Actin-dependent movement of bacterial pathogens

Direct targeting of host microtubule and actin cytoskeletons by a chlamydial pathogenic effector protein

To propagate within a eukaryotic cell, pathogenic bacteria hijack and remodulate host cell functions. The Gram-negative obligate intracellular Chlamydiaceae, which pose a serious threat to human and animal health, attach to host cells and inject effector proteins that reprogram host cell machineries. Members of the conserved chlamydial TarP family have been characterized as major early effectors that bind to and remodel the host actin cytoskeleton. We now describe a new function for the Chlamydia pneumoniae TarP member CPn0572, namely the ability to bind and alter the microtubule cytoskeleton. Thus, CPn0572 is unique in being the only prokaryotic protein that directly modulates both dynamic cytoskeletons of a eukaryotic cell. Ectopically expressed GFP-CPn0572 associates in a dose-independent manner with either cytoskeleton singly or simultaneously. In vitro, CPn0572 binds directly to microtubules. Expression of a microtubule-only CPn0572 variant resulted in the formation of an aberrantly thick, stabilized microtubule network. Intriguingly, during infection, secreted CPn0572 also colocalized with altered microtubules, suggesting that this protein also affects microtubule dynamics during infection. Our analysis points to a crosstalk between actin and microtubule cytoskeletons via chlamydial CPn0572.

Bacterial Pathogenesis: Assessment of Intracellular Positioning of Pathogen-Containing Vacuoles During Infection

Intracellular bacterial pathogens implement a diverse array of strategies to target host cells and establish infection. For vacuolar pathogens, the process of pathogen-containing vacuole movement within host cells, termed intracellular trafficking, is central to both pathogen survival and infection progression. Typically a process mediated by secreted virulence factors that manipulate the host cytoskeletal machinery, internalized pathogen-containing vacuoles traffic to the site of replication to establish a unique replicative niche, and if applicable, traffic back toward the host cell periphery for cell-to-cell spread. As such, the intracellular positioning of pathogen-containing vacuoles represents a fundamental measure of infection progression. Here, we describe a fluorescence microscopy-based method to quantitatively assess bacterial intracellular positioning, using Salmonella enterica serovar Typhimurium infection of epithelial cells as a model. This experimental approach can be modified to study infection in diverse host cell types, and with a broad array of pathogens. The system can also be adapted to examine the kinetics of infection, identify secreted virulence factors that mediate host trafficking, investigate host factors that are targeted by the pathogen for trafficking, and assess functional domains within a virulence factor responsible for mediating the phenotype. Collectively, these tools can provide fundamental insight into the pathogenesis of a diverse array of intracellular bacterial pathogens, and new host factors that are hijacked to mediate infection. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Culture and preparation of host cells Alternate Protocol: Culture and preparation of host cells to assess host factor contribution to bacterial positioning Basic Protocol 2: Infection of epithelial cells with S. Typhimurium Basic Protocol 3: Fluorescence staining for analysis of bacterial positioning Basic Protocol 4: Fluorescence microscopy analysis of bacterial positioning.

Bacterial Pathogen-Mediated Suppression of Host Trafficking to Lysosomes: Fluorescence Microscopy-Based DQ-Red BSA Analysis

Intracellular bacterial pathogens have evolved to be adept at manipulating host cellular function for the benefit of the pathogen, often by means of secreted virulence factors that target host pathways for modulation. The lysosomal pathway is an essential cellular response pathway to intracellular pathogens and, as such, represents a common target for bacterial-mediated evasion. Here, we describe a method to quantitatively assess bacterial pathogen-mediated suppression of host cell trafficking to lysosomes, using Salmonella enterica serovar Typhimurium infection of epithelial cells as a model. This live-cell imaging assay involves the use of a BODIPY TR-X conjugate of BSA (DQ-Red BSA) that traffics to and fluoresces in functional lysosomes. This method can be adapted to study infection with a broad array of pathogens in diverse host cell types. It is capable of being applied to identify secreted virulence factors responsible for a phenotype of interest as well as domains within the bacterial protein that are important for mediating the phenotype. Collectively, these tools can provide invaluable insight into the mechanisms of pathogenesis of a diverse array of pathogenic bacteria, with the potential to uncover virulence factors that may be suitable targets for therapeutic intervention. Key features • Infection-based analysis of bacterial-mediated suppression of host trafficking to lysosomes, using Salmonella enterica serovar Typhimurium infection of human epithelial cells as a model. • Live microscopy-based analysis allows for the visualization of individually infected host cells and is amenable to phenotype quantification. • Assay can be adapted to a broad array of pathogens and diverse host cell types. • Assay can identify virulence factors mediating a phenotype and protein domains that mediate a phenotype.

Armeniacae semen amarum: a review on its botany, phytochemistry, pharmacology, clinical application, toxicology and pharmacokinetics

Armeniacae semen amarum-seeds of Prunus armeniaca L. (Rosaceae) (ASA), also known as Kuxingren in Chinese, is a traditional Chinese herbal drug commonly used for lung disease and intestinal disorders. It has long been used to treat coughs and asthma, as well as to lubricate the colon and reduce constipation. ASA refers to the dried ripe seed of diverse species of Rosaceae and contains a variety of phytochemical components, including glycosides, organic acids, amino acids, flavonoids, terpenes, phytosterols, phenylpropanoids, and other components. Extensive data shows that ASA exhibits various pharmacological activities, such as anticancer activity, anti-oxidation, antimicrobial activity, anti-inflammation, protection of cardiovascular, neural, respiratory and digestive systems, antidiabetic effects, and protection of the liver and kidney, and other activities. In clinical practice, ASA can be used as a single drug or in combination with other traditional Chinese medicines, forming ASA-containing formulas, to treat various afflictions. However, it is important to consider the potential adverse reactions and pharmacokinetic properties of ASA during its clinical use. Overall, with various bioactive components, diversified pharmacological actions and potent efficacies, ASA is a promising drug that merits in-depth study on its functional mechanisms to facilitate its clinical application.

Machine learning-based motion tracking reveals an inverse correlation between adhesivity and surface motility of the leptospirosis spirochete

Bacterial motility is often a crucial virulence factor for pathogenic species. A common approach to study bacterial motility is fluorescent labeling, which allows detection of individual bacterial cells in a population or in host tissues. However, the use of fluorescent labeling can be hampered by protein expression stability and/or interference with bacterial physiology. Here, we apply machine learning to microscopic image analysis for label-free motion tracking of the zoonotic bacterium Leptospira interrogans on cultured animal cells. We use various leptospiral strains isolated from a human patient or animals, as well as mutant strains. Strains associated with severe disease, and mutant strains lacking outer membrane proteins (OMPs), tend to display fast mobility and reduced adherence on cultured kidney cells. Our method does not require fluorescent labeling or genetic manipulation, and thus could be applied to study motility of many other bacterial species.

ActuAtor, a Listeria-inspired molecular tool for physical manipulation of intracellular organizations through de novo actin polymerization

Form and function are often interdependent throughout biology. Inside cells, mitochondria have particularly attracted attention since both their morphology and functionality are altered under pathophysiological conditions. However, directly assessing their causal relationship has been beyond reach due to the limitations of manipulating mitochondrial morphology in a physiologically relevant manner. By engineering a bacterial actin regulator, ActA, we developed tools termed "ActuAtor" that inducibly trigger actin polymerization at arbitrary subcellular locations. The ActuAtor-mediated actin polymerization drives striking deformation and/or movement of target organelles, including mitochondria, Golgi apparatus, and nucleus. Notably, ActuAtor operation also disperses non-membrane-bound entities such as stress granules. We then implemented ActuAtor in functional assays, uncovering the physically fragmented mitochondria being slightly more susceptible to degradation, while none of the organelle functions tested are morphology dependent. The modular and genetically encoded features of ActuAtor should enable its application in studies of the form-function interplay in various intracellular contexts.

Fungal mechanisms of intracellular survival: what can we learn from bacterial pathogens?

