Primary Amoebic Meningoencephalitis by Naegleria fowleri: Pathogenesis and Treatments

Naegleria fowleri is a free-living amoeba (FLA) that is commonly known as the "brain-eating amoeba." This parasite can invade the central nervous system (CNS), causing an acute and fulminating infection known as primary amoebic meningoencephalitis (PAM). Even though PAM is characterized by low morbidity, it has shown a mortality rate of 98%, usually causing death in less than two weeks after the initial exposure. This review summarizes the most recent information about N. fowleri, its pathogenic molecular mechanisms, and the neuropathological processes implicated. Additionally, this review includes the main therapeutic strategies described in case reports and preclinical studies, including the possible use of immunomodulatory agents to decrease neurological damage.

Copper detoxification machinery of the brain-eating amoeba Naegleria fowleri involves copper-translocating ATPase and the antioxidant system

Copper is a trace metal that is necessary for all organisms but toxic when present in excess. Different mechanisms to avoid copper toxicity have been reported to date in pathogenic organisms such as Cryptococcus neoformans and Candida albicans. However, little if anything is known about pathogenic protozoans despite their importance in human and veterinary medicine. Naegleria fowleri is a free-living amoeba that occurs naturally in warm fresh water and can cause a rapid and deadly brain infection called primary amoebic meningoencephalitis (PAM). Here, we describe the mechanisms employed by N. fowleri to tolerate high copper concentrations, which include various strategies such as copper efflux mediated by a copper-translocating ATPase and upregulation of the expression of antioxidant enzymes and obscure hemerythrin-like and protoglobin-like proteins. The combination of different mechanisms efficiently protects the cell and ensures its high copper tolerance, which can be advantageous both in the natural environment and in the host. Nevertheless, we demonstrate that copper ionophores are potent antiamoebic agents; thus, copper metabolism may be considered a therapeutic target.

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N. fowleri employs the combination of copper efflux and antioxidant system to ensure a high copper tolerance.

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Copper efflux in N. fowleri is mediated by a copper-translocating P-type ATPase.

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Copper ionophores have amoebicidal effect against N. fowleri and thus may be potentially used as antiamoebic agents.

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Iron-binding proteins hemerythrin and protoglobin are highly upregulated in N. fowleri under copper overload.

Growth dynamic of Naegleria fowleri in a microbial freshwater biofilm

The presence of pathogenic free-living amoebae (FLA) such as Naegleria fowleri in freshwater environments is a potential public health risk. Although its occurrence in various water sources has been well reported, its presence and associated factors in biofilm remain unknown. In this study, the density of N. fowleri in biofilms spontaneously growing on glass slides fed by raw freshwater were followed at 32 °C and 42 °C for 45 days. The biofilms were collected with their substrata and characterized for their structure, numbered for their bacterial density, thermophilic free-living amoebae, and pathogenic N. fowleri. The cell density of N. fowleri within the biofilms was significantly affected both by the temperature and the nutrient level (bacteria/amoeba ratio). At 32 °C, the density remained constantly low (1-10 N. fowleri/cm(2)) indicating that the amoebae were in a survival state, whereas at 42 °C the density reached 30-900 N. fowleri/cm(2) indicating an active growth phase. The nutrient level, as well, strongly affected the apparent specific growth rate (μ) of N. fowleri in the range of 0.03-0.23 h(-1). At 42 °C a hyperbolic relationship was found between μ and the bacteria/amoeba ratio. A ratio of 10(6) to 10(7) bacteria/amoeba was needed to approach the apparent μ(max) value (0.23 h(-1)). Data analysis also showed that a threshold for the nutrient level of close to 10(4) bacteria/amoeba is needed to detect the growth of N. fowleri in freshwater biofilm. This study emphasizes the important role of the temperature and bacteria as prey to promote not only the growth of N. fowleri, but also its survival.

The immune response toNaegleria fowleriamebae and pathogenesis of infection

The genus Naegleria is comprised of a group of free-living ameboflagellates found in diverse habitats worldwide. Over 30 species have been isolated from soil and water but only Naegleria fowleri (N. fowleri) has been associated with human disease. Naegleria fowleri causes primary amebic meningoencephalitis (PAM), a fatal disease of the central nervous system. The pathogenesis of PAM and the role of host immunity to N. fowleri are poorly understood. Strategies for combating infection are limited because disease progression is rapid and N. fowleri has developed strategies to evade the immune system. The medical significance of these free-living ameboflagellates should not be underestimated, not only because they are agents of human disease, but also because they can serve as reservoirs of pathogenic bacteria.

