Caenorhabditis elegans as a model for intracellular pathogen infection

The genetically tractable nematode Caenorhabditis elegans is a convenient host for studies of pathogen infection. With the recent identification of two types of natural intracellular pathogens of C. elegans, this host now provides the opportunity to examine interactions and defence against intracellular pathogens in a whole‐animal model for infection. C. elegans is the natural host for a genus of microsporidia, which comprise a phylum of fungal‐related pathogens of widespread importance for agriculture and medicine. More recently, C. elegans has been shown to be a natural host for viruses related to the Nodaviridae family. Both microsporidian and viral pathogens infect the C. elegans intestine, which is composed of cells that share striking similarities to human intestinal epithelial cells. Because C. elegans nematodes are transparent, these infections provide a unique opportunity to visualize differentiated intestinal cells in vivo during the course of intracellular infection. Together, these two natural pathogens of C. elegans provide powerful systems in which to study microbial pathogenesis and host responses to intracellular infection.     

Here,there, and everywhere: How pathogenic Escherichia coli sense and respond to gastrointestinal biogeography

Gastrointestinal (GI) pathogens enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC), and related mouse pathogen Citrobacter rodentium, are referred to as attaching and effacing (AE) pathogens for the lesions they form upon colonisation of the host epithelium. EPEC, EHEC, and C. rodentium are well known to use a type III secretion system to intimately attach to intestinal cells and secrete bacterial effectors to manipulate host cell processes. Less well known is the ability of AE pathogens to overcome significant physiological and microbial barriers and target specific gut niches for initial colonisation of the host epithelium. This review considers recent work highlighting the biogeography of the GI tract as it applies to colonisation by enteric pathogens, including environmental barriers to enteric infection, signals sensed by AE pathogens for navigation of the GI tract, and the tools AE pathogens use to respond to the changing host environment.     

Variation in resistance to multiple pathogen species: anther smuts of Silene uniflora

The occurrence of multiple pathogen species on a shared host species is unexpected when they exploit the same micro‐niche within the host individual. One explanation for such observations is the presence of pathogen‐specific resistances segregating within the host population into sites that are differentially occupied by the competing pathogens. This study used experimental inoculations to test whether specific resistances may contribute to the maintenance of two species of anther‐smut fungi, Microbotryum silenes‐inflatae and Microbotryum lagerheimii, in natural populations of Silene uniflora in England and Wales. Overall, resistance to the two pathogens was strongly positively correlated among host populations and to a lesser degree among host families within populations. A few instances of specific resistance were also observed and confirmed by replicated inoculations. The results suggest that selection for resistance to one pathogen may protect the host from the emergence via host shifts of related pathogen species, and conversely that co‐occurrence of two species of pathogens may be dependent on the presence of host genotypes susceptible to both.     

Infection experiments reveal broad host ranges of Eurychasma dicksonii (Oomycota) and Chytridium polysiphoniae (Chytridiomycota), two eukaryotic parasites in marine brown algae (Phaeophyceae)

Unialgal cultures of the brown alga Pylaiella littoralis (L.) Kjellman infected by either Eurychasma dicksonii (Wright) Magnus (Oomycota) or Chytridium polysiphoniae (Cohn) H. E. Petersen (Chytridiomycota) were used to elaborate the host ranges of these pathogens. Infection experiments with 48 host species covering 13 orders of the Phaeophyceae showed that 45 species were susceptible to attack by Eurychasma and 23 to Chytridium. The two pathogens showed host-specific differences in generation times: while in Pylaiella the shortest cycles were 16 days for Eurychasma and 6 days for Chytridium, one and five days more, respectively, were required in Acinetospora. Heavy parasite attack on the microscopic stages of host species with heteromorphic life histories, like kelps (Laminariales), is documented and discussed as a potential regulatory factor for the population dynamics of macroalgae.     

