Free-living amoebae: pathogenicity and immunity

Lactoferrin and Lysozyme Inhibit the Proteolytic Activity and Cytopathic Effect of Naegleria fowleri Enzymes

Naegleria fowleri is a ubiquitous free-living amoeba that causes primary amoebic meningoencephalitis. As a part of the innate immune response at the mucosal level, the proteins lactoferrin (Lf) and lysozyme (Lz) are secreted and eliminate various microorganisms. We demonstrate that N. fowleri survives the individual and combined effects of bovine milk Lf (bLf) and chicken egg Lz (cLz). Moreover, amoebic proliferation was not altered, even at 24 h of co-incubation with each protein. Trophozoites' ultrastructure was evaluated using transmission electron microscopy, and these proteins did not significantly alter their organelles and cytoplasmic membranes. Protease analysis using gelatin-zymograms showed that secreted proteases of N. fowleri were differentially modulated by bLf and cLz at 3, 6, 12, and 24 h. The bLf and cLz combination resulted in the inhibition of N. fowleri-secreted proteases. Additionally, the use of protease inhibitors on bLf-zymograms demonstrated that secreted cysteine proteases participate in the degradation of bLf. Nevertheless, the co-incubation of trophozoites with bLf and/or cLz reduced the cytopathic effect on the MDCK cell line. Our study suggests that bLf and cLz, alone or together, inhibited secreted proteases and reduced the cytopathic effect produced by N. fowleri; however, they do not affect the viability and proliferation of the trophozoites.

Acanthamoeba castellanii Genotype T4: Inhibition of Proteases Activity and Cytopathic Effect by Bovine Apo-Lactoferrin

Acanthamoeba castellanii genotype T4 is a clinically significant free-living amoeba that causes granulomatous amoebic encephalitis and amoebic keratitis in human beings. During the initial stages of infection, trophozoites interact with various host immune responses, such as lactoferrin (Lf), in the corneal epithelium, nasal mucosa, and blood. Lf plays an important role in the elimination of pathogenic microorganisms, and evasion of the innate immune response is crucial in the colonization process. In this study, we describe the resistance of A. castellanii to the microbicidal effect of bovine apo-lactoferrin (apo-bLf) at different concentrations (25, 50, 100, and 500 µM). Acanthamoeba castellanii trophozoites incubated with apo-bLf at 500 µM for 12 h maintained 98% viability. Interestingly, despite this lack of effect on viability, our results showed that the apo-bLf inhibited the cytopathic effect of A. castellanii in MDCK cells culture, and analysis of amoebic proteases by zymography showed significant inhibition of cysteine and serine proteases by interaction with the apo-bLf. From these results, we conclude that bovine apo-Lf influences the activity of A. castellanii secretion proteases, which in turn decreases amoebic cytopathic activity.

Histological Changes in the Kidneys and Heart in Experimental Acanthamoebiasis in Immunocompetent and Immunosuppressed Hosts

The course of Acanthamoeba spp. infection depends on the age and immune status of the host, and the virulence of the Acanthamoeba spp. strain. Some strains of free-living amoebae exhibit organ specificity, during the course of infection, while others may cause changes in many organs or completely lose pathogenicity. The aim of the current study was to investigate the pathological properties of Acanthamoeba spp. isolated from a patient with acute myeloid leukemia and atypical pneumonia (AM22). Moreover, the objective was to investigate the histopathological changes in the kidneys and heart of immunocompetent and immunosuppressed mice infected with Acanthamoeba spp. Amoebae were re-isolated from both the kidneys and hearts of the inoculated mice, although no cysts or trophozoites of the amoebae were detected in microscopic slides of the fragments of these organs. Acanthamoeba spp. induced changes in the kidney and heart weight of infected mice. In immunocompetent and immunosuppressed Acanthamoeba spp. infected mice, we found some histopathological changes, including areas with less acidic cytoplasm and a relaxation of muscle fibers. In further studies, it is important to analyze changes in gene and protein expressions in the heart and kidneys of hosts with disseminated acanthamoebiasis to better understand the course of infection in these organs, because the results of histological analysis varied depending on the immune status and duration of infection.

Corneal Changes in Acanthamoeba Keratitis at Various Levels of Severity: An In Vivo Confocal Microscopic Study

Purpose

To investigate the relationship between Acanthamoeba cysts and inflammatory cells in Acanthamoeba keratitis (AK) by in vivo confocal microscopy (IVCM).

Methods

A case-control study included 30 patients with AK and 20 normal controls. The severity of the AK was divided into mild, moderate, and severe. The central cornea and four standard quadrants of the peripheral cornea were imaged by IVCM. The cyst infiltration and dendritic cell (DC) density and maturity (size, length, field, and number of dendrites) were quantified. The relationship between clinical severity, cyst density, and DC alterations was characterized by Spearman correlation analysis.