Pathogenic and opportunistic free-living amoebae: Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri, and Sappinia diploidea

Among the many genera of free-living amoebae that exist in nature, members of only four genera have an association with human disease: Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri and Sappinia diploidea. Acanthamoeba spp. and B. mandrillaris are opportunistic pathogens causing infections of the central nervous system, lungs, sinuses and skin, mostly in immunocompromised humans. Balamuthia is also associated with disease in immunocompetent children, and Acanthamoeba spp. cause a sight-threatening infection, Acanthamoeba keratitis, mostly in contact-lens wearers. Of more than 30 species of Naegleria, only one species, N. fowleri, causes an acute and fulminating meningoencephalitis in immunocompetent children and young adults. In addition to human infections, Acanthamoeba, Balamuthia and Naegleria can cause central nervous system infections in animals. Because only one human case of encephalitis caused by Sappinia diploidea is known, generalizations about the organism as an agent of disease are premature. In this review we summarize what is known of these free-living amoebae, focusing on their biology, ecology, types of disease and diagnostic methods. We also discuss the clinical profiles, mechanisms of pathogenesis, pathophysiology, immunology, antimicrobial sensitivity and molecular characteristics of these amoebae.

In Vitro Antiproliferative Effects of Neuroleptics, Antimycotics and Antibiotics on the Human Pathogens Acanthamoeba polyphaga and Naegleria fowleri

BACKGROUND

Using reproducible conditions in vitro, the aim of this study was to obtain a comparative evaluation of the efficacies of several tricyclic neuroleptics, antimycotics and antibiotics with antiproliferative activities against Acanthamoeba polyphaga and Naegleria fowleri trophozoites.

METHODS

We used reproducible conditions in vitro to obtain results.

RESULTS

In the case of A.polyphaga, the tricyclic neuroleptics trifluoperazine and chlorpromazine had the best inhibitory (IC50) effects followed by mepacrine, ketoconazole, pentamidine, miconazole, amphotericin B, and metronidazole. Of all, rifampicin was the least effective. Mepacrine was the most effective compound with the minimum inhibitory concentration (MIC100) against A.polyphaga [corrected] The most effective drugs against N. fowleri expressed as (IC50) were as follows: the antimycotics ketoconazole and amphotericin B, followed by trifluoperazine, mepacrine, chlorpromazine, miconazole, and metronidazole. The least effectives were rifampicin and pentamidine. The most potent growth inhibitors (MIC100) against N. fowleri were the antimycotics amphotericin B and ketoconazole and the neuroleptic trifluoperazine. It was clear that there are major differences between the two amebas in their susceptibility to some of the drugs.

CONCLUSIONS

The drugs with the minimal inhibitory concentration (MIC) values could be considered alone or in combination as potential anti-amebic agents for the treatment of the diseases produced by these amebas.

The involvement of an integrin-like protein and protein kinase C in amoebic adhesion to fibronectin and amoebic cytotoxicity

Adherence of a pathogen to the host cell is one of the critical steps in microbial infections. Naegleria fowleri, a causative agent of primary amoebic meningoencephalitis in humans, is expected to interact with extracellular components of the host, such as fibronectin, in a receptor-mediated mode. In this study, we investigated the interaction between N. fowleri and fibronectin to understand its cytopathology. In binding assays using immobilized fibronectin, the number of amoebae bound to fibronectin was increased compared to the controls, and was dependent on the amount of coated fibronectin present. A fibronectin binding protein of 60 kDa was found in extracts of N. fowleri. Western blot and immunolocalization assays using integrin alpha(5)/FnR antibodies showed that a 60 kDa protein reacted with the antibodies in extracts of N. fowleri, which was localized on the surface of N. fowleri. Preincubation of N. fowleri with the integrin antibodies significantly inhibited amoebic binding to fibronectin and cytotoxicity to the CHO cells. Additionally, protein kinase C activity was detected in the extract of N. fowleri. When N. fowleri was pretreated with protein kinase C activator or inhibitor, the abilities of amoebic adhesion to fibronectin and cytotoxicity to the host cells were markedly affected compared to untreated amoebae. These results suggest that an amoebic integrin-like receptor and protein kinase C play important roles in amoebic cellular processes in response to fibronectin.

