Phagocytosis in Acanthamoeba: I. A mannose receptor is responsible for the binding and phagocytosis of yeast

Yeast-amoeba interaction influences murine cryptococcosis

The virulence of Cryptococcus spp. is modulated in the natural environment through interaction with abiotic and biotic factors, and this can occasionally have implications for the progression of cryptococcosis in mammals. Hence, we evaluated whether the prior interaction of highly virulent Cryptococcus gattii strain R265 with Acanthamoeba castellanii influenced the progression of cryptococcosis. The influence of the capsule on endocytosis was evaluated using amoeba and yeast morphometrics. Mice were intratracheally infected with yeast re-isolated from the amoeba (Interaction), yeast without prior contact with the amoeba (Non-Interaction), or sterile phosphate-buffered saline (SHAM). Morbidity signs and symptoms were monitored during the survival curve, while cytokine and fungal burden measurements and histopathological analysis were performed on the 10th day post infection. Morbidity and mortality parameters in experimental cryptococcosis were influenced by the prior interaction of yeast with amoeba, which led to phenotypic changes in the cryptococcal cells, polysaccharide secretion, and their tolerance to oxidative stress. Our results suggest that a prior yeast-amoeba interaction modulates yeast virulence, which is associated with a greater tolerance to oxidative stress related to the exo-polysaccharide content and influences the progression of cryptococcal infection.

Taurine, a Component of the Tear Film, Exacerbates the Pathogenic Mechanisms of Acanthamoeba castellanii in the Ex Vivo Amoebic Keratitis Model

Acanthamoeba spp. is the etiological agent of amoebic keratitis. In this study, the effect of taurine in physiological concentrations in tears (195 μM) on trophozoites of Acanthamoeba castellanii through the ex vivo amoebic keratitis model was evaluated. Trophozoites were coincubated with the Syrian golden hamster cornea (Mesocricetus auratus) for 3 and 6 h. Group 1: Control (-). Corneas coincubated with amoebic culture medium and taurine. Group 2: Control (+). Corneas coincubated with trophozoites without taurine. Group 3: Corneas coincubated with taurine 15 min before adding trophozoites. Group 4: Trophozoites coincubated 15 min with taurine before placing them on the cornea. Group 5: Corneas coincubated for 15 min with trophozoites; subsequently, taurine was added. Results are similar for both times, as evaluated by scanning electron microscopy. As expected, in the corneas of Group 1, no alterations were observed in the corneal epithelium. In the corneas of Group 2, few adhered trophozoites were observed on the corneal surface initiating migrations through cell junctions as previously described; however, in corneas of Groups 3, 4 and 5, abundant trophozoites were observed, penetrating through different corneal cell areas, emitting food cups and destabilizing corneal surface in areas far from cell junctions. Significant differences were confirmed in trophozoites adherence coincubated with taurine (p < 0.05). Taurine does not prevent the adhesion and invasion of the amoebae, nor does it favor its detachment once these have adhered to the cornea, suggesting that taurine in the physiological concentrations found in tears stimulates pathogenic mechanisms of A. castellanii.

On the origin of the functional versatility of macrophages

Macrophages represent the most functionally versatile cells in the animal body. In addition to recognizing and destroying pathogens, macrophages remove senescent and exhausted cells, promote wound healing, and govern tissue and metabolic homeostasis. In addition, many specialized populations of tissue-resident macrophages exhibit highly specialized functions essential for the function of specific organs. Sometimes, however, macrophages cease to perform their protective function and their seemingly incomprehensible response to certain stimuli leads to pathology. In this study, we address the question of the origin of the functional versatility of macrophages. To this end, we have searched for the evolutionary origin of macrophages themselves and for the emergence of their characteristic properties. We hypothesize that many of the characteristic features of proinflammatory macrophages evolved in the unicellular ancestors of animals, and that the functional repertoire of macrophage-like amoebocytes further expanded with the evolution of multicellularity and the increasing complexity of tissues and organ systems. We suggest that the entire repertoire of macrophage functions evolved by repurposing and diversification of basic functions that evolved early in the evolution of metazoans under conditions barely comparable to that in tissues of multicellular organisms. We believe that by applying this perspective, we may find an explanation for the otherwise counterintuitive behavior of macrophages in many human pathologies.

Biological characteristics and pathogenicity of Acanthamoeba

Acanthamoeba is an opportunistic protozoa, which exists widely in nature and is mainly distributed in soil and water. Acanthamoeba usually exists in two forms, trophozoites and cysts. The trophozoite stage is one of growth and reproduction while the cyst stage is characterized by cellular quiescence, commonly resulting in human infection, and the lack of effective monotherapy after initial infection leads to chronic disease. Acanthamoeba can infect several human body tissues such as the skin, cornea, conjunctiva, respiratory tract, and reproductive tract, especially when the tissue barriers are damaged. Furthermore, serious infections can cause Acanthamoeba keratitis, granulomatous amoebic encephalitis, skin, and lung infections. With an increasing number of Acanthamoeba infections in recent years, the pathogenicity of Acanthamoeba is becoming more relevant to mainstream clinical care. This review article will describe the etiological characteristics of Acanthamoeba infection in detail from the aspects of biological characteristic, classification, disease, and pathogenic mechanism in order to provide scientific basis for the diagnosis, treatment, and prevention of Acanthamoeba infection.

