The role of glyceraldehyde 3-phosphate dehydrogenase (GapA-1) in <Emphasis Type="Italic">Neisseria meningitidis</Emphasis> adherence to human cells

Background  

Glyceraldehyde 3-phosphate dehydrogenases (GAPDHs) are cytoplasmic glycolytic enzymes, which although lacking identifiable secretion signals, have also been found localized to the surface of several bacteria (and some eukaryotic organisms); where in some cases they have been shown to contribute to the colonization and invasion of host tissues. Neisseria meningitidis is an obligate human nasopharyngeal commensal which can cause life-threatening infections including septicaemia and meningitis. N. meningitidis has two genes, gapA-1 and gapA-2, encoding GAPDH enzymes. GapA-1 has previously been shown to be up-regulated on bacterial contact with host epithelial cells and is accessible to antibodies on the surface of capsule-permeabilized meningococcal cells. The aims of this study were: 1) to determine whether GapA-1 was expressed across different strains of N. meningitidis; 2) to determine whether GapA-1 surface accessibility to antibodies was dependant on the presence of capsule; 3) to determine whether GapA-1 can influence the interaction of meningococci and host cells, particularly in the key stages of adhesion and invasion.     

Ultrastructure at the Host-Parasite Interface of Phytophthora capsici in Roots and Stems of Capsicum annuum

The infecting hyphae of Phytophthora capsici grew intercellularly in infected tissues of roots and stems of pepper (Capsicum annuum). The vascular tissues were not markedly disorganized even when heavily infected. Intercellularly growing hyphae penetrated the host cells by forming haustorium-like bodies. The consistent features of ultrastructural changes in infected tissues of pepper roots and stems were degeneration of cell organelles and dissolution of host cell walls. The cytoplasm detached from the cell wall aggregated abundantly around some haustorium-like bodies or the penetration sites of fungal hyphae. The host cell walls were palely stained, thinned and swollen, possibly being biochemically altered by the action of fungal macerating enzymes. Electron-dense, wall-like material was apposed on the outer wall of xylem vessel contacted by fungal hyphae. The infecting hyphae were also surrounded by granular, dark-staining cytoplasm. Characteristics of host cell responses to the invading P. capsici were the deposition of papilla-like material on host cell walls next to hyphae and the encasement of haustorium-like bodies with wall appositions.     

Settlement of the diazotrophic,phytoeffective bacterial strain Pantoea agglomerans on and within winter wheat: An investigation using ELISA and transmission electron microscopy

The aim of this study was to investigate the ability of Pantoea agglomerans, a plant growth-promoting bacterium, to colonize various regions and tissues of the wheat plant (Triticum aestivum L.) by using different inoculation methods and inoculum concentrations. In addition, the enzyme-linked immunosorbent assay (ELISA) and transmission electron microscopy (TEM) were used to determine: (a) the ability of the bacterial cells to grow and survive both on the surface and within internal tissue of the plant and (b) the response of the plant to bacterial infection. After inoculation, cells of the diazotrophic bacterial strain P. agglomerans were found to be located in roots, stems and leaves. Colony development of bacterial cells was only detected within intercellular spaces of the root and on the root surface. However, single bacterial cells were observed in leaves and stems on the surface of the epidermis, in the vicinity to stomatal cells, within intercellular spaces of the mesophyll and within xylem vessels. Inoculated bacterial cells were found to be able to enter host tissues, to multiply in the plant and to maintain a delicate relationship between endophyte and host. The density of bacterial settlement in the plant in all experiments was about 10⁶ to 10⁷ cells per mL root or shoot sap. Establishment was confirmed by a low coefficient of variation of ELISA means at these concentrations.     

Ultrastructural and cytochemical studies on the infection of wheat spikes by<Emphasis Type="Italic">Fusarium culmorum</Emphasis> as well as on degradation of cell wall components and localization of mycotoxins in the host tissue

The infection process and pathway of spreading ofFusarium culmorum in wheat spikes was examined by means of light, scanning and transmission electron microscopy after spray inoculation and single spikelet inoculation. Macroconidia of the pathogen germinated on the host surfaces, however, hyphal development and penetration of host tissues normally occurred on the inner surfaces of the lemma, glume and palea as well as on the ovary. The pathogen spread downward to the rachilla and rachis node by inter- and intracellular growth from the glume, lemma, palea and ovary. The pathogen extended in the rachis in upward and downward direction by inter- and intracellular growth inside and outside of the vascular bundles of the rachis. The spreading of the hyphae in the host tissues was associated with pronounced alterations including disintegration and digestion of host cell walls, suggesting production of cell wall degrading enzymes during infection and spreading in the host tissues. Immunogold labelling studies revealed that accumulation ofFusarium toxins in infected wheat spike tissue showed a close relationship to pathological changes in the host cells, symptom appearance and pathogen colonisation of the host tissue.Fusarium toxins may play an important role in wheat head blight development.     

