Identification of a Gal/GalNAc lectin in the protozoan Hartmannella vermiformis as a potential receptor for attachment and invasion by the Legionnaires' disease bacterium

Molecular regulation of virulence in Legionella pneumophila

Legionella pneumophila is a gram-negative bacteria found in natural and anthropogenic aquatic environments such as evaporative cooling towers, where it reproduces as an intracellular parasite of cohabiting protozoa. If L. pneumophila is aerosolized and inhaled by a susceptible person, bacteria may colonize their alveolar macrophages causing the opportunistic pneumonia Legionnaires' disease. L. pneumophila utilizes an elaborate regulatory network to control virulence processes such as the Dot/Icm Type IV secretion system and effector repertoire, responding to changing nutritional cues as their host becomes depleted. The bacteria subsequently differentiate to a transmissive state that can survive in the environment until a replacement host is encountered and colonized. In this review, we discuss the lifecycle of L. pneumophila and the molecular regulatory network that senses nutritional depletion via the stringent response, a link to stationary phase-like metabolic changes via alternative sigma factors, and two-component systems that are homologous to stress sensors in other pathogens, to regulate differentiation between the intracellular replicative phase and more transmissible states. Together, we highlight how this prototypic intracellular pathogen offers enormous potential in understanding how molecular mechanisms enable intracellular parasitism and pathogenicity.

Revisiting Drug Development Against the Neglected Tropical Disease, Amebiasis

Amebiasis is a neglected tropical disease which is caused by the protozoan parasite Entamoeba histolytica. This disease is one of the leading causes of diarrhea globally, affecting largely impoverished residents in developing countries. Amebiasis also remains one of the top causes of gastrointestinal diseases in returning international travellers. Despite having many side effects, metronidazole remains the drug of choice as an amebicidal tissue-active agent. However, emergence of metronidazole resistance in pathogens having similar anaerobic metabolism and also in laboratory strains of E. histolytica has necessitated the identification and development of new drug targets and therapeutic strategies against the parasite. Recent research in the field of amebiasis has led to a better understanding of the parasite’s metabolic and cellular pathways and hence has been useful in identifying new drug targets. On the other hand, new molecules effective against amebiasis have been mined by modifying available compounds, thereby increasing their potency and efficacy and also by repurposing existing approved drugs. This review aims at compiling and examining up to date information on promising drug targets and drug molecules for the treatment of amebiasis.

Evolution of the Arsenal of Legionella pneumophila Effectors To Modulate Protist Hosts

Within the human host, Legionella pneumophila replicates within alveolar macrophages, leading to pneumonia. However, L. pneumophila is an aquatic generalist pathogen that replicates within a wide variety of protist hosts, including amoebozoa, percolozoa, and ciliophora. The intracellular lifestyles of L. pneumophila within the two evolutionarily distant hosts macrophages and protists are remarkably similar. Coevolution with numerous protist hosts has shaped plasticity of the genome of L. pneumophila, which harbors numerous proteins encoded by genes acquired from primitive eukaryotic hosts through interkingdom horizontal gene transfer. The Dot/Icm type IVb translocation system translocates ∼6,000 effectors among Legionella species and >320 effector proteins in L. pneumophila into host cells to modulate a plethora of cellular processes to create proliferative niches. Since many of the effectors have likely evolved to modulate cellular processes of primitive eukaryotic hosts, it is not surprising that most of the effectors do not contribute to intracellular growth within human macrophages. Some of the effectors may modulate highly conserved eukaryotic processes, while others may target protist-specific processes that are absent in mammals. The lack of studies to determine the role of the effectors in adaptation of L. pneumophila to various protists has hampered the progress to determine the function of most of these effectors, which are routinely studied in mouse or human macrophages. Since many protists restrict L. pneumophila, utilization of such hosts can also be instrumental in deciphering the mechanisms of failure of L. pneumophila to overcome restriction of certain protist hosts. Here, we review the interaction of L. pneumophila with its permissive and restrictive protist environmental hosts and outline the accomplishments as well as gaps in our knowledge of L. pneumophila-protist host interaction and L. pneumophila's evolution to become a human pathogen.

Phagocytosis: what's on the menu? 1

Phagocytosis is an evolutionarily conserved process. In Protozoa, phagocytosis fulfills a feeding mechanism, while in Metazoa, phagocytosis diversified to play multiple organismal roles, including immune defence, tissue homeostasis, and remodeling. Accordingly, phagocytes display a high level of plasticity in their capacity to recognize, engulf, and process targets that differ in composition and morphology. Here, we review how phagocytosis adapts to its multiple roles and discuss in particular the effect of target morphology in phagocytic uptake and phagosome maturation.

From Many Hosts, One Accidental Pathogen: The Diverse Protozoan Hosts of Legionella

The 1976 outbreak of Legionnaires' disease led to the discovery of the intracellular bacterial pathogen Legionella pneumophila. Given their impact on human health, Legionella species and the mechanisms responsible for their replication within host cells are often studied in alveolar macrophages, the primary human cell type associated with disease. Despite the potential severity of individual cases of disease, Legionella are not spread from person-to-person. Thus, from the pathogen's perspective, interactions with human cells are accidents of time and space—evolutionary dead ends with no impact on Legionella's long-term survival or pathogenic trajectory. To understand Legionella as a pathogen is to understand its interaction with its natural hosts: the polyphyletic protozoa, a group of unicellular eukaryotes with a staggering amount of evolutionary diversity. While much remains to be understood about these enigmatic hosts, we summarize the current state of knowledge concerning Legionella's natural host range, the diversity of Legionella-protozoa interactions, the factors influencing these interactions, the importance of avoiding the generalization of protozoan-bacterial interactions based on a limited number of model hosts and the central role of protozoa to the biology, evolution, and persistence of Legionella in the environment.

Ingestion and digestion studies in Tetrahymena pyriformis based on chemically modified microparticles

Recognition of food and, in consequence, ingestion of digestible particles is a prerequisite for energy metabolism in Tetrahymena pyriformis. Understanding why some particles are ingested and digested, whereas others are not, is important for many fields of research, e.g. survival of pathogens in single-celled organisms or establishment of endosymbiotic relationships. We offered T. pyriformis synthetical bovine-serum-albumin (BSA)-methacrylate microparticles of approximately 5.5 μm diameter and studied the ciliates' ingestion and digestion behaviour. Different staining techniques as well as co-feeding with a transformant strain of Escherichia coli revealed that T. pyriformis considers these particles as natural food source and shows no feeding preference. Further, they are ingested at normal rates and may serve as sole food source. A pivotal advantage of these particles is the convenient modification of their surface by binding different ligands resulting in defined surface properties. Ingestion rate of modified microparticles either increased (additional BSA, enzymes) or decreased (amino acids). Furthermore, we investigated glycosylation patterns by lectin binding. By binding different substances to the surface in combination with various staining techniques, we provide a versatile experimental tool for elucidating details on food recognition and digestion that may allow to study evading digestion by pathogens or potential endosymbionts, too.

