Salmonella interactions with host cells: type III secretion at work

The bacterial pathogen Salmonella enterica has evolved a very sophisticated functional interface with its vertebrate hosts. At the center of this interface is a specialized organelle, the type III secretion system, that directs the translocation of bacterial proteins into the host cell. Salmonella spp. encode two such systems that deliver a remarkable array of bacterial proteins capable of modulating a variety of cellular functions, including actin cytoskeleton dynamics, nuclear responses, and endocytic trafficking. Many of these bacterial proteins operate by faithful mimicry of host proteins, in some cases representing the result of extensive molecular tinkering and convergent evolution. The coordinated action of these type III secreted proteins secures the replication and survival of the bacteria avoiding overt damage to the host. The study of this remarkable pathogen is not only illuminating general paradigms in microbial pathogenesis but is also providing valuable insight into host cell functions.

Cytoskeletal rearrangements accompanying salmonella entry into epithelial cells

Salmonella bacteria can enter (invade) eukaryotic cells, and exist as intracellular parasites. Confocal, light immunofluorescence and electron microscopy were used to examine various cytoskeletal components of cultured Madin Darby canine kidney (MDCK) and HeLa epithelial cells after infection with Salmonella typhimurium. These bacteria entered and remained within membrane-bound vacuoles and were surrounded by large (5–10 microns) dense structures composed of various cytoskeletal components. These structures consisted of extensive aggregations of polymerized actin, alpha-actinin and tropomyosin above and beside the invading bacterium in both epithelial cell lines. These structures were evident soon after bacterial addition (maximal at 20 min for HeLa cells, 60 min for MDCK cells), and disappeared later in the infection as the cytoskeletal components returned to a more normal distribution after bacterial internalization. Surprisingly, tubulin also aggregated above internalized Salmonella although bacterial entry or penetration through polarized monolayers was not disrupted by the microtubule-inhibiting agent nocadazole (this treatment actually enhanced tubulin accumulation around these organisms). There were little if any rearrangements in intermediate filaments composed of keratin or vimentin. Large amounts of talin also accumulated above and around invading Salmonella, but there was only a minor accumulation of vinculin around a few organisms. Pretreatment of epithelial cells with the microfilament inhibitor cytochalasin D blocked bacterial internalization but did not prevent accumulation of polymerized actin and alpha-actinin directly beneath uninternalized bacteria, yet prevented accumulation of the other cytoskeletal components. These results suggest that Salmonella bind to the surface and trigger a signal in epithelial cells that causes marked rearrangements in various cytoskeletal components, including recruitment of actin filaments and alpha-actinin, which then generates the force necessary for bacterial uptake.

Salmonella entry into host cells: the work in concert of type III secreted effector proteins

Upon contact with intestinal epithelial cells, Salmonella enterica serovar spp. inject a set of bacterial proteins into host cells via the bacterial SPI-1 type III secretion system. SopE, SopE2 and SopB, activate CDC42 and Rac to initiate actin cytoskeleton rearrangements. SipA and SipC, two Salmonella actin-binding proteins, directly modulate host actin dynamics to facilitate bacterial uptake. SptP promotes the recovery of the actin cytoskeleton rearrangements by antagonizing CDC42 and Rac. Therefore, Salmonella-induced reversible actin cytoskeleton rearrangements are the result of two coordinated steps: (i) stimulation of host signal transduction to indirectly promote actin rearrangements and (ii) direct modulation of actin dynamics.

Microbial pathogenesis and cytoskeletal function

Pathogenic microbes subvert normal host-cell processes to create a specialized niche, which enhances their survival. A common and recurring target of pathogens is the host cell's cytoskeleton, which is utilized by these microbes for purposes that include attachment, entry into cells, movement within and between cells, vacuole formation and remodelling, and avoidance of phagocytosis. Our increased understanding of these processes in recent years has not only contributed to a greater comprehension of the molecular causes of infectious diseases, but has also revealed fundamental insights into normal functions of the cytoskeleton. From the use of bacterial toxins to investigate Rho family GTPases to in vitro studies of actin polymerization using Listeria and Shigella, the study of pathogenesis has provided important tools to probe cytoskeletal function.