Characterization of CagI in the Cag Pathogenicity Island of <Emphasis Type="Italic">Helicobacter pylori</Emphasis>

Helicobacter pylori is a highly successful human-specific gastric pathogen that infects up to 50% of the world’s population. Virulent H. pylori isolates harbor the cytotoxin-associated genes pathogenicity island (cag-PAI), which encodes a type IV secretion system that translocates bacterial effector (e.g., CagA oncoprotein) molecules into host cells. Although some cag-PAI genes are shown to be required for CagA delivery or localization, the majority have no known function. In the current study, the authors performed a cell components fractionation assay and showed that CagI, one of the cag-PAI proteins located in the bacterial membrane, was not translocated into host cells. The homologous recombination method then was used to construct the isogenic mutant of H. pylori cagI, and the translocation assay was performed. The results showed that the isogenic mutant of H. pylori NCTC 11637 cagI could cause a reduction in the degree of CagA translocation. Overall, the results suggested that CagI might be an accessory component of the CagA secretion system not translocated into host cells and that it is located in the bacterial membrane.     

A dual role for proline iminopeptidase in the regulation of bacterial motility and host immunity

During plant–pathogen interactions, pathogenic bacteria have evolved multiple strategies to cope with the sophisticated defence systems of host plants. Proline iminopeptidase (PIP) is essential to Xanthomonas campestris pv. campestris (Xcc) virulence, and is conserved in many plant‐associated bacteria, but its pathogenic mechanism remains unclear. In this study, we found that disruption of pip in Xcc enhanced its flagella‐mediated bacterial motility by decreasing intracellular bis‐(3′,5′)‐cyclic dimeric guanosine monophosphate (c‐di‐GMP) levels, whereas overexpression of pip in Xcc restricted its bacterial motility by elevating c‐di‐GMP levels. We also found that PIP is a type III secretion system‐dependent effector capable of eliciting a hypersensitive response in non‐host, but not host plants. When we transformed pip into the host plant Arabidopsis, higher bacterial titres were observed in pip‐overexpressing plants relative to wild‐type plants after Xcc inoculation. The repressive function of PIP on plant immunity was dependent on PIP's enzymatic activity and acted through interference with the salicylic acid (SA) biosynthetic and regulatory genes. Thus, PIP simultaneously regulates two distinct regulatory networks during plant–microbe interactions, i.e. it affects intracellular c‐di‐GMP levels to coordinate bacterial behaviour, such as motility, and functions as a type III effector translocated into plant cells to suppress plant immunity. Both processes provide bacteria with the regulatory potential to rapidly adapt to complex environments, to utilize limited resources for growth and survival in a cost‐efficient manner and to improve the chances of bacterial survival by helping pathogens to inhabit the internal tissues of host plants.     

Role of host GTPases in infection by Listeria monocytogenes

The bacterial pathogen Listeria monocytogenes induces internalization into mammalian cells and uses actin‐based motility to spread within tissues. Listeria accomplishes this intracellular life cycle by exploiting or antagonizing several host GTPases. Internalization into human cells is mediated by the bacterial surface proteins InlA or InlB. These two modes of uptake each require a host actin polymerization pathway comprised of the GTPase Rac1, nucleation promotion factors, and the Arp2/3 complex. In addition to Rac1, InlB‐mediated internalization involves inhibition of the GTPase Arf6 and participation of Dynamin and septin family GTPases. After uptake, Listeria is encased in host phagosomes. The bacterial protein GAPDH inactivates the human GTPase Rab5, thereby delaying phagosomal acquisition of antimicrobial properties. After bacterial‐induced destruction of the phagosome, cytosolic Listeria uses the surface protein ActA to stimulate actin‐based motility. The GTPase Dynamin 2 reduces the density of microtubules that would otherwise limit bacterial movement. Cell‐to‐cell spread results when motile Listeria remodel the host plasma membrane into protrusions that are engulfed by neighbouring cells. The human GTPase Cdc42, its activator Tuba, and its effector N‐WASP form a complex with the potential to restrict Listeria protrusions. Bacteria overcome this restriction through two microbial factors that inhibit Cdc42‐GTP or Tuba/N‐WASP interaction.     