Comparative transcriptome combined with morphophysiological analyses revealed the molecular mechanism underlying Tetrahymena thermophila predation-induced antiphage defense in Aeromonas hydrophila

Protozoan predation has been demonstrated to be a strong driving force for bacterial defence strategies in the environment. Our previous study demonstrated that Aeromonas hydrophila NJ-35, which evolved small-colony variants (SCVs), displayed various adaptive traits in response to Tetrahymena thermophila predation, such as enhanced phage resistance. However, the evolutionary mechanisms are largely unknown. In this study, we performed a genome- and transcriptome-wide analysis of the SCV1, representing one strain of the SCVs, for identification of the genes of mutation and altered expression underlying this phage resistance phenotype. Our study demonstrated that phage resistance caused by T. thermophila predation was due to the downregulation of a flagellar biosynthesis regulator, flhF, in SCV1. Interestingly, we confirmed that phage resistance in SCV1 was not straightforwardly attributable to the absence of flagella but to FlhF-mediated secretion of extracellular protein that hinders phage adsorption. This finding improves our understanding of the mechanisms by which A. hydrophila lowers the susceptibility to phage infection under predation pressure, and highlights an important contribution of bacterium–protozoan interactions in driving the adaptive evolution of pathogens in complex environments.

Acanthamoeba Mannose and Laminin Binding Proteins Variation across Species and Genotypes

Acanthamoeba is a ubiquitous free-living amoeba capable of being an opportunistic pathogen in humans and animals. A critical step in infection is the adhesion of the amoeba to host cells and tissues, and two major parasite adhesins, mannose-binding protein (MBP) and laminin-binding protein (LBP), are known to recognize the cell surface glycoproteins and those of the extracellular matrix, respectively. In this study, the available genomes of Acanthamoeba were analysed to recover the sequences of MBP and LBP using previously published genetic data. Genes for both proteins were successfully obtained from strains belonging to various genotypes (T4A, T4D, T4G, T4F, T2, T5, T10, T22, T7 and T18), resulting in a single gene for LBP but identifying two types of MBP, MBP1 and MBP2. Phylogenetic analysis based on deduced amino acid sequences shows that both MBP and LBP have a branching pattern that is consistent with that based on 18S rDNA, indicating that changes in both proteins occurred during diversification of Acanthamoeba lines. Notably, all MBPs possess a conserved motif, shared with some bacterial C-type lectins, which could be the recognition site for mannose binding.

Commensals Serve as Natural Barriers to Mammalian Cells during Acanthamoeba castellanii Invasion

Acanthamoeba castellanii is a free-living, pathogenic ameba found in the soil and water. It invades the body through ulcerated skin, the nasal passages, and eyes and can cause blinding keratitis and granulomatous encephalitis. However, the mechanisms underlying the opportunistic pathogenesis of A. castellanii remain unclear. In this study, we observed that commensal bacteria significantly reduced the cytotoxicity of the ameba on mammalian cells. This effect occurred in the presence of both Gram-positive and Gram-negative commensals. Additionally, commensals mitigated the disruption of cell junctions. Ex vivo experiments on mouse eyeballs further showed that the commensals protected the corneal epithelial layer. Together, these findings indicate that A. castellanii is pathogenic to individuals with a dysbiosis of the microbiota at infection sites, further highlighting the role of commensals as a natural barrier during parasite invasion. IMPORTANCE Acanthamoeba castellanii, an opportunistic protozoan widely present in the environment, can cause Acanthamoeba keratitis and encephalitis in humans. However, only a few reports describe how the ameba acts as an opportunistic pathogen. Our study showed that the normal microbiota interfered with the cytotoxicity of Acanthamoeba, persevered during Acanthamoeba invasion, and reduced corneal epithelium peeling in the mouse eyeball model. This suggests that commensals may act as a natural barrier against Acanthamoeba invasion. In future, individuals who suffer from Acanthamoeba keratitis should be examined for microbiota absence or dysbiosis to reduce the incidence of Acanthamoeba infection in clinical settings.

War of the microbial world: Acanthamoeba spp. interactions with microorganisms

Acanthamoeba is known to interact with a plethora of microorganisms such as bacteria, fungi and viruses. In these interactions, the amoebae can be predatory in nature, transmission vehicle or an incubator. Amoebae consume microorganisms, especially bacteria, as food source to fulfil their nutritional needs by taking up bacteria through phagocytosis and lysing them in phagolysosomes and hence play an eminent role in the regulation of bacterial density in the nature and accountable for eradication of around 60% of the bacterial population in the environment. Acanthamoeba can also act as a “Trojan horse” for microbial transmission in the environment. Additionally, Acanthamoeba may serve as an incubator-like reservoir for microorganisms, including those that are pathogenic to humans, where the microorganisms use amoebae’s defences to resist harsh environment and evade host defences and drugs, whilst growing in numbers inside the amoebae. Furthermore, amoebae can also be used as a “genetic melting pot” where exchange of genes as well as adaptation of microorganisms, leading to higher pathogenicity, may arise. Here, we describe bacteria, fungi and viruses that are known to interact with Acanthamoeba spp.

Amoeba Predation of Cryptococcus neoformans Results in Pleiotropic Changes to Traits Associated with Virulence

