

Salmonella enterica is an important enteric pathogen of humans and a variety of domestic and wild animals. Infection is initiated in the intestinal tract, and severe disease produces widespread destruction of the intestinal mucosa. Salmonella strains can also disseminate from the intestine and produce serious, sometimes fatal infections with considerable cytopathology in a number of systemic organs. A combination of bacterial genetic and cell biology studies have shown that Salmonella uses specific virulence mechanisms to induce host cell death during infection. Salmonella produces one set of virulence proteins to promote invasion of the intestine and a different set to mediate systemic disease. Significantly, each set of virulence factors mediates a distinct mechanism of host cell death. The Salmonella pathogenicity island-1 (SPI-1) locus encodes a type III protein secretion system (TTSS) that delivers effector proteins required for intestinal invasion and the production of enteritis. The SPI-1 effector SipB activates caspase-1 in macrophages, releasing IL-1beta and IL-18 and inducing rapid cell death by a mechanism that has features of both apoptosis and necrosis. Caspase-1 is required for Salmonella to infect Peyer's patches and disseminate to systemic tissues in mice. Progressive Salmonella infection in mice requires the SPI-2 TTSS and associated effector proteins as well as the SpvB cytotoxin. Apoptosis of macrophages in the liver is found during systemic infection. In cell culture, Salmonella strains induce delayed apoptosis dependent on SPI-2 function in macrophages from a variety of sources. This delayed apoptosis also requires activation of TLR4 on macrophages by the bacterial LPS. Downstream activation of kinase pathways leads to balanced pro- and antiapoptotic regulatory factors in the cell. NF-kappaB and p38 mitogen-activated protein kinase (MAPK) are particularly important for the induction of antiapoptotic factors, whereas the kinase PKR is required for bacterial-induced apoptosis. The Salmonella SPI-2 TTSS is essential for altering the balance in favor of apoptosis during intracellular infection, but the effectors involved remain poorly characterized. The SpvB cytotoxin has been shown to play a role in apoptosis in human macrophages by depolymerizing the actin cytoskeleton. A model for the role of bacteria-induced host cell death in Salmonella pathogenesis is proposed. In the intestine, the Salmonella SPI-1 TTSS and SipB mediate macrophage death by caspase-1 activation, which also releases IL-1beta and IL-18, promoting inflammation and subsequent phagocytosis by incoming macrophages and leading to dissemination to systemic tissues. Intracellular secretion of virulence effector proteins by the SPI-2 TTSS facilitates growth of Salmonella in these macrophages and the delayed onset of apoptosis in extraintestinal tissues. These infected, apoptotic cells are targeted for engulfment by incoming macrophages, thus perpetuating the cycle of cell-to-cell spread that is the hallmark of systemic Salmonella infection.

Burkholderia cepacia complex isolates survive intracellularly without replication within acidic vacuoles of Acanthamoeba polyphaga

We have previously demonstrated that isolates of the Burkholderia cepacia complex can survive intracellularly in murine macrophages and in free-living Acanthamoeba. In this work, we show that the clinical isolates B. vietnamiensis strain CEP040 and B. cenocepacia H111 survived but did not replicate within vacuoles of A. polyphaga. B. cepacia-containing vacuoles accumulated the fluid phase marker Lysosensor Blue and displayed strong blue fluorescence, indicating that they had low pH. In contrast, the majority of intracellular bacteria within amoebae treated with the V-ATPse inhibitor bafilomycin A1 localized in vacuoles that did not fluoresce with Lysosensor Blue. Experiments using bacteria fluorescently labelled with chloromethylfluorescein diacetate demonstrated that intracellular bacteria remained viable for at least 24 h. In contrast, Escherichia coli did not survive within amoebae after 2 h post infection. Furthermore, intracellular B. vietnamiensis CEP040 retained green fluorescent protein within the bacterial cytoplasm, while this protein rapidly escaped from the cytosol of phagocytized heat-killed bacteria into the vacuolar lumen. Transmission electron microscopy analysis confirmed that intracellular Burkholderia cells were structurally intact. In addition, both Legionella pneumophila- and B. vietnamiensis-containing vacuoles did not accumulate cationized ferritin, a compound that localizes within the lysosome. Thus, our observations support the notion that B. cepacia complex isolates can use amoebae as a reservoir in the environment by surviving without intracellular replication within an acidic vacuole that is distinct from the lysosomal compartment.