Sympatry and interference of divergent Microbotryum pathogen species

The impact of infectious diseases in natural ecosystems is strongly influenced by the degree of pathogen specialization and by the local assemblies of potential host species. This study investigated anther‐smut disease, caused by fungi in the genus Microbotryum, among natural populations of plants in the Caryophyllaceae. A broad geographic survey focused on sites of the disease on multiple host species in sympatry. Analysis of molecular identities for the pathogens revealed that sympatric disease was most often due to co‐occurrence of distinct, host‐specific anther‐smut fungi, rather than localized cross‐species disease transmission. Flowers from sympatric populations showed that the Microbotryum spores were frequently moved between host species. Experimental inoculations to simulate cross‐species exposure to the pathogens in these plant communities showed that the anther‐smut pathogen was less able to cause disease on its regular host when following exposure of the plants to incompatible pathogens from another host species. These results indicate that multi‐host/multi‐pathogen communities are common in this system and they involve a previously hidden mechanism of interference between Microbotryum fungi, which likely affects both pathogen and host distributions.     

Moonlighting activity of the epigenetic machinery restrains infection

Intracellular bacterial pathogens have evolved a range of mechanisms, including manipulation of the host cell epigenetic machinery and host cell gene expression rewiring, to parasitize and thrive inside host phagocytes. A new study in The EMBO Journal (Yaseen et al, 2018 ) reports that, conversely, host macrophages can use epigenetic modulators to modify the cell surface of invading pathogens and counteract infection. This study opens new avenues to better understand host–pathogen interactions and to develop novel, more effective antimicrobial strategies.     

Regulation of Caenorhabditis elegans and Pseudomonas aeruginosa machinery during interactions

The amenability of Caenorhabditis elegans against pathogen provides a valuable tool for studying host–pathogen interactions. Physiological experiments revealed that the P. aeruginosa was able to kill C. elegans efficiently. The effects of P. aeruginosa PA14, PAO1 and their isolated lipopolysaccharide (LPS) on the host system were analyzed. The LPS at higher concentrations (≥2 mg/ml) was toxic to the host animals. Kinetic studies using qPCR revealed the regulation of host-specific candidate antimicrobial genes during pathogen-mediated infections. In addition, the pathogen-specific virulent gene, exoT expression, was anlyzed and found to be varied during the interactions with the host system. Ability of the pathogens to modify their internal machinery in the presence of the host was analyzed by XRD, FTIR and PCA. LPS isolated from pathogens upon exposure to C. elegans showed modifications at their functional regions. LPS from PAO1 showed difference in d-spacing angle (Å) and °2Th position. FTIR spectra revealed alterations in polysaccharide (1,200–900 cm⁻¹) and fatty acid (3,000–2,800 cm⁻¹) regions of LPS from P. aeruginosa PAO1 exposed to the host system. These data provide additional insights on how the pathogens subvert its own and host machinery during interactions.     

The focal complex of epithelial cells provides a signalling platform for interleukin‐8 induction in response to bacterial pathogens

Bacterial pathogens can induce an inflammatory response from epithelial tissues due to secretion of the pro‐inflammatory chemokine interleukin‐8 (IL‐8). Many bacterial pathogens manipulate components of the focal complex (FC) to induce signalling events in host cells. We examined the interaction of several bacterial pathogens with host cells, including Campylobacter jejuni, to determine if the FC is required for induction of chemokine signalling in response to bacterial pathogens. Our data indicate that secretion of IL‐8 is triggered by C. jejuni, Helicobacter pylori and Salmonella enterica serovar Typhimurium in response to engagement of β₁ integrins. Additionally, we found that the secretion of IL‐8 from C. jejuni infected epithelial cells requires FAK, Src and paxillin, which in turn are necessary for Erk 1/2 recruitment and activation. Targeting the FC component paxillin with siRNA prevented IL‐8 secretion from cells infected with several bacterial pathogens, including C. jejuni, Helicobacter pylori, Salmonella enterica serovar Typhimurium, Staphylococcus aureus, Pseudomonas aeruginosa, and Vibrio parahaemolyticus. Our findings indicate that maximal IL‐8 secretion from epithelial cells in response to bacterial infection is dependent on the FC. Based on the commonality of the host response to bacterial pathogens, we propose that the FC is a signalling platform for an epithelial cell response to pathogenic organisms.     