Amoeboid predators, such as amoebae, are proposed to select for survival traits in soil microbes such as Cryptococcus neoformans; these traits can also function in animal virulence by defeating phagocytic immune cells, such as macrophages. Consistent with this notion, incubation of various fungal species with amoebae enhanced their virulence, but the mechanisms involved are unknown. In this study, we exposed three strains of C. neoformans (1 clinical and 2 environmental) to predation by Acanthamoeba castellanii for prolonged times and then analyzed surviving colonies phenotypically and genetically. Surviving colonies comprised cells that expressed either pseudohyphal or yeast phenotypes, which demonstrated variable expression of traits associated with virulence, such as capsule size, urease production, and melanization. Phenotypic changes were associated with aneuploidy and DNA sequence mutations in some amoeba-passaged isolates, but not in others. Mutations in the gene encoding the oligopeptide transporter (CNAG_03013; OPT1) were observed among amoeba-passaged isolates from each of the three strains. Isolates derived from environmental strains gained the capacity for enhanced macrophage toxicity after amoeba selection and carried mutations on the CNAG_00570 gene encoding Pkr1 (AMP-dependent protein kinase regulator) but manifested reduced virulence in mice because they elicited more effective fungal-clearing immune responses. Our results indicate that C. neoformans survival under constant amoeba predation involves the generation of strains expressing pleiotropic phenotypic and genetic changes. Given the myriad potential predators in soils, the diversity observed among amoeba-selected strains suggests a bet-hedging strategy whereby variant diversity increases the likelihood that some will survive predation.IMPORTANCECryptococcus neoformans is a ubiquitous environmental fungus that is also a leading cause of fatal fungal infection in humans, especially among immunocompromised patients. A major question in the field is how an environmental yeast such as C. neoformans becomes a human pathogen when it has no need for an animal host in its life cycle. Previous studies showed that C. neoformans increases its pathogenicity after interacting with its environmental predator amoebae. Amoebae, like macrophages, are phagocytic cells that are considered an environmental training ground for pathogens to resist macrophages, but the mechanism by which C. neoformans changes its virulence through interactions with protozoa is unknown. Our study indicates that fungal survival in the face of amoeba predation is associated with the emergence of pleiotropic phenotypic and genomic changes that increase the chance of fungal survival, with this diversity suggesting a bet-hedging strategy to ensure that some forms survive.

Paradigms of Protist/Bacteria Symbioses Affecting Human Health: Acanthamoeba species and Trichomonas vaginalis

Ever since the publication of the seminal paper by Lynn Margulis in 1967 proposing the theory of the endosymbiotic origin of organelles, the study of the symbiotic relationships between unicellular eukaryotes and prokaryotes has received ever-growing attention by microbiologists and evolutionists alike. While the evolutionary significance of the endosymbiotic associations within protists has emerged and is intensively studied, the impact of these relationships on human health has been seldom taken into account. Microbial endosymbioses involving human eukaryotic pathogens are not common, and the sexually transmitted obligate parasite Trichomonas vaginalis and the free-living opportunistic pathogen Acanthamoeba represent two unique cases in this regard, to date. The reasons of this peculiarity for T. vaginalis and Acanthamoeba may be due to their lifestyles, characterized by bacteria-rich environments. However, this characteristic does not fully explain the reason why no bacterial endosymbiont has yet been detected in unicellular eukaryotic human pathogens other than in T. vaginalis and Acanthamoeba, albeit sparse and poorly investigated examples of morphological identification of bacteria-like microorganisms associated with Giardia and Entamoeba were reported in the past. In this review article we will present the body of experimental evidences revealing the profound effects of these examples of protist/bacteria symbiosis on the pathogenesis of the microbial species involved, and ultimately their impact on human health.

Free-living amoebae promote Candida auris survival and proliferation in water

Candida auris is an emerging species responsible for life-threatening infections. Its ability to be resistant to most systemic antifungal classes and its capacity to persist in a hospital environment have led to health concerns. Currently, data about environmental reservoirs are limited but remain essential in control of C. auris spread. The aim of our study was to explore the interactions between C. auris and two free-living amoeba (FLA) species, Vermamoeba vermiformis and Acanthamoeba castellanii, potentially found in the same water environment. Candida auris was incubated with FLA trophozoites or their culture supernatants. The number of FLA and yeasts was determined at different times and transmission electron microscopy (TEM) was performed. Supernatants of FLAs promoted yeast survival and proliferation. Internalization of viable C. auris within both FLA species was also evidenced by TEM. A water environmental reservoir of C. auris can therefore be considered through FLAs and contamination of the hospital water networks would consequently be possible.

Acanthamoeba castellanii as an alternative interaction model for the dermatophyte Trichophyton rubrum

BACKGROUND

Trichophyton rubrum (Tr) is the main aetiological agent of human dermatophytosis, being isolated from the environment and keratinised tissues. In the environment, Tr can interact with other organisms, such as free-living amoebas (FLA), which can act as an alternative host system to study the interaction between microbes and phagocytic cells.

OBJECTIVES

To characterise the Acanthamoeba castellanii (ALX)-Tr interaction.

METHODS

Interaction was characterised in three conditions: trophozoites (PYG), late (PYG/NES) and early (NES) encystation stimulus, evaluating encystation kinetics, phagocytosis, exocytosis and fungicidal activity dynamics.

RESULTS

Tr was able to induce ALX encystation and be internalised by ALX. The number of internalised conidia was high at 1 hour, and ALX presented fungicidal activity with increased intracellular ROS production and exocytosis. In PYG/NES, phagocytosis and ROS production were reduced, with decreased ALX's fungicidal activity. However, in NES there was an increased fungal engulfment, and a reduced ROS production and higher fungal burden. Furthermore, exogenous mannose decreased phagocytosis of Tr conidia, and divalent cations induced ROS production and increased ALX's fungicidal activity. Interestingly, phagocytosis was reduced in the presence of cytoskeleton inhibitor, but exocytosis was increased, suggesting that Tr conidia may have alternative pathways to escape ALX's cells.

CONCLUSION

A castellanii is a proper model for studying Tr-FLA interaction, since ALX can engulf, produce ROS and kill Tr, and all these parameters are influenced by an encystation stimulus and divalent cations. Moreover, this interaction is likely to occur in the environment implicating in the adaptation to environmental stressful conditions in both organisms.