Fungal infections represent a major, albeit neglected, public health threat with serious medical and economic burdens globally. With unacceptably high mortality rates, invasive fungal pathogens are responsible for millions of deaths each year, with a steadily increasing incidence primarily in immunocompromised individuals. The poor therapeutic options and rise of antifungal drug resistance pose further challenges in controlling these infections. These fungal pathogens have adapted to survive within mammalian hosts and can establish intracellular niches to promote survival within host immune cells. To do that, they have developed diverse methods to circumvent the innate immune system attack. This includes strategies such as altering their morphology, counteracting macrophage antimicrobial action, and metabolic adaptation. This is reminiscent of how bacterial pathogens have adapted to survive within host cells and cause disease. However, relative to the great deal of information available concerning intracellular bacterial pathogenesis, less is known about the mechanisms fungal pathogens employ. Therefore, here we review our current knowledge and recent advances in our understanding of how fungi can evade and persist within host immune cells. This review will focus on the major fungal pathogens, including Cryptococcus neoformans, Candida albicans, and Aspergillus fumigatus, among others. As we discover and understand the strategies used by these fungi, similarities with their bacterial counterparts are becoming apparent, hence we can use the abundant information from bacteria to guide our studies in fungi. By understanding these strategies, new lines of research will open that can improve the treatments of these devastating fungal diseases.

Polarized SCAR and the Arp2/3 complex regulate apical cortical remodeling in asymmetrically dividing neuroblasts

Although the formin-nucleated actomyosin cortex has been shown to drive the changes in cell shape that accompany animal cell division in both symmetric and asymmetric cell divisions, the mitotic role of cortical Arp2/3-nucleated actin networks remain unclear. Here using asymmetrically dividing Drosophila neural stem cells as a model system, we identify a pool of membrane protrusions that form at the apical cortex of neuroblasts as they enter mitosis. Strikingly, these apically localized protrusions are enriched in SCAR, and depend on SCAR and Arp2/3 complexes for their formation. Because compromising SCAR or the Arp2/3 complex delays the apical clearance of Myosin II at the onset of anaphase and induces cortical instability at cytokinesis, these data point to a role for an apical branched actin filament network in fine-tuning the actomyosin cortex to enable the precise control of cell shape changes during an asymmetric cell division.

A Tale about Shigella: Evolution, Plasmid, and Virulence

Shigella spp. cause hundreds of millions of intestinal infections each year. They target the mucosa of the human colon and are an important model of intracellular bacterial pathogenesis. Shigella is a pathovar of Escherichia coli that is characterized by the presence of a large invasion plasmid, pINV, which encodes the characteristic type III secretion system and icsA used for cytosol invasion and cell-to-cell spread, respectively. First, we review recent advances in the genetic aspects of Shigella, shedding light on its evolutionary history within the E. coli lineage and its relationship to the acquisition of pINV. We then discuss recent insights into the processes that allow for the maintenance of pINV. Finally, we describe the role of the transcription activators VirF, VirB, and MxiE in the major virulence gene regulatory cascades that control the expression of the type III secretion system and icsA. This provides an opportunity to examine the interplay between these pINV-encoded transcriptional activators and numerous chromosome-encoded factors that modulate their activity. Finally, we discuss novel chromosomal genes icaR, icaT, and yccE that are regulated by MxiE. This review emphasizes the notion that Shigella and E. coli have walked the fine line between commensalism and pathogenesis for much of their history.

Synaptopodin is necessary for Shigella flexneri intercellular spread

For many intracellular pathogens, their virulence depends on an ability to spread between cells of an epithelial layer. For intercellular spread to occur, these pathogens deform the plasma membrane into a protrusion structure that is engulfed by the neighboring cell. Although the polymerization of actin is essential for spread, how these pathogens manipulate the actin cytoskeleton in a manner that enables protrusion formation is still incompletely understood. Here, we identify the mammalian actin binding protein synaptopodin as required for efficient intercellular spread. Using a model cytosolic pathogen, Shigella flexneri , we show that synaptopodin contributes to organization of actin around bacteria and increases the length of the actin tail at the posterior pole of the bacteria. We show that synaptopodin presence enables protrusions to form and to resolve at a greater rate, indicating that greater stability of the actin tail enables the bacteria to push against the membrane with greater force. We demonstrate that synaptopodin recruitment around bacteria requires the bacterial protein IcsA, and we show that this recruitment is further enhanced in a type 3 secretion system dependent manner. These data establish synaptopodin as required for intracellular bacteria to reprogram the actin cytoskeleton in a manner that enables efficient protrusion formation and enhance our understanding of the cellular function of synaptopodin.

Authors Summary

Intercellular spread is essential for many cytosolic dwelling pathogens during their infectious life cycle. Despite knowing the steps required for intercellular spread, relatively little is known about the host-pathogen interactions that enable these steps to occur. Here, we identify a requirement for the actin binding protein synaptopodin during intercellular spread by cytosolic bacteria. We show synaptopodin is necessary for the stability and recruitment of polymerized actin around bacteria. We also demonstrate synaptopodin is necessary to form plasma membrane structures known as protrusions that are necessary for the movement of these bacteria between cells. Thus, these findings implicate synaptopodin as an important actin-binding protein for the virulence of intracellular pathogens that require the actin cytoskeleton for their spread between cells.

The Attenuation Mechanism and Live Vaccine Potential of a Low-Virulence Edwardsiella ictaluri Strain Obtained by Rifampicin Passaging Culture

The rifampicin-resistant strain E9-302 of Edwardsiella ictaluri strain 669 (WT) was generated by continuous passage on BHI agar plates containing increasing concentrations of rifampicin. E9-302 was attenuated significantly by 119 times to zebrafish Danio rerio compared to WT in terms of the 50% lethal dose (LD₅₀). Zebrafish vaccinated with E9-302 via intraperitoneal (IP) injection at a dose of 1 × 10³ CFU/fish had relative percentage survival (RPS) rates of 85.7% when challenged with wild-type E. ictaluri via IP 14 days post-vaccination (dpv). After 14 days of primary vaccination with E9-302 via immersion (IM) at a dose of 4 × 10⁷ CFU/ml, a booster IM vaccination with E9-302 at a dose of 2 × 10⁷ CFU/ml exhibited 65.2% RPS against challenge with wild-type E. ictaluri via IP 7 days later. These results indicated that the rifampicin-resistant attenuated strain E9-302 had potential as a live vaccine against E. ictaluri infection. A previously unreported amino acid site change at position 142 of the RNA polymerase (RNAP) β subunit encoded by the gene rpoB associated with rifampicin resistance was identified. Analysis of the whole-genome sequencing results revealed multiple missense mutations in the virulence-related genes esrB and sspH2 in E9-302 compared with WT, and a 189 bp mismatch in one gene, whose coding product was highly homologous to glycosyltransferase family 39 protein. This study preliminarily explored the molecular mechanism underlying the virulence attenuation of rifampicin-resistant strain E9-302 and provided a new target for the subsequent study of the pathogenic mechanism of E. ictaluri.

ArpC5 isoforms regulate Arp2/3 complex-dependent protrusion through differential Ena/VASP positioning

Regulation of the Arp2/3 complex is required for productive nucleation of branched actin networks. An emerging aspect of regulation is the incorporation of subunit isoforms into the Arp2/3 complex. Specifically, both ArpC5 subunit isoforms, ArpC5 and ArpC5L, have been reported to fine-tune nucleation activity and branch junction stability. We have combined reverse genetics and cellular structural biology to describe how ArpC5 and ArpC5L differentially affect cell migration. Both define the structural stability of ArpC1 in branch junctions and, in turn, by determining protrusion characteristics, affect protein dynamics and actin network ultrastructure. ArpC5 isoforms also affect the positioning of members of the Ena/Vasodilator-stimulated phosphoprotein (VASP) family of actin filament elongators, which mediate ArpC5 isoform-specific effects on the actin assembly level. Our results suggest that ArpC5 and Ena/VASP proteins are part of a signaling pathway enhancing cell migration.