Manipulation of the host cell death pathway by Shigella

Host cells deploy multiple defences against microbial infection. One prominent host defence mechanism, the death of infected cells, plays a pivotal role in clearing damaged cells, eliminating pathogens, removing replicative niches, exposing intracellular bacterial pathogens to extracellular immune surveillance and presenting bacteria‐derived antigens to the adaptive immune system. Although cell death can occur under either physiological or pathophysiological conditions, it acts as an innate defence mechanism against bacterial pathogens by limiting their persistent colonization. However, many bacterial pathogens, including Shigella, have evolved mechanisms that manipulate host cell death for their own benefit.     

Landscape and oceanic barriers shape dispersal and population structure in the island nematode Pristionchus pacificus

In this study evolutionary host plant patterns at ranks from order to species were analysed using spatial evolutionary and ecological vicariance analysis (SEEVA), based on a multigene phylogeny of 45 ascomycete fungal species. The objective was to understand speciation events and host associations in Ophiognomonia (Gnomoniaceae). Species of this genus are perithecial fungi that occur as endophytes, pathogens, and latent saprobes on plants in the families of Betulaceae, Fagaceae, Juglandaceae, Lauraceae, Malvaceae, Platanaceae, Rosaceae, Salicaceae, and Sapindaceae. A second objective was to determine whether speciation events are influenced by host conservatism, host specialization, or host switching at different taxonomic host ranks. Host differences between sister clades were interpreted using the divergence index (D) from the SEEVA analysis, ranging from 0 for no divergence to 1 for maximum possible divergence. Several fungal subclades showed clear patterns of host order/family conservatism (D = 1.00) for hosts in Betulaceae, Fagaceae, Juglandaceae, and Rosaceae. Clear trends of host specialization at host genus and species ranks (D = 1.00) were suggested within these host families. Independent host jumps were observed for two species at the family rank and three at the order rank. As a result of this study, host specificity and specialization is hypothesized as a mechanism that can strongly contribute to speciation patterns in fungal pathogens. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 1–16.     

Resistance and Susceptibility of Plants to Fungal Pathogens

Plants are under continuous threat of infection by pathogens endowed with diverse strategies to colonize their host. Comprehensive biochemical and genetic approaches are now starting to reveal the complex signaling pathways that mediate plant disease resistance. Initiation of defense signaling often involves specific recognition of invading pathogens by the products of specialized host resistance (R) genes. Potential resistance signaling components have been identified by mutational analyses to be required for specific resistance in the model Arabidopsis and some crop species. Strikingly, many of the components share similarity to that of innate immune systems in animals. Evidence is also accumulating that plant pathogens have a number of ways to evade host defenses during the early stages of infection, similar to animal pathogens. These strategies are becoming much better understood in a number of plant–pathogen interactions. In this review, we focus on the current knowledge of host factors that control plant resistance and susceptibility to fungal pathogens. The knowledge accumulated in these studies will serve a fundamental basis for combating diseases in strategic molecular agriculture.     

The recycling endosome and bacterial pathogens

Bacterial pathogens have developed a wide range of strategies to survive within human cells. A number of pathogens multiply in a vacuolar compartment, whereas others can rupture the vacuole and replicate in the host cytosol. A common theme among many bacterial pathogens is the use of specialised secretion systems to deliver effector proteins into the host cell. These effectors can manipulate the host's membrane trafficking pathways to remodel the vacuole into a replication‐permissive niche and prevent degradation. As master regulators of eukaryotic membrane traffic, Rab GTPases are principal targets of bacterial effectors. This review highlights the manipulation of Rab GTPases that regulate host recycling endocytosis by several bacterial pathogens, including Chlamydia pneumoniae, Chlamydia trachomatis, Shigella flexneri, Salmonella enterica serovar Typhimurium, Uropathogenic Escherichia coli, and Legionella pneumophila. Recycling endocytosis plays key roles in a variety of cellular aspects such as nutrient uptake, immunity, cell division, migration, and adhesion. Though much remains to be understood about the molecular basis and the biological relevance of bacterial pathogens exploiting Rab GTPases, current knowledge supports the notion that endocytic recycling Rab GTPases are differentially targeted to avoid degradation and support bacterial replication. Thus, future studies of the interactions between bacterial pathogens and host endocytic recycling pathways are poised to deepen our understanding of bacterial survival strategies.     

