A glance at Listeria and Salmonella cell invasion: different strategies to promote host actin polymerization

Subversion strategies of lysosomal killing by intracellular pathogens

Many pathogenic organisms need to reach either an intracellular compartment or the cytoplasm of a target cell for their survival, replication or immune system evasion. Intracellular pathogens frequently penetrate into the cell through the endocytic and phagocytic pathways (clathrin-mediated endocytosis, phagocytosis and macropinocytosis) that culminates in fusion with lysosomes. However, several mechanisms are triggered by pathogenic microorganisms - protozoan, bacteria, virus and fungus - to avoid destruction by lysosome fusion, such as rupture of the phagosome and thereby release into the cytoplasm, avoidance of autophagy, delaying in both phagolysosome biogenesis and phagosomal maturation and survival/replication inside the phagolysosome. Here we reviewed the main data dealing with phagosome maturation and evasion from lysosomal killing by different bacteria, protozoa, fungi and virus.

Modeling the invasion of human small intestinal epithelial-like cells by Salmonella enterica Typhimurium and Listeria monocytogenes using Bayesian inference

In order to develop a mechanistic bacterial dose-response model, based on the concept of Key Events Dose-Response Framework (KEDRF), this study aimed to investigate the invasion of intestinal model cells (Caco-2) by Salmonella Typhimurium and Listeria monocytogenes and described the behavior of both pathogens as a mathematical model using Bayesian inference. Monolayer-cultured Caco-2 cells (approximately 10⁵ cells) were co-cultured with various concentrations (10³ - 10⁷ CFU·ml^-1 ) of S. Typhimurium and L. monocytogenes for up to 9 h to investigate the invasion of the pathogens into the Caco-2 cells. While an exposure of ≥ 10³ CFU·ml^-1 of S. Typhimurium initiated the invasion of Caco-2 cells within 3 h, much less exposure (10² CFU·ml^-1 ) of L. monocytogenes was sufficient for invasion within the same period. Furthermore, while the maximum number of invading S. Typhimurium cells reached by approximately 10³ CFU·cm^-2 for 6 h exposure , the invading maximum numbers of L. monocytogenes cells increased by approximately 10⁶ CFU·cm^-2 for the same exposure period. The invasion kinetics of both the pathogens was successfully described as an asymptotic exponential mathematical model using Bayesian inference. The developed pathogen invasion model allowed the estimation of probability of S. Typhimurium and L. monocytogenes infection, based on the physiological natures of digestion process, which was comparable to the published dose-response relationship. The invasion models developed in the present study will play a key role in the development of an alternative pathogen dose-response models based on KEDRF concept.

Myosins, an Underestimated Player in the Infectious Cycle of Pathogenic Bacteria

Myosins play a key role in many cellular processes such as cell migration, adhesion, intracellular trafficking and internalization processes, making them ideal targets for bacteria. Through selected examples, such as enteropathogenic E. coli (EPEC), Neisseria, Salmonella, Shigella, Listeria or Chlamydia, this review aims to illustrate how bacteria target and hijack host cell myosins in order to adhere to the cell, to enter the cell by triggering their internalization, to evade from the cytosolic autonomous cell defense, to promote the biogenesis of intracellular replicative niche, to disseminate in tissues by cell-to-cell spreading, to exit out the host cell, and also to evade from macrophage phagocytosis. It highlights the diversity and sophistication of the strategy evolved by bacteria to manipulate one of their privileged targets, the actin cytoskeleton.

Bacterial Actin-Specific Endoproteases Grimelysin and Protealysin as Virulence Factors Contributing to the Invasive Activities of Serratia