Internalization of Leptospira interrogans via diverse endocytosis mechanisms in human macrophages and vascular endothelial cells

Leptospirosis, one of the leading global causes of morbidity and mortality, is an emerging public health problem, particularly in large urban centers of developing countries. Leptospirosis results from infection with an organism belonging to the Leptospira genus L. interrogans. The extensive invasive ability has previously been documented, however a mechanism that describes how the organism is internalized by human macrophages and transmigrates through human blood vessel remains poorly understood. In the present study, we utilized a human macrophage and vascular endothelial cell line to study the diverse invasive mechanisms by which L. interrogans infections occur. We found that THP-1 and HUVEC had a diverse expression of cell receptors and L. interrogans entered THP-1 and HUVEC by different pathways. In the macrophage model cell line, ITGB1/FAK-signaling mediated microfilament dependent endocytosis with lysosome fusion, whereas ITGB1/CAV-1/PI3K-signaling mediated microfilament dependent endocytosis and transcytosis without lysosome fusion in the endothelial cell model. Shedding of pathogenic leptospires from HUVEC displayed higher viability than those from THP-1. The monolayer of HUVEC maintained integrity during the infection, while 3D imaging showed that leptospires were transmigrated both intra- and intercellularly. These results indicate that endocytosis of leptospires in human macrophages and human vascular endothelial cells are quite different, macrophages are responsible for eliminating leptospires in the human body during the infection while vascular endothelial cells facilitate dissemination of leptospires from blood vessels into target organs where they cause injury.

Leptospirosis is a zoonotic bacterial disease which causes 1.03 million cases and 58,900 deaths each year. Human infections occur when the primary reservoir hosts, such as rodents, contaminate food and water with leptospires. Unlike other bacterial pathogens, leptospires invade the human body through mucosal barriers and enter the bloodstream, which can result in septicemia. Left untreated, leptospirosis can spread into multiple organs and tissues such as lungs, liver and kidneys. Pathological features of this disease include high fever, myalgia, lymphadenectasis, hemorrhaging and jaundice. Human macrophages and vascular endothelial cells play important roles in eliminating and preventing the transmission of this pathogen. We speculated that leptospires could be neutralized in macrophages and subsequently transported by vascular endothelial cells throughout the human body. In this research, the diverse mechanisms of human macrophages and vascular endothelial cells infected by leptospires were explored. Our findings can be used to improve the treatment, prevention, and supervision regarding leptospire transmission and infection.

Intelligent design of polymersomes for antibacterial and anticancer applications

Polymersomes (or polymer vesicles) have attracted much attention for biomedical applications in recent years because their lumen can be used for drug delivery and their coronas and membrane can be modified with a variety of functional groups. Thus, polymersomes are very suitable for improved antibacterial and anticancer therapy. This review mainly highlighted recent advances in the synthetic protocols and design principles of intelligent antibacterial and anticancer polymersomes. Antibacterial polymersomes are divided into three categories: polymersomes as antibiotic nanocarriers, intrinsically antibacterial polymersomes, and antibacterial polymersomes with supplementary means including photothermal and photodynamic therapy. Similarly, the anticancer polymersomes are divided into two categories: polymersomes-based delivery systems and anticancer polymersomes with supplementary means. In addition, the bilateral relationship between bacteria and cancer is addressed, since more and more evidences show that bacteria may cause cancer or promote cancer progression. Finally, prospective on next-generation antibacterial and anticancer polymersomes are discussed. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Biology-Inspired Nanomaterials > Lipid-Based Structures.

Regulator of Actin-Based Motility (RoaM) Downregulates Actin Tail Formation by Rickettsia rickettsii and Is Negatively Selected in Mammalian Cell Culture

The etiological agent of Rocky Mountain spotted fever, Rickettsia rickettsii, is an obligately intracellular pathogen that induces the polymerization of actin filaments to propel the bacterium through the cytoplasm and spread to new host cells. Cell-to-cell spread via actin-based motility is considered a key virulence determinant for spotted fever group rickettsiae, as interruption of sca2, the gene directly responsible for actin polymerization, has been shown to reduce fever in guinea pigs. However, little is known about how, or if, motility is regulated by the bacterium itself. We isolated a hyperspreading variant of R. rickettsii Sheila Smith that produces actin tails at an increased rate. A1G_06520 (roaM [regulator of actin-based motility]) was identified as a negative regulator of actin tail formation. Disruption of RoaM significantly increased the number of actin tails compared to the wild-type strain but did not increase virulence in guinea pigs; however, overexpression of RoaM dramatically decreased the presence of actin tails and moderated fever response. Localization experiments suggest that RoaM is not secreted, while reverse transcription-quantitative PCR (RT-qPCR) data show that various levels of RoaM do not significantly affect the expression of the known rickettsial actin-regulating proteins sca2, sca4, and rickA. Taken together, the data suggest a previously unrecognized level of regulation of actin-based motility in spotted fever group rickettsiae. Although this gene is intact in many isolates of spotted fever, transitional, and ancestral group Rickettsia spp., it is often ablated in highly passaged laboratory strains. Serial passage experiments revealed strong negative selection of roaM in Vero 76 cells.

Effects of intra-articular inoculation with Mycoplasma bovis on immunological responses in calf joints

Mycoplasma arthritis that caused by Mycoplasma bovis exhibit severe lameness. This disease is difficult to cure with antibiotics, but the detailed pathological mechanisms have not been fully clarified. In this study, we examined the effects of intra-articular inoculation with M. bovis on immunological responses in calf joints. We inoculated three calves each with M. bovis or phosphate buffer saline (control) into the right stifle joint and dissected them at 15 days postinoculation. Mycoplasma bovis-inoculated calves exhibited swelling of the stifle joint, increases in synovial fluid, fibrin deposition, and cartilage thinning. Intracellular M. bovis was detected in synovial tissues analyzed by immunohistochemistry and transmission electron microscopy. Messenger RNA expressions of interleukin (IL)-1β, IL-6, IL-8, IL-12p40, and IL-17A in synovial fluid cells and synovial tissues from M. bovis-inoculated calves were significantly higher than those from control calves. Protein levels of these cytokines in synovial fluid from M. bovis-inoculated calves were markedly higher than those from control calves. Our study clarified that inoculation with M. bovis into the stifle joint induced the production of inflammatory cytokines by synovial fluid cells and synovial tissues, causing a severe inflammatory response in joints. Additionally, M. bovis could invade cells in synovial tissues, which may have aided it in evading antibiotics and host immune surveillance.

Bacterial motility: machinery and mechanisms

Bacteria have developed a large array of motility mechanisms to exploit available resources and environments. These mechanisms can be broadly classified into swimming in aqueous media and movement over solid surfaces. Swimming motility involves either the rotation of rigid helical filaments through the external medium or gyration of the cell body in response to the rotation of internal filaments. On surfaces, bacteria swarm collectively in a thin layer of fluid powered by the rotation of rigid helical filaments, they twitch by assembling and disassembling type IV pili, they glide by driving adhesins along tracks fixed to the cell surface and, finally, non-motile cells slide over surfaces in response to outward forces due to colony growth. Recent technological advances, especially in cryo-electron microscopy, have greatly improved our knowledge of the molecular machinery that powers the various forms of bacterial motility. In this Review, we describe the current understanding of the physical and molecular mechanisms that allow bacteria to move around.

Chimeric Antigen Receptor Design and Efficacy in Ovarian Cancer Treatment

Our increased understanding of tumour biology gained over the last few years has led to the development of targeted molecular therapies, e.g., vascular endothelial growth factor A (VEGF-A) antagonists, poly[ADP-ribose] polymerase 1 (PARP1) inhibitors in hereditary breast and ovarian cancer syndrome (BRCA1 and BRCA2 mutants), increasing survival and improving the quality of life. However, the majority of ovarian cancer (OC) patients still do not have access to targeted molecular therapies that would be capable of controlling their disease, especially resistant or relapsed. Chimeric antigen receptors (CARs) are recombinant receptor constructs located on T lymphocytes or other immune cells that change its specificity and functions. Therefore, in a search for a successful solid tumour therapy using CARs the specific cell surface antigens identification is crucial. Numerous in vitro and in vivo studies, as well as studies on humans, prove that targeting overexpressed molecules, such as mucin 16 (MUC16), annexin 2 (ANXA2), receptor tyrosine-protein kinase erbB-2 (HER2/neu) causes high tumour cells toxicity and decreased tumour burden. CARs are well tolerated, side effects are minimal and they inhibit disease progression. However, as OC is heterogenic in its nature with high mutation diversity and overexpression of different receptors, there is a need to consider an individual approach to treat this type of cancer. In this publication, we would like to present the history and status of therapies involving the CAR T cells in treatment of OC tumours, suggest potential T cell-intrinsic determinants of response and resistance as well as present extrinsic factors impacting the success of this approach.