Hsp90‐dependent regulatory circuitry controlling temperature‐dependent fungal development and virulence

The pathogenic fungi Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans are an increasing cause of human mortality, especially in immunocompromised populations. During colonization and adaptation to various host environments, these fungi undergo morphogenetic alterations that allow for survival within the host. One key environmental cue driving morphological changes is external temperature. The Hsp90 chaperone protein provides one mechanism to link temperature with the signalling cascades that regulate morphogenesis, fungal development and virulence. Candida albicans is a model system for understanding the connections between morphogenesis and Hsp90. Due to the high degree of conservation in Hsp90, many of the connections in C. albicans may be extrapolated to other fungal pathogens or parasites. Examining the role of Hsp90 during development and morphogenesis in these three major fungal pathogens may provide insight into key aspects of adaptation to the host, leading to additional avenues for therapy.     

Host preferences of Palaearctic Culicoides biting midges: implications for transmission of orbiviruses

Feeding success depends on host availability, host defensive reactions and host preferences. Host choice is a critical determinant of the intensity at which pathogens are transmitted. The aim of the current study was to describe host preferences of Palaearctic Culicoides species (Diptera: Ceratopogonidae) Latreille using traps baited with the five different host species of poultry, horse, cattle, sheep and goat. Collections were carried out nightly in July and August 2009 in western France with three replicates of a 5 × 5 randomized Latin square (five sites, five hosts). Moreover, an ultraviolet (UV) light/suction trap was operated during host‐baited collections to correlate Culicoides biting rates and UV light/suction trap catches. A total of 660 Culicoides belonging to 12 species, but comprised mainly of Culicoides scoticus Downes and Kettle, Culicoides dewulfi Goetghebuer and Culicoides obsoletus Meigen, were collected on animal baits. Abundance was highest for the horse, which accounted for 95% of all Culicoides caught, representing 10 species. The horse, the largest bait, was the most attractive host, even when abundance data were corrected by weight, body surface or Kleiber's scaling factor. Culicoides obsoletus was the only dominant species attracted by birds. Both C. scoticus and C. dewulfi were collected mainly from the upper body of the horse. Finally, the quantification of host preferences allows for discussion of implications for the transmission of Culicoides‐borne pathogens such as bluetongue virus.     

Individual and combined effects of multiple pathogens on Pacific treefrogs

In nature, individual hosts often encounter multiple pathogens simultaneously, which can lead to additive, antagonistic, or synergistic effects on hosts. Synergistic effects on infection prevalence or severity could greatly affect host populations. However, ecologists and managers often overlook the influence of pathogen combinations on hosts. This is especially true in amphibian conservation, even though multiple pathogens coexist within amphibian populations, and several pathogens have been implicated in amphibian population declines and extinctions. Using an amphibian host, Pseudacris regilla (Pacific treefrog), we experimentally investigated interactive effects among three pathogens: the trematode Ribeiroia sp. (hereafter, Ribeiroia), the fungus Batrachochytrium dendrobatidis (hereafter, BD), and the water mold Achlya flagellata. We detected no effects of A. flagellata, but did find effects of Ribeiroia and BD that varied depending on context. Low doses of Ribeiroia caused relatively few malformations, while higher Ribeiroia doses caused numerous deformities dominated by missing and reduced limbs and limb elements. Exposure to low doses of BD accelerated larval host development, despite there being no detectable BD infections, while exposure to higher BD doses caused infection but did not alter developmental rate. Hosts exposed to both Ribeiroia and BD exhibited the highest mortality, although overall evidence of interactive effects of multiple pathogens was limited. We suggest further research on the influence of multi-pathogen assemblages on amphibians, particularly under a variety of ecological conditions and with a wider diversity of hosts and pathogens.     