Giant virus vs amoeba: fight for supremacy

Since the discovery of mimivirus, numerous giant viruses associated with free-living amoebae have been described. The genome of giant viruses can be more than 2.5 megabases, and virus particles can exceed the size of many bacteria. The unexpected characteristics of these viruses have made them intriguing research targets and, as a result, studies focusing on their interactions with their amoeba host have gained increased attention. Studies have shown that giant viruses can establish host-pathogen interactions, which have not been previously demonstrated, including the unprecedented interaction with a new group of small viruses, called virophages, that parasitize their viral factories. In this brief review, we present recent advances in virophage-giant virus-host interactions and highlight selected studies involving interactions between giant viruses and amoebae. These unprecedented interactions involve the giant viruses mimivirus, marseillevirus, tupanviruses and faustovirus, all of which modulate the amoeba environment, affecting both their replication and their spread to new hosts.

The 'Amoeboid Predator-Fungal Animal Virulence' Hypothesis

The observation that some aspects of amoeba-fungal interactions resemble animal phagocytic cell-fungal interactions, together with the finding that amoeba passage can enhance the virulence of some pathogenic fungi, has stimulated interest in the amoeba as a model system for the study of fungal virulence. Amoeba provide a relatively easy and cheap model system where multiple variables can be controlled for the study of fungi-protozoal (amoeba) interactions. Consequently, there have been significant efforts to study fungal⁻amoeba interactions in the laboratory, which have already provided new insights into the origin of fungal virulence as well as suggested new avenues for experimentation. In this essay we review the available literature, which highlights the varied nature of amoeba-fungal interactions and suggests some unsolved questions that are potential areas for future investigation. Overall, results from multiple independent groups support the 'amoeboid predator⁻fungal animal virulence hypothesis', which posits that fungal cell predation by amoeba can select for traits that also function during animal infection to promote their survival and thus contribute to virulence.

Tupanvirus-infected amoebas are induced to aggregate with uninfected cells promoting viral dissemination

The discovery of giant viruses in the last years has fascinated the scientific community due to virus particles size and genome complexity. Among such fantastic discoveries, we have recently described tupanviruses, which particles present a long tail, and has a genome that contains the most complete set of translation-related genes ever reported in the known virosphere. Here we describe a new kind of virus-host interaction involving tupanvirus. We observed that tupanvirus-infected amoebas were induced to aggregate with uninfected cells, promoting viral dissemination and forming giant host cell bunches. Even after mechanical breakdown of bunches, amoebas reaggregated within a few minutes. This remarkable interaction between infected and uninfected cells seems to be promoted by the expression of a mannose receptor gene. Our investigations demonstrate that the pre-treatment of amoebas with free mannose inhibits the formation of bunches, in a concentration-dependent manner, suggesting that amoebal-bunch formation correlates with mannose receptor gene expression. Finally, our data suggest that bunch-forming cells are able to interact with uninfected cells promoting the dissemination and increase of tupanvirus progeny.

Bentonite clay supplemented diet on immunity in stinging catfish, Heteropneustes fossilis against Aeromonas hydrophila

The effect of Sodium Bentonite (SB) enriched diet on growth performance, innate immune response, and disease resistance in stinging catfish, Heteropneustes fossilis against Aeromonas hydrophila is reported. The infected fish fed with 5% SB had the maximum weight gain diet (PWG %) and specific growth rate (SGR %) were 26% and 29% when compared to 14% and 17% with 10% diet. Similarly the phagocytic activity increased significantly when infected fish were fed with 5% or 10% SB diets during the experimental period; the complement, respiratory burst and lysozyme activities were also significantly enhanced on weeks 2 and 4. The lower cumulative mortality (10% and 15%) was observed when the infected fish were fed with 5% and 10% SB diets for 30 days. The results suggest that the infected H. fossilis after administration of 5% and 10% SB enriched diets for 30 days had significantly improved growth performance, innate immunity, and disease resistance against A. hydrophilla. Hence, sodium bentonite can be used as a feed additive to stimulate immunity and for disease resistance in the effective production of economically valuable freshwater catfish, H. fossilis.

Fungi that Infect Humans

Fungi must meet four criteria to infect humans: growth at human body temperatures, circumvention or penetration of surface barriers, lysis and absorption of tissue, and resistance to immune defenses, including elevated body temperatures. Morphogenesis between small round, detachable cells and long, connected cells is the mechanism by which fungi solve problems of locomotion around or through host barriers. Secretion of lytic enzymes, and uptake systems for the released nutrients, are necessary if a fungus is to nutritionally utilize human tissue. Last, the potent human immune system evolved in the interaction with potential fungal pathogens, so few fungi meet all four conditions for a healthy human host. Paradoxically, the advances of modern medicine have made millions of people newly susceptible to fungal infections by disrupting immune defenses. This article explores how different members of four fungal phyla use different strategies to fulfill the four criteria to infect humans: the Entomophthorales, the Mucorales, the Ascomycota, and the Basidiomycota. Unique traits confer human pathogenic potential on various important members of these phyla: pathogenic Onygenales comprising thermal dimorphs such as Histoplasma and Coccidioides; the Cryptococcus spp. that infect immunocompromised as well as healthy humans; and important pathogens of immunocompromised patients-Candida, Pneumocystis, and Aspergillus spp. Also discussed are agents of neglected tropical diseases important in global health such as mycetoma and paracoccidiomycosis and common pathogens rarely implicated in serious illness such as dermatophytes. Commensalism is considered, as well as parasitism, in shaping genomes and physiological systems of hosts and fungi during evolution.