Development and validation of a high-content screening assay for inhibitors of enteropathogenic E. coli adhesion

Enteropathogenic E. coli (EPEC) causes intestinal infections leading to severe diarrhea. EPEC attaches to the host cell causing lesions to the intestinal epithelium coupled with the effacement of microvilli. In the process, actin accumulates into a pedestal-like structure under bacterial microcolonies. We designed an automated fluorescence microscopy-based screening method for discovering compounds capable of inhibiting EPEC adhesion and virulence using aurodox, a type three secretion system (T3SS) inhibitor, as a positive control. The screening assay employs an EPEC strain (2348/69) expressing a fluorescent protein and actin staining for monitoring the bacteria and their pedestals respectively, analyzing these with a custom image analysis pipeline. The assay allows for the discovery of compounds capable of preventing the formation of pathogenic actin rearrangements. These compounds may be interfering with virulence-related molecular pathways relevant for developing antivirulence leads.

Mechanical competition triggered by innate immune signaling drives the collective extrusion of bacterially infected epithelial cells

Intracellular pathogens alter their host cells' mechanics to promote dissemination through tissues. Conversely, host cells may respond to the presence of pathogens by altering their mechanics to limit infection. Here, we monitored epithelial cell monolayers infected with intracellular bacterial pathogens, Listeria monocytogenes or Rickettsia parkeri, over days. Under conditions in which these pathogens trigger innate immune signaling through NF-κB and use actin-based motility to spread non-lytically intercellularly, we found that infected cell domains formed three-dimensional mounds. These mounds resulted from uninfected cells moving toward the infection site, collectively squeezing the softer and less contractile infected cells upward and ejecting them from the monolayer. Bacteria in mounds were less able to spread laterally in the monolayer, limiting the growth of the infection focus, while extruded infected cells underwent cell death. Thus, the coordinated forceful action of uninfected cells actively eliminates large domains of infected cells, consistent with this collective cell response representing an innate immunity-driven process.

Avian intestinal ultrastructure changes provide insight into the pathogenesis of enteric diseases and probiotic mode of action

Epithelial damage and loss of barrier integrity occur following intestinal infections in humans and animals. Gut health was evaluated by electron microscopy in an avian model that exposed birds to subclinical necrotic enteritis (NE) and fed them a diet supplemented with the probiotic Bacillus amyloliquefaciens strain H57 (H57). Scanning electron microscopy of ileal mucosa revealed significant villus damage, including focal erosions of epithelial cells and villous atrophy, while transmission electron microscopy demonstrated severe enterocyte damage and loss of cellular integrity in NE-exposed birds. In particular, mitochondria were morphologically altered, appearing irregular in shape or swollen, and containing electron-lucent regions of matrix and damaged cristae. Apical junctional complexes between adjacent enterocytes were significantly shorter, and the adherens junction was saccular, suggesting loss of epithelial integrity in NE birds. Segmented filamentous bacteria attached to villi, which play an important role in intestinal immunity, were more numerous in birds exposed to NE. The results suggest that mitochondrial damage may be an important initiator of NE pathogenesis, while H57 maintains epithelium and improves the integrity of intestinal mucosa. Potential actions of H57 are discussed that further define the mechanisms responsible for probiotic bacteria’s role in maintaining gut health.

During host cell traversal and cell-to-cell passage, Toxoplasma gondii sporozoites inhabit the parasitophorous vacuole and posteriorly release dense granule protein-associated membranous trails

Toxoplasma gondii has a worldwide distribution and infects virtually all warm-blooded animals, including humans. Ingestion of the environmentally resistant oocyst stage, excreted only in the feces of cats, is central to transmission of this apicomplexan parasite. There is vast literature on the host and T. gondii tachyzoite (proliferative stage of the parasite) but little is known of the host-parasite interaction and conversion of the free-living stage (sporozoite inside the oocyst) to the parasitic stage. Here, we present events that follow invasion of host cells with T. gondii sporozoites by using immunofluorescence (IF) and transmission electron microscopy (TEM). Several human type cell cultures were infected with T. gondii sporozoites of the two genotypes (Type II, ME49 and Type III, VEG) most prevalent worldwide. For the first known time, using anti-rhoptry neck protein 4 (RON4) antibodies, the moving junction was visualized in sporozoites during the invasion process and shortly after its completion. Surprisingly, IF and TEM evaluation revealed that intracellular sporozoites release, at their posterior end, long membranous tails, herein named sporozoite-specific trails (SSTs). Differential permeabilization and IF experiments showed that the SSTs are associated with several dense granule proteins (GRAs) and that their membranous component is of parasite origin. Furthermore, TEM observations demonstrated that SST-associated sporozoites are delimited by a typical parasitophorous vacuole, which is retained during parasite exit from the host cell and during cell-to-cell passage. Our data strongly suggest that host cell traversal by T. gondii sporozoites relies on a novel force-producing mechanism, based on the massive extrusion at the parasite posterior pole of GRA-associated membranous material derived from the same pool of membranes forming the intravacuolar network.

Tree of motility - A proposed history of motility systems in the tree of life

Motility often plays a decisive role in the survival of species. Five systems of motility have been studied in depth: those propelled by bacterial flagella, eukaryotic actin polymerization and the eukaryotic motor proteins myosin, kinesin and dynein. However, many organisms exhibit surprisingly diverse motilities, and advances in genomics, molecular biology and imaging have showed that those motilities have inherently independent mechanisms. This makes defining the breadth of motility nontrivial, because novel motilities may be driven by unknown mechanisms. Here, we classify the known motilities based on the unique classes of movement‐producing protein architectures. Based on this criterion, the current total of independent motility systems stands at 18 types. In this perspective, we discuss these modes of motility relative to the latest phylogenetic Tree of Life and propose a history of motility. During the ~4 billion years since the emergence of life, motility arose in Bacteria with flagella and pili, and in Archaea with archaella. Newer modes of motility became possible in Eukarya with changes to the cell envelope. Presence or absence of a peptidoglycan layer, the acquisition of robust membrane dynamics, the enlargement of cells and environmental opportunities likely provided the context for the (co)evolution of novel types of motility.

Crossing the kingdom border: Human diseases caused by plant pathogens

Interactions between pathogenic microorganisms and their hosts are varied and complex, encompassing open-field scale interactions to interactions at the molecular level. The capacity of plant pathogenic bacteria and fungi to cause diseases in human and animal systems was, until recently, considered of minor importance. However, recent evidence suggests that animal and human infections caused by plant pathogenic fungi, bacteria and viruses may have critical impacts on human and animal health and safety. This review analyses previous research on plant pathogens as causal factors of animal illness. In addition, a case study involving disruption of type III effector-mediated phagocytosis in a human cell line upon infection with an opportunistic phytopathogen, Pseudomonas syringae pv. tomato, is discussed. Further knowledge regarding the molecular interactions between plant pathogens and human and animal hosts is needed to understand the extent of disease incidence and determine mechanisms for disease prevention.

Human Melioidosis

The causative agent of melioidosis, Burkholderia pseudomallei, a tier 1 select agent, is endemic in Southeast Asia and northern Australia, with increased incidence associated with high levels of rainfall. Increasing reports of this condition have occurred worldwide, with estimates of up to 165,000 cases and 89,000 deaths per year. The ecological niche of the organism has yet to be clearly defined, although the organism is associated with soil and water. The culture of appropriate clinical material remains the mainstay of laboratory diagnosis. Identification is best done by phenotypic methods, although mass spectrometric methods have been described. Serology has a limited diagnostic role. Direct molecular and antigen detection methods have limited availability and sensitivity. Clinical presentations of melioidosis range from acute bacteremic pneumonia to disseminated visceral abscesses and localized infections. Transmission is by direct inoculation, inhalation, or ingestion. Risk factors for melioidosis include male sex, diabetes mellitus, alcohol abuse, and immunosuppression. The organism is well adapted to intracellular survival, with numerous virulence mechanisms. Immunity likely requires innate and adaptive responses. The principles of management of this condition are drainage and debridement of infected material and appropriate antimicrobial therapy. Global mortality rates vary between 9% and 70%. Research into vaccine development is ongoing.