Results

The maximum cyst density in the mild, moderate, and severe groups was 31.3 cysts/mm2 (17.2-32.8), 62.5 cysts/mm2 (59.3-103.1), and 162.5 cysts/mm2 (65.6-215.6), respectively. Compared to normal participants, a significant increase in the mean corneal DC density was detected in patients with AK (290.2 ± 97.0 vs. 25.3 ± 8.3 cells/mm2; P < 0.001). Patients with AK presented an increase in median DC size (178.3 vs. 63.6 µm2/cell, P < 0.001), median DC field (518.1 vs. 256.6 µm2/cell, P = 0.008), and median DC dendrite length (42.3 vs. 14.7 µm/cell, P < 0.001). Increased AK severity was correlated with an increase in cyst density, DC size, and dendrite length (all P < 0.05). An increase in cyst density was significantly correlated with an increase in DC density (β = 0.484, P = 0.026) and DC size (β = 0.557, P = 0.009).

Conclusions

Cyst density and depth of infiltration as well as maturity of the surrounding DC increased significantly with the severity of AK.

Translational Relevance

Quantitative analysis of cyst density and DC maturity may provide a new method of evaluating the severity of AK.

Schwann Cell Autophagy and Necrosis as Mechanisms of Cell Death by Acanthamoeba

Amoebae of the genus Acanthamoeba are etiological agents of granulomatous amoebic encephalitis (GAE). Recently, through an in vivo GAE model, Acanthamoeba trophozoites were immunolocalized in contact with the peripheral nervous system (PNS) cells—Schwann cells (SC). In this study, we analyzed in greater detail the in vitro early morphological events (1, 2, 3, and 4 h) during the interaction of A. culbertsoni trophozoites (ATCC 30171) with SC from Rattus norvegicus (ATCC CRL-2941). Samples were processed for scanning and transmission electron microscopy as well as confocal microscopy. After 1 h of interaction, amoebae were observed to be adhered to the SC cultures, emitting sucker-like structures associated with micro-phagocytic channels. In addition, evidence of necrosis was identified since edematous organelles as well as multivesicular and multilamellar bodies characteristics of autophagy were detected. At 2 h, trophozoites migrated beneath the SC culture in which necrosis and autophagy persisted. By 3 and 4 h, extensive lytic zones were observed. SC necrosis was confirmed by confocal microscopy. We reported for the first time the induction of autophagic and necrotic processes in PNS cells, associated in part with the contact-dependent pathogenic mechanisms of A. culbertsoni trophozoites.

Molecular Driving Forces in Peptide Adsorption to Metal Oxide Surfaces

Molecular recognition between peptides and metal oxide surfaces is a fundamental process in biomineralization, self-assembly, and biocompatibility. Yet, the underlying driving forces and dominant mechanisms remain unclear, bringing obstacles to understand and control this process. To elucidate the mechanism of peptide/surface recognition, specifically the role of serine phosphorylation, we employed molecular dynamics simulation and metadynamics-enhanced sampling to study five artificial peptides, DDD, DSS, DpSpS, DpSpSGKK, and DpSKGpSK, interacting with two surfaces: rutile TiO₂ and quartz SiO₂. On both surfaces, we observe that phosphorylation increases the binding energy. However, the interfacial peptide conformation reveals a distinct binding mechanism on each surface. We also study the impact of peptide sequence to binding free energy and interfacial conformation on both surfaces, specifically the impact on the behavior of phosphorylated serine. Finally, the results are discussed in context of prior studies investigating the role of serine phosphorylation in peptide binding to silica.

Mouse neutrophils release extracellular traps in response to Naegleria fowleri

Naegleria fowleri is a free-living amoeba, which is able to infect humans through the nasal mucosa causing a disease in the central nervous system known as primary amoebic meningoencephalitis (PAM). Polymorphonuclear cells (PMNs) play a critical role in the early phase of N fowleri infection. Recently, a new biological defence mechanism called neutrophil extracellular traps (NETs) has been attracting attention. These structures represent an important strategy to immobilize and kill invading microorganisms. In this work, we evaluate the capacity of N fowleri to induce the NETs release by PMNs cells in mice in vitro and in vivo. In vitro: Neutrophils from bone marrow were cocultured with N fowleri trophozoites. In vivo: we employed a mouse model of PAM. We evaluated DNA, histone and myeloperoxidase (MPO) and the formation of NETs by confocal microscopy. Our results showed N fowleri induce both NETs and MPO release by PMNs cells in mice after trophozoite exposure, which increased through time, in vitro and in vivo. These results demonstrate that NETs are somehow associated with the amoebas. We suggest PMNs release their traps trying to avoid N fowleri attachment at the apical side of the nasal epithelium.