Rapid detection and enumeration of Naegleria fowleri in surface waters by solid-phase cytometry

A new method for the rapid and accurate detection of pathogenic Naegleria fowleri amoebae in surface environmental water was developed. The method is based on an immunofluorescent assay combined with detection by solid-phase cytometry. In this study we developed and compared two protocols using different reporter systems conjugated to antibodies. The monoclonal antibody Ac5D12 was conjugated with biotin and horseradish peroxidase, and the presence of cells was revealed with streptavidin conjugated to both R-phycoerythrin and cyanine Cy5 (RPE-Cy5) and tyramide-fluorescein isothiocyanate, respectively. The RPE-Cy5 protocol was the most efficient protocol and allowed the detection of both trophozoite and cyst forms in water. The direct counts obtained by this new method were not significantly different from those obtained by the traditional culture approach, and results were provided within 3 h. The sensitivity of the quantitative method is 200 cells per liter. The limit is due only to the filtration capacity of the membrane used.

A light microscopy study of the migration of Naegleria fowleri from the nasal submucosa to the central nervous system during the early stage of primary amebic meningoencephalitis in mice

The migratory pathway of Naegleria fowleri from the nasal submucosa to the central nervous system (CNS) during the early stage of primary amebic meningoencephalitis (PAM) was investigated in mice. Twenty-one-day-old CD-1 mice were inoculated by intranasal instillation of 1 x 10(6) amebas. Animals were divided into 3 groups of 5 and, after being anesthetized, were killed at intervals of 24, 32, and 48 hr postinoculation by transcardial perfusion with formaldehyde, acetic acid, and methanol. The heads were decalcified, divided in the midsagittal plane, and the area of the cribriform plate removed and embedded in paraffin. Serial sections were cut at 8 microm and stained with a combination of celestin blue, Harris' hematoxylin, and acid fuchsin for light microscopy. Focal inflammation and amebas were observed in the submucosal nerve plexus, olfactory nerves penetrating the cribriform plate, and the olfactory bulb of the brain as early as 24 hr postinoculation. The time periods selected assured that the disease process would not obliterate soft tissue structures. Earlier studies used moribund mice in which the inflammation and the number of amebas were overwhelming. The present study provides convincing evidence that amebas gain initial access to the CNS through olfactory nerves within the cribriform plate during the early stages of PAM.

Naegleria fowleri: a free living amoeba of emerging medical importance

Naegleria fowleri, a free-living amoeba is ubiquitous and word-wide in distribution. Infection is due to inhalation or aspiration of aerosols containing cysts found in the environment. Of late, the amoeba is emerging as a pathogen of medical importance causing primary amoebic meningoencephalitis (PAM) in humans. The diagnosis of the condition is mainly parasitic which depends on the detection and identification of Naegleria trophozoites in the cerebro-spinal fluid (CSF) or biopsied brain tissue. Serological tests are not useful in the diagnosis of PAM. Most cases are fatal and various amoebicidal agents have been tried unsuccessfully. The present paper provides a review of the recent information on the biology and epidemiology of the disease caused by the amoeba Approaches in the diagnosis, pathophysiology and treatment of the condition are also discussed.

[Human pathology caused by free-living amoebae]

Naegleria fowleri, Acanthamoeba spp. and Balamuthia mandrillaris are free-living amoebae that occasionally may induce pathology in human beings. CNS disease due to N. fowleri, called "primary" amoebic meningoencephalitis, is acquired after exposure to polluted waters in swimming pools, rivers, and lakes. The clinical course is acute, often fulminant and characterized pathologically by necrotizing hemorrhagic meningoencephalitis, involving mainly the base of the brain, brainstem and cerebellum. In contrast, some Acanthamoeba spp. and B. mandrillaris cause opportunistic, chronic "granulomatous" encephalitis in subjects pathologically or iatrogenically immunocompromised. There are, most likely, foci of protozoa in lung and skin reaching the CNS by hematogenous route. Only Acanthamoeba spp. can also produce severe, subacute keratitis, mainly today in contact lens wearers.