Epidemiology of Nucleus-Dwelling Holospora: Infection, Transmission, Adaptation, and Interaction with Paramecium

The chapter describes the exceptional symbiotic associations formed between the ciliate Paramecium and Holospora, highly infectious bacteria residing in the host nuclei. Holospora and Holospora-like bacteria (Alphaproteobacteria) are characterized by their ability for vertical and horizontal transmission in host populations, a complex biphasic life cycle, and pronounced preference for host species and colonized cell compartment. These bacteria are obligate intracellular parasites; thus, their metabolic repertoire is dramatically reduced. Nevertheless, they perform complex interactions with the host ciliate. We review ongoing efforts to unravel the molecular adaptations of these bacteria to their unusual lifestyle and the host's employment in the symbiosis. Furthermore, we summarize current knowledge on the genetic and genomic background of Paramecium-Holospora symbiosis and provide insights into the ecological and evolutionary consequences of this interaction. The diversity and occurrence of symbioses between ciliates and Holospora-like bacteria in nature is discussed in connection with transmission modes of symbionts, host specificity and compatibility of the partners. We aim to summarize 50 years of research devoted to these symbiotic systems and conclude trying to predict some perspectives for further studies.

Edwardsiella induces microtubule-severing in host epithelial cells

Edwardsiella bacteria cause economic losses to a variety of commercially important fish globally. Human infections are rare and result in a gastroenteritis-like illness. Because these bacteria are evolutionarily related to other Enterobacteriaceae and the host cytoskeleton is a common target of enterics, we hypothesized that Edwardsiella may cause similar phenotypes. Here we use HeLa and Caco-2 infection models to show that microtubules are severed during the late infections. This microtubule alteration phenotype was not dependant on the type III or type VI secretion system (T3SS and T6SS) of the bacteria as ΔT3SS and ΔT6SS mutants of E. piscicida EIB202 and E. tarda ATCC15947 that lacks both also caused microtubule disassembly. Immunolocalization experiments showed the host katanin catalytic subunits A1 and A like 1 proteins at regions of microtubule severing, suggesting their involvement in the microtubule disassembly events. To identify bacterial components involved in this phenotype, we screened a 2,758 transposon library of E. piscicida EIB202 and found that 4 single mutations in the atpFHAGDC operon disrupted microtubule disassembly in HeLa cells. We then constructed three atp deletion mutants; they all could not disassemble host microtubules. This work provides the first clear evidence of host cytoskeletal alterations during Edwardsiella infections.

Type IV Pili Can Mediate Bacterial Motility within Epithelial Cells

Pseudomonas aeruginosa is among bacterial pathogens capable of twitching motility, a form of surface-associated movement dependent on type IV pili (T4P). Previously, we showed that T4P and twitching were required for P. aeruginosa to cause disease in a murine model of corneal infection, to traverse human corneal epithelial multilayers, and to efficiently exit invaded epithelial cells. Here, we used live wide-field fluorescent imaging combined with quantitative image analysis to explore how twitching contributes to epithelial cell egress. Results using time-lapse imaging of cells infected with wild-type PAO1 showed that cytoplasmic bacteria slowly disseminated throughout the cytosol at a median speed of >0.05 μm s-1 while dividing intracellularly. Similar results were obtained with flagellin (fliC) and flagellum assembly (flhA) mutants, thereby excluding swimming, swarming, and sliding as mechanisms. In contrast, pilA mutants (lacking T4P) and pilT mutants (twitching motility defective) appeared stationary and accumulated in expanding aggregates during intracellular division. Transmission electron microscopy confirmed that these mutants were not trapped within membrane-bound cytosolic compartments. For the wild type, dissemination in the cytosol was not prevented by the depolymerization of actin filaments using latrunculin A and/or the disruption of microtubules using nocodazole. Together, these findings illustrate a novel form of intracellular bacterial motility differing from previously described mechanisms in being directly driven by bacterial motility appendages (T4P) and not depending on polymerized host actin or microtubules.IMPORTANCE Host cell invasion can contribute to disease pathogenesis by the opportunistic pathogen Pseudomonas aeruginosa Previously, we showed that the type III secretion system (T3SS) of invasive P. aeruginosa strains modulates cell entry and subsequent escape from vacuolar trafficking to host lysosomes. However, we also showed that mutants lacking either type IV pili (T4P) or T4P-dependent twitching motility (i) were defective in traversing cell multilayers, (ii) caused less pathology in vivo, and (iii) had a reduced capacity to exit invaded cells. Here, we report that after vacuolar escape, intracellular P. aeruginosa can use T4P-dependent twitching motility to disseminate throughout the host cell cytoplasm. We further show that this strategy for intracellular dissemination does not depend on flagellin and resists both host actin and host microtubule disruption. This differs from mechanisms used by previously studied pathogens that utilize either host actin or microtubules for intracellular dissemination independently of microbe motility appendages.

[Mechanism of bacterial motility]

Bacteria, life living at microscale, can spread only by thermal fluctuation. However, the ability of directional movement, such as swimming by rotating flagella, gliding over surfaces via mobile cell-surface adhesins, and actin-dependent movement, could be useful for thriving through searching more favorable environments, and such motility is known to be related to pathogenicity. Among diverse migration mechanisms, perhaps flagella-dependent motility would be used by most species. The bacterial flagellum is a molecular nanomachine comprising a helical filament and a basal motor, which is fueled by an electrochemical gradient of cation across the cell membrane (ion motive force). Many species, such as Escherichia coli, possess flagella on the outside of the cell body, whereas flagella of spirochetes reside within the periplasmic space. Flagellar filaments or helical spirochete bodies rotate like a screw propeller, generating propulsive force. This review article describes the current knowledge of the structure and operation mechanism of the bacterial flagellum, and flagella-dependent motility in highly viscous environments.

Structural basis of Toxoplasma gondii perforin-like protein 1 membrane interaction and activity during egress

Intracellular pathogens must egress from the host cell to continue their infectious cycle. Apicomplexans are a phylum of intracellular protozoans that have evolved members of the membrane attack complex and perforin (MACPF) family of pore forming proteins to disrupt cellular membranes for traversing cells during tissue migration or egress from a replicative vacuole following intracellular reproduction. Previous work showed that the apicomplexan Toxoplasma gondii secretes a perforin-like protein (TgPLP1) that contains a C-terminal Domain (CTD) which is necessary for efficient parasite egress. However, the structural basis for CTD membrane binding and egress competency remained unknown. Here, we present evidence that TgPLP1 CTD prefers binding lipids that are abundant in the inner leaflet of the lipid bilayer. Additionally, solving the high-resolution crystal structure of the TgPLP1 APCβ domain within the CTD reveals an unusual double-layered β-prism fold that resembles only one other protein of known structure. Three direct repeat sequences comprise subdomains, with each constituting a wall of the β-prism fold. One subdomain features a protruding hydrophobic loop with an exposed tryptophan at its tip. Spectrophotometric measurements of intrinsic tryptophan fluorescence are consistent with insertion of the hydrophobic loop into a target membrane. Using CRISPR/Cas9 gene editing we show that parasite strains bearing mutations in the hydrophobic loop, including alanine substitution of the tip tryptophan, are equally deficient in egress as a strain lacking TgPLP1 altogether. Taken together our findings suggest a crucial role for the hydrophobic loop in anchoring TgPLP1 to the membrane to support its cytolytic activity and egress function.