Host tissues as microhabitats for Wolbachia and quantitative insights into the bacterial community in terrestrial isopods

Animal–bacterial symbioses are highly dynamic in terms of multipartite interactions, both between the host and its symbionts as well as between the different bacteria constituting the symbiotic community. These interactions will be reflected by the titres of the individual bacterial taxa, for example via host regulation of bacterial loads or competition for resources between symbionts. Moreover, different host tissues represent heterogeneous microhabitats for bacteria, meaning that host‐associated bacteria might establish tissue‐specific bacterial communities. Wolbachia are widespread endosymbiotic bacteria, infecting a large number of arthropods and filarial nematodes. However, relatively little is known regarding direct interactions between Wolbachia and other bacteria. This study represents the first quantitative investigation of tissue‐specific Wolbachia–microbiota interactions in the terrestrial isopod Armadillidium vulgare. To this end, we obtained a more complete picture of the Wolbachia distribution patterns across all major host tissues, integrating all three feminizing Wolbachia strains (wVulM, wVulC, wVulP) identified to date in this host. Interestingly, the different Wolbachia strains exhibited strain‐specific tissue distribution patterns, with wVulM reaching lower titres in most tissues. These patterns were consistent across different host genetic backgrounds and might reflect different co‐evolutionary histories between the Wolbachia strains and A. vulgare. Moreover, Wolbachia‐infected females carried higher total bacterial loads in several, but not all, tissues, irrespective of the Wolbachia strain. Taken together, this quantitative approach indicates that Wolbachia is part of a potentially more diverse bacterial community, as exemplified by the presence of highly abundant bacterial taxa in the midgut caeca of several A. vulgare populations.     

丁香假单胞菌中的III型分泌系统及其调控机制研究进展

丁香假单胞菌(Pseudomonas syringae)是引起许多作物病害的一种革兰氏阴性病原细菌。该细菌入侵寄主植物细胞主要通过其III型分泌系统(type III secretion system,T3SS)将效应蛋白转入到寄主真核细胞内,抑制寄主免疫功能,以达到成功侵染和定殖的目的。III型分泌系统的主调控因子RhpR/S通过感受环境信号的变化直接调控hrpR/S及其他毒力相关通路。同时III型分泌系统基因的表达也受到其他调控因子的影响,包括σ因子HrpL、双组分系统GacA/S、Lon蛋白酶、第二信使分子和环境信号等。本文在简要介绍丁香假单胞菌III型分泌系统组成和功能的基础上,综述丁香假单胞菌III型分泌系统调控机制的最新研究进展,以期为深入探究病原菌的致病机制提供参考和思路。     

Induction of apoptosis by cooperative bacteria in the morphogenesis of host epithelial tissues

 Associations with pathogenic bacteria have recently been shown to initiate apoptotic programs in the cells of their animal hosts, where host cell death is hypothesized to be a response of the immune system, either initiated as a mechanism of host defense or bacterial offense. In this study, we present evidence that bacterial initiation of apoptosis is neither restricted to pathogenesis nor to the initation of an immune response. In the cooperative association between the sepiolid squid Euprymna scolopes and the luminous bacterium Vibrio fischeri, the bacteria induce a dramatic morphogenesis of the host tissues during the first few days of interaction between these partners. The most striking change is the bacteria-triggered loss of an extensive superficial epithelium that potentiates the infection process. Our analyses of these tissues revealed that the bacteria induce apoptosis in the cells that comprise this epithelium within hours of the interaction with bacteria. Ultrastructural analysis revealed that after 24 h the integrity of the epithelium had been lost, i.e., the basement membrane had degenerated and the majority of the cells exhibited signs of apoptosis, most notably chromatin condensation. Analysis of these tissues with probes that reveal intracellular acidification showed that the cells first undergo an initial acidification beginning about 6–8 h after exposure to V. fischeri. As determined by end-labeling of DNA fragments, extensive endonuclease activity was detected at approximately 16–20 h post-infection. These data provide evidence that cooperative bacteria can participate in the remodeling of host tissues through the induction of host apoptotic programs. Received: 10 November 1997 / Accepted: 22 April 1998