Phagocytosis in Teleosts. Implications of the New Cells Involved

Phagocytosis is the process by which cells engulf some solid particles to form internal vesicles known as phagosomes. Phagocytosis is in fact a specific form of endocytosis involving the vesicular interiorization of particles. Phagocytosis is essentially a defensive reaction against infection and invasion of the body by foreign substances and, in the immune system, phagocytosis is a major mechanism used to remove pathogens and/or cell debris. For these reasons, phagocytosis in vertebrates has been recognized as a critical component of the innate and adaptive immune responses to pathogens. Furthermore, more recent studies have revealed that phagocytosis is also crucial for tissue homeostasis and remodeling. Professional phagocytes in teleosts are monocyte/macrophages, granulocytes and dendritic cells. Nevertheless, in recent years phagocytic properties have also been attributed to teleost lymphocytes and thrombocytes. The possible implications of such cells on this important biological process, new factors affecting phagocytosis, evasion of phagocytosis or new forms of phagocytosis will be considered and discussed.

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.

Looking in amoebae as a source of mycobacteria

Mycobacteria exhibit various relationships with amoebae, ranging from the killing of one partner by the other one, to amoebae hosting mycobacteria in trophozoites and cysts. This observation indicates that poorly described biological factors affect the relationships, including mycobacterial cell-wall glycolipids and the size of the mycobacteria. Experimental observations indicate that a majority of environmental, opportunistic mycobacteria but also obligate pathogens including Mycobacterium tuberculosis, Mycobacterium leprae and Mycobacterium ulcerans are inter-amoebal organisms. Amoebae may give opportunities for genetic exchanges between mycobacteria, sympatric intra-amoebal organisms and the amoebae themselves. Amoebae clearly protect opportunistic mycobacterial pathogens during their environmental life but their role for obligate mycobacterial infection remains to be established. Accordingly, water was the source for emerging, community-acquired and health care-associated infection with amoeba-resisting mycobacteria of the Mycobacterium avium, Mycobacterium abscessus and Mycobacterium fortuitum groups, among others. Amoebae are organisms where mycobacteria can be found and, accordingly, amoeba co-culture can be used for the isolation of mycobacteria from environmental and clinical specimens. Looking in amoebae may help recovering new species of mycobacteria.

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.

Signaling at neuro/immune synapses

Immunological and neural synapses share properties such as the synaptic cleft, adhesion molecules, stability, and polarity. However, the mismatch in scale has limited the utility of these comparisons. The discovery of phosphatase micro-exclusion from signaling elements in immunological synapses and innate phagocytic synapses define a common functional unit at a common sub-micron scale across synapse types. Bundling of information from multiple antigen receptor microclusters by an immunological synapse has parallels to bundling of multiple synaptic inputs into a single axonal output by neurons, allowing integration and coincidence detection. Bonafide neuroimmune synapses control the inflammatory reflex. A better understanding of the shared mechanisms between immunological and neural synapses could aid in the development of new therapeutic modalities for immunological, neurological, and neuroimmunological disorders alike.

Receptor signaling clusters in the immune synapse

Signaling processes between various immune cells involve large-scale spatial reorganization of receptors and signaling molecules within the cell-cell junction. These structures, now collectively referred to as immune synapses, interleave physical and mechanical processes with the cascades of chemical reactions that constitute signal transduction systems. Molecular level clustering, spatial exclusion, and long-range directed transport are all emerging as key regulatory mechanisms. The study of these processes is drawing researchers from physical sciences to join the effort and represents a rapidly growing branch of biophysical chemistry. Recent advances in physical and quantitative analyses of signaling within the immune synapses are reviewed here.

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.

The type III secretion system is involved in Escherichia coli K1 interactions with Acanthamoeba

The type III secretion system among Gram-negative bacteria is known to deliver effectors into host cell to interfere with host cellular processes. The type III secretion system in Yersina, Pseudomonas and Enterohemorrhagic Escherichia coli have been well documented to be involved in the bacterial pathogenicity. The existence of type III secretion system has been demonstrated in neuropathogenic E. coli K1 strains. Here, it is observed that the deletion mutant of type III secretion system in E. coli strain EC10 exhibited defects in the invasion and intracellular survival in Acanthamoeba castellanii (a keratitis isolate) compared to its parent strain. Next, it was determined whether type III secretion system plays a role in E. coli K1 survival inside Acanthamoeba during the encystment process. Using encystment assays, our findings revealed that the type III secretion system-deletion mutant exhibited significantly reduced survival inside Acanthamoeba cysts compared with its parent strain, EC10 (P<0.01). This is the first demonstration that the type III secretion system plays an important role in E. coli interactions with Acanthamoeba. A complete understanding of how amoebae harbor bacterial pathogens will help design strategies against E. coli transmission to the susceptible hosts.

Identification, molecular characterization and expression analysis of a mucosal C-type lectin in the eastern oyster, Crassostrea virginica

Lectins are well known to actively participate in the defense functions of vertebrates and invertebrates where they play an important role in the recognition of foreign particles. They have also been reported to be involved in other processes requiring carbohydrate-lectin interactions such as symbiosis or fertilization. In this study, we report a novel putative C-type lectin (CvML) from the eastern oyster Crassostrea virginica and we investigated its involvement in oyster physiology. The cDNA of this lectin is 610 bp long encoding for a 161-residue protein. CvML presents a signal peptide and a single carbohydrate recognition domain (CRD) which contains a YPD motif and two putative conserved sites, WID and DCM, for calcium binding. CvML transcripts were expressed in mucocytes lining the epithelium of the digestive gland and the pallial organs (mantle, gills, and labial palps) but were not detected in other tissues including hemocytes. Its expression was significantly up-regulated following starvation or bacterial bath exposure but not after injection of bacteria into oyster's adductor muscle. These results highlight the potential role of CvML in the interactions between oyster and waterborne microorganisms at the pallial interfaces with possible involvement in physiological functions such as particle capture or mucosal immunity.