Mechanisms associated with phagocytosis of Arcobacter butzleri by Acanthamoeba castellanii

Acanthamoeba castellanii is a free-living amoeba widely found in environmental matrices such as soil and water. Arcobacter butzleri is an emerging potential zoonotic pathogen that can be isolated from environmental water sources, where they can establish endosymbiotic relationships with amoebas. The aim of this study was to describe the implication of mannose-binding proteins and membrane-associated receptors of glucose and galactose present in the amoebic membrane, during the attachment of Arcobacter butzleri by blocking with different saccharides. Another objective was to describe the signaling pathways involved in phagocytosis of these bacteria using specific inhibitors and analyze the implication of phagolysosome formation on the survival of Arcobacter butzleri inside the amoeba. We infer that the attachment of Arcobacter butzleri to the amoeba is a process which involves the participation of mannose-binding proteins and membrane-associated receptors of glucose and galactose present in the amoeba. We also demonstrated an active role of protozoan actin polymerization in the phagocytosis of Arcobacter butzleri and a critical involvement of PI3K and RhoA pathways. Further, we demonstrated that the tyrosine kinase-induced actin polymerization signal is essential in Acanthamoeba-mediated bacterial uptake. Through phagolysosomal formation analysis, we conclude that the survival of Arcobacter butzleri inside the amoeba could be related with the ability to remain inside vacuoles not fused with lysosomes, or with the ability to retard the fusion between these structures. All these results help the understanding of the bacterial uptake mechanisms used by Acanthamoeba castellanii and contribute to evidence of the survival mechanisms of Arcobacter butzleri.

Cellular microbiology and molecular ecology of Legionella-amoeba interaction

Legionella pneumophila is an aquatic organism that interacts with amoebae and ciliated protozoa as the natural hosts, and this interaction plays a central role in bacterial ecology and infectivity. Upon transmission to humans, L. pneumophila infect and replicate within alveolar macrophages causing pneumonia. Intracellular proliferation of L. pneumophila within the two evolutionarily distant hosts is facilitated by bacterial exploitation of evolutionarily conserved host processes that are targeted by bacterial protein effectors injected into the host cell by the Dot/Icm type VIB translocation system. Although cysteine is semi-essential for humans and essential for amoeba, it is a metabolically favorable source of carbon and energy generation by L. pneumophila. To counteract host limitation of cysteine, L. pneumophila utilizes the AnkB Dot/Icm-translocated F-box effector to promote host proteasomal degradation of polyubiquitinated proteins within amoebae and human cells. Evidence indicates ankB and other Dot/Icm-translocated effector genes have been acquired through inter-kingdom horizontal gene transfer.

From amoeba to macrophages: exploring the molecular mechanisms of Legionella pneumophila infection in both hosts

Legionella pneumophila is a Gram-negative bacterium and the causative agent of Legionnaires' disease. It replicates within amoeba and infects accidentally human macrophages. Several similarities are seen in the L. pneumophila-infection cycle in both hosts, suggesting that the tools necessary for macrophage infection may have evolved during co-evolution of L. pneumophila and amoeba. The establishment of the Legionella-containing vacuole (LCV) within the host cytoplasm requires the remodeling of the LCV surface and the hijacking of vesicles and organelles. Then L. pneumophila replicates in a safe intracellular niche in amoeba and macrophages. In this review we will summarize the existing knowledge of the L. pneumophila infection cycle in both hosts at the molecular level and compare the factors involved within amoeba and macrophages. This knowledge will be discussed in the light of recent findings from the Acanthamoeba castellanii genome analyses suggesting the existence of a primitive immune-like system in amoeba.

Host signal transduction and protein kinases implicated in Legionella infection

Modulation of the phosphorylation status of proteins by both kinases and phosphatases plays an important role in cellular signal transduction. Challenge of host cells by Legionella pneumophila manipulates the phosphorylation state of multiple host factors. These changes play roles in bacterial uptake, vacuole modification, cellular survival, and the immune response. In addition to modification by host cell kinases in response to the bacterium, L. pneumophila translocates bacterial kinases into the host cell that may contribute to further signaling modifications. Proper regulation of host cell signaling by L. pneumophila is necessary for its ability to replicate intracellulary, while avoiding host defenses.

Interaction of Escherichia coli K1 and K5 with Acanthamoeba castellanii trophozoites and cysts

The existence of symbiotic relationships between Acanthamoeba and a variety of bacteria is well-documented. However, the ability of Acanthamoeba interacting with host bacterial pathogens has gained particular attention. Here, to understand the interactions of Escherichia coli K1 and E. coli K5 strains with Acanthamoeba castellanii trophozoites and cysts, association assay, invasion assay, survival assay, and the measurement of bacterial numbers from cysts were performed, and nonpathogenic E. coli K12 was also applied. The association ratio of E. coli K1 with A. castellanii was 4.3 cfu per amoeba for 1 hr but E. coli K5 with A. castellanii was 1 cfu per amoeba for 1 hr. By invasion and survival assays, E. coli K5 was recovered less than E. coli K1 but still alive inside A. castellanii. E. coli K1 and K5 survived and multiplied intracellularly in A. castellanii. The survival assay was performed under a favourable condition for 22 hr and 43 hr with the encystment of A. castellanii. Under the favourable condition for the transformation of trophozoites into cysts, E. coli K5 multiplied significantly. Moreover, the pathogenic potential of E. coli K1 from A. castellanii cysts exhibited no changes as compared with E. coli K1 from A. castellanii trophozoites. E. coli K5 was multiplied in A. castellanii trophozoites and survived in A. castellanii cysts. Therefore, this study suggests that E. coli K5 can use A. castellanii as a reservoir host or a vector for the bacterial transmission.

Modulation of host cell function by Legionella pneumophila type IV effectors

Macrophages and protozoa ingest bacteria by phagocytosis and destroy these microbes using a conserved pathway that mediates fusion of the phagosome with lysosomes. To survive within phagocytic host cells, bacterial pathogens have evolved a variety of strategies to avoid fusion with lysosomes. A virulence strategy used by the intracellular pathogen Legionella pneumophila is to manipulate host cellular processes using bacterial proteins that are delivered into the cytosolic compartment of the host cell by a specialized secretion system called Dot/Icm. The proteins delivered by the Dot/Icm system target host factors that play evolutionarily conserved roles in controlling membrane transport in eukaryotic cells, which enables L. pneumophila to create an endoplasmic reticulum-like vacuole that supports intracellular replication in both protozoan and mammalian host cells. This review focuses on intracellular trafficking of L. pneumophila and describes how bacterial proteins contribute to modulation of host processes required for survival within host cells.

Importance of type II secretion for survival of Legionella pneumophila in tap water and in amoebae at low temperatures

Legionella pneumophila type II secretion mutants showed reduced survival in both tap water at 4 to 17 degrees C and aquatic amoebae at 22 to 25 degrees C. Wild-type supernatants stimulated the growth of these mutants, indicating that secreted factors promote low-temperature survival. There was a correlation between low-temperature survival and secretion function when 12 additional Legionella species were examined.