Salmonella enterica serotype Choleraesuis: epidemiology, pathogenesis, clinical disease, and treatment

Nontyphoid Salmonella strains are important causes of reportable food-borne infection. Among more than 2,000 serotypes, Salmonella enterica serotype Choleraesuis shows the highest predilection to cause systemic infections in humans. The most feared complication of serotype Cholearesuis bacteremia in adults is the development of mycotic aneurysm, which previously was almost uniformally fatal. The advances in diagnostic techniques, surgical care, and antimicrobial therapy have greatly improved the survival of these patients. However, the recent emergence of serotype Choleraesuis that is resistant to ampicillin, chloramphenicol, trimethoprim-sulfamethoxazole, and, notably, fluoroquinolone antibiotics has aroused concern about the use of these agents for the empirical treatment of systemic infection caused by this organism. In view of the serious implications of the situation, the chain of transmission and mechanism of resistance should be carefully studied to reduce the spread of infection and threat to human health. To date, there are no vaccines available to prevent serotype Choleraesuis infections in humans. The availability, in the near future, of the genome sequence of serotype Cholearesuis will facilitate the development of effective vaccines as well as the discovery of new targets for novel antimicrobial agents.

Pretreatment of Pseudomonas aeruginosa with a sub-MIC of imipenem enhances bactericidal activity of neutrophils

The influence of the pretreatment of Pseudomonas aeruginosa strain O1 (PAO-1) with a sub-minimum inhibitory concentration (MIC) of imipenem on bactericidal activity, phagocytosis, the production of oxygen radical intermediates, and the induction of apoptosis in murine peritoneal neutrophils, as well as the catalase activity in the bacteria in comparison with that of ceftazidime-treated bacteria were studied. Bacteria treated with imipenem at (1/4) MIC were killed at significantly higher rates by neutrophils than ceftazidime-treated and nontreated bacteria. However, antibiotic-treated bacteria showed similar numbers of bacteria-phagocytized neutrophils to those in untreated bacteria. Imipenem pretreatment of bacteria led to an increase in the production of oxygen radical intermediates by neutrophils and the inhibition of neutrophilic apoptosis following incubation, whereas these features did not occur in neutrophils incubated with nontreated and ceftazidime-treated bacteria. The catalase activity of bacteria was not suppressed by pretreatment with either antibiotic at (1/4) MIC. These findings suggest that the exposure of P. aeruginosa to a sub-MIC of imipenem enhances the susceptibility of the bacteria to neutrophilic killing and effectively modifies the physiological activities of neutrophils, but does not decrease bacterial catalase activity. These actions may account for the postantibiotic leukocyte enhancement (PALE) effect of a sub-MIC of imipenem in the host.

Translating tissue culture results into animal models: the case of Salmonella typhimurium

Investigators use both in vitro and in vivo models to better understand infectious disease processes. Both models are extremely useful in research, but there exists a significant gap in complexity between the highly controlled reductionist in vitro systems and the largely undefined, but relevant variability encompassing in vivo animal models. In an effort to understand how Salmonella initiates disease at the intestinal epithelium, in vitro models have served a useful purpose in allowing investigators to identify molecular mechanisms responsible for Salmonella invasion of host cells and stimulation of host inflammatory responses. Identification of these molecular mechanisms has generated hypotheses that are now being tested using in vivo models. Translating the in vitro findings into the context of an animal model and subsequently to human disease remains a difficult challenge for any disease process.

Neuromodulation of enteropathogen internalization in Peyer's patches from porcine jejunum

Jejunal Peyer's patches (JPP) are innervated sites of immune induction and enteropathogen infection. We investigated the role of enteric nerves in modulating pathogen entry into porcine JPP. Presumptive norepinephrine (NE)-containing nerve fibers were localized in JPP domes and follicle-associated villi by secondary immunofluorescence histochemistry. NE or the neuronal conduction blocker saxitoxin increased intracellular internalization of pathogenic Salmonella choleraesuis and Escherichia coli O157:H7, but not nonpathogenic E. coli, into isolated JPP mucosa. NE action was prevented by the alpha-adrenergic antagonist phentolamine. Withdrawal of enteric neural activity or NE administration appears to modulate JPP interactions with pathogenic bacteria.