Type 2 diabetes mellitus BALB/c mice are more susceptible to granulomatous amoebic encephalitis: Immunohistochemical study

Granulomatous amoebic encephalitis (GAE) is a chronic, difficult to resolve infection caused by amphizoic amoebae of the genus Acanthamoeba, which in most cases occurs in immunosuppressed persons or with chronic diseases such as diabetes. In this study, we describe the early events of A. culbertsoni infection of GAE in diabetic mice model. Diabetes was induced in male BALB/c mice, with a dose of streptozotocin (130 mg/kg). Healthy and diabetic mice were inoculated via intranasal with 1 × 10⁶ trophozoites of A. culbertsoni. Then were sacrificed and fixed by perfusion at 24, 48, 72 and 96 h post-inoculation, the brains and nasopharyngeal meatus were processed to immunohistochemical analysis. Invasion of trophozoites in diabetic mice was significantly greater with respect to inoculated healthy mice. Trophozoites and scarce cysts were immunolocalized in respiratory epithelial adjacent bone tissue, olfactory nerve packets, Schwann cells and the epineurium base since early 24 h post-inoculation. After 48 h, trophozoites were observed in the respiratory epithelium, white matter of the brain, subcortical central cortex and nasopharyngeal associated lymphoid tissue (NALT). At 72 h, cysts and trophozoites were immunolocalized in the olfactory bulb with the presence of a low inflammatory infiltrate characterized by polymorphonuclear cells. Scarce amoebae were observed in the granular layer of the cerebellum without evidence of inflammation or tissue damage. No amoebas were observed at 96 h after inoculation, suggesting penetration to other tissues at this time. In line with this, no inflammatory infiltrate was observed in the surrounding tissues where the amoebae were immunolocalized, which could contribute to the rapid spread of infection, particularly in diabetic mice. All data suggest that trophozoites invade the tissues by separating the superficial cells, penetrating between the junctions without causing cytolytic effect in the adjacent cells and subsequently reaching the CNS, importantly, diabetes increases the susceptibility to amoebae infection, which could favor the GAE development.

Disseminated Acanthamoeba Infection Masquerading as Bacillary Angiomatosis in a Patient With AIDS

Disseminated Acanthamoeba infections and bacillary angiomatosis are among the un usual opportunistic infections encountered in patients with AIDS. We report a fatal case of disseminated Acanthamoeba palestinensis infection involving the skin, palate, brain, lungs, and testes in a patient with AIDS in whom the mucocutaneous lesions were initially interpreted clinically and pathologically as bacillary angiomatosis. Int J Surg Pathol 2(1):11-16, 1994

Neutrophils extracellular traps damage Naegleria fowleri trophozoites opsonized with human IgG

Naegleria fowleri infects humans through the nasal mucosa causing a disease in the central nervous system known as primary amoebic meningoencephalitis (PAM). Polymorphonuclear cells (PMNs) play a critical role in the early phase of N. fowleri infection. Recently, a new biological defence mechanism called neutrophil extracellular traps (NETs) has been attracting attention. NETs are composed of nuclear DNA combined with histones and antibacterial proteins, and these structures are released from the cell to direct its antimicrobial attack. In this work, we evaluate the capacity of N. fowleri to induce the liberation of NETs by human PMN cells. Neutrophils were cocultured with unopsonized or IgG-opsonized N. fowleri trophozoites. DNA, histone, myeloperoxidase (MPO) and neutrophil elastase (NE) were stained, and the formation of NETs was evaluated by confocal microscopy and by quantifying the levels of extracellular DNA. Our results showed N. fowleri induce the liberation of NETs including release of MPO and NE by human PMN cells as exposure interaction time is increased, but N. fowleri trophozoites evaded killing. However, when trophozoites were opsonized, they were susceptible to the neutrophils activity. Therefore, our study suggests that antibody-mediated PMNs activation through NET formation may be crucial for antimicrobial responses against N. fowleri.

Overview of the effect and epidemiology of parasitic central nervous system infections in African children

Infections of the central nervous system are a significant cause of neurologic dysfunction in resource-limited countries, especially in Africa. The prevalence is not known and is most likely underestimated because of the lack of access to accurate diagnostic screens. For children, the legacy of subsequent neurodisability, which affects those who survive, is a major cause of the burden of disease in Africa. Of the parasitic infections with unique effect in Africa, cerebral malaria, neurocysticercosis, human African trypanosomiasis, toxoplasmosis, and schistosomiasis are largely preventable conditions, which are rarely seen in resource-equipped settings. This article reviews the current understandings of these parasitic and other rarer infections, highlighting the specific challenges in relation to prevention, diagnosis, treatment, and the complications of coinfection.

Acanthamoeba spp. in domestic tap water in houses of contact lens wearers in the metropolitan area of Mexico City

A survey was carried out in the metropolitan area of Mexico City to determine the presence of Acanthamoeba in the tap water of houses of contact lens wearers. Water samples were taken from the mains water entry, bathroom sinks and storage containers (roof tanks, cisterns) of 27 houses; and from the solution contained in the contact lens cases. Samples were filtered and cultured onto NNE medium. The isolates were identified based on their morphological features and pathogenicity. Total and fecal coliforms, water temperature, pH, dissolved oxygen and residual free-chlorine were measured by standard methods. Forty five isolates of Acanthamoeba from 200 water samples were obtained. The highest number of amoebae was isolated from cisterns and roof tanks. Most Acanthamoeba isolates were non-pathogenic, however, their presence in tap water is a potential hazard since some species can cause Acanthamoeba keratitis and granulomatous amoebic encephalitis.