Pelagicoccus croceus sp. nov., a novel marine member of the family Puniceicoccaceae within the phylum 'Verrucomicrobia' isolated from seagrass

An obligately aerobic, spherical, non-motile, pale-yellow pigmented bacterium was isolated from a piece of leaf of seagrass, Enhalus acoroides (L.f.) Royle, grown in Okinawa, Japan and was subjected to a polyphasic taxonomic study. Phylogenetic analyses based on 16S rRNA gene sequences revealed that the novel isolate N5FB36-5(T) shared approximately 96-98 % sequence similarity with the species of the genus Pelagicoccus of the family Puniceicoccaceae within the phylum 'Verrucomicrobia'. The DNA-DNA relatedness values of strain N5FB36-5(T) with Pelagicoccus mobilis 02PA-Ca-133(T) and Pelagicoccus albus YM14-201(T) were below 70 %, which is accepted as the phylogenetic definition of a novel species. beta-Lactam antibiotic susceptibility test and amino acid analysis of the cell wall hydrolysates indicated the absence of muramic acid and diaminopimelic acid in the cell walls, which suggested that this strain lacks an ordinary Gram-negative type of peptidoglycan in the cell wall. The DNA G+C content of strain N5FB36-5(T) was 51.6 mol%; MK-7 was the major menaquinone; and the presence of C(16 : 0), C(16 : 1)omega7c and anteiso-C(15 : 0) as the major cellular fatty acids supported the identification of the novel isolate as a member of the genus Pelagicoccus. On the basis of polyphasic taxonomic data, it was concluded that this strain should be classified as a novel species of the genus Pelagicoccus, for which the name Pelagicoccus croceus sp. nov. is proposed. The type strain is N5FB36-5(T) (=MBIC08283(T)=KCTC [corrected] 12903(T)).

Receptor-mediated uptake of Legionella pneumophila by Acanthamoeba castellanii and Naegleria lovaniensis

AIMS

Investigation of the attachment and uptake of Legionella pneumophila by Acanthamoeba castellanii and Naegleria lovaniensis, as these are two critical steps in the subsequent bacterial survival in both amoeba hosts.

METHODS AND RESULTS

Initially, the mode of Legionella uptake was examined using inhibitors of microfilament-dependent and receptor-mediated uptake phagocytosis. Secondly, the minimum saccharide structure to interfere with L. pneumophila uptake was determined by means of selected saccharides. Bacterial attachment and uptake by each of the amoeba species occurred through a receptor-mediated endocytosis, which required de novo synthesis of host proteins. Legionella pneumophila showed a high affinity to the alpha1-3D-mannobiose domain of the mannose-binding receptor located on A. castellanii. In contrast, L. pneumophila bacteria had a high affinity for the GalNAcbeta1-4Gal domain of the N-acetyl-D-galactosamine receptor of N. lovaniensis.

CONCLUSIONS

Our data pointed to a remarkable adaptation of L. pneumophila to invade different amoeba hosts, as the uptake by both amoeba species is mediated by two different receptor families.

SIGNIFICANCE AND IMPACT OF THE STUDY

The fact that L. pneumophila is taken up by two different amoeba species using different receptor families adds further complexity to the host-parasite interaction process, as 14 amoeba species are known to be appropriate Legionella hosts.

Environmental predators as models for bacterial pathogenesis

Environmental bacteria are constantly threatened by bacterivorous predators such as free-living protozoa and nematodes. In the course of their coevolution with environmental predators, some bacteria developed sophisticated defence mechanisms, including the secretion of toxins, or the capacity to avoid lysosomal killing and to replicate intracellularly within protozoa. To analyse the interactions with bacterial pathogens on a molecular, cellular or organismic level, protozoa and other non-mammalian hosts are increasingly used. These include amoebae, as well as genetically tractable hosts, such as the social amoeba Dictyostelium discoideum, the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. Using these hosts, the virulence mechanisms of opportunistic pathogenic bacteria such as Legionella, Mycobacterium, Pseudomonas or Vibrio were found to be not only relevant for the interactions of the bacteria with protozoa, nematodes and insect phagocytes, but also with mammalian hosts including humans. Thus, non-mammalian model hosts provide valuable insight into the pathogenesis of environmental bacteria.

Cell surface lectin-binding glycoconjugates on marine planktonic protists

Carbohydrate-protein interactions appear to play an important role in the phagocytosis of microbial prey by free-living protozoa. The present study utilizes FITC-labelled plant lectins to investigate the presence and localization of cell surface glycoconjugates on live and fixed planktonic protists (Dunaliella primolecta, Oxyrrhis marina, Goniomonas amphinema, Paraphysomonas vestita and Euplotes vannus). With live flagellate preparations, lectins primarily bound to external cell surfaces, with minimal internal staining observed. In contrast, cell fixation permeabilized cell membranes, allowing lectins to bind to internal structures, such as nuclear membranes and food vacuoles, interfering with the characterization of cell surface glycoconjugates. The method developed to label cell surface sugar moieties of live planktonic protists successfully overcomes the problems associated with fixation, and thus provides a useful protocol for future studies on protistan cell surface carbohydrate characterization.

Oligosaccharide receptor mimics inhibit Legionella pneumophila attachment to human respiratory epithelial cells

Legionnaire's disease is caused by the intracellular pathogen Legionella pneumophila, presenting as an acute pneumonia. Attachment is the key step during infection, often relying on an interaction between host cell oligosaccharides and bacterial adhesins. Inhibition of this interaction by receptor mimics offers possible novel therapeutic treatments. L. pneumophila attachment to the A549 cell line was significantly reduced by treatment with tunicamycin (73.6%) and sodium metaperiodate (63.7%). This indicates the importance of cell surface oligosaccharide chains in adhesion. A number of putative anti-adhesion compounds inhibited attachment to the A549 and U937 cell lines. The most inhibitory compounds were polymeric saccharides, GalNAcbeta1-4Gal, Galbeta1-4GlcNAc and para-nitrophenol. These compounds inhibited adhesion to a range of human respiratory cell lines, including nasal epithelial, bronchial epithelial and alveolar epithelial cell lines and the human monocytic cell line, U937. Some eukaryotic receptors for L. pneumophila were determined to be the glycolipids, asialo-GM1 and asialo-GM2 that contain the inhibitory saccharide moiety, GalNAcbeta1-4Gal. The identified compounds have the potential to be used as novel treatments for Legionnaire's disease.

Investigation of mechanisms involved in phagocytosis of Legionella pneumophila by human cells

Legionella pneumophila, the causative agent of Legionnaires' disease, is able to survive and multiply efficiently in a variety of mammalian cells. By using in vitro assays, the uptake of L. pneumophila into monocytes has shown to be mediated, at least in part, through attachment of complement-coated bacteria to complement receptors, but complement-independent phagocytosis could also be demonstrated. Since complement levels in the human lung are normally low, the role of complement-dependent phagocytosis in the pathogenesis of Legionnaires' disease is doubtful. However, the contribution of other potential phagocytosis-related host cell surface molecules to the phagocytosis of L. pneumophila has never been investigated. We therefore analyzed the role of complement receptors 1 (CD35) and 3 (CD11b/18), the lipopolysaccharide (LPS) receptor (CD14), the beta(1)-integrin chain of the fibronectin receptor (CD29), the intercellular adhesion molecule 1 (ICAM-1, CD54) and the transferrin receptor (CD71) in the complement-independent uptake of L. pneumophila. To exclude any influence of culture conditions onto phagocytosis rates, we compared a fresh clinical isolate with an agar-adapted isolate of L. pneumophila. In addition, we used three different host cell types (MM6, HeLa and Jurkat cells) expressing different rates of complement receptors. We could show that both strains of L. pneumophila were phagocytized by the three host cell lines to the same extent, but intracellular multiplication was only found in MM6 and, although to a much lesser degree, in Jurkat cells. Preincubation of MM6 cells with monoclonal antibodies directed against the above cited phagocytosis-related receptors did not result in inhibition of L. pneumophila uptake. We therefore conclude that typical phagocytosis-related cell surface receptors are not involved in the complement-independent phagocytosis of L. pneumophila.