Cdc42 and Rac1 regulate late events in Salmonella typhimurium-induced interleukin-8 secretion from polarized epithelial cells

Salmonella typhimurium colonization of the intestinal epithelium initiates biochemical cross-talk between pathogen and host that results in the secretion of chemokines, such as interleukin (IL)-8, that direct neutrophil migration to the site of infection. In nonpolarized cells, Rac1 and Cdc42 have been shown to regulate both bacterial invasion and signaling events leading to nuclear responses and IL-8 secretion. However, because the underlying actin cytoskeleton and the associated signaling machinery are distributed much differently in polarized epithelial cells, we used polarized Madin-Darby canine kidney monolayers to investigate the role of Rac1 and Cdc42 in S. typhimurium-induced pro-inflammatory responses in the more physiologically relevant polarized state. In Madin-Darby canine kidney monolayers expressing dominant-negative Rac1 or Cdc42, both Salmonella- and tumor necrosis factor alpha-induced activation of NFkappaB and mitogen-activated protein kinase signaling cascades proceeded normally, but IL-8 secretion was inhibited. We found that Rac1 and Cdc42 were not involved in early pro-inflammatory signaling events, as in nonpolarized cells, but rather regulated the basolateral exocytosis and secretion of IL-8. In contrast, dominant-negative Rac1 inhibited apical actin pedestal formation, indicating that pedestal formation and nuclear signaling for pro-inflammatory activation are not linked. These findings indicate that there are significant differences in the requirements of pathogen-induced host cell signaling pathways in polarized and nonpolarized cells.

Rapid translocation of polarized MDCK cell monolayers by Leptospira interrogans, an invasive but nonintracellular pathogen

Pathogenic spirochetes of the genus Leptospira are a major cause of human zoonotic infectious disease worldwide. After gaining entry through the skin, the organism causes disease by hematogenously disseminating to multiple organs. The mechanism by which it penetrates the mammalian cell barriers to disseminate is not well understood. In this study, we used a low-passage-number isolate of Leptospira interrogans to elucidate the invasive potential of this spirochete. Quantification of bacteria by dark-field microscopy revealed that pathogenic spirochetes were able to translocate through polarized MDCK cell monolayers at a rate significantly greater than that of nonpathogenic Leptospira or a recognized invasive bacterial pathogen, Salmonella: In contrast to Salmonella, L. interrogans did not alter transepithelial electrical resistance during cell translocation. Both transmission and scanning electron microscopy revealed tight association of the extracellular spirochetes with the host cell plasma membrane, without membrane perturbations suggestive of cytoskeletal rearrangement. Spirochetes were not observed within intercellular junctions or membrane-bound compartments inside cells. They were found within the cytoplasm of only 8% of the counted cells. These results indicate that Leptospira is an invasive but not a facultative intracellular organism. We propose that the rapid translocation of mammalian cells by pathogenic Leptospira is a mechanism designed to evade killing by host cells that permits the organism to quickly reach the bloodstream and disseminate to multiple organs.

Multidrug efflux systems play an important role in the invasiveness of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an important opportunistic human pathogen. Certain strains can transmigrate across epithelial cells, and their invasive phenotype is correlated with capacity to cause invasive human disease and fatal septicemia in mice. Four multidrug efflux systems have been described in P. aeruginosa, however, their contribution to virulence is unclear. To clarify the role of efflux systems in invasiveness, P. aeruginosa PAO1 wild-type (WT) and its efflux mutants were evaluated in a Madin-Darby canine kidney (MDCK) epithelial cell monolayer system and in a murine model of endogenous septicemia. All efflux mutants except a deltamexCD-oprJ deletion demonstrated significantly reduced invasiveness compared with WT. In particular, a deltamexAB-oprM deletion strain was compromised in its capacity to invade or transmigrate across MDCK cells, and could not kill mice, in contrast to WT which was highly invasive (P < 0.0006) and caused fatal infection (P < 0.0001). The other mutants, including deltamexB and deltamexXY mutants, were intermediate between WT and the deltamexAB-oprM mutant in invasiveness and murine virulence. Invasiveness was restored to the deltamexAB-oprM mutant by complementation with mexAB-oprM or by addition of culture supernatant from MDCK cells infected with WT. We conclude that the P. aeruginosa MexAB-OprM efflux system exports virulence determinants that contribute to bacterial virulence.