Temporal analysis of protozoan lysis in a microfluidic device

A microfluidic device was fabricated and characterized for studying cell lysis of Arcella vulgaris, a nonpathogenic amoeba, over time. The device contains a series of chambers which capture cells allowing them to be subsequently exposed to a constant flow of biocidal agent. With this microfluidic system, individual cells are observed as they undergo lysis. This allows high-throughput measurements of individual lysis events, which are not possible with conventional techniques. Differences in lysis and decay times for Arcella were seen at different flow rates and concentrations of benzalkonium chloride, a biocidal detergent. The efficacy of benzalkonium chloride, chlorhexidine digluconate, phenol, sodium dodecyl sulfate, and Triton X-100 were compared, revealing information on their mechanisms of action. The presented device allows cell capture, controlled exposure to chemical biocides, and observation of lysis with single-cell resolution. Observations at the single cell level give insight into the mechanistic details of the lysis of individual Arcella cells vs. the population; decay times for individual Arcella cells were much shorter when compared to a population of 15 cells.

[Isolation and identification of potentially pathogenic free-living amoebae in samples from environments in a public hospital in the city of Porto Alegre, Rio Grande do Sul]

A study on the presence of free-living amoebae in a public hospital was developed in the city of Porto Alegre, State of Rio Grande do Sul. Dust and biofilms were collected using sterile swabs that had been prepared for this study, from 15 hospital environments, including the intensive care center, pediatric intensive care unit, kitchen, emergency room, outpatient surgical center, clinical surgical center, water storage tanks, taps and six drinking fountains for general use, every month from July 2004 to March 2005. The FLAs were isolated by culturing, using non-nutrient agar medium with the addition of heat-killed Escherichia coli. The protozoa were identified by morphological observation of cysts and trophozoites, in accordance with Page's morphological criteria (1988). Among the 135 samples collected from the 15 environments, 47 (35%) were positive for FLAs. Of these, thirty-four percent presented morphological characteristics particular to the genus Acanthamoeba.

Primary amebic meningoencephalitis: a silent killer

One week after swimming in a man-made lake, a 9-year-old boy presented to the emergency department with headache, vomiting and lethargy. He had neck pain upon flexion and was unable to stand or walk. Cerebrospinal fluid examination revealed trophozoite and diflagellate forms consistent with Naegleria fowleri, an ameba species known to cause primary amebic meningoencephalitis. Despite aggressive management with amphotericin B and rifampin, he died 2 days later. This case report describes the clinical presentation, diagnostic findings and management of this uncommon but lethal entity.

Granulomatous cutaneous centrofacial and meningocerebral amebiasis

A 7-year-old, otherwise healthy Peruvian boy presented with a 3-month history of an indurated centrofacial plaque. Histologic examination revealed a granuloma containing free-living amebae tentatively identified as Balamuthia mandrillaris. The patient failed to respond to tentative treatment. He was admitted to the intensive care unit 7 months later with neurologic manifestations of granulomatous amebic encephalitis, which proved fatal. The difficulty in diagnosing this rare presentation of cutaneous amebiasis, the challenge of treating the condition, and the morbidity and high mortality associated with cerebral involvement are discussed.

Acanthamoeba castellanii promotion of in vitro survival and transmission of coxsackie b3 viruses

This work was undertaken to determine whether Acanthamoeba could play a role in the survival and transmission of coxsackieviruses and focused on in vitro interactions between Acanthamoeba castellanii and coxsackie B3 viruses (CVB-3). Residual virus titer evaluations and immunofluorescence experiments revealed a remarkable CVB-3 adsorption on amoeba surfaces and accumulation inside cells. The survival of viruses was independent of the dynamics of amoeba replication and encystment. In addition, our results indicated that virus-infected amoebas can release infectious viruses during interaction with human macrophages. On the basis of these data, Acanthamoeba appears to be a potential promoter of the survival of coxsackieviruses and their transmission to human hosts.

Neuroparasitic infections: cestodes, trematodes, and protozoans

Parasitic infection of the nervous system can produce a variety of symptoms and signs. Because symptoms of infection are often mild or nonspecific, diagnosis can be difficult. Familiarity with basic epidemiological characteristics and distinguishing radiographic findings can increase the likelihood of detection and proper treatment of parasitic infection of the nervous system. This article discusses the clinical presentation, diagnosis, and treatment for some of the more common infections of the nervous system caused by cestodes, trematodes and protozoans: Echinococcus spp., Spirometra spp. (sparganosis), Paragonimus spp., Schistosoma spp., Trypanosoma spp., Naegleria fowlerii, Acanthamoeba histolytica, and Balamuthia mandrillaris.