Structure and function of the Entamoeba histolytica Gal/GalNAc lectin

Gal/GalNAc lectin is a novel multifunctional virulence factor of the human parasite Entamoeba histolytica. The native protein is a 260-kDa heterodimer consisting of a type 1 membrane protein disulfide bonded to a lipid-anchored protein. Each subunit has several isoforms that may form functionally different heterodimers, analogous to the integrin family of proteins. Recently a second 150-kDa Gal/GalNAc lectin has been identified in E. histolytica that associates with the 260-kDa lectin. The functions of the 260-kDa lectin have been characterized using specific monoclonal antibodies. This lectin plays roles in many of the critical aspects of this parasite's pathogenicity including adherence, cytolysis, invasion, resistance to lysis by complement, and also perhaps encystment. Current knowledge regarding both the structure and function of this unique multifunctional virulence factor are discussed.

Legionella and Legionnaires' disease: 25 years of investigation

There is still a low level of clinical awareness regarding Legionnaires' disease 25 years after it was first detected. The causative agents, legionellae, are freshwater bacteria with a fascinating ecology. These bacteria are intracellular pathogens of freshwater protozoa and utilize a similar mechanism to infect human phagocytic cells. There have been major advances in delineating the pathogenesis of legionellae through the identification of genes which allow the organism to bypass the endocytic pathways of both protozoan and human cells. Other bacteria that may share this novel infectious process are Coxiella burnetti and Brucella spp. More than 40 species and numerous serogroups of legionellae have been identified. Most diagnostic tests are directed at the species that causes most of the reported human cases of legionellosis, L. pneumophila serogroup 1. For this reason, information on the incidence of human respiratory disease attributable to other species and serogroups of legionellae is lacking. Improvements in diagnostic tests such as the urine antigen assay have inadvertently caused a decrease in the use of culture to detect infection, resulting in incomplete surveillance for legionellosis. Large, focal outbreaks of Legionnaires' disease continue to occur worldwide, and there is a critical need for surveillance for travel-related legionellosis in the United States. There is optimism that newly developed guidelines and water treatment practices can greatly reduce the incidence of this preventable illness.

Legionella and Legionnaires' disease: 25 years of investigation

There is still a low level of clinical awareness regarding Legionnaires' disease 25 years after it was first detected. The causative agents, legionellae, are freshwater bacteria with a fascinating ecology. These bacteria are intracellular pathogens of freshwater protozoa and utilize a similar mechanism to infect human phagocytic cells. There have been major advances in delineating the pathogenesis of legionellae through the identification of genes which allow the organism to bypass the endocytic pathways of both protozoan and human cells. Other bacteria that may share this novel infectious process are Coxiella burnetti and Brucella spp. More than 40 species and numerous serogroups of legionellae have been identified. Most diagnostic tests are directed at the species that causes most of the reported human cases of legionellosis, L. pneumophila serogroup 1. For this reason, information on the incidence of human respiratory disease attributable to other species and serogroups of legionellae is lacking. Improvements in diagnostic tests such as the urine antigen assay have inadvertently caused a decrease in the use of culture to detect infection, resulting in incomplete surveillance for legionellosis. Large, focal outbreaks of Legionnaires' disease continue to occur worldwide, and there is a critical need for surveillance for travel-related legionellosis in the United States. There is optimism that newly developed guidelines and water treatment practices can greatly reduce the incidence of this preventable illness.

Actin machinery of phagocytic cells: universal target for bacterial attack

Uptake of microorganisms by eukaryotic cells depends on proper functioning of the actin machinery. It creates a driving force for the cell membrane deformations necessary for ingestion and killing of microbes by phagocytes. Therefore, specific alterations in the activity of the actin apparatus could be favorable for pathogenic bacteria, representing an efficient mechanism in their virulence. Such alterations are supposed to be achieved in two principle ways. One is accomplished via binding of bacterial ligands to certain surface receptors, which initiate subsequent actin cytoskeleton rearrangements. Another is to introduce cytoskeleton-targeted products directly into eukaryotic cells and in this way modulate the activity of the actin apparatus. Indeed, Legionella and some other intracellular parasites possess ligands able to stimulate certain receptors on the surface of phagocytes and possess devices suitable for translocation of effector molecules into eukaryotic cytoplasm. The results of such events could be increased uptake of these microbes and their subsequent transportation to permit multiplication in their intracellular niche. On the contrary, representatives of Clostridium and a number of other extracellular pathogens create products which penetrate eukaryotic cells and disorganize the actin cytoskeleton network, thus making uptake of these pathogens by phagocytes impossible.

Legionella pneumophila: an aquatic microbe goes astray

Legionella pneumophila is naturally found in fresh water were the bacteria parasitize within protozoa. It also survives planctonically in water or biofilms. Upon aerosol formation via man-made water systems, L. pneumophila can enter the human lung and cause a severe form of pneumonia, called Legionnaires' disease. The pathogenesis of Legionnaires' disease is largely due to the ability of L. pneumophila to invade and grow within macrophages. An important characteristic of the intracellular survival strategy is the replication within the host vacuole that does not fuse with endosomes or lysosomes. In recent times a great number of bacterial virulence factors which affect growth of L. pneumophila in both macrophages and protozoa have been identified. The ongoing Legionella genome project and the use of genetically tractable surrogate hosts are expected to significantly contribute to the understanding of bacterium-host interactions and the regulation of virulence traits during the infection cycle. Since person-to-person transmission of legionellosis has never been observed, the measures for disease prevention have concentrated on eliminating the pathogen from water supplies. In this respect detection and analysis of Legionella in complex environmental consortia become increasingly important. With the availability of new molecular tools this area of applied research has gained new momentum.

Role of the Legionella pneumophila rtxA gene in amoebae

Legionella pneumophila infects humans, causing Legionnaires' disease, from aerosols generated by domestic and environmental water sources. In aquatic environments L. pneumophila is thought to replicate primarily in protozoa. A 'repeats in structural toxin' (RTX) gene, rtxA, from L. pneumophila was identified recently that plays a role in entry and replication in human macrophages and also has the ability to infect mice. However, the role of this gene in the interaction of L. pneumophila with environmental protozoa and its distribution in different Legionella species has not been examined. Southern analyses demonstrated that rtxA is present in all L. pneumophila isolates tested and correlates with species that have been shown to cause disease in humans. To evaluate the importance of rtxA in the interaction with protozoa a series of studies was carried out in an environmental host for L. pneumophila, Acanthamoeba castellanii. The L. pneumophila rtxA gene plays a role in both adherence and entry into A. castellanii similar to that observed in human monocytic cells. Furthermore, it was found that rtxA is involved in intracellular survival and trafficking. In addition to demonstrating involvement of rtxA in the interaction of L. pneumophila with host cells, these data support a role for this gene both during disease in humans and in environmental reservoirs.