Helicobacter pylori attachment to gastric cells induces cytoskeletal rearrangements and tyrosine phosphorylation of host cell proteins

The consequences of Helicobacter pylori attachment to human gastric cells were examined by transmission electron microscopy and immunofluorescence microscopy. H. pylori attachment resulted in (i) effacement of microvilli at the site of attachment, (ii) cytoskeletal rearrangement directly beneath the bacterium, and (iii) cup/pedestal formation at the site of attachment. Double-immunofluorescence studies revealed that the cytoskeletal components actin, alpha-actinin, and talin are involved in the process. Immunoblot analysis showed that binding of H. pylori to AGS cells induced tyrosine phosphorylation of two host cell proteins of 145 and 105 kDa. These results indicate that attachment of H. pylori to gastric epithelial cells resembles that of enteropathogenic Escherichia coli. Coccoid H. pylori, which are thought to be terminally differentiated bacterial forms, are capable of binding and inducing cellular changes of the same sort as spiral H. pylori, including tyrosine phosphorylation of host proteins.

Striking a balance: modulation of the actin cytoskeleton by Salmonella

Salmonella spp. have evolved the ability to enter into cells that are normally nonphagocytic. The internalization process is the result of a remarkable interaction between the bacteria and the host cells. Immediately on contact, Salmonella delivers a number of bacterial effector proteins into the host cell cytosol through the function of a specialized organelle termed the type III secretion system. Initially, two of the delivered proteins, SopE and SopB, stimulate the small GTP-binding proteins Cdc42 and Rac. SopE is an exchange factor for these GTPases, and SopB is an inositol polyphosphate phosphatase. Stimulation of Cdc42 and Rac leads to marked actin cytoskeleton rearrangements, which are further enhanced by SipA, a Salmonella protein also delivered into the host cell by the type III secretion system. SipA lowers the critical concentration of G-actin, stabilizes F-actin at the site of bacterial entry, and increases the bundling activity of the host-cell protein T-plastin (fimbrin). The cellular responses stimulated by Salmonella are short-lived; therefore, immediately after bacterial entry, the cell regains its normal architecture. Remarkably, this process is mediated by SptP, another target of the type III secretion system. SptP exert its function by serving as a GTPase-activating protein for Cdc42 and Rac, turning these G proteins off after their stimulation by the bacterial effectors SopE and SopB. The balanced interaction of Salmonella with host cells constitutes a remarkable example of the sophisticated nature of a pathogen/host relationship shaped by evolution through a longstanding coexistence.

Intracellular pathogens and the actin cytoskeleton

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

Infection by tubercular mycobacteria is spread by nonlytic ejection from their amoeba hosts

To generate efficient vaccines and cures for Mycobacterium tuberculosis, we need a far better understanding of its modes of infection, persistence, and spreading. Host cell entry and the establishment of a replication niche are well understood, but little is known about how tubercular mycobacteria exit host cells and disseminate the infection. Using the social amoeba Dictyostelium as a genetically tractable host for pathogenic mycobacteria, we discovered that M. tuberculosis and M. marinum, but not M. avium, are ejected from the cell through an actin-based structure, the ejectosome. This conserved nonlytic spreading mechanism requires a cytoskeleton regulator from the host and an intact mycobacterial ESX-1 secretion system. This insight offers new directions for research into the spreading of tubercular mycobacteria infections in mammalian cells.

Surfing pathogens and the lessons learned for actin polymerization

A number of unrelated bacterial species as well as vaccinia virus (ab)use the process of actin polymerization to facilitate and enhance their infection cycle. Studies into the mechanism by which these pathogens hijack and control the actin cytoskeleton have provided many interesting insights into the regulation of actin polymerization in migrating cells. This review focuses on what we have learnt from the actin-based motilities of Listeria, Shigella and vaccinia and discusses what we would still like to learn from our nasty friends, including enteropathogenic Escherichia coli and Rickettsia

Intercellular spreading of Porphyromonas gingivalis infection in primary gingival epithelial cells

Porphyromonas gingivalis, an important periodontal pathogen, is an effective colonizer of oral tissues. The organism successfully invades, multiplies in, and survives for extended periods in primary gingival epithelial cells (GECs). It is unknown whether P. gingivalis resides in the cytoplasm of infected cells throughout the infection or can spread to adjacent cells over time. We developed a technique based on flow cytofluorometry and fluorescence microscopy to study propagation of the organism at different stages of infection of GECs. Results showed that P. gingivalis spreads cell to cell and that the amount of spreading increases gradually over time. There was a very low level of propagation of bacteria to uninfected cells early in the infection (3 h postinfection), but there were 20-fold and 45-fold increases in the propagation rate after 24 h and 48 h, respectively, of infection. Immunofluorescence microscopy of infected cells suggested that intercellular translocation of P. gingivalis may be mediated through actin-based membrane protrusions, bypassing the need for release of bacteria into extracellular medium. Consistent with these observations, cytochalasin D treatment of infected cells resulted in significant inhibition of bacterial spreading. This study shows for the first time that P. gingivalis disseminates from cell to cell without passing through the extracellular space. This mechanism of spreading may allow P. gingivalis to colonize oral tissues without exposure to the humoral immune response.