Disruption of the Salmonella-containing vacuole leads to increased replication of Salmonella enterica serovar typhimurium in the cytosol of epithelial cells

Salmonella enterica serovar Typhimurium is a facultative intracellular pathogen that inhabits a vacuolar compartment, called the Salmonella-containing vacuole (SCV), in infected host cells. Maintenance of the SCV is accomplished by SifA, and mutants of this Salmonella pathogenicity island 2 type III effector replicate more efficiently in epithelial cells. Here we demonstrate that enhanced replication of sifA mutants occurs in the cytosol of these cells. Increased replication of wild-type bacteria was also observed in cells treated with wortmannin or expressing Rab5 Q79L or Rab7 N125I, all of which caused a loss of SCV integrity. Our findings demonstrate the requirement of the host cell endosomal system for maintenance of the SCV and that loss of this compartment allows increased replication of serovar Typhimurium in the cytosol of epithelial cells.

Identification of Salmonella enteritidis outer membrane proteins expressed during attachment to human intestinal epithelial cells

AIMS

To express and identify the Salmonella enterica ser. Enteritidis outer membrane proteins that are involved in the attachment to human intestinal epithelial cells.

METHODS AND RESULTS

Salmonella Enteritidis attachment proteins were expressed by infection of human intestinal epithelial cells (int-407) with Salm. Enteritidis strain SE28, followed by pulse labelling with [35S]-methionine. Outer membrane proteins were separated by TritonX-114 phase partitioning, and were detected by Salm. Enteritidis-specific polyclonal antibody. Outer membrane proteins of molecular weights 82.3, 75.6, 49.3, 35.5 and 19.3 kDa were newly synthesized. Expression of 31.2 and 16 kDa proteins was up-regulated, whereas the expression of 40.7 kDa protein was down-regulated. Polyclonal antibodies against the 82.3 and 75.6 kDa proteins significantly (P < 0.05) reduced the binding of Salm. Enteritidis to int-407 cells in vitro.

CONCLUSIONS

Outer membrane proteins 82.3 and 75.6 kDa are potentially involved in the attachment of Salm. Enteritidis to the intestinal mucosa.

SIGNIFICANCE AND IMPACT OF THE STUDY

Outer membrane proteins 82.3 and 75.6 kDa identified in this study could be used as potential vaccines to block or reduce Salm. Enteritidis colonization in chickens.

Interaction of bacterial pathogens with polarized epithelium

Many pathogens must surmount an epithelial cell barrier in order to establish an infection. While much has been learned about the interaction of bacterial pathogens with cultured epithelial cells, the influence of cell polarity on these events has only recently been appreciated. This review outlines bacterial-host epithelial cell interactions in the context of the distinct apical and basolateral surfaces of the polarized epithelium that lines the lumens of our organs.

Hepatocyte growth factor alters renal epithelial cell susceptibility to uropathogenic Escherichia coli

The urinary tract is frequently the source of Escherichia coli bacteremia. Bacteria from the urinary tract must cross an epithelial layer to enter the bloodstream. Hepatocyte growth factor (HGF) alters the polarity of Madin-Darby canine kidney (MDCK) epithelial cells. The role of cell polarity in determining renal epithelial resistance to Escherichia coli invasion is not well known. A model of polarized and HGF-treated MDCK epithelial cells grown on filters was used to study the role of epithelial cell polarity during the interaction of nonvirulent (XL1-Blue) and uropathogenic (J96) strains of Escherichia coli with renal epithelium. Basolateral exposure of MDCK cells to J96, but not XL1-Blue, resulted in loss of transepithelial resistance (TER), which was due to epithelial cytotoxicity and not degradation of epithelial junctional proteins by bacterial proteases. Apical exposure to both J96 and XL1-Blue did not alter TER. Pretreatment of polarized MDCK cell monolayers with HGF renders the cells sensitive to loss of TER and cytotoxicity by apical exposure to J96. Analysis by confocal microscopy demonstrated that HGF treatment of MDCK cell monolayers also greatly enhances adherence of J96 to the apical surface of the cell monolayer. These data demonstrate that the basolateral surface of polarized epithelia is more susceptible to J96 cytotoxicity. The data also support the hypothesis that processes that alter epithelial cell polarity increase sensitivity of epithelia to bacterial injury and adherence from the apical compartment.