Quantification and Characterization of Phagocytosis in the Soil Amoeba Acanthamoeba castellanii by Flow Cytometry

Phagocytosis in the common grazing soil amoeba Acanthamoeba castellanii was characterized by flow cytometry. Uptake of fluorescently labelled latex microbeads by cells was quantified by appropriate setting of thresholds on light scatter channels and, subsequently, on fluorescence histograms. Confocal laser scanning microscopy was used to verify the effectiveness of sodium azide as a control for distinguishing between cell surface binding and internalization of beads. It was found that binding of beads at the cell surface was complete within 5 min and 80% of cells had beads associated with them after 10 min. However, the total number of phagocytosed beads continued to rise up to 2 h. The prolonged increase in numbers of beads phagocytosed was due to cell populations containing increasing numbers of beads peaking at increasing time intervals from the onset of phagocytosis. Fine adjustment of thresholds on light scatter channels was used to fractionate cells according to cell volume (cell cycle stage). Phagocytotic activity was approximately threefold higher in the largest (oldest) than in the smallest (newly divided) cells of A. castellanii and showed some evidence of periodicity. At no stage in the cell cycle did phagocytosis cease. Binding and phagocytosis of beads were also markedly influenced by culture age and rate of rotary agitation of cell suspensions. Saturation of phagocytosis (per cell) at increasing bead or decreasing cell concentrations occurred at bead/cell ratios exceeding 10:1. This was probably a result of a limitation of the vacuolar uptake system of A. castellanii, as no saturation of bead binding was evident. The advantages of flow cytometry for characterization of phagocytosis at the single-cell level in heterogeneous protozoal populations and the significance of the present results are discussed.

Microalgal cell surface carbohydrates as recognition sites for particle sorting in suspension-feeding bivalves

Cell surface carbohydrates play important roles in cell recognition mechanisms. Recently, we provided evidence that particle selection by suspension-feeding bivalves can be mediated by interactions between carbohydrates associated with the particle surface and lectins present in mucus covering bivalve feeding organs. In this study, we used lectins tagged with fluorescein isothiocyanate (FITC) to characterize carbohydrate moieties on the surface of microalgal species and evaluate the effect of oyster mucus on lectin binding. These analyses revealed that concanavalin A (Con A), one of six lectins tested, bound to Isochrysis sp., while Nitzschia closterium reacted with Pisum sativum agglutinin (PNA) and peanut agglutinin (PEA). The cell surface of Rhodomonas salina bound with PNA and Con A, and Tetraselmis maculata cell surface was characterized by binding with PNA, PEA, and Con A. Pre-incubation of microalgae with oyster pallial mucus significantly decreased the binding of FITC-labeled lectins, revealing that lectins present in mucus competitively blocked binding sites. This decrease was reversed by washing mucus-coated microalgae with specific carbohydrates. These results were used to design a feeding experiment to evaluate the effect of lectins on sorting of microalgae by oysters. Crassostrea virginica fed with an equal ratio of Con A-labeled Isochrysis sp. and unlabeled Isochrysis sp. produced pseudofeces that were significantly enriched in Con A-labeled Isochrysis sp. and depleted in unlabeled microalgae. Selection occurred even though two physical-chemical surface characteristics of the cells in each treatment did not differ significantly. This work confirms the involvement of carbohydrate-lectin interaction in the particle sorting mechanism in oysters, and provides insights into the carbohydrate specificity of lectins implicated in the selection of microalgal species.

Free-living amoebae, a training field for macrophage resistance of mycobacteria

Mycobacterium species evolved from an environmental recent common ancestor by reductive evolution and lateral gene transfer. Strategies selected through evolution and developed by mycobacteria resulted in resistance to predation by environmental unicellular protists, including free-living amoebae. Indeed, mycobacteria are isolated from the same soil and water environments as are amoebae, and experimental models using Acanthamoeba spp. and Dictyostelium discoideum were exploited to analyse the mechanisms for intracellular survival. Most of these mechanisms have been further reproduced in macrophages for mycobacteria regarded as opportunistic and obligate pathogens. Amoebal cysts may protect intracellular mycobacteria against adverse conditions and may act as a vector for mycobacteria. The latter hypothesis warrants further environmental and clinical studies to better assess the role of free-living amoebae in the epidemiology of infections caused by mycobacteria.

Mechanism involved in phagocytosis and killing of Listeria monocytogenes by Acanthamoeba polyphaga

Intra-cellular pathogen, Listeria monocytogenes, is capable of invasion and survival within mammalian cells. However, Acanthamoeba polyphaga trophozoites phagocytose and rapidly degrade Listeria cells. In order to provide more information on amoeba phagocytosis and killing mechanisms, this study used several inhibitor agents known to affect the phagocytosis and killing of bacteria by eukaryotes. Amoebae were pre-treated with mannose, cytochalasin D, wortmannin, suramin, ammonium chloride, bafilomycin A and monensin followed by co-culture with bacteria. Phagocytosis and killing of bacterial cells by amoeba trophozoites was assessed using plate counting methods and microscopy. The data presented indicates that actin polymerisation and cytoskeletal rearrangement are involved in phagocytosis of L. monocytogenes cells by A. polyphaga trophozoites. Further, both phagosomal acidification and phagosome-lysosome fusion are involved in killing and degradation of L. monocytogenes cells by A. polyphaga. However, the mannose-binding protein receptor does not play an important role in uptake of bacteria by amoeba trophozoites. In conclusion, this data reveals the similar principles of molecular mechanisms used by different types of eukaryotes in uptake and killing of bacteria.