Modulation of Immune Responses by Particle Size and Shape

The immune system has to cope with a wide range of irregularly shaped pathogens that can actively move (e.g., by flagella) and also dynamically remodel their shape (e.g., transition from yeast-shaped to hyphal fungi). The goal of this review is to draw general conclusions of how the size and geometry of a pathogen affect its uptake and processing by phagocytes of the immune system. We compared both theoretical and experimental studies with different cells, model particles, and pathogenic microbes (particularly fungi) showing that particle size, shape, rigidity, and surface roughness are important parameters for cellular uptake and subsequent immune responses, particularly inflammasome activation and T cell activation. Understanding how the physical properties of particles affect immune responses can aid the design of better vaccines.

Chlamydia trachomatis induces remodeling of the actin cytoskeleton during attachment and entry into HeLa cells

To elucidate the host cell machinery utilized by Chlamydia trachomatis to invade epithelial cells, we examined the role of the actin cytoskeleton in the internalization of chlamydial elementary bodies (EBs). Treatment of HeLa cells with cytochalasin D markedly inhibited the internalization of C. trachomatis serovar L2 and D EBs. Association of EBs with HeLa cells induced localized actin polymerization at the site of attachment, as visualized by either phalloidin staining of fixed cells or the active recruitment of GFP-actin in viable infected cells. The recruitment of actin to the specific site of attachment was accompanied by dramatic changes in the morphology of cell surface microvilli. Ultrastructural studies revealed a transient microvillar hypertrophy that was dependent upon C. trachomatis attachment, mediated by structural components on the EBs, and cytochalasin D sensitive. In addition, a mutant CHO cell line that does not support entry of C. trachomatis serovar L2 did not display such microvillar hypertrophy following exposure to L2 EBs, which is in contrast to infection with serovar D, to which it is susceptible. We propose that C. trachomatis entry is facilitated by an active actin remodeling process that is induced by the attachment of this pathogen, resulting in distinct microvillar reorganization throughout the cell surface and the formation of a pedestal-like structure at the immediate site of attachment and entry.

Co-ordinate regulation of distinct host cell signalling pathways by multifunctional enteropathogenic Escherichia coli effector molecules

Enteropathogenic Escherichia coli (EPEC) is a major cause of paediatric diarrhoea and a model for the family of attaching and effacing (A/E) pathogens. A/E pathogens encode a type III secretion system to transfer effector proteins into host cells. The EPEC Tir effector protein acts as a receptor for the bacterial surface protein intimin and is involved in the formation of Cdc42-independent, actin-rich pedestal structures beneath the adhered bacteria. In this paper, we demonstrate that EPEC binding to HeLa cells also induces Tir-independent, cytoskeletal rearrangement evidenced by the early, transient formation of filopodia-like structures at sites of infection. Filopodia formation is dependent on expression of the EPEC Map effector molecule - a protein that targets mitochondria and induces their dysfunction. We show that Map-induced filopodia formation is independent of mitochondrial targeting and is abolished by cellular expression of the Cdc42 inhibitory WASP-CRIB domain, demonstrating that Map has at least two distinct functions in host cells. The transient nature of the filopodia is related to an ability of EPEC to downregulate Map-induced cell signalling that, like pedestal formation, was dependent on both Tir and intimin proteins. The ability of Tir to downregulate filopodia was impaired by disrupting a putative GTPase-activating protein (GAP) motif, suggesting that Tir may possess such a function, with its interaction with intimin triggering this activity. Furthermore, we also found that Map-induced cell signalling inhibits pedestal formation, revealing that the cellular effects of Tir and Map must be co-ordinately regulated during infection. Possible implications of the multifunctional nature of EPEC effector molecules in pathogenesis are discussed.