布鲁氏菌逃逸宿主的抗感染免疫机制

布鲁氏菌病是由布鲁氏菌引发的世界范围的人兽共患传染病。布鲁氏菌为兼性胞内寄生菌,无典型的毒力因子,但却有很强的致病性,常引发人和动物的慢性感染。逃逸宿主的抗感染免疫反应是慢性感染的先决条件,这种能力对于布鲁氏菌的毒力来说似乎也越来越关键。作为成功的致病性病原菌,布鲁氏菌采用"隐秘的"策略以逃避或抑制固有免疫、调节适应性免疫,从而在宿主细胞内建立长期的持续性感染。本文将围绕布鲁氏菌逃逸宿主的抗感染免疫的分子机制进行阐述,旨为阐明布鲁氏菌毒力的新见解,这很可能为布病的预防开辟新的途径。     

Role of the ESCRT‐III complex in controlling integrity of the Salmonella‐containing vacuole

Intracellular pathogens need to establish specialised niches for survival and proliferation in host cells. The enteropathogen Salmonella enterica accomplishes this by extensive reorganisation of the host endosomal system deploying the SPI2‐encoded type III secretion system (SPI2‐T3SS). Fusion events of endosomal compartments with the Salmonella‐containing vacuole (SCV) form elaborate membrane networks within host cells enabling intracellular nutrition. However, which host compartments exactly are involved in this process and how the integrity of Salmonella‐modified membranes is accomplished are not fully resolved. An RNA interference knockdown screen of host factors involved in cellular logistics identified the ESCRT (endosomal sorting complex required for transport) system as important for proper formation and integrity of the SCV in infected epithelial cells. We demonstrate that subunits of the ESCRT‐III complex are specifically recruited to the SCV and membrane network. To investigate the role of ESCRT‐III for the intracellular lifestyle of Salmonella, a CHMP3 knockout cell line was generated. Infected CHMP3 knockout cells formed amorphous, bulky SCV. Salmonella within these amorphous SCV were in contact with host cell cytosol, and the attenuation of an SPI2‐T3SS‐deficient mutant strain was partially abrogated. ESCRT‐dependent endolysosomal repair mechanisms have recently been described for other intracellular pathogens, and we hypothesise that minor damages of the SCV during bacterial proliferation are repaired by the action of ESCRT‐III recruitment in Salmonella‐infected host cells.     

In Vitro Selection for Improved Plant Resistance to Toxin-Producing Pathogens

Simultaneously with the progress in plant biotechnology since the 1980s, new methods in plant pathology have been developed. This review summarizes papers that cover basic research on the effects of selective agents on in vitro cultures of host plants, as well as applications of agents in regeneration systems that result in lines with increased variability in resistance or susceptibility. The first part of the study deals with theoretical aspects of the interactions between plants and toxin‐producing pathogens, mode of phytotoxic action, and host‐ and non‐host‐selective toxins. The second part lists and describes various agents used for selections in vitro. In the last two decades more than 100 publications focused on these selections for the improvement of resistance to plant pathogens. Over 30 plant species were examined to utilise various selection agents extracted from about 40 plant pathogens. The review covers basic research studies and methods that elucidate the relationships between in vitro and in vivo mechanisms of resistance, but also try to develop practical applications to obtain resistant breeding lines. Such methods often utilise some type of explant cultures of the host plants that are treated with various selective agents (culture filtrates, toxins, elicitors), which then elicit typical reactions that parallel those by the pathogens. Their application successfully resulted in resistant lines in banana, carnation, grapevine, strawberry and wheat. Nowadays, these techniques are an important complement to classical breeding methods.