The intracellular parasite Toxoplasma gondii infects many hosts including humans. Infected people with a weak immune system can suffer severe disease when the parasite replicates uncontrolled via repeated cycles of cell invasion, intracellular growth, and exit, resulting in cell death. Previous studies showed that T. gondii encodes a pore-forming protein, TgPLP1, which contains an unusual domain that is crucial for efficient exit from both the parasite containing vacuole and the host cell. However, how TgPLP1 recognizes and binds to the appropriate membrane is unclear. Here we use a combination of biochemistry, structural biology, and parasitology to identify a preference of TgPLP1 for specific lipids and show that a loop within the structure of the membrane-binding domain inserts into the target membrane and is necessary for exit from the parasite containing vacuole. Our study sheds light into the determinants of membrane binding in TgPLP1 and may inform the overall mechanism of pore formation in similar systems.

Pseudomonas syringae evades phagocytosis by animal cells via type III effector-mediated regulation of actin filament plasticity

Certain animal and plant pathogenic bacteria have developed virulence factors including effector proteins that enable them to overcome host immunity. A plant pathogen, Pseudomonas syringae pv. tomato (Pto) secretes a large repertoire of effectors via a type III secretory apparatus, thereby suppressing plant immunity. Here, we show that Pto causes sepsis in mice. Surprisingly, the effector HopQ1 disrupted animal phagocytosis by inhibiting actin rearrangement via direct interaction with the LIM domain of the animal target protein LIM kinase, a key regulator of actin polymerization. The results provide novel insight into animal host-plant pathogen interactions. In addition, the current study firstly demonstrates that certain plant pathogenic bacteria such as Pto evade phagocytosis by animal cells due to cross-kingdom suppression of host immunity.

Pathways of host cell exit by intracellular pathogens

Host cell exit is a critical step in the life-cycle of intracellular pathogens, intimately linked to barrier penetration, tissue dissemination, inflammation, and pathogen transmission. Like cell invasion and intracellular survival, host cell exit represents a well-regulated program that has evolved during host-pathogen co-evolution and that relies on the dynamic and intricate interplay between multiple host and microbial factors. Three distinct pathways of host cell exit have been identified that are employed by three different taxa of intracellular pathogens, bacteria, fungi and protozoa, namely (i) the initiation of programmed cell death, (ii) the active breaching of host cellderived membranes, and (iii) the induced membrane-dependent exit without host cell lysis. Strikingly, an increasing number of studies show that the majority of intracellular pathogens utilize more than one of these strategies, dependent on life-cycle stage, environmental factors and/or host cell type. This review summarizes the diverse exit strategies of intracellular-living bacterial, fungal and protozoan pathogens and discusses the convergently evolved commonalities as well as system-specific variations thereof. Key microbial molecules involved in host cell exit are highlighted and discussed as potential targets for future interventional approaches.

From hitchhiker to hijacker: pathogen exploitation of endosomal phosphoinositides 1

Signalling through phosphoinositide lipids is essential for regulating many cellular processes, including endosomal trafficking. A number of intracellular pathogens have found ways to subvert host trafficking pathways via exploitation of endosomal phosphoinositides. This review will discuss how pathogens such as bacteria, viruses, and eukaryotic parasites depend on endosomal phosphoinositides for infection as well as the mechanisms through which some are able to actively manipulate these signalling lipids to facilitate invasion, survival, replication, and immune evasion.

Wnt5A Signaling Promotes Defense Against Bacterial Pathogens by Activating a Host Autophagy Circuit

Bacterial pathogens are associated with severe infections (e.g., sepsis) and exacerbation of debilitating conditions such as chronic obstructive pulmonary disease (COPD). The interactions of bacterial pathogens with macrophages, a key component of innate immunity and host defense, are not clearly understood and continue to be intensively studied. Having previously demonstrated a role of Wnt5A signaling in phagocytosis, we proceeded to decipher the connection of Wnt5A signaling with infection by pathogenic bacteria, namely Pseudomonas aeruginosa (PA) and Streptococcus pneumoniae (SP), which are related with the progression of COPD and sepsis. We found that during the initial hours of infection with PA and SP, there is decrease in the steady state levels of the Wnt5A protein in macrophages. Suppression of Wnt5A signaling, moreover, impairs macrophage clearance of the bacterial infection both in vitro and in vivo. Activation of Wnt5A signaling, on the other hand, enhances clearance of the infection. Macrophage-mediated containment of bacterial infection in our study is dependant on Wnt5A-induced Rac1/Disheveled activation and cytochalasin D inhibitable actin assembly, which is associated with ULK1 kinase activity and LC3BII accumulation. Our experimental findings are consistent with Wnt5A signaling-dependent induction of autophagic killing (xenophagy) of PA and SP, as further substantiated by transmission electron microscopy. Overall, our study unveils the prevalence of a Wnt5A-Rac1-Disheveled-mediated actin-associated autophagy circuit as an important component of innate immunity in host macrophages that may turn out crucial for restricting infection by leading bacterial pathogens.

Loss of Actin-Based Motility Impairs Ectromelia Virus Release In Vitro but Is Not Critical to Spread In Vivo

Ectromelia virus (ECTV) is an orthopoxvirus and the causative agent of mousepox. Like other poxviruses such as variola virus (agent of smallpox), monkeypox virus and vaccinia virus (the live vaccine for smallpox), ECTV promotes actin-nucleation at the surface of infected cells during virus release. Homologs of the viral protein A36 mediate this function through phosphorylation of one or two tyrosine residues that ultimately recruit the cellular Arp2/3 actin-nucleating complex. A36 also functions in the intracellular trafficking of virus mediated by kinesin-1. Here, we describe the generation of a recombinant ECTV that is specifically disrupted in actin-based motility allowing us to examine the role of this transport step in vivo for the first time. We show that actin-based motility has a critical role in promoting the release of virus from infected cells in vitro but plays a minor role in virus spread in vivo. It is likely that loss of microtubule-dependent transport is a major factor for the attenuation observed when A36R is deleted.

Cellular Exit Strategies of Intracellular Bacteria

The coevolution of intracellular bacteria with their eukaryotic hosts has presented these pathogens with numerous challenges for their evolutionary progress and survival. Chief among these is the ability to exit from host cells, an event that is fundamentally linked to pathogen dissemination and transmission. Recent years have witnessed a major expansion of research in this area, and this chapter summarizes our current understanding of the spectrum of exit strategies that are exploited by intracellular pathogens. Clear themes regarding the mechanisms of microbial exit have emerged and are most easily conceptualized as (i) lysis of the host cell, (ii) nonlytic exit of free bacteria, and (iii) release of microorganisms into membrane-encased compartments. The adaptation of particular exit strategies is closely linked with additional themes in microbial pathogenesis, including host cell death, manipulation of host signaling pathways, and coincident activation of proinflammatory responses. This chapter will explore the molecular determinants used by intracellular pathogens to promote host cell escape and the infectious advantages each exit pathway may confer, and it will provide an evolutionary framework for the adaptation of these mechanisms.

Bacterial effector NleL promotes enterohemorrhagic E. coli-induced attaching and effacing lesions by ubiquitylating and inactivating JNK

As a major diarrheagenic human pathogen, enterohemorrhagic Escherichia coli (EHEC) produce attaching and effacing (A/E) lesions, characterized by the formation of actin pedestals, on mammalian cells. A bacterial T3SS effector NleL from EHEC O157:H7 was recently shown to be a HECT-like E3 ligase in vitro, but its biological functions and host targets remain elusive. Here, we report that NleL is required to effectively promote EHEC-induced A/E lesions and bacterial infection. Furthermore, human c-Jun NH2-terminal kinases (JNKs) were identified as primary substrates of NleL. NleL-induced JNK ubiquitylation, particularly mono-ubiquitylation at the Lys 68 residue of JNK, impairs JNK’s interaction with an upstream kinase MKK7, thus disrupting JNK phosphorylation and activation. This subsequently suppresses the transcriptional activity of activator protein-1 (AP-1), which modulates the formation of the EHEC-induced actin pedestals. Moreover, JNK knockdown or inhibition in host cells complements NleL deficiency in EHEC infection. Thus, we demonstrate that the effector protein NleL enhances the ability of EHEC to infect host cells by targeting host JNK, and elucidate an inhibitory role of ubiquitylation in regulating JNK phosphorylation.