In vitro cell invasion of Mycoplasma gallisepticum

The ability of the widespread avian pathogen Mycoplasma gallisepticum to invade cultured human epithelial cells (HeLa-229) and chicken embryo fibroblasts (CEF) was investigated by using the gentamicin invasion assay and a double immunofluorescence microscopic technique for accurate localization of cell-associated mycoplasmas. The presence of intracellular mycoplasmas in both cell lines was clearly demonstrated, with organisms entering the eukaryotic cells within 20 min. Internalized mycoplasmas have the ability to leave the cell, but also to survive within the intracellular space over a 48-h period. Frequencies of invasion were shown to differ between the two cell lines, but were also considerably dependent on the mycoplasma input population. Of the prototype strain R, a low-passage population in artificial medium, R(low), was capable of active cell invasion, while a high-passage population, R(high), showed adherence to but nearly no uptake into HeLa-229 and CEF. By passaging R(low) and R(high) multiple times through HeLa-229 cells, the invasion frequency was significantly increased. Taken together, these findings demonstrate that M. gallisepticum has the capability of entering nonphagocytic host cells that may provide this pathogen with the opportunity for resisting host defenses and selective antibiotic therapy, establishing chronic infections, and passing through the respiratory mucosal barrier to cause systemic infections.

Involvement of actin filaments in budding of measles virus: studies on cytoskeletons of infected cells

Cytoskeletons were prepared from measles virus infected HeLa cells to investigate the involvement of cytoskeletal filaments in virus budding at the plasma membrane. The cytoskeletons retained nearly 80% of measles virus hemagglutinin, the major viral polypeptides, including P, NP, and M, and 2 to 12% of the total cell bound infectivity. As demonstrated with platinum- and carbon-shadowed cytoskeletons, all stages of budding, i.e., virus specific strands, stub-like protrusions, and completely rounded virus particles, are associated with actin filaments composing the outer part of the cytoskeletal network. As shown with ultrathin sections of flat embedded extracted cells, actin filaments identified with heavy meromyosin almost exclusively protrude into virus particles with their barbed ends and are in close association with viral nucleocapsids. The data support previous suggestions that actin is involved in virus budding and show that budding itself is possibly the result of a vectorial growth of actin filaments.

Bacterial entry into epithelial cells: the paradigm of Shigella

Shigella flexneri is a model for the entry of bacterial pathogens into nonphagocytic epithelial cells. On contact with the epithelial cell surface, the Ipa proteins are secreted from the bacterium. The Ipa complex then triggers a reorganization of the host-cell cytoskeleton leading to the formation of membrane ruffles, which engulf the bacterium.

The cell biology of infection by intracellular bacterial pathogens

Listeria monocytogenes and Shigella flexneri are unrelated bacterial pathogens that have independently evolved similar strategies of survival within an infected host animal. Bacteria coming into contact with the surface of an epithelial cell induce cytoskeletal rearrangements resulting in phagocytosis. They then secrete enzymes that degrade the phagosomal membrane, releasing the bacteria into the host cytoplasm. Intracytoplasmic bacteria move rapidly, in association with a "comet tail" made up of host cell actin filaments. When moving bacteria reach the cell margin, they push out long protrusions with the bacteria at the tips that are then taken up by neighboring cells, allowing the infection to spread from cell to cell. This review summarizes what is currently known about the interactions between the bacteria and the host at each stage of the infection and discusses what mammalian cell biologists can learn by studying bacterial pathogens.

High-frequency intracellular invasion of epithelial cells by serotype M1 group A streptococci: M1 protein-mediated invasion and cytoskeletal rearrangements

A clonal variant of serotype M1 group A streptococcus (designated M1inv+) has been linked to severe and invasive infections, including sepsis, necrotizing fasciitis and toxic shock. High frequency internalization of cultured epithelial cells by the M1inv+ strain 90-226 is dependent upon the M1 protein. Invasion of HeLa cells was blocked by an anti-M1 antibody, invasion by an M1- strain (90-226 emm1::km) was greatly reduced, and latex beads bound to M1 protein were readily internalized by HeLa cells. Beads coated with a truncated M1 protein were internalized far less frequently. Scanning electron microscopy indicated that streptococci invade by a zipper-like mechanism, that may be mediated by interactions with host cell microvilli. Initially, internalized streptococci and streptococci undergoing endocytosis are associated with polymerized actin. Later in the internalization process, streptococcal-containing vacuoles are associated with the lysosomal membrane glycoprotein, LAMP-1.