The C-terminus of IcmT is essential for pore formation and for intracellular trafficking of Legionella pneumophila within Acanthamoeba polyphaga

We have shown previously that the five rib (release of intracellular bacteria) mutants of Legionella pneumophila are competent for intracellular replication but defective in pore formation-mediated cytolysis and egress from protozoan and mammalian cells. The rib phenotype results from a point mutation (deletion) DeltaG544 in icmT that is predicted to result in the expression of a protein truncated by 32 amino acids from the C-terminus. In contrast to the rib mutants that are capable of intracellular replication, an icmT null mutant was completely defective in intracellular replication within mammalian and protozoan cells, in addition to its defect in pore formation-mediated cytolysis. The icmT wild-type allele complemented the icmT null mutant for both defects of intracellular replication and pore formation-mediated cytolysis and egress from mammalian cells. In contrast, the icmTDeltaG544 allele complemented the icmT null mutant for intracellular growth, but not for the pore-forming activity. Consistent with their defect in pore formation-mediated cytotoxicity in vitro, both mutants failed to cause pulmonary inflammation in A/J mice. Interestingly, the rib mutant was severely defective in intracellular growth within Acanthamoeba polyphaga. Confocal laser scanning and electron microscopy confirmed that the rib mutant and the icmT null mutant were severely and completely defective, respectively, in intracellular growth in A. polyphaga, and the respective defects correlated with fusion of the bacterial phagosomes to lysosomes. Taken together, the data showed that the C-terminus domain of IcmT is essential for the pore-forming activity and is required for intracellular trafficking and replication within A. polyphaga, but not within mammalian cells.

Icm/dot-dependent upregulation of phagocytosis by Legionella pneumophila

Legionella pneumophila is the causative agent of Legionnaires' disease, a severe pneumonia. Dependent on the icm/dot loci, L. pneumophila survives and replicates in macrophages and amoebae within a specialized phagosome that does not fuse with lysosomes. Here, we report that phagocytosis of wild-type L. pneumophila is more efficient than uptake of icm/dot mutants. Compared with the wild-type strain JR32, about 10 times fewer icm/dot mutant bacteria were recovered from HL-60 macrophages in a gentamicin protection assay. The defect in phagocytosis of the mutants could be complemented by supplying the corresponding genes on a plasmid. Using fluorescence microscopy and green fluorescent protein (GFP)-expressing strains, 10-20 times fewer icm/dot mutant bacteria were found to be internalized by HL-60 cells and human monocyte-derived macrophages (HMMPhi). Compared with icm/dot mutants, wild-type L. pneumophila infected two to three times more macrophages and yielded a population of highly infected host cells (15-70 bacteria per macrophage) that was not observed with icm/dot mutant strains. Wild-type and icmT mutant bacteria were found to adhere similarly and compete for binding to HMMPhi. In addition, wild-type L. pneumophila was also phagocytosed more efficiently by Acanthamoeba castellanii, indicating that the process is independent of adherence receptor(s). Wild-type L. pneumophila enhanced phagocytosis of an icmT mutant strain in a synchronous co-infection, suggesting that increased phagocytosis results from (a) secreted effector(s) acting in trans.

Legionella pneumophila is internalized by a macropinocytotic uptake pathway controlled by the Dot/Icm system and the mouse Lgn1 locus

The products of the Legionella pneumophila dot/icm genes enable the bacterium to replicate within a macrophage vacuole. This study demonstrates that the Dot/Icm machinery promotes macropinocytotic uptake of L. pneumophila into mouse macrophages. In mouse strains harboring a permissive Lgn1 allele, L. pneumophila promoted formation of vacuoles that were morphologically similar to macropinosomes and dependent on the presence of an intact Dot/Icm system. Macropinosome formation appeared to occur during, rather than after, the closure of the plasma membrane about the bacterium, since a fluid-phase marker preloaded into the macrophage endocytic path failed to label the bacterium-laden macropinosome. The resulting macropinosomes were rich in GM1 gangliosides and glycosylphosphatidylinositol-linked proteins. The Lgn1 allele restrictive for L. pneumophila intracellular replication prevented dot/icm-dependent macropinocytosis, with the result that phagosomes bearing the microorganism were targeted into the endocytic network. Analysis of macrophages from recombinant inbred mouse strains support the model that macropinocytotic uptake is controlled by the Lgn1 locus. These results indicate that the products of the dot/icm genes and Lgn1 are involved in controlling an internalization route initiated at the time of bacterial contact with the plasma membrane.

Flagellum of Legionella pneumophila positively affects the early phase of infection of eukaryotic host cells

Legionella pneumophila, the etiologic agent of Legionnaires' disease, contains a single, monopolar flagellum which is composed of one major subunit, the FlaA protein. To evaluate the role of the flagellum in the pathogenesis and ecology of Legionella, the flaA gene of L. pneumophila Corby was mutagenized by introduction of a kanamycin resistance cassette. Immunoblots with antiflagellin-specific polyclonal antiserum, electron microscopy, and motility assays confirmed that the specific flagellar mutant L. pneumophila Corby KH3 was nonflagellated. The redelivery of the intact flaA gene into the chromosome (L. pneumophila Corby CD10) completely restored flagellation and motility. Coculture studies showed that the invasion efficiency of the flaA mutant was moderately reduced in amoebae and severely reduced in HL-60 cells. In contrast, adhesion and the intracellular rate of replication remained unaffected. Taking these results together, we have demonstrated that the flagellum of L. pneumophila positively affects the establishment of infection by facilitating the encounter of the host cell as well as by enhancing the invasion capacity.

HtrA homologue of Legionella pneumophila: an indispensable element for intracellular infection of mammalian but not protozoan cells

Legionella pneumophila replicates within alveolar macrophages, and possibly, alveolar epithelial cells and also within protozoa in the aquatic environment. Here we characterize an L. pneumophila mutant defective in the HtrA/DegP stress-induced protease/chaperone homologue and show that HtrA is indispensable for intracellular replication within mammalian macrophages and alveolar epithelial cells and for intrapulmonary replication in A/J mice. Importantly, amino acid substitutions of two conserved residues in the catalytic domain of (H103mapstoR and S212mapstoA) and in-frame deletions of either or both of the two conserved PDZ domains of HtrA abolish its function. Interestingly, the htrA mutant exhibits a parental-type phenotype in intracellular replication within the protozoan host Acanthamoeba polyphaga. We used a promoterless lacZ fusion to the htrA promoter to probe the phagosomal microenvironment harboring L. pneumophila within macrophages and within A. polyphaga for the exposure to stress stimuli. The data show that expression through the htrA promoter is induced by 12,000- to 20,000-fold throughout the intracellular infection of macrophages but its induction is by 120- to 500-fold within protozoa compared to in vitro expression. Data derived from confocal laser scanning microscopy reveal that in contrast to the parental strain, phagosomes harboring the htrA mutant within U937 macrophages colocalize with the late endosomal-lysosomal marker LAMP-2, similar to killed L. pneumophila. Coinfection experiments examined by confocal laser scanning microscopy show that in communal phagosomes harboring both the parental strain and the htrA mutant, replication of the mutant is not rescued, while replication of a dotA mutant control, which is normally trafficked into a phagolysosome, is rescued by the parental strain. Our data show, for the first time, that the stress response by L. pneumophila (mediated, at least in part, by HtrA) is indispensable for intracellular replication within mammalian but not protozoan cells.