Enterohemorrhagic Escherichia coli (EHEC) can cause attaching and effacing (A/E) lesions to form in the colons of animals and humans, contributing to severe bacterial infection. NleL, an E3 ubiquitin ligase from EHEC O157:H7 is one of the bacterial type III secretion effectors that may be involved in the regulation of A/E lesions. However, NleL’s exact host targets and the detailed mechanistic actions are still unclear. Here, we report that the effector protein NleL effectively promotes EHEC-induced A/E lesions and bacterial infection by targeting the host JNK protein. Specifically, we find that NleL-mediated JNK ubiquitylation abolishes phosphorylation and activation of host JNK, subsequently suppressing the host JNK/AP-1 signaling pathway to favor the formation of EHEC-mediated actin pedestals on the surface of mammalian cells. Collectively, our work has not only discovered the A/E lesion-promoting function of NleL during EHEC infection, but also revealed a novel regulatory mechanism of host JNK protein.

Increased Neurotropic Threat from Burkholderia pseudomallei Strains with a B. mallei-like Variation in the bimA Motility Gene, Australia

These strains have heightened pathogenic potential for rapid dissemination to multiple tissues, including the central nervous system.

Neurologic melioidosis is a serious, potentially fatal form of Burkholderia pseudomallei infection. Recently, we reported that a subset of clinical isolates of B. pseudomallei from Australia have heightened virulence and potential for dissemination to the central nervous system. In this study, we demonstrate that this subset has a B. mallei–like sequence variation of the actin-based motility gene, bimA. Compared with B. pseudomallei isolates having typical bimA alleles, isolates that contain the B. mallei–like variation demonstrate increased persistence in phagocytic cells and increased virulence with rapid systemic dissemination and replication within multiple tissues, including the brain and spinal cord, in an experimental model. These findings highlight the implications of bimA variation on disease progression of B. pseudomallei infection and have considerable clinical and public health implications with respect to the degree of neurotropic threat posed to human health.

Pediatric melioidosis in Sarawak, Malaysia: Epidemiological, clinical and microbiological characteristics

Background

Melioidosis is a serious, and potentially fatal community-acquired infection endemic to northern Australia and Southeast Asia, including Sarawak, Malaysia. The disease, caused by the usually intrinsically aminoglycoside-resistant Burkholderia pseudomallei, most commonly affects adults with predisposing risk factors. There are limited data on pediatric melioidosis in Sarawak.

Methods

A part prospective, part retrospective study of children aged <15 years with culture-confirmed melioidosis was conducted in the 3 major public hospitals in Central Sarawak between 2009 and 2014. We examined epidemiological, clinical and microbiological characteristics.

Findings

Forty-two patients were recruited during the 6-year study period. The overall annual incidence was estimated to be 4.1 per 100,000 children <15 years, with marked variation between districts. No children had pre-existing medical conditions. Twenty-three (55%) had disseminated disease, 10 (43%) of whom died. The commonest site of infection was the lungs, which occurred in 21 (50%) children. Other important sites of infection included lymph nodes, spleen, joints and lacrimal glands. Seven (17%) children had bacteremia with no overt focus of infection. Delays in diagnosis and in melioidosis-appropriate antibiotic treatment were observed in nearly 90% of children. Of the clinical isolates tested, 35/36 (97%) were susceptible to gentamicin. Of these, all 11 isolates that were genotyped were of a single multi-locus sequence type, ST881, and possessed the putative B. pseudomallei virulence determinants bimA_Bp, fhaB3, and the YLF gene cluster.

Conclusions

Central Sarawak has a very high incidence of pediatric melioidosis, caused predominantly by gentamicin-susceptible B. pseudomallei strains. Children frequently presented with disseminated disease and had an alarmingly high death rate, despite the absence of any apparent predisposing risk factor.

Melioidosis is a serious, and often fatal community-acquired infection endemic to Southeast Asia and northern Australia. It is caused by the environmental saprophyte Burkholderia pseudomallei, a bacterium that is intrinsically resistant to many commonly used antibiotics. Its presence in Sarawak, Malaysian Borneo, has been documented, and recently, a novel gentamicin-susceptible strain discovered. However, there are limited data on the burden and clinical characteristics of melioidosis in Sarawak, both in adults and in children. In this study, we comprehensively investigated all pediatric melioidosis cases in Central Sarawak over a 6-year period. We found that this region has a very high incidence of pediatric melioidosis, and that children frequently presented with disseminated disease and had high fatality rates, despite the absence of any predisposing risk factor. We confirmed that these infections were caused predominantly by gentamicin-susceptible B. pseudomallei strains. We also highlighted other undescribed epidemiological, clinical and microbiological features, which may help in the overall understanding of B. pseudomallei infections. We emphasized the importance of improving the awareness and recognition of melioidosis in children, both in Sarawak and in other endemic regions.

Explicit Modeling of siRNA-Dependent On- and Off-Target Repression Improves the Interpretation of Screening Results

RNAi is broadly used to map gene regulatory networks, but the identification of genes that are responsible for the observed phenotypes is challenging, as small interfering RNAs (siRNAs) simultaneously downregulate the intended on targets and many partially complementary off targets. Additionally, the scarcity of publicly available control datasets hinders the development and comparative evaluation of computational methods for analyzing the data. Here, we introduce PheLiM (https://github.com/andreariba/PheLiM), a method that uses predictions of siRNA on- and off-target downregulation to infer gene-specific contributions to phenotypes. To assess the performance of PheLiM, we carried out siRNA- and CRISPR/Cas9-based genome-wide screening of two well-characterized pathways, bone morphogenetic protein (BMP) and nuclear factor κB (NF-κB), and we reanalyzed publicly available siRNA screens. We demonstrate that PheLiM has the overall highest accuracy and most reproducible results compared to other available methods. PheLiM can accommodate various methods for predicting siRNA off targets and is broadly applicable to the identification of genes underlying complex phenotypes.

Identification of Candidate Host Cell Factors Required for Actin-Based Motility of Burkholderia pseudomallei

Intracellular actin-based motility of the melioidosis pathogen Burkholderia pseudomallei requires the bacterial factor BimA. Located at one pole of the bacterium, BimA recruits and polymerizes cellular actin to promote bacterial motility within and between cells. Here, we describe an affinity approach coupled with mass spectrometry to identify cellular proteins recruited to BimA-expressing bacteria under conditions that promote actin polymerization. We identified a group of cellular proteins that are recruited to the B. pseudomallei surface in a BimA-dependent manner, a subset of which were independently validated with specific antisera including the ubiquitous scaffold protein Ras GTPase-activating-like protein (IQGAP1). IQGAP1 integrates several key cellular signaling pathways including those involved in actin dynamics and has been shown to be involved in the adhesion of attaching and effacing Escherichia coli to infected cells and invasion of host cells by Salmonella enterica serovar Typhimurium. Although a direct interaction between BimA and IQGAP1 could not be detected using either conventional pulldown or yeast two hybrid techniques, confocal microscopy revealed that IQGAP1 is recruited to B. pseudomallei actin tails in infected cells, and siRNA-mediated knockdown highlighted a role for this protein in controlling the length and actin density of B. pseudomallei actin tails.

Viruses That Exploit Actin-Based Motility for Their Replication and Spread

The actin cytoskeleton is a crucial part of the eukaryotic cell. Viruses depend on host cells for their replication, and, as a result, many have developed ways of manipulating the actin network to promote their spread. This chapter reviews the various ways in which viruses utilize the actin cytoskeleton at discrete steps in their life cycle, from entry into the host cell, replication, and assembly of new progeny to virus release. Various actin inhibitors that function in different ways to affect proper actin dynamics can be used to parse the role of actin at these steps.