Early signaling events involved in the entry of Rickettsia conorii into mammalian cells

Rickettsia conorii, the causative agent of Mediterranean spotted fever, is able to attach to and invade a variety of cell types both in vitro and in vivo. Although previous studies show that entry of R. conorii into non-phagocytic cells relies on actin polymerization, little else is known about the molecular details governing Rickettsia-host cell interactions and actin rearrangements. We determined that R. conorii recruits the Arp2/3 complex to the site of entry foci and that expression of an Arp 2/3 binding derivative of the WASP-family member, Scar, inhibited bacterial entry into Vero cells, establishing that Arp2/3 is an active component of this process. Using transient transfection with plasmids expressing dominant negative versions of small GTPases, we showed that Cdc42, but not Rac1 is involved in R. conorii invasion into Vero cells. Using pharmacological approaches, we show that this invasion is dependent on phosphoinositide (PI) 3-kinase and on protein tyrosine kinase (PTK) activities, in particular Src-family kinases. C-Src and its downstream target, p80/85 cortactin, colocalize at entry sites early in the infection process. R. conorii internalization correlated with the tyrosine phosphorylation of several other host proteins, including focal adhesion kinase (FAK), within minutes of R. conorii infection. Our results reveal that R. conorii entry into nonphagocytic cells is dependent on the Arp2/3 complex and that the interplay of pathways involving Cdc42, PI 3-kinase, c-Src, cortactin and tyrosine-phosphorylated proteins regulates Arp2/3 activation leading to the localized actin rearrangements observed during bacterial entry. This is the first report that documents the mechanism of entry of a rickettsial species into mammalian cells.

Release of bluetongue virus-like particles from insect cells is mediated by BTV nonstructural protein NS3/NS3A

Recombinant baculoviruses and immunoelectron microscopy have been used to demonstrate the association of virus-like particles (VLPs) of bluetongue virus (i.e., particles composed of BTV, VP2, VP3, VP5, and VP7 proteins) and the cell cytoskeleton, as well as the release of such particles from infected cells in the presence of BTV NS3/NS3A, but not when BTV NS1 protein was coexpressed. Examination of cells infected with recombinants that express core-like particles (CLPs) composed of VP3 and VP7 showed that CLPs were present within the soluble fraction of the cells and not associated with the cell cytoskeleton. The simultaneous expression of the nonstructural NS1 or NS3/NS3A proteins with CLPs did not lead to the association of such particles with the cytoskeleton, nor to their release from cells. The failure of CLPs synthesized in the presence of VP2 or VP5 to attach to the cytoskeleton indicated that both outer coat proteins are required for a stable virus-cytoskeleton interaction. The ultrastructural, immunoelectron microscopical, and biochemical examinations, showed conclusively that the presence of NS3/NS3A is required for the budding and subsequent release of VLPs from infected cells.

Chlamydia trachomatis utilizes the host cell microtubule network during early events of infection

The host cell cytoskeleton is known to play a vital role in the life cycles of several pathogenic intracellular microorganisms by providing the basis for a successful invasion and by promoting movement of the pathogen once inside the host cell cytoplasm. McCoy cells infected with Chlamydia trachomatis serovars E or L2 revealed, by indirect immunofluorescence microscopy, collocation of microtubules and Chlamydia-containing vesicles during the process of migration from the host cell surface to a perinuclear location. The vast majority of microtubule-associated Chlamydia vesicles also collocated with tyrosine-phosphorylated McCoy cell proteins. After migration, the Chlamydia-containing vesicles were positioned exactly at the centre of the microtubule network, indicating a microtubule-dependent mode of chlamydial redistribution. Inhibition of host cell dynein, a microtubule-dependent motor protein known to be involved in directed vesicle transport along microtubules, was observed to have a pronounced effect on C. trachomatis infectivity. Furthermore, dynein was found to collocate with perinuclear aggregates of C. trachomatis E and L2 but not C. pneumoniae VR-1310, indicating a marked difference in the cytoskeletal requirements for C. trachomatis and C. pneumoniae during early infection events. In support of this view, C. pneumoniae VR-1310 was shown to induce much less tyrosine phosphorylation of HeLa cell proteins during uptake than that seen for C. trachomatis.