Synthesis and activity of 1H-benzimidazole and 1H-benzotriazole derivatives as inhibitors of Acanthamoeba castellanii

Chloro-, bromo- and methyl- analogues of 1H-benzimidazole and 1H-benzotriazole and their N-alkyl derivatives have been synthesized and tested in vitro against the protozoa Acanthamoeba castellanii. The results indicate that 5,6-dimethyl-1H-benzotriazole (11) and 5,6-dibromo-1H-benzotriazole (14) have higher efficacy than the antiprotozoal agent chlorohexidine.

In vitro evaluation of the effectiveness of the macrolide rokitamycin and chlorpromazine against Acanthamoeba castellanii

The present study demonstrates the in vitro effectiveness of the macrolide rokitamycin and the phenothiazine compound chlorpromazine against Acanthamoeba castellanii. Growth curve evaluations revealed that both drugs inhibit trophozoite growth in dose- and time-dependent ways. The effects of both drugs when they were used at the MICs at which 100% of isolates are inhibited were amoebistatic, but at higher doses they were amoebicidal as well as cysticidal. Experiments showed that when rokitamycin was associated with chlorpromazine or amphotericin B, rokitamycin enhanced their activities. Furthermore, low doses of rokitamycin and chlorpromazine, alone or in combination, blocked the cytopathic effect of A. castellanii against WKD cells derived from the human cornea. These results may have important significance in the development of new anti-Acanthamoeba compounds.

Post-mortem culture of Balamuthia mandrillaris from the brain and cerebrospinal fluid of a case of granulomatous amoebic meningoencephalitis, using human brain microvascular endothelial cells

The first isolation in the UK of Balamuthia mandrillaris amoebae from a fatal case of granulomatous amoebic meningoencephalitis is reported. Using primary cultures of human brain microvascular endothelial cells (HBMECs), amoebae were isolated from the brain and cerebrospinal fluid (CSF). The cultures showed a cytopathic effect at 20-28 days, but morphologically identifiable B. mandrillaris amoebae were seen in cleared plaques in subcultures at 45 days. The identification of the organism was later confirmed using PCR on Chelex-treated extracts. Serum taken while the patient was still alive reacted strongly with slide antigen prepared from cultures of the post-mortem isolate, and also with those from a baboon B. mandrillaris strain at 1:10,000 in indirect immunofluorescence, but with Acanthamoeba castellanii (Neff) at 1:160, supporting B. mandrillaris to be the causative agent. If the presence of amoebae in the post-mortem CSF reflects the condition in life, PCR studies on CSF and on biopsies of cutaneous lesions may also be a valuable tool. The role of HBMECs in understanding the interactions of B. mandrillaris with the blood-brain barrier is discussed.

Intranasal coadministration of the Cry1Ac protoxin with amoebal lysates increases protection against Naegleria fowleri meningoencephalitis

Cry1Ac protoxin has potent mucosal and systemic adjuvant effects on antibody responses to proteins or polysaccharides. In this work, we examined whether Cry1Ac increased protective immunity against fatal Naegleria fowleri infection in mice, which resembles human primary amoebic meningoencephalitis. Higher immunoglobulin G (IgG) than IgA anti-N. fowleri responses were elicited in the serum and tracheopulmonary fluids of mice immunized by the intranasal or intraperitoneal route with N. fowleri lysates either alone or with Cry1Ac or cholera toxin. Superior protection against a lethal challenge with 5 x 10(4) live N. fowleri trophozoites was achieved for immunization by the intranasal route. Intranasal immunization of N. fowleri lysates coadministered with Cry1Ac increased survival to 100%; interestingly, immunization with Cry1Ac alone conferred similar protection to that achieved with amoebal lysates alone (60%). When mice intranasally immunized with Cry1Ac plus lysates were challenged with amoebae, both IgG and IgA mucosal responses were rapidly increased, but only the increased IgG response persisted until day 60 in surviving mice. The brief rise in the level of specific mucosal IgA does not exclude the role that this isotype may play in the early defense against this parasite, since higher IgA responses were detected in nasal fluids of mice intranasally immunized with lysates plus either Cry1Ac or cholera toxin, which, indeed, were the treatments that provided the major protection levels. In contrast, serum antibody responses do not seem to be related to the protection level achieved. Both acquired and innate immune systems seem to play a role in host defense against N. fowleri infection, but further studies are required to elucidate the mechanisms involved in protective effects conferred by Cry1Ac, which may be a valuable tool to improve mucosal vaccines.