Essential role for the Legionella pneumophila rep helicase homologue in intracellular infection of mammalian cells

We have previously isolated 32 mutants of Legionella pneumophila that are defective in the infection of mammalian cells but not protozoa. The mutated loci have been designated macrophage-specific infectivity (mil) loci. In this study we characterized the mil mutant GK11. This mutant was incapable of growth within U937 macrophage-like cells and WI-26 alveolar epithelial cells. This defect in intracellular replication correlated with a defect in cytopathogenicity to these cells. Sequence analysis of the GK11 locus revealed it to be highly similar to rep helicase genes of other bacteria. Since helicase mutants of Escherichia coli are hypersensitive to thymine starvation, we examined the sensitivity of GK11 to thymineless death (TLD). In the absence of thymine and thymidine, mutant GK11 did not undergo TLD but was defective for in vitro growth, and the defect was partially restored when these compounds were added to the growth medium. In addition, supplementation with thymidine or thymine partially restored the ability of GK11 to grow within and kill U937 macrophage-like cells. The data suggested that the low levels of thymine or thymidine in the L. pneumophila phagosome contributed to the defect of GK11 within macrophages. Using confocal laser scanning microscopy, we determined the effect of the mutation in the Rep helicase homologue on the intracellular trafficking of GK11 within macrophages. In contrast to the wild-type strain, phagosomes harboring GK11 colocalized with several late endosomal/lysosomal markers, including LAMP-1, LAMP-2, and cathepsin D. In addition, only 50% of the GK11 phagosomes colocalized with the endoplasmic reticulum marker BiP 4 h postinfection. Colocalization of BiP with GK11 phagosomes was absent 6 h postinfection, while 90% of the wild-type phagosomes colocalized with this marker at both time points. We propose that the low level of thymine within the L. pneumophila phagosome in combination with simultaneous exposure to multiple stress stimuli results in deleterious mutations that cannot be repaired in the rep helicase homologue mutant, rendering it defective in intracellular replication.

Hijacking of apoptotic pathwaysby bacterial pathogens

Increasing evidence indicates that apoptosis of the host cell may constitute a defense mechanism to confine the infection by bacterial pathogens. Certain pathogens have developed elegant mechanisms to modulate the fate of the host cell, which include induction or blockage of apoptosis. These studies will promote our understanding of the pathogenesis of infectious diseases and aid the development of means for therapeutic intervention.

Temporal pore formation-mediated egress from macrophages and alveolar epithelial cells by Legionella pneumophila

Legionella pneumophila does not induce apoptosis in the protozoan host, but induces pore formation-mediated cytolysis after termination of intracellular replication (L.-Y. Gao and Y. Abu Kwaik, Environ. Microbiol. 2:79-90, 2000). In contrast to this single mode of killing of protozoa, we have recently proposed a biphasic model by which L. pneumophila kills macrophages, in which the first phase is manifested through the induction of apoptosis during early stages of the infection, followed by an independent and temporal induction of necrosis during late stages of intracellular replication. Here we show that, similar to the protozoan host, the induction of necrosis and cytolysis of macrophages by L. pneumophila is mediated by the pore-forming toxin or activity. This activity is temporally and maximally expressed only upon termination of bacterial replication and correlates with cytolysis of macrophages and alveolar epithelial cells in vitro. We have identified five L. pneumophila mutants defective in the pore-forming activity. The phagosomes harboring the mutants do not colocalize with the late endosomal or lysosomal marker Lamp-1, and the mutants replicate intracellularly similar to the parental strain. Interestingly, despite their prolific intracellular replication, the mutants are defective in cytotoxicity and are "trapped" within and fail to lyse and egress from macrophages and alveolar epithelial cells upon termination of intracellular replication. However, the mutants are subsequently released from the host cell, most likely due to apoptotic death of the host cell. Data derived from cytotoxicity assays, confocal laser scanning microscopy, and electron microscopy confirm the defect in the mutants to induce necrosis of macrophages and the failure to egress from the host cell. Importantly, the mutants are completely defective in acute lethality (24 to 48 h) to intratracheally inoculated A/J mice. We conclude that the pore-forming activity of L. pneumophila is not required for phagosomal trafficking or for intracellular replication. This activity is expressed upon termination of bacterial replication and is essential to induce cytolysis of infected macrophages to allow egress of intracellular bacteria. In addition, this activity plays a major role in pulmonary immunopathology in vivo.

Signal transduction in the protozoan host Hartmannella vermiformis upon attachment to Legionella pneumophila

Intracellular replication of the Legionnaires' disease bacterium, Legionella pneumophila, within protozoa plays a major role in bacterial ecology and pathogenesis. Invasion of the protozoan host Hartmannella vermiformis by L. pneumophila is mediated by attachment to the Gal/GalNAc lectin receptor, which is similar to the beta(2) integrin transmembrane receptors of mammalian cells. Bacterial invasion is associated with induction of a protein tyrosine phosphatase (PTPase) activity in H. vermiformis that results in tyrosine dephosphorylation of the lectin receptor and several cytoskeletal proteins. In this report, we show that entry of L. pneumophila into H. vermiformis is not required to induce tyrosine dephosphorylation of one of the cytoskeletal proteins, paxillin. Tyrosine dephosphorylation of paxillin is mediated at the level of bacterial attachment to the lectin receptor, and is blocked by inhibiting bacterial attachment to the lectin receptor. Attachment of L. pneumophila to the lectin receptor is not mediated by the type IV pilus, which is one of the bacterial ligands involved in attachment to protozoa. Interestingly, the lectin receptor in resting H. vermiformis is associated with several phosphorylated proteins that are dissociated upon bacterial attachment and invasion. We show that the L. pneumophila-induced PTPase activity in H. vermiformis and the associated tyrosine dephosphorylation of host proteins can be mimicked by the cytoskeletal disrupting agent, cytochalasin D. Taken together, our data indicate that attachment of L. pneumophila to the lectin receptor of H. vermiformis induces a PTPase activity, tyrosine dephosphorylation of the lectin and cytoskeletal proteins, dissociation of the lectin from its associated phosphorylated proteins, and most probably disassembly of the cytoskeleton. This novel L. pneumophila-protozoa interaction may be a bacterial strategy to invade protozoa and to be trafficked into a replicative 'niche', or to block differentiation of the protozoan host into a cyst in which L. pneumophila cannot replicate.

Identification of novel loci involved in entry by Legionella pneumophila

Legionella pneumophila is primarily an intracellular pathogen during infection; thus, the mechanisms of entry into host cells are likely to be important for pathogenesis. Several L. pneumophila mutants that display an enhanced-entry (Enh) phenotype were isolated by selecting for bacteria that enter host cells at a higher frequency than wild-type. In the course of characterizing the genetic basis of one of these mutants, C3, a strategy was developed for the isolation of laboratory-media-repressed virulence determinants from L. pneumophila. Screens for dominant mutations using a genomic DNA library from C3 resulted in the isolation of three cosmids that confer an Enh phenotype to wild-type L. pneumophila. Transposon mutagenesis of these cosmids allowed identification of three loci that affect entry. Analysis of the putative proteins encoded by these loci, designated rtxA and enhC, demonstrated similarity to repeats in the structural toxin protein and the secreted Sel-1 protein from Caenorhabditis elegans, respectively. L. pneumophila rtxA and enhC mutants display significantly reduced entry into host cells, compared to wild-type bacteria. The phenotype that the cosmids containing these loci confer is most likely due to elevated expression resulting from their presence on multicopy vectors. The use of increased gene copy number to overexpress genes that are normally repressed under laboratory growth conditions is generally applicable to the isolation of virulence determinants from L. pneumophila and other bacterial pathogens.