Association of bluetongue virus with the cytoskeleton

Analysis of the distribution of [35S]methionine-labeled virus proteins following lysis of bluetongue virus (BTV)-infected cells with nonionic detergents showed that a major proportion of the virus-specific proteins was located in the insoluble nuclear-cytoskeletal fraction. Neither the proportion nor the species of virus protein associated with the cytoskeleton was altered following treatment of infected cells with microtubule or microfilament disrupting drugs (colchicine and cytochalasin B, respectively). Electron microscopic examination of BTV-infected cells revealed cytoplasmic virus-specified tubules, viral inclusion bodies (VIB), and progeny virus particles. Whole-mount transmission electron microscopy of nonionic detergent-extracted cells demonstrated the association of VIB, virus particles, and tubules with the cytoskeleton. The identity of virus particles was confirmed with an immunogold labeling technique using a neutralizing monoclonal antibody to BTV protein VP2. Several lines of evidence indicate that virus particles, VIB, and tubules bind to intermediate filaments in BTV-infected cells. These structures remained cytoskeleton associated in infected cells treated with colchicine or cytochalasin B. Linear arrays of filament-associated virus particles were formed around VIB following colchicine-induced juxtanuclear aggregation of intermediate filaments. Virus particles were associated with filaments approximately 10 nm in diameter. Filaments associated with virus particles reacted with an anti-vimentin monoclonal antibody in an immunogold labeling procedure.

Shigella applies molecular mimicry to subvert vinculin and invade host cells

Shigella flexneri, the causative agent of bacillary dysentery, injects invasin proteins through a type III secretion apparatus upon contacting the host cell, which triggers pathogen internalization. The invasin IpaA is essential for S. flexneri pathogenesis and binds to the cytoskeletal protein vinculin to facilitate host cell entry. We report that IpaA harbors two vinculin-binding sites (VBSs) within its C-terminal domain that bind to and activate vinculin in a mutually exclusive fashion. Only the highest affinity C-terminal IpaA VBS is necessary for efficient entry and cell–cell spread of S. flexneri, whereas the lower affinity VBS appears to contribute to vinculin recruitment at entry foci of the pathogen. Finally, the crystal structures of vinculin in complex with the VBSs of IpaA reveal the mechanism by which IpaA subverts vinculin's functions, where S. flexneri utilizes a remarkable level of molecular mimicry of the talin–vinculin interaction to activate vinculin. Mimicry of vinculin's interactions may therefore be a general mechanism applied by pathogens to infect the host cell.

RNAi screen reveals an Abl kinase-dependent host cell pathway involved in Pseudomonas aeruginosa internalization

Internalization of the pathogenic bacterium Pseudomonas aeruginosa by non-phagocytic cells is promoted by rearrangements of the actin cytoskeleton, but the host pathways usurped by this bacterium are not clearly understood. We used RNAi-mediated gene inactivation of ∼80 genes known to regulate the actin cytoskeleton in Drosophila S2 cells to identify host molecules essential for entry of P. aeruginosa. This work revealed Abl tyrosine kinase, the adaptor protein Crk, the small GTPases Rac1 and Cdc42, and p21-activated kinase as components of a host signaling pathway that leads to internalization of P. aeruginosa. Using a variety of complementary approaches, we validated the role of this pathway in mammalian cells. Remarkably, ExoS and ExoT, type III secreted toxins of P. aeruginosa, target this pathway by interfering with GTPase function and, in the case of ExoT, by abrogating P. aeruginosa–induced Abl-dependent Crk phosphorylation. Altogether, this work reveals that P. aeruginosa utilizes the Abl pathway for entering host cells and reveals unexpected complexity by which the P. aeruginosa type III secretion system modulates this internalization pathway. Our results furthermore demonstrate the applicability of using RNAi screens to identify host signaling cascades usurped by microbial pathogens that may be potential targets for novel therapies directed against treatment of antibiotic-resistant infections.

Mortality from Pseudomonas aeruginosa infections, one of the leading causes of hospital acquired infections, approaches 40%, and multiple drug resistant infections are common and increasing. Internalization of P. aeruginosa by the host cell appears to play a fundamental role in the pathogenesis of this opportunistic bacterium, but the host cell factors involved in this process are incompletely understood. We used a targeted RNAi screen in Drosophila S2 cells to identify a subset of regulators of the host actin cytoskeleton that contribute to bacterial entry and confirmed their involvement in infection of mammalian cells. We found that P. aeruginosa can modulate this internalization pathway in a complex manner by injecting the bacterial toxins ExoS and ExoT into the host cell via its type III secretion system. The identified host cell molecules may serve as targets for novel drugs to treat infections resistant to conventional antibiotics and may be applicable to a wide range of pathogens.