Health effects of Acanthamoeba spp. and its potential for waterborne transmission

Risk from Acanthamoeba keratitis is complex, depending upon the virulence of the particular strain, exposure, trauma, or other stress to the eye, and host immune response. Bacterial endosymbionts may also play a factor in the pathogenicity of Acanthamoeba. Which factor(s) may be the most important is not clear. The ability of the host to produce IgA antibodies in tears may be a significant factor. The immune response of the host is a significant risk factor for GAE infection. If so, then a certain subpopulation with an inability to produce IgA in the tears may be at greatest risk. There was no sufficient data on the occurrence or types of Acanthamoeba in tapwater in the U.S. Published work on amoebal presence in tapwater does not provide information on the type of treatment the water received or the level of residual chlorine. Assessment of the pathogenicity by cell culture and molecular methods of Acanthamoeba in tapwater would also be useful in the risk assessment process for drinking water. The possibility that Acanthamoeba spp. might serve as vectors for bacterial infections from water sources also should be explored. The bacterial endosymbionts include an interesting array of pathogens such as Vibrio cholerae and Legionella pneumophila, both of which are well recognized waterborne/water-based pathogens. Work is needed to determine if control of Acanthamoeba spp. is needed to control water-based pathogens in water supplies.

Neuropathological and Ultrastructural Features of Amebic Encephalitis Caused bySappinia diploidea

Here we present the neuropathological, ultrastructural, and radiological features of Sappinia diploidea, a newly recognized human pathogen. The patient was a 38-year-old man with visual disturbances, headache, and a seizure. Brain images showed a solitary mass in the posterior left temporal lobe. The mass was composed of necrotizing hemorrhagic inflammation that contained free-living amebae. Immunofluorescence microscopy showed that the organism was not a species of ameba previously known to cause encephalitis. Trophozoites had a highly distinctive double nucleus, and transmission electron microscopy confirmed that they contained 2 nuclei closely apposed along a flattened surface. The 2 nuclei were attached to each other by distinctive connecting perpendicular filaments. This and several other unique structural features led to the diagnosis of S. diploidea encephalitis. The patient was treated postoperatively with a sequential regimen of anti-amebic drugs (azithromycin, pentamidine, itraconazole, and flucytosine) and is alive after 5 years. Guidelines to recognize future cases of S. diploidea encephalitis are as follows. 1) It presented as a tumor-like cerebral mass without an abscess wall. 2) It had central necrotic and hemorrhagic inflammation that contained acute and chronic inflammatory cells without granulomas or eosinophils. 3) It contained trophozoites (40-70 microm diameter) that contained a distinctive double nucleus. 4) Cyst forms in the host were not excluded or definitely evident. 5) Trophozoites engulfed host blood cells and were stained brightly with Giemsa and periodic acid-Schiff. 6) Trophozoites often were present in viable brain parenchyma on the periphery of the mass without inflammatory response. 7) The prognosis after surgical excision and medical treatment was favorable in this instance.

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.

Trichomonas vaginalis: identification of a phospholipase A-dependent hemolytic activity in a vesicular subcellular fraction

Trichomonad total extracts (TTE), or vesicular (P30) and soluble (530) subcellular fractions from 3 pathogenic Trichomonas vaginalis strains (GT-3. GT-13. and GT-15), lysed both human and Sprague-Dawley rat erythrocytes in a time- and dose-dependent manner. The entire hemolytic activity of TTE was located in P30, showing 2 peaks of maximum activity, one at pH 6.0 and another at pH 8.0. in the presence of 1 mM Ca2+. Hemolytic activity on rat erythrocytes was greater at pH 6.0 16.71 +/- 0.33 hemolytic units IHU]/mg/hr to 11.60 +/- 0.24 HU/mg/hr) than at pH 8.0 (3.81 +/- 0.30 HU/mg/hr to 5.75 +/- 0.65 HU/mg/hr). and it was greater than that on human red blood cells at pH 6.0 (2.67 +/- 0.19 HU/mg/hr to 4.08 +/- 0.15 HU/mg/hr) or pH 8.0 (2.24 +/- 0.0 9 HU/mg/hr to 2.81 +/- 0.06 HU/mg/hr). The alkaline and acidic hemolytic activity diminished (60-93% at pH 6.0 and 78-93% at pH 8.0) by the effect of 80 microM Rosenthal's inhibitor, which also inhibited 27-45% and 29-54% trichomonad alkaline and acidic phospholipase A activities, respectively. Vesicles, vacuoles, and hydrogenosomes were rich in P30. Trichomonas vaginalis has a hemolytic PLA, which could be involved in its cytopathogenic mechanism.

Assessing the risk of primary amoebic meningoencephalitis from swimming in the presence of environmental Naegleria fowleri

Free-living Naegleria fowleri amoebae cause primary amoebic meningoencephalitis (PAM). Because of the apparent conflict between their ubiquity and the rarity of cases observed, we sought to develop a model characterizing the risk of PAM after swimming as a function of the concentration of N. fowleri. The probability of death from PAM as a function of the number of amoebae inhaled is modeled according to results obtained from animals infected with amoeba strains. The calculation of the probability of inhaling one or more amoebae while swimming is based on a double hypothesis: that the distribution of amoebae in the water follows a Poisson distribution and that the mean quantity of water inhaled while swimming is 10 ml. The risk of PAM for a given concentration of amoebae is then obtained by summing the following products: the probability of inhaling n amoebae x the probability of PAM associated with inhaling these n amoebae. We chose the lognormal model to assess the risk of PAM because it yielded the best analysis of the studentized residuals. Nonetheless, the levels of risk thereby obtained cannot be applied to humans without correction, because they are substantially greater than those indicated by available epidemiologic data. The curve was thus adjusted by a factor calculated with the least-squares method. This provides the PAM risk in humans as a function of the N. fowleri concentration in the river. For example, the risk is 8.5 x 10(-8) at a concentration of 10 N. fowleri amoebae per liter.