The article reviews the discovery, properties and functional activities of new bacterial enzymes, proteases grimelysin (ECP 32) of Serratia grimesii and protealysin of Serratia proteamaculans, characterized by both a highly specific "actinase" activity and their ability to stimulate bacterial invasion. Grimelysin cleaves the only polypeptide bond Gly42-Val43 in actin. This bond is not cleaved by any other proteases and leads to a reversible loss of actin polymerization. Similar properties were characteristic for another bacterial protease, protealysin. These properties made grimelysin and protealysin a unique tool to study the functional properties of actin. Furthermore, bacteria Serratia grimesii and Serratia proteamaculans, producing grimelysin and protealysin, invade eukaryotic cells, and the recombinant Escherichia coli expressing the grimelysin or protealysins gene become invasive. Participation of the cellular c-Src and RhoA/ROCK signaling pathways in the invasion of eukaryotic cells by S. grimesii was shown, and involvement of E-cadherin in the invasion has been suggested. Moreover, membrane vesicles produced by S. grimesii were found to contain grimelysin, penetrate into eukaryotic cells and increase the invasion of bacteria into eukaryotic cells. These data indicate that the protease is a virulence factor, and actin can be a target for the protease upon its translocation into the host cell.

Invasion of vaginal epithelial cells by uropathogenic Escherichia coli

Host-associated reservoirs account for the majority of recurrent and oftentimes recalcitrant infections. Previous studies established that uropathogenic E. coli - the primary cause of urinary tract infections (UTIs) - can adhere to vaginal epithelial cells preceding UTI. Here, we demonstrate that diverse urinary E. coli isolates not only adhere to, but also invade vaginal cells. Intracellular colonization of the vaginal epithelium is detected in acute and chronic murine UTI models indicating the ability of E. coli to reside in the vagina following UTI. Conversely, in a vaginal colonization model, E. coli are detected inside vaginal cells and the urinary tract, indicating that vaginal colonization can seed the bladder. More critically, bacteria are identified inside vaginal cells from clinical samples from women with a history of recurrent UTI. These findings suggest that E. coli can establish a vaginal intracellular reservoir, where it may reside safely from extracellular stressors prior to causing an ascending infection.

The Ethanolamine Permease EutH Promotes Vacuole Adaptation of Salmonella enterica and Listeria monocytogenes during Macrophage Infection

Ethanolamine is a ubiquitous and essential molecule within a host. Significantly, bacterial pathogens exploit ethanolamine during infection to promote growth and regulate virulence. The ethanolamine permease EutH is dispensable for growth in vitro under standard conditions, whereas EutH is required for ethanolamine utilization at low pH. These findings suggested a model in which EutH facilitates diffusion of ethanolamine into the bacterial cell in acidic environments. To date, the ecological significance of this model has not been thoroughly investigated, and the importance of EutH to bacterial growth under physiologically relevant conditions is not known. During infection, immune cells internalize invading bacteria within an acidic, nutrient-depleted vacuole called the phagosome. Here, we investigated the hypothesis that EutH promotes bacterial survival following phagocytosis. Our findings indicate that EutH is important for survival and replication of the facultative intracellular pathogens Salmonella enterica serovar Typhimurium and Listeria monocytogenes during prolonged or transient exposure to the phagosome, respectively. Furthermore, in agreement with EutH being important in the acidic environment, neutralization of the vacuole abolished the requirement for EutH. Significantly, consistent with a role for EutH in promoting intramacrophage survival, EutH was not required during S Typhimurium local intestinal infection but specifically conferred an advantage upon dissemination to peripheral organs. These findings reveal a physiologically relevant and conserved role for EutH in spatiotemporal niche adaptation during infection.

[Research advances on the molecular mechanism of autophagy regulated by Porphyromonas gingivalis]

Autophagy is an intracellular conservative degradation pathway. This event has been considered as a key step in host defense against bacterial infection. However, Porphyromonas gingivalis, as one of the evidence-sufficient periodontal pathogens, can utilize self-induced autophagy to achieve persistent intracellular survival and proliferation, which enable this organism to escape from host immune surveillance. This review focuses on molecular mechanism of P. gingivalis internalization and autophagy to illuminate its pathogenesis and to further explore the relationship between P. gingivalis and systemic diseases.