From protozoa to mammalian cells: a new paradigm in the life cycle of intracellular bacterial pathogens

It is becoming apparent that several intracellular bacterial pathogens of humans can also survive within protozoa. This interaction with protozoa may protect these pathogens from harsh conditions in the extracellular environment and enhance their infectivity in mammals. This relationship has been clearly established in the case of the interaction between Legionella pneumophila and its protozoan hosts. In addition, the adaptation of bacterial pathogens to the intracellular life within the primitive eukaryotic protozoa may have provided them with the means to infect the more evolved mammalian cells. This is evident from the existence of several similarities, at both the phenotypic and the molecular levels, between the infection of mammalian and protozoan cells by L. pneumophila. Thus, protozoa appear to play a central role in the transition of bacteria from the environment to mammals. In essence, protozoa may be viewed as a 'biological gym', within which intracellular bacterial pathogens train for their encounters with the more evolved mammalian cells. Thus, intracellular bacterial pathogens have benefited from the structural and biochemical conservation of cellular processes in eukaryotes. The interaction of intracellular bacterial pathogens and protozoa highlights this conservation and may constitute a simplified model for the study of these pathogens and the evolution of cellular processes in eukaryotes. Furthermore, in addition to being environmental reservoirs for known intracellular pathogens of humans and animals, protozoa may be sources of emerging pathogenic bacteria. It is thus critical to re-examine the relationship between bacteria and protozoa to further our understanding of current human bacterial pathogenesis and, possibly, to predict the appearance of emerging pathogens.

Cell polarization and adhesion in a motile pathogenic protozoan: role and fate of the Entamoeba histolytica Gal/GalNAc lectin

The human pathogenic protozoan Entamoeba histolytica is a motile cell polarized into a front pseudopod and a rear uroid. The amoebic Gal/GalNAc surface lectin is a major adhesion molecule composed of an immunodominant 170-kDa heavy subunit, mostly extracellular except for a short cytoplasmic tail, and of an extracellular light subunit. The binding of multivalent ligands triggers lectin capping and recruitment to the uroid. The properties of the Gal/GalNAc lectin and its role in amoeba adhesion and uroid polarization are reviewed in the context of the molecular mechanisms underlying cell polarization and locomotion.

The mechanism of killing and exiting the protozoan host Acanthamoeba polyphaga by Legionella pneumophila

The ability of Legionella pneumophila to cause legionnaires' disease is dependent on its capacity to replicate within cells in the alveolar spaces. The bacteria kill mammalian cells in two phases: induction of apoptosis during the early stages of infection, followed by an independent and rapid necrosis during later stages of the infection, mediated by a pore-forming activity. In the environment, L. pneumophila is a parasite of protozoa. The molecular mechanisms by which L. pneumophila kills the protozoan cells, after their exploitation for intracellular proliferation, are not known. In an effort to decipher these mechanisms, we have examined induction of both apoptosis and necrosis in the protozoan Acanthamoeba polyphaga upon infection by L. pneumophila. Our data show that, although A. polyphaga undergoes apoptosis following treatment with actinomycin D, L. pneumophila does not induce apoptosis in these cells. Instead, intracellular L. pneumophila induces necrotic death in A. polyphaga, which is mediated by the pore-forming activity. Mutants of L. pneumophila defective in expression of the pore-forming activity are indistinguishable from the parental strain in intracellular replication within A. polyphaga. The parental strain bacteria cause necrosis-mediated lysis of all the A. polyphaga cells within 48 h after infection, and all the intracellular bacteria are released into the tissue culture medium. In contrast, all cells infected by the mutants remain intact, and the intracellular bacteria are 'trapped' within A. polyphaga after the termination of intracellular replication. Failure to exit the host cell after termination of intracellular replication results in a gradual decline in the viability of the mutant strain bacteria within A. polyphaga starting 48h after infection. Our data show that the pore-forming activity of L. pneumophila is not required for intracellular bacterial replication within A. polyphaga but is required for killing and exiting the protozoan host upon termination of intracellular replication.

Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages

Legionella pneumophila first commanded attention in 1976, when investigators from the Centers for Disease Control and Prevention identified it as the culprit in a massive outbreak of pneumonia that struck individuals attending an American Legion convention (). It is now clear that this gram-negative bacterium flourishes naturally in fresh water as a parasite of amoebae, but it can also replicate within alveolar macrophages. L. pneumophila pathogenesis is discussed using the following model as a framework. When ingested by phagocytes, stationary-phase L. pneumophila bacteria establish phagosomes which are completely isolated from the endosomal pathway but are surrounded by endoplasmic reticulum. Within this protected vacuole, L. pneumophila converts to a replicative form that is acid tolerant but no longer expresses several virulence traits, including factors that block membrane fusion. As a consequence, the pathogen vacuoles merge with lysosomes, which provide a nutrient-rich replication niche. Once the amino acid supply is depleted, progeny accumulate the second messenger guanosine 3',5'-bispyrophosphate (ppGpp), which coordinates entry into the stationary phase with expression of traits that promote transmission to a new phagocyte. A number of factors contribute to L. pneumophila virulence, including type II and type IV secretion systems, a pore-forming toxin, type IV pili, flagella, and numerous other factors currently under investigation. Because of its resemblance to certain aspects of Mycobacterium, Toxoplasma, Leishmania, and Coxiella pathogenesis, a detailed description of the mechanism used by L. pneumophila to manipulate and exploit phagocyte membrane traffic may suggest novel strategies for treating a variety of infectious diseases. Knowledge of L. pneumophila ecology may also inform efforts to combat the emergence of new opportunistic macrophage pathogens.

The Legionella pneumophila prp locus; required during infection of macrophages and amoebae

Transposon mutagenesis was performed using mTn 10phoA to identify Legionella pneumophila genes that are expressed under certain in vitro conditions, and are required for intracellular replication. Of the 1653 PhoA fusions examined, 19 PhoA(+)fusion mutants were isolated and screened for differential expression of fusion proteins after growth at 30 or 37 degrees C, in the presence of low iron, or increased magnesium concentrations. The mutants were examined for their cytopathogenicity and intracellular replication within U937 macrophage-like cells and the protozoan Hartmannella vermiformis. One of the mutants generated, BS10, was defective in its multiplication within U937 macrophage-like cells and H. vermiformis. The defect in BS10 was complemented with a cosmid clone containing the wild type locus. The open reading frame interrupted by the insertion was homologous to prpD of Salmonella typhimurium and mmgE of Bacillus subtilis.