Differential infection of polarized epithelial cell lines by sialic acid-dependent and sialic acid-independent rotavirus strains

Infection of epithelial cells by some animal rotaviruses, but not human or most animal rotaviruses, requires the presence of N-acetylneuraminic (sialic) acid (SA) on the cell surface for efficient infectivity. To further understand how rotaviruses enter susceptible cells, six different polarized epithelial cell lines, grown on permeable filter membrane supports containing 0.4-microm pores, were infected apically or basolaterally with SA-independent or SA-dependent rotaviruses. SA-independent rotaviruses applied apically or basolaterally were capable of efficiently infecting both sides of the epithelium of all six polarized cell lines tested, while SA-dependent rotaviruses only infected efficiently through the apical surface of five of the polarized cell lines tested. Regardless of the route of virus entry, SA-dependent and SA-independent rotaviruses were released almost exclusively from the apical domain of the plasma membrane of polarized cells before monolayer disruption or cell lysis. The transepithelial electrical resistance (TER) of cells decreased at the same time, irrespective of whether infection with SA-independent rotaviruses occurred apically or basolaterally. The TER of cells infected apically with SA-dependent rotaviruses decreased earlier than that of cells infected basolaterally. Rotavirus infection decreased TER before the appearance of cytopathic effect and cell death and resulted in an increase in the paracellular permeability to [(3)H]inulin as a function of loss of TER. The presence of SA residues on either the apical or basolateral side was determined using a Texas Red-conjugated lectin, wheat germ agglutinin (WGA), which binds SA residues. WGA bound exclusively to SA residues on the apical surface of the cells, confirming the requirement for SA residues on the apical cell membrane for efficient infectivity of SA-dependent rotaviruses. These results indicate that the rotavirus SA-independent cellular receptor is present on both sides of the epithelium, but SA-dependent and SA-independent rotavirus strains infect polarized epithelial cells by different mechanisms, which may be relevant for pathogenesis and selection of vaccine strains. Finally, rotavirus-induced alterations of the epithelial barrier and paracellular permeability suggest that common mechanisms of pathogenesis may exist between viral and bacterial pathogens of the intestinal tract.

The GTPase Rac1 selectively regulates Salmonella invasion at the apical plasma membrane of polarized epithelial cells

The bacterial pathogen Salmonella typhimurium colonizes its animal hosts by inducing its internalization into intestinal epithelial cells. This process requires reorganization of the actin cytoskeleton of the apical plasma membrane into elaborate membrane ruffles that engulf the bacteria. Members of the Ρ family of small GTPases are critical regulators of actin structure, and in nonpolarized cells, the GTPase Cdc42 has been shown to modulate Salmonella entry. Because the actin architecture of epithelial cells is organized differently from that of nonpolarized cells, we examined the role of two ‘Rgr; family GTPases, Cdc42 and Rac1, in invasion of polarized monolayers of MDCK cells by S. typhimurium. Surprisingly, we found that endogenous Rac1, but not Cdc42, was activated during bacterial entry at the apical pole, and that this activation required the bacterial effector protein SopE. Furthermore, expression of dominant inhibitory Rac1 but not Cdc42 significantly inhibited apical internalization of Salmonella, indicating that Rac1 activation is integral to the bacterial entry process. In contrast, during basolateral internalization, both Cdc42 and Rac1 were activated; however, neither GTPase was required for entry. These findings, which differ significantly from previous observations in nonpolarized cells, indicate that the host cell signaling pathways activated by bacterial pathogens may vary with cell type, and in epithelial tissues may further differ between plasma membrane domains.