The role of biofilms and protozoa in Legionella pathogenesis: implications for drinking water

Current models to study Legionella pathogenesis include the use of primary macrophages and monocyte cell lines, various free-living protozoan species and murine models of pneumonia. However, there are very few studies of Legionella spp. pathogenesis aimed at associating the role of biofilm colonization and parasitization of biofilm microbiota and release of virulent bacterial cell/vacuoles in drinking water distribution systems. Moreover, the implications of these environmental niches for drinking water exposure to pathogenic legionellae are poorly understood. This review summarizes the known mechanisms of Legionella spp. proliferation within Acanthamoeba and mammalian cells and advocates the use of the amoeba model to study Legionella pathogenicity because of their close association with Legionella spp. in the aquatic environment. The putative role of biofilms and amoebae in the proliferation, development and dissemination of potentially pathogenic Legionella spp. is also discussed. Elucidating the mechanisms of Legionella pathogenicity development in our drinking water systems will aid in elimination strategies and procedural designs for drinking water systems and in controlling exposure to Legionella spp. and similar pathogens.

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.

Biochemical prey recognition by planktonic protozoa

Planktonic flagellates and ciliates are the major consumers of phytoplankton and bacterioplankton in aquatic environments, playing a pivotal role in carbon cycling and nutrient regeneration. Despite certain unicellular predators using chemosensory responses to locate and select their prey, the biochemical mechanisms behind prey reception and selection have not been elucidated. Here we identify a Ca(2+)-dependent, mannose-binding lectin on the marine dinoflagellate Oxyrrhis marina, which is used as a feeding receptor for recognizing prey. Blocking the receptor using 20 microM mannose-BSA inhibited ingestion of phytoplankton prey, Isochrysis galbana, by 60%. In prey selection studies, O. marina ingested twice as many 6 mum diameter beads coated with mannose-BSA as those coated with galNac-BSA. When pre-incubated with mannose-BSA, O. marina was no longer able to discriminate between different sugar-coated beads. Thus, these findings reveal molecular mechanisms of protozoan prey recognition. Our results also indicate the functional similarity between cellular recognition used by planktonic protozoa to discriminate between different prey items, and those used by metazoan phagocytic blood cells to recognize invading microorganisms.

Periodontopathogenic bacteria multiply in the environmental amoeba Acanthamoeba castellani

Porphyromonas gingivalis and Prevotella intermedia are obligate anaerobe gram-negative periodontopathogenic bacteria. Free-living amoebae, such as Acanthamoeba spp., are well known as environmental hosts of several human pathogens, such as Franciscella tularensis, Chlamydia pneumoniae, Legionella pneumophila and Mycobacteria spp. This study tested the ability of P. gingivalis and P. intermedia to become internalized, to survive and replicate in Acanthamoeba castellani. Our results show for the first time that P. gingivalis and P. intermedia isolated from periodontitis patients are capable of infecting A. castellani cells in vitro and are able to survive and multiply intracellularly. From our experimental data it can be suggested that periodontopathogenic bacteria might be conditioned in the evolution by amoebae and free-living amoebae can act as an environmental reservoir for these pathogens.

Characterization of Acanthamoeba–microsphere association by multiparameter flow cytometry and confocal microscopy

BACKGROUND

Acanthamoebae, in common with other protozoa, readily endocytose particulate material, which in turn may lead to the spread of infectious disease.

METHODS

Evaluation and quantification of plain and carboxylate FITC-microsphere association with acanthamoebal trophzoites was undertaken using a combination of flow cytometry and confocal microscopy. Trophozoites from strains and species of Acanthamoeba were exposed to plain and carboxylate FITC-microspheres. Microsphere size and aspects such as trophozoite starvation, maturity, and exposure to metabolic inhibitors were assessed.

RESULTS

All species and strains of Acanthamoeba readily endocytosed plain and carboxylate microspheres. Starving trophozoites significantly increased binding and potential ingestion of microspheres, whereas trophozoites of increasing maturity lost such abilities. Trophozoites showed a significant preference for 2.0- and 3.0-microm-diameter microspheres when compared with other sizes, which in turn could occupy much of the cytoplasm. The physiological inhibitors sodium azide, 2,4-dinitrophenol, and cytochalasin B reduced microsphere association with trophozoites; however, some microspheres still bound and associated with trophozoites after inhibitor exposure, a manifestation of both active and inactive agent involvement in microsphere endocytosis.

CONCLUSIONS

Even though the origins of microsphere binding by acanthamoebal trophozoite remains shrouded, the combination of flow cytometry and confocal microscopy supported synergistic quantification and qualification of trophozoite-microsphere endocytosis.

Porins limit the intracellular persistence of Mycobacterium smegmatis

The genus Mycobacterium comprises highly pathogenic as well as opportunistic or apathogenic species exhibiting a great variability with respect to their ability to persist or multiply within monocytic host cells. The impact of the permeability of the mycobacterial outer membrane on intracellular persistence was studied. For this purpose, a Mycobacterium smegmatis mutant with a deletion of the major porin gene mspA and a second mutant lacking mspA and the homologous porin gene mspC were used. Deletion of mspA together with mspC significantly enhanced intracellular persistence in murine bone marrow macrophages, the mouse macrophage cell line J774A.1 and Acanthamoeba castellanii. Complementation of mspA in the porin mutant strains resulted in restoration of the wild-type phenotype with respect to intracellular persistence. This is the first report to show that the deletion of porins of mycobacteria results in improved persistence in eukaryotic cells, demonstrating that the intracellular persistence of M. smegmatis depends upon the permeability of the outer membrane.