By releasing ADP, Acanthamoeba castellanii causes an increase in the cytosolic free calcium concentration and apoptosis in wish cells

The role played by soluble molecules that may participate in acanthamoebal cytopathogenicity has yet to be fully characterized. We demonstrate here that Acanthamoeba castellanii trophozoites constitutively release ADP in the medium. Cell-free supernatants prepared from A. castellanii, by interaction with specific P(2y2) purinoceptors expressed on the Wish cell membrane, caused a biphasic rise in [Ca(2+)](i), extensive cell membrane blebbing, cytoskeletal disorganization, and the breakdown of nuclei. Cell damage induced by amoebic supernatants was blocked by the P(2y2) inhibitor Suramin. The same results were found in Wish cells exposed to purified ADP. These findings suggest that pathogenic free-living A. castellanii may have a cytopathic effect on human epithelial cells through ADP release, by a process that begins with a rise of cytosolic free-calcium concentration, and culminates in apoptosis.

Correlations between morphological, molecular biological, and physiological characteristics in clinical and nonclinical isolates of Acanthamoeba spp

Eleven Acanthamoeba isolates, obtained from Acanthamoeba keratitis patients, from contact lens cases of non-Acanthamoeba keratitis patients, from asymptomatic individuals, from necrotic tissue, and from tap water and two reference strains were investigated by morphological, molecular biological, and physiological means in order to discriminate clinically relevant and nonrelevant isolates. All clinically relevant isolates showed Acanthamoeba sp. group II morphology. 18S ribosomal DNA sequencing revealed sequence type T4 to be the most prevalent group among the isolates and also the group recruiting most of the pathogenic strains. Interestingly, within T4 the strains of no clinical relevance clustered together. Moreover, physiological properties appeared to be highly consistent with initial pathogenicity and with sequence clustering. Altogether, the results of our study indicate a correlation between the phylogenetic relationship and pathogenicity.

Granulomatous amebic encephalitis in a patient with AIDS: isolation of acanthamoeba sp. Group II from brain tissue and successful treatment with sulfadiazine and fluconazole

A patient with AIDS, treated with highly active antiretroviral therapy and trimethoprim-sulfamethoxazole, presented with confusion, a hemifield defect, and a mass lesion in the right occipital lobe. A brain biopsy confirmed granulomatous amebic encephalitis (GAE) due to Acanthamoeba castellanii. The patient was treated with fluconazole and sulfadiazine, and the lesion was surgically excised. This is the first case of AIDS-associated GAE responding favorably to therapy. The existence of a solitary brain lesion, absence of other sites of infection, and intense cellular response in spite of a very low CD4 count conditioned the favorable outcome. We review and discuss the diagnostic microbiologic options for the laboratory diagnosis of infections due to free-living amebae.

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.

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.

Acanthamoeba castellanii metabolites increase the intracellular calcium level and cause cytotoxicity in wish cells

Previous studies have shown that trophozoites of the pathogenic free-living amoeba Acanthamoeba castellanii rapidly lyse a variety of cells in vitro. However, the role played by cytolitic molecules that may participate in Acanthamoebal cytopathogenicity has yet to be completely elucidated. The aim of this work was to study whether soluble molecules released by A. castellanii trophozoites could induce cytopathic effect in human epithelial cells in vitro. The results obtained indicate that A. castellanii trophozoites constitutively elaborate and release soluble factors that immediately elicit a cytosolic free-calcium increase in target cells. This phenomenon is induced by low molecular weight amoebic metabolites and depends on a transmembrane influx of extracellular calcium. Morphological changes, cytoskeletal damage, cell death and cytolysis followed the elevation of cytosolic free-calcium levels. Calcium ions are very important for cell homeostasis, in fact, they control the functions of a variety of cellular responses, including secretion, cell proliferation and apoptosis. Our results suggest that the substained elevation of the cytosolic free-calcium in response to A. castellanii metabolites might play a fundamental role in target cell damage during Acanthamoeba infections.

Primary amoebic meningoencephalitis with Naegleria fowleri: clinical review

Two children with primary amoebic meningoencephalitis secondary to Naegleria fowleri are reported. Both children died, and the causative agent was identified at autopsy. Presentation and outcome conformed to the usual course of primary amoebic meningoencephalitis and reaffirm the gravity and rapid progression of this infection. The epidemiology, microbiology, diagnostic considerations, and treatment are discussed. Primary amoebic meningoencephalitis should be considered in the differential diagnosis of children with meningitis or encephalitis.