Lack of intracellular replication of M. tuberculosis and M. bovis BCG caused by delivering bacilli to lysosomes in murine brain microvascular endothelial cells

Invasion and traversal of the blood-brain barrier (BBB) by Mycobacterium tuberculosis cause meningeal tuberculosis (TB) in the central nervous system (CNS). Meningeal TB is a serious, often fatal disease that disproportionately affects young children. The mechanisms involved in CNS invasion by M. tuberculosis bacilli are poorly understood. In this study, we microscopically examined endosomal trafficking and measured survival of M. tuberculosis and M. bovis Bacille Calmette-Guérin (BCG) bacilli in murine brain microvascular endothelial cells (BMECs). The results show that both species internalize but do not replicate in BMECs in the absence of a cytotoxic response. Confocal microscopy indicates that bacilli-containing vacuoles are associated with the early endosomal marker, Rab5, late endosomal marker, Rab7, and lysosomal marker, LAMP2, suggesting that bacilli-containing endosomes mature into endolysosomes in BMECs. Our data also show that a subset of intracellular M. tuberculosis, but not BCG bacilli, escape into the cytoplasm to avoid rapid lysosomal killing. However, the intracellular mycobacteria examined cannot spread cell-to-cell in BMECs. Taken together, these data show that with the exception of the small terminal cytoplasmic population of bacilli, M. tuberculosis does not modulate intracellular trafficking in BMECs as occurs in macrophages and lung epithelial and endothelial cells.

Manipulation of host membranes by the bacterial pathogens Listeria, Francisella, Shigella and Yersinia

Bacterial pathogens display an impressive arsenal of molecular mechanisms that allow survival in diverse host niches. Subversion of plasma membrane and cytoskeletal functions are common themes associated to infection by both extracellular and intracellular pathogens. Moreover, intracellular pathogens modify the structure/stability of their membrane-bound compartments and escape degradation from phagocytic or autophagic pathways. Here, we review the manipulation of host membranes by Listeria monocytogenes, Francisella tularensis, Shigella flexneri and Yersinia spp. These four bacterial model pathogens exemplify generalized strategies as well as specific features observed during bacterial infection processes.

Morphological plasticity of bacteria-Open questions

Morphological plasticity of bacteria is a cryptic phenomenon, by which bacteria acquire adaptive benefits for coping with changing environments. Some environmental cues were identified to induce morphological plasticity, but the underlying molecular mechanisms remain largely unknown. Physical and chemical factors causing morphological changes in bacteria have been investigated and mostly associated with potential pathways linked to the cell wall synthetic machinery. These include starvation, oxidative stresses, predation effectors, antimicrobial agents, temperature stresses, osmotic shock, and mechanical constraints. In an extreme scenario of morphological plasticity, bacteria can be induced to be shapeshifters when the cell walls are defective or deficient. They follow distinct developmental pathways and transform into assorted morphological variants, and most of them would eventually revert to typical cell morphology. It is suggested that phenotypic heterogeneity might play a functional role in the development of morphological diversity and/or plasticity within an isogenic population. Accordingly, phenotypic heterogeneity and inherited morphological plasticity are found to be survival strategies adopted by bacteria in response to environmental stresses. Here, microfluidic and nanofabrication technology is considered to provide versatile solutions to induce morphological plasticity, sort and isolate morphological variants, and perform single-cell analysis including transcriptional and epigenetic profiling. Questions such as how morphogenesis network is modulated or rewired (if epigenetic controls of cell morphogenesis apply) to induce bacterial morphological plasticity could be resolved with the aid of micro-nanofluidic platforms and optimization algorithms, such as feedback system control.

Implications of Spatiotemporal Regulation of Shigella flexneri Type Three Secretion Activity on Effector Functions: Think Globally, Act Locally

Shigella spp. are Gram-negative bacterial pathogens that infect human colonic epithelia and cause bacterial dysentery. These bacteria express multiple copies of a syringe-like protein complex, the Type Three Secretion apparatus (T3SA), which is instrumental in the etiology of the disease. The T3SA triggers the plasma membrane (PM) engulfment of the bacteria by host cells during the initial entry process. It then enables bacteria to escape the resulting phagocytic-like vacuole. Freed bacteria form actin comets to move in the cytoplasm, which provokes bacterial collision with the inner leaflet of the PM. This phenomenon culminates in T3SA-dependent secondary uptake and vacuolar rupture in neighboring cells in a process akin to what is observed during entry and named cell-to-cell spread. The activity of the T3SA of Shigella flexneri was recently demonstrated to display an on/off regulation during the infection. While the T3SA is active when bacteria are in contact with PM-derived compartments, it switches to an inactive state when bacteria are released within the cytosol. These observations indicate that effector proteins transiting through the T3SA are therefore translocated in a highly time and space constrained fashion, likely impacting on their cellular distribution. Herein, we present what is currently known about the composition, the assembly and the regulation of the T3SA activity and discuss the consequences of the on/off regulation of T3SA on Shigella effector properties and functions during the infection. Specific examples that will be developed include the role of effectors IcsB and VirA in the escape from LC3/ATG8-positive vacuoles formed during cell-to-cell spread and of IpaJ protease activity against N-miristoylated proteins. The conservation of a similar regulation of T3SA activity in other pathogens such as Salmonella or Enteropathogenic Escherichia coli will also be briefly discussed.

Common Themes in Cytoskeletal Remodeling by Intracellular Bacterial Effectors

Bacterial pathogens interact with various types of tissues to promote infection. Because it controls the formation of membrane extensions, adhesive processes, or the junction integrity, the actin cytoskeleton is a key target of pathogens during infection. We will highlight common and specific functions of the actin cytoskeleton during bacterial infections, by first reviewing the mechanisms of intracellular motility of invasive Shigella, Listeria, and Rickettsia. Through the models of EPEC/EHEC, Shigella, Salmonella, and Chlamydia spp., we will illustrate various strategies of diversion of actin cytoskeletal processes used by these bacteria to colonize or breach epithelial/endothelial barriers.

Integration of linear and dendritic actin nucleation in Nck-induced actin comets

The role of the Nck adaptor protein in balancing linear versus branched actin nucleation in comet tails is evaluated. Nck recruits both linear and branched nucleation-promoting factors, both of which are necessary for the formation of actin comets. The findings highlight a novel role for Nck in pathogen-like actin motility.

The Nck adaptor protein recruits cytosolic effectors such as N-WASP that induce localized actin polymerization. Experimental aggregation of Nck SH3 domains at the membrane induces actin comet tails—dynamic, elongated filamentous actin structures similar to those that drive the movement of microbial pathogens such as vaccinia virus. Here we show that experimental manipulation of the balance between unbranched/branched nucleation altered the morphology and dynamics of Nck-induced actin comets. Inhibition of linear, formin-based nucleation with the small-molecule inhibitor SMIFH2 or overexpression of the formin FH1 domain resulted in formation of predominantly circular-shaped actin structures with low mobility (actin blobs). These results indicate that formin-based linear actin polymerization is critical for the formation and maintenance of Nck-dependent actin comet tails. Consistent with this, aggregation of an exclusively branched nucleation-promoting factor (the VCA domain of N-WASP), with density and turnover similar to those of N-WASP in Nck comets, did not reconstitute dynamic, elongated actin comets. Furthermore, enhancement of branched Arp2/3-mediated nucleation by N-WASP overexpression caused loss of the typical actin comet tail shape induced by Nck aggregation. Thus the ratio of linear to dendritic nucleation activity may serve to distinguish the properties of actin structures induced by various viral and bacterial pathogens.

Mechanisms of Disease: Host-Pathogen Interactions between Burkholderia Species and Lung Epithelial Cells

Members of the Burkholderia species can cause a range of severe, often fatal, respiratory diseases. A variety of in vitro models of infection have been developed in an attempt to elucidate the mechanism by which Burkholderia spp. gain entry to and interact with the body. The majority of studies have tended to focus on the interaction of bacteria with phagocytic cells with a paucity of information available with regard to the lung epithelium. However, the lung epithelium is becoming more widely recognized as an important player in innate immunity and the early response to infections. Here we review the complex relationship between Burkholderia species and epithelial cells with an emphasis on the most pathogenic species, Burkholderia pseudomallei and Burkholderia mallei. The current gaps in knowledge in our understanding are highlighted along with the epithelial host-pathogen interactions that offer potential opportunities for therapeutic intervention.