Evidence that hsp90 is involved in the altered interactions of Acanthamoeba castellanii variants with bacteria

There are many similarities between the interactions of environmental protozoa with pathogenic bacterial species and those observed in mammalian macrophages. Since single-celled protozoa predate mammalian hosts, it is likely that interactions in environmental biofilms have selected for many of the bacterial virulence mechanisms responsible for human disease. In order to better understand bacterial-phagocyte interactions, we developed a selection for Acanthamoeba castellanii variants that are more resistant to killing by bacterial pathogens. We identified four amoebal clones that display decreased phagocytosis of bacteria but no difference in uptake of latex beads compared to wild-type amoebae. These amoebal variants display differences in cellular morphology, partial resistance to killing by bacteria, more bactericidal activity, and higher frequencies of lysosome fusion with the bacterial vacuole. Three proteins are present at lower levels in these variants than in wild-type amoebae, and matrix-assisted laser desorption ionization-time of flight mass spectrometry allowed identification of two of them as actin and hsp90. We found that specific inhibitors of hsp90 produce a similar phenotypic effect in macrophages. These data suggest that hsp90 plays a role in phagocytic and, possibly, bactericidal pathways that affect interactions of phagocytic cells with bacteria.

Mechanisms associated with Acanthamoeba castellanii (T4) phagocytosis

Using fluorescein isothiocyanate (FITC)-labelled Escherichia coli, phagocytosis in Acanthamoeba is studied. This assay is based on the quenching effect of trypan blue on FITC-labelled E. coli. Only intracellular E. coli retain their fluorescence, which are easily discriminated from non-fluorescent adherent bacteria. Acanthamoeba uptake of E. coli is significantly reduced in the presence of genistein, a protein tyrosine kinase inhibitor. In contrast, sodium orthovanadate (protein tyrosine phosphatase inhibitor) increases bacterial uptake by Acanthamoeba. Treatment of Acanthamoeba with cytochalasin D (actin polymerization inhibitor) abolished the ability of Acanthamoeba to phagocytose E. coli suggesting that tyrosine kinase-mediated signaling may play a role in Acanthamoeba phagocytosis. In addition, we showed that phosphatidylinositol 3-kinase (PI3K) plays an important role in Acanthamoeba uptake of E. coli. Role of mannose-binding protein in Acanthamoeba phagocytosis is discussed further.

Pathogenesis of Acanthamoeba infections

Acanthamoeba are free-living, harmless organisms, however, given the opportunity and the appropriate conditions, they can cause painful, sight-threatening as well as fatal infections and, thus, are considered opportunistic pathogens. Acanthamoeba infections have become increasingly important in the past few years due to increasing populations of contact lens users and AIDS patients. The mechanisms associated with the pathogenesis of Acanthamoeba tend to be highly complex, depending on parasite, host and the environmental factors. Elucidation of the biochemical, cellular and molecular basis of the pathogenesis of diseases caused by Acanthamoeba may lead to the development of therapeutic interventions.

Acanthamoeba spp. as agents of disease in humans

Acanthamoeba spp. are free-living amebae that inhabit a variety of air, soil, and water environments. However, these amebae can also act as opportunistic as well as nonopportunistic pathogens. They are the causative agents of granulomatous amebic encephalitis and amebic keratitis and have been associated with cutaneous lesions and sinusitis. Immuno compromised individuals, including AIDS patients, are particularly susceptible to infections with Acanthamoeba. The immune defense mechanisms that operate against Acanthamoeba have not been well characterized, but it has been proposed that both innate and acquired immunity play a role. The ameba's life cycle includes an active feeding trophozoite stage and a dormant cyst stage. Trophozoites feed on bacteria, yeast, and algae. However, both trophozoites and cysts can retain viable bacteria and may serve as reservoirs for bacteria with human pathogenic potential. Diagnosis of infection includes direct microscopy of wet mounts of cerebrospinal fluid or stained smears of cerebrospinal fluid sediment, light or electron microscopy of tissues, in vitro cultivation of Acanthamoeba, and histological assessment of frozen or paraffin-embedded sections of brain or cutaneous lesion biopsy material. Immunocytochemistry, chemifluorescent dye staining, PCR, and analysis of DNA sequence variation also have been employed for laboratory diagnosis. Treatment of Acanthamoeba infections has met with mixed results. However, chlorhexidine gluconate, alone or in combination with propamidene isethionate, is effective in some patients. Furthermore, effective treatment is complicated since patients may present with underlying disease and Acanthamoeba infection may not be recognized. Since an increase in the number of cases of Acanthamoeba infections has occurred worldwide, these protozoa have become increasingly important as agents of human disease.

SA. Phagocytosis affects biguanide sensitivity of Acanthamoeba spp

The incidence of Acanthamoeba keratitis, a disease associated with contact lens wear, has been in apparent decline with the advent of multipurpose contact lens solutions. The concentrations of the biguanides chlorhexidine digluconate (CHX) and particularly polyhexamethylene biguanide (PHMB) included in multipurpose solutions (MPSs) are sublethal for amoebae. We evaluated by flow cytometry the effects of these two biguanides on phagocytosis of particles and the survival of trophozoites of Acanthamoeba castellanii and A. polyphaga. Trophozoites of A. castellanii and A. polyphaga (10(6)/ml) were exposed to solutions of 5 and 50 microg of PHMB and CHX per ml in the presence and absence of particles (i.e., heat-killed yeasts and bacteria and latex beads). In addition, trophozoites were exposed to particles treated with these concentrations of the two biguanides. In the absence of particles, trophozoites of A. polyphaga appeared to be more resistant to the biguanides than those of A. castellanii. In the presence of particles, the rates of survival of both species were decreased. In most instances, particles treated with sublethal concentrations of both biguanides that were adsorbed onto the particles reduced the incidence of phagocytosis. Particles present in MPSs in contact lens cases may be involved in the decreased incidence of Acanthamoeba keratitis.

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.