Activation of caspase 3 during Legionella pneumophila-induced apoptosis

The hallmark of Legionnaires' disease is replication of Legionella pneumophila within cells in the alveolar spaces. The mechanisms by which L. pneumophila replicates intracellularly and kills the host cell are largely not understood. We have recently shown that within 3 h of initiation of the infection and prior to intracellular replication, L. pneumophila induces apoptosis in macrophages, alveolar epithelial cells, and peripheral blood monocytes, which correlates with cytopathogenicity (L.-Y. Gao and Y. Abu Kwaik, Infect. Immun. 67:862-870, 1999). In this report, we show that the ability of L. pneumophila to induce apoptosis is, largely, not growth phase regulated. We demonstrate that the induction of apoptosis by L. pneumophila in macrophages is mediated through the activation of caspase 3. The enzymatic activity of caspase 3 to cleave a specific synthetic substrate in vitro is detected in L. pneumophila-infected macrophages at 2 h after infection and is maximal at 3 h, with over 900% increase in activity. The activity of caspase 3 to cleave a specific substrate [poly(ADP-ribose) polymerase, or PARP] in vivo is also detected at 2 h and is maximal at 3 h postinfection. The activity of caspase 3 to cleave the synthetic substrate in vitro and PARP in vivo is blocked by a specific inhibitor of caspase 3. The kinetics of caspase 3 activation correlates with that of L. pneumophila-induced nuclear apoptosis. Inhibition of caspase 3 activity blocks L. pneumophila-induced nuclear apoptosis and cytopathogenicity during early stages of the infection. Consistent with the ability to induce apoptosis, extracellular L. pneumophila also activates caspase 3. Three dotA/icmWXYZ mutants of L. pneumophila that are defective in inducing apoptosis do not induce caspase 3 activation, suggesting that expression and/or export of the apoptosis-inducing factor(s) is regulated by the dot/icm virulence system. This is the first description of the role of caspase 3 activation in induction of nuclear apoptosis in the host cell infected by a bacterial pathogen.

Legionella: from environmental habitats to disease pathology, detection and control

Studies on Legionella show a continuum from environment to human disease. Legionellosis is caused by Legionella species acquired from environmental sources, principally water sources such as cooling towers, where Legionella grows intracellularly in protozoa within biofilms. Aquatic biofilms, which are widespread not only in nature, but also in medical and dental devices, are ecological niches in which Legionella survives and proliferates and the ultimate sources to which outbreaks of legionellosis can be traced. Invasion and intracellular replication of L. pneumophila within protozoa in the environment play a major role in the transmission of Legionnaires' disease. Protozoa provide the habitats for the environmental survival and reproduction of Legionella species. L. pneumophila proliferates intracellularly in various species of protozoa within vacuoles studded with ribosomes, as it also does within macrophages. Growth within protozoa enhances the environmental survival capability and the pathogenicity (virulence) of Legionella. The growth requirements of Legionella, the ability of Legionella to enter a viable non-culturable state, the association of Legionella with protozoa and the occurrence of Legionella within biofilms complicates the detection of Legionella and epidemiological investigations of legionellosis. Polymerase chain reaction (PCR) methods have been developed for the molecular detection of Legionella and used in environmental and epidemiological studies. Various physical and chemical disinfection methods have been developed to eliminate Legionella from environmental sources, but gaining control of Legionella in environmental waters, where they are protected from disinfection by growing within protozoa and biofilms, remains a challenge, and one that must be overcome in order to eliminate sporadic outbreaks of legionellosis.

Dissection of the impact of various intracellular signaling pathways on stable cell aggregate formation of rat thymocytes after initial lectin-dependent cell association of using a plant lectin as model and target-selective inhibitors

Bivalent lectins as bridging molecules between cells or cell surface lectins as docking points are involved in mediation of cell adhesion by specific recognition of suitable glycoconjugates on an opposing surface. The initial contact formation by a lectin can lead to intracellular post-binding events which effect stable cell association even in the presence of the haptenic sugar. To delineate the participation of intracellular signaling pathways in the cascade of reactions to establish firm association, reagents with proven inhibitory capacity on certain biochemical targets provide suitable tools. Using this approach with rat thymocytes and the galactoside-binding lectin from mistletoe (Viscum album L. agglutinin, VAA) as a model, a panel of 27 inhibitors with impact on e.g. several types of kinases, tyrosine phosphatases, NO synthases, G proteins, enzymes of arachidonate and cyclic nucleotide metabolism and calmodulin was systematically tested with respect to their capacity to impair the formation of lactose-resistant cell aggregates. In addition to the recently reported effectiveness of N-ethylmaleimide, nordihydroguaiaretic acid, and trifluoperazine the agents diacylglycerol kinase inhibitor II, emodin, D609, DPI, KT5720, KT5926, MK-886, bisindolylmaleimide I, and (+/-)methoxyverapamil were able to reduce aggregate stability in the presence of the haptenic sugar. Thus, various types of kinases including p561lck tyrosine kinase, lipoxygenases, phosphatidylcholine-specific phospholipase C as well as calmodulin and Ca(2+)-currents, but not modulators of the metabolism of cyclic nucleotides, NO synthases, MAP kinases, tyrosine phosphatases and phospholipase A (preferentially group II) and C can play a role in eliciting contact stability. More than one principal signaling pathway appears to be linked to the measurable parameter, since inhibitory substances show additive properties in co-incubation assays and differentially affect two lectin-elicited cellular activities, i.e. intracellular movement of Ca(2+)-ions and H2O2-generation, which can accompany cell adhesion and aggregation. Pronounced differences in the extent of modulation of H2O2-generation in human neutrophils by the same set of substances emphasizes that general conclusions on the post-binding effects for a certain lectin in different cell types are definitely precluded. In aggregate, the approach to employ inhibitors with target selectivity intimates an involvement of protein kinases A, C, Ca2+/calmodulin-dependent protein kinase II, p56lck tyrosine kinase, leukotrienes and/or hydroxyeicosatetraenoic acids, phosphatidylcholine-specific phospholipase C and Ca(2+)-fluxes in events following initial binding of a galactoside-specific plant lectin to rat thymocytes which establish firm cell contacts.

The prepilin peptidase is required for protein secretion by and the virulence of the intracellular pathogen Legionella pneumophila

Prepilin peptidases cleave, among other substrates, the leader sequences from prepilin-like proteins that are required for type II protein secretion in Gram-negative bacteria. To begin to assess the importance of type II secretion for the virulence of an intracellular pathogen, we examined the effect of inactivating the prepilin peptidase (pilD) gene of Legionella pneumophila. Although the pilD mutant and its parent grew similarly in bacteriological media, they did differ in colony attributes and recoverability from late stationary phase. Moreover, at least three proteins were absent from the mutant's supernatant, indicating that PilD is necessary for the secretion of Legionella proteins. The absence of both the major secreted protein and a haemolytic activity from the mutant signalled that the L. pneumophila zinc metalloprotease is excreted via type II secretion. Most interestingly, the pilD mutant was greatly impaired in its ability to grow within Hartmannella vermiformis amoebae and the human macrophage-like U937 cells. As reintroduction of pilD into the mutant restored inefectivity and as a mutant lacking type IV pilin replicated like wild type, these data suggested that the intracellular growth of L. pneumophila is promoted by proteins secreted via a type II pathway. Intratracheal inoculation of guinea pigs revealed that the LD50 for the pilD mutant is at least 100-fold greater than that for its parent, and the culturing of bacteria from infected animals showed a rapid clearance of the mutant from the lungs. This is the first study to indicate a role for PilD and type II secretion in intracellular parasitism.