Naegleria fowleri: characterization of a secreted histolytic cysteine protease

Naegleria fowleri is the etiologic agent of primary amebic meningoencephalitis, a rare but rapidly fatal disease of humans. It invades the central nervous system via nasal mucosa and cribriform plate. Once in brain tissue, the organism induces an acute hemorrhagic, necrotizing meningoencephalitis. We hypothesize that a protease released by the parasite contributes to tissue destruction and facilitates host invasion. Analysis of conditioned media of N. fowleri cultures revealed a major 30-kDa protease with substrate and inhibitor specificity consistent with cysteine proteases. Amino-terminal amino acid sequence of the purified enzyme showed it to be a thiol protease with homology to cathepsin L. It catalyzed the in vitro degradation of extracellular matrix and had a cytopathic effect on mammalian cells. Both ameba-induced matrix degradation and the cytopathic effect are inhibited by Z-Phe-Ala-fluoromethyl ketone, an irreversible cysteine protease inhibitor. Our results indicate that N. fowleri secretes a cysteine protease with the capacity to destroy host tissue. Naegleria gruberi, a nonpathogenic species, expresses a similar protease but, unlike its pathogenic relative, is not thermotolerant to temperatures above 30 degrees C.

Modulation of biological functions of Naegleria fowleri amoebae by growth medium

Two strains of Naegleria fowleri amoebae were studied when the amoebae were maintained in the same growth medium or in two different media. A weakly pathogenic strain of N. fowleri, LEE, and a highly pathogenic strain, LEEmpC1, were compared for growth properties, the presence or absence of surface structures termed food cups, cytopathogenicity, cellular locomotion, susceptibility to complement-mediated lysis and immunological relatedness by western immunoblot analysis when grown in Nelson medium or in Cline medium. The two different strains of N. fowleri, LEE and LEEmpC1, were more similar in protein profiles and functional activity when both strains were grown in the same nutritional medium. Differences in growth, proteins synthesized, cytopathogenicity, susceptibility to complement lysis and rate of locomotion were noted when the same strain was grown in different media. Naegleria fowleri grown in Cline medium demonstrated an increased rate of growth, an increase in its rate of locomotion, an increased resistance to complement lysis, and destroyed target nerve cells by contact-dependent lysis. In contrast, the same strain of amoeba grown in Nelson medium showed slower growth, destroyed target cells by trogocytosis, and was less resistant to complement-mediated lysis.

Antagonistic action of the bacterium Bacillus licheniformis M-4 toward the amoeba Naegleria fowleri

Free-living amoebae belonging to the species Naegleria fowleri are known to be the etiological agents for a form of fulminant meningoencephalitis that is generally fatal (primary amoebic meningoencephalitis). In a broad bacterial screening from soil and water we have isolated three strains (M-4, D-13 and A-12) belonging to the species Bacillus licheniformis that have remarkable amoebicidal activity against Naegleria sp. and also against different Gram-positive and Gram-negative bacteria. Physical-chemical characteristics, partial purification and biological activities of a substance produced by the M-4 strain have been investigated. This substance (m-4) is stable at high temperature (up to 100 degrees C) and extremes of pH (2.5-9.5) and also at -20 degrees C for months. Its production is greatly influenced by oxygenation of the cultures and is probably related to the sporulation process of the bacterium. Scanning electron microscope observations reveal that amoebae are lysed after a few minutes contact with m-4.

A method for assessing the migratory response of Naegleria fowleri utilizing [3H]uridine-labeled amoebae

A new procedure is described to assay the migratory response of Naegleria fowleri (ATCC 30894) amoebae to potential chemoattractants. The method utilizes a blind-well Boyden chemotaxis chamber, two micropore filters of different construction, and amoebae-labeled with [3H]uridine. The technique was standardized by determining the influence of incubation time, filter construction, filter pore size and geometry, amoebae to filter pore ratio, and chemoattractant concentration. Radiolabeled amoebae were placed in Boyden chambers that contained the combination of an upper polycarbonate filter with distinct pores with a diameter of 8 microns and a lower filter of nitrocellulose with a 150-micron depth to separate the wells. A ratio of two amoebae to one filter pore and a 2-h incubation period were chosen to obtain optimal migration conditions. Nerve cell extract was used as the chemoattractant. The migratory responses of both highly pathogenic and weakly pathogenic strains of N. fowleri to nerve cell extract were compared using either the radiolabel procedure or the conventional single filter, leading-front method. Using either method, a highly pathogenic cloned strain of N. fowleri amoebae moved in a directional manner (chemotactically) in vitro to B103 rat neuroblastoma cell extract. In contrast, a weakly pathogenic strain of amoebae responded in a nondirectional manner (chemokinetically) to nerve cell extract. While both the leading-front assay and the radiolabel assay give accurate results, the measurement of radiolabeled cells allows one to test a greater number of attractants in one assay and the procedure eliminates observer bias.