Cell surface control of differentiation in Acanthamoeba

Acanthamoeba castellanii (Neff) is a free-living soil amoeba with close relatives that are opportunistic pathogens. Trophozoites differentiate into cysts when deprived of nutrients; cysts convert into trophozoites, leaving the wall behind, in the presence of nutrients. The data presented here, which includes immunoaffinity purification of the receptor, indicate that cell surface molecular signals also control Acanthamoeba differentiation in both directions. Monoclonal antibodies that bind specifically to a 40 kD trophozoite protein initiate the encystment of trophozoites. When bound to cysts the same monoclonal antibodies prevent excystment. Washing away the antibody allows both trophozoites and cysts to resume normal activity. One of these monoclonal antibodies inhibits pinocytosis, while another has no effect on pinocytosis.

Granulomatous amebic encephalitis: a review and report of a spontaneous case from Venezuela

Granulomatous amebic encephalitis (GAE), or meningoencephalitis due to Acanthamoeba spp. and leptomyxid ameba are uncommon CNS infections that generally occur in immunocompromised hosts. We describe a case of GAE caused by Balamuthia mandrillaris previously designated as a leptomyxid ameba, in an apparently healthy 14-year-old Venezuelan boy. This case was characterized by sudden onset of seizures, focal neurologic signs and by a prolonged clinical course (from November 1992 to March 1993). Neuroimaging studies showed cerebral hypodense lesions in cerebral hemispheres, brain stem and cerebellum. Microscopically, we found a chronic granulomatous inflammatory reaction with necrotizing angiitis, large numbers of amebic trophozoites and few cysts in perivascular spaces and within necrotic CNS tissue. The amebas were identified as B. mandrillaris based on their immunofluorescence reactivity with the anti-B. mandrillaris serum. So far, 30 cases of GAE due to B. mandrillaris have been recognized in humans, two in AIDS patients. No visceral involvement by free-living amebas or any other significant abnormality was observed. This patient developed "spontaneous" GAE, but it remains possible that an undiagnosed abnormality in cell-mediated immunity or a deficient humoral immune response may explain the susceptibility of this patient to this opportunistic infection.

Acanthamoeba-specific human T-cell clones isolated from healthy individuals

T-cell responses to pathogenic free-living amoebae, Acanthamoeba sp., were analyzed in healthy Japanese individuals. Of 20 healthy subjects, 10 (50%) showed significant proliferative responses of peripheral blood mononuclear cells to the soluble amoebic antigens in vitro. The antigens used were not mitogenic, and no evidence of amoebic superantigens was available. We established human T-cell clones reactive to Acanthamoeba, all of which were CD3- and CD4-positive, CD8-negative, and TCR-alpha beta-positive. We isolated two strains of Acanthamoeba from two patients, one from a patient with meningoencephalitis (CSF strain) and the other from a patient with keratitis (K strain). Of 13 clones, 11 were reactive to the K-strain as well as to the CSF-strain antigen under human leukocyte antigen (HLA)-DR restriction, whereas the other two were specific for the K-strain antigen. All but one clone tested showed TH1-equivalent functions because these cells produced interferon (IFN)-gamma in response to the amoebic antigen but produced no detectable level of interleukin 4 (IL-4). These results suggest that immunocompetent hosts might have acquired protective immunity mediated by Acanthamoeba-specific T-cells during natural sensitization.

[Interleukin-2 production and alteration of T cell subsets in mice infected with Naegleria fowleri]

Naegleria fowleri is the cause of primary amoebic meningoencephalitis in man. IL-2 levels after stimulation of T lymphocytes by PHA or N. fowleri lysates, the amounts of T lymphocyte subsets and the blastogenic responses of T lymphocytes in mice after infected with pathogenic N. fowleri were studied comparing between two study groups, one 1 x 10(4) trophozoites inoculated mice and the other 1 x 10(5) trophozoites inoculated mice. All experimental samples were obtained on the day 7, 14 and 24 after inoculation. The mice inoculated with 1 x 10(4) trophozoites showed a 14.3% mortality rate, and 72.2% in the mice inoculated with 1 x 10(5) trophozoites. The IL-2 levels on day 14 of two experimental groups were significantly decreased as compared with the control group. Thy 1.2+ T cells in the total spleen lymphocytes of 1 x 10(5) trophozoites inoculated group on day 7 were significantly increased compared with the control group. There was no significant difference between 1 x 10(4) trophozoites inoculated group and the control group. L3T4+ T cells and Ly2+ T cells in the total spleen lymphocytes of 1 x 105 trophozoites inoculated group on day 7 were significantly increased compared with the control group. The DNA S fraction of T cells in the spleen of 1 x 10(5) trophozoites inoculated group was significantly increased on day 7. The amount of S fractions of DNA were sequentially decreased on day 14 and 24, but they were also significantly increased compared with the control group.(ABSTRACT TRUNCATED AT 250 WORDS)