YopJ Family Effectors Promote Bacterial Infection through a Unique Acetyltransferase Activity

Gram-negative bacterial pathogens rely on the type III secretion system to inject virulence proteins into host cells. These type III secreted "effector" proteins directly manipulate cellular processes to cause disease. Although the effector repertoires in different bacterial species are highly variable, the Yersinia outer protein J (YopJ) effector family is unique in that its members are produced by diverse animal and plant pathogens as well as a nonpathogenic microsymbiont. All YopJ family effectors share a conserved catalytic triad that is identical to that of the C55 family of cysteine proteases. However, an accumulating body of evidence demonstrates that many YopJ effectors modify their target proteins in hosts by acetylating specific serine, threonine, and/or lysine residues. This unique acetyltransferase activity allows the YopJ family effectors to affect the function and/or stability of their targets, thereby dampening innate immunity. Here, we summarize the current understanding of this prevalent and evolutionarily conserved type III effector family by describing their enzymatic activities and virulence functions in animals and plants. In particular, the molecular mechanisms by which representative YopJ family effectors subvert host immunity through posttranslational modification of their target proteins are discussed.

Crucial roles of TNFAIP8 protein in regulating apoptosis and Listeria infection

TNF-α-induced protein 8 (TNFAIP8 or TIPE) is a newly described regulator of cancer and infection. However, its precise roles and mechanisms of actions are not well understood. We report in this article that TNFAIP8 regulates Listeria monocytogenes infection by controlling pathogen invasion and host cell apoptosis in a RAC1 GTPase-dependent manner. TNFAIP8-knockout mice were resistant to lethal L. monocytogenes infection and had reduced bacterial load in the liver and spleen. TNFAIP8 knockdown in murine liver HEPA1-6 cells increased apoptosis, reduced bacterial invasion into cells, and resulted in dysregulated RAC1 activation. TNFAIP8 could translocate to plasma membrane and preferentially associate with activated RAC1-GTP. The combined effect of reduced bacterial invasion and increased sensitivity to TNF-α-induced clearance likely protected the TNFAIP8-knockout mice from lethal listeriosis. Thus, by controlling bacterial invasion and the death of infected cells through RAC1, TNFAIP8 regulates the pathogenesis of L. monocytogenes infection.

In vitro protective effect of lactic acid bacteria on Listeria monocytogenes adhesion and invasion of Caco-2 cells

The adhesion of Listeria monocytogenes to intestinal endothelial cells is a crucial step in the infection process, which is not well understood. In this study, we evaluated the potential ability of bacteriocin-producing Enterococcus faecium, Leuconostoc mesenteroides and Lactobacillus sakei strains to prevent the adhesion and invasion of eukaryotic cells by ten different L. monocytogenes isolates. The results showed that E. faecium 130 co-cultured with L. monocytogenes was the most effective in preventing infection of Caco-2 cells, as the vast majority of isolates showed significantly lower adhesion counts and invasion rates below the quantification limit of the method (<30 cfu/plate). L. sakei 1 was the least effective strain in preventing L. monocytogenes infection; only one isolate presented a lower adhesion rate and two isolates reduced the invasion rate of Caco-2 cells. Fluorescence in situ hybridisation (FISH) assay was shown to be an effective tool to illustrate and identify species in co-culture with L. monocytogenes during the adhesion process to Caco-2 cells.

Identification of host proteins involved in rickettsial invasion of tick cells

Tick-borne spotted fever group (SFG) Rickettsia species are obligate intracellular bacteria capable of infecting both vertebrate and invertebrate host cells, an essential process for subsequent bacterial survival in distinct hosts. The host cell signaling molecules involved in the uptake of Rickettsia into mammalian and Drosophila cells have been identified; however, invasion into tick cells is understudied. Considering the movement of SFG Rickettsia between vertebrate and invertebrate hosts, the hypothesis is that conserved mechanisms are utilized for host cell invasion. The current study employed biochemical inhibition assays to determine the tick proteins involved in Rickettsia montanensis infection of tick-derived cells from a natural host, Dermacentor variabilis. The results revealed several tick proteins important for rickettsial invasion, including actin filaments, actin-related protein 2/3 complex, phosphatidylinositol-3'-kinase, protein tyrosine kinases (PTKs), Src family PTK, focal adhesion kinase, Rho GTPase Rac1, and neural Wiskott-Aldrich syndrome protein. Delineating the molecular mechanisms of rickettsial infection is critical to a thorough understanding of rickettsial transmission in tick populations and the ecology of tick-borne rickettsial diseases.

Parasite fate and involvement of infected cells in the induction of CD4+ and CD8+ T cell responses to Toxoplasma gondii

During infection with the intracellular parasite Toxoplasma gondii, the presentation of parasite-derived antigens to CD4⁺ and CD8⁺ T cells is essential for long-term resistance to this pathogen. Fundamental questions remain regarding the roles of phagocytosis and active invasion in the events that lead to the processing and presentation of parasite antigens. To understand the most proximal events in this process, an attenuated non-replicating strain of T. gondii (the cpsII strain) was combined with a cytometry-based approach to distinguish active invasion from phagocytic uptake. In vivo studies revealed that T. gondii disproportionately infected dendritic cells and macrophages, and that infected dendritic cells and macrophages displayed an activated phenotype characterized by enhanced levels of CD86 compared to cells that had phagocytosed the parasite, thus suggesting a role for these cells in priming naïve T cells. Indeed, dendritic cells were required for optimal CD4⁺ and CD8⁺ T cell responses, and the phagocytosis of heat-killed or invasion-blocked parasites was not sufficient to induce T cell responses. Rather, the selective transfer of cpsII-infected dendritic cells or macrophages (but not those that had phagocytosed the parasite) to naïve mice potently induced CD4⁺ and CD8⁺ T cell responses, and conferred protection against challenge with virulent T. gondii. Collectively, these results point toward a critical role for actively infected host cells in initiating T. gondii-specific CD4⁺ and CD8⁺ T cell responses.

CD4⁺ and CD8⁺ T cells are critical for controlling many infections. To generate a T cell response during infection, T cells must encounter the microbial peptides that they recognize bound to MHC molecules on the surfaces of other cells, such as dendritic cells. It is currently unclear how dendritic cells acquire the antigens they present to T cells during infection with many intracellular pathogens. It is possible that these antigens are phagocytosed and processed by dendritic cells, or antigens may be presented by cells that are infected by pathogens such as Toxoplasma gondii, which invades host cells independently of phagocytosis. To differentiate these pathways, we developed a novel technique to track the fate of T. gondii in vivo that distinguishes actively infected cells from those that phagocytosed parasites. This technique was used to examine each of these cell populations. We also used pharmacological inhibitors of parasite invasion, and the transfer of sort-purified infected or uninfected dendritic cells and macrophages to determine what roles phagocytosis and active invasion have in the initiation of T cell responses. Our results demonstrate that phagocytosis of parasites is not sufficient to induce CD4⁺ or CD8⁺ T cell responses, whereas infected cells are critical for this process.

Quantitative insights into actin rearrangements and bacterial target site selection from Salmonella Typhimurium infection of micropatterned cells

Reorganization of the host cell actin cytoskeleton is crucial during pathogen invasion. We established micropatterned cells as a standardized infection model for cell invasion to quantitatively study actin rearrangements triggered by Salmonella Typhimurium (S. Tm). Micropatterns of extracellular matrix proteins force cells to adopt a reproducible shape avoiding strong cell-to-cell variations, a major limitation in classical cell culture conditions. S. Tm induced F-actin-rich ruffles and invaded micropatterned cells similar to unconstrained cells. Yet, standardized conditions allowed fast and unbiased comparison of cellular changes triggered by the SipA and SopE bacterial effector proteins. Intensity measurements in defined regions revealed that the content of pre-existing F-actin remained unchanged during infection, suggesting that newly polymerized F-actin in bacteria-triggered ruffles originates from the G-actin pool. Analysing bacterial target sites, we found that bacteria did not show any preferences for the local actin cytoskeleton specificities. Rather, invasion was constrained to a specific 'cell height', due to flagella-mediated near-surface swimming. We found that invasion sites were similar to bacterial binding sites, indicating that S. Tm can induce a permissive invasion site wherever it binds. As micropatterned cells can be infected by many different pathogens they represent a valuable new tool for quantitative analysis of host-pathogen interactions.

Host actin remodeling and protection from malaria by hemoglobinopathies

Many intracellular pathogens remodel the actin of their host cells, and the human malaria parasite Plasmodium falciparum is no exception to this rule. The surprising finding is that several hemoglobinopathies that protect carriers from severe malaria may do so by interfering with host actin reorganization. Here we discuss our current understanding of actin remodeling in P. falciparum-infected erythrocytes, how hemoglobinopathies interfere with this process, and how impaired host actin remodeling affects the virulence of P. falciparum.