Effect of antimicrobial compounds on Balamuthia mandrillaris encystment and human brain microvascular endothelial cell cytopathogenicity

Diagnosing Balamuthia mandrillaris amebic meningoencephalitis in a 64-year-old woman from the Southwest of China

Balamuthia mandrillaris amebic encephalitis (BAE) can cause a fatal condition if diagnosis is delayed or effective treatment is lacking. Patients with BAE have been previously reported in 12 provinces of China, with skin lesions being the primary symptom and encephalitis developing after several years. However, a significantly lower number of cases has been reported in Southwest China. Here we report an aggressive BAE case of a 64-year-old woman farmer with a history of skin lesions on her left hand. She was admitted to our hospital due to symptoms of dizziness, headache, cough, vomiting, and gait instability. She was initially diagnosed with syphilitic meningoencephalitis and received a variety of empirical treatment that failed to improve her symptoms. Finally, she was diagnosed with BAE combined with amebic pneumonia using next-generation sequencing (NGS), qRT-PCR, sequence analysis, and imaging studies. She died approximately 3 weeks after the onset. This case highlights that the rapid development of encephalitis can be a prominent clinical manifestation of Balamuthia mandrillaris infection.

Balamuthia mandrillaris trophozoites ingest human neuronal cells via a trogocytosis-independent mechanism

BACKGROUND

Environmental protozoa need an adaptation mechanism to survive drastic changes in niches in the human body. In the brain parenchyma, Balamuthia mandrillaris trophozoites, which are causative agents of fatal brain damage, must acquire nutrients through the ingestion of surrounding cells. However, the mechanism deployed by the trophozoites for cellular uptake remains unknown.

METHODS

Amoebic ingestion of human neural cell components was investigated using a coculture system of clinically isolated B. mandrillaris trophozoites and human neuroblastoma SH-SY5Y cells. Cell-to-cell interactions were visualized in a three-dimensional manner using confocal and holotomographic microscopes.

RESULTS

The B. mandrillaris trophozoites first attached themselves to human neuroblastoma SH-SY5Y cells and then twisted themselves around the cytoplasmic bridge. Based on fluorescence-based cell tracking, the B. mandrillaris trophozoites then inserted invadopodia into the cytoplasm of the human cells. Subsequently, the human protein-enriched components were internalized into the trophozoites in the form of nonmembranous granules, whereas the human lipids were dispersed in the cytoplasm. Intervention of trogocytosis, a process involving nibbling on parts of the target cells, failed to inhibit this cellular uptake.

CONCLUSIONS

Human cell ingestion by B. mandrillaris trophozoites likely differs from trogocytosis, suggesting that a pathogen-specific strategy can be used to ameliorate brain damage.

Balamuthia mandrillaris: An opportunistic, free-living ameba - An updated review

Balamuthia mandrillaris is an opportunistic, free-living ameba that is pathogenic to humans. It has a worldwide distribution but is mainly detected in warmer regions. Balamuthia infections are rare but have been reported in both immunocompetent and immunocompromised individuals of all ages. B. mandrillaris can enter through wounds on the skin or the nose and cause cutaneous lesions and the usually fatal Balamuthia amebic encephalitis (BAE). Infection usually spreads from the lungs or through nerve fibers, and attacks the central nervous system, forming granulomatous lesions and necrosis in the brain. Balamuthia infection is usually chronic, and patients initially present with nonspecific symptoms, including headache, nausea, myalgia, and low-grade fever. As the disease progresses, the patient becomes paralyzed and comatose, often leading to death. Lack of knowledge of predisposing factors, specific treatment, and standardized detection tools have resulted in a nearly cent percent fatality rate. Although only about 200 cases have been reported worldwide since its characterization in the 1990s, the number of reported cases has increased over the years. BAE is an emerging disease and a major health concern. Few patients have survived Balamuthia infections with antimicrobial treatment that has largely been empirical. Early diagnosis is the key and requires familiarity with the disease and a high degree of suspicion on the part of the diagnostician. There are currently no specific treatment and prevention recommendations. This review highlights our current understanding of B. mandrillaris in terms of its pathogenicity, genomics, and novel diagnostic and therapeutic approaches against BAE infections.

Opportunistic free-living amoebal pathogens

Pathogenic free-living amoebae affecting the central nervous system are known to cause granulomatous amoebic encephalitis (GAE) or primary amoebic meningoencephalitis (PAM). Although hosts with impaired immunity are generally at a higher risk of severe disease, amoebae such as Naegleria fowleri and Balamuthia mandrillaris can instigate disease in otherwise immunocompetent individuals, whereas Acanthamoeba species mostly infect immunocompromised people. Acanthamoeba also cause a sight-threatening eye infection, mostly in contact lens wearers. Although infections due to pathogenic amoebae are considered rare, recently, these deadly amoebae were detected in water supplies in the USA. This is of particular concern, especially with global warming further exacerbating the problem. Herein, we describe the epidemiology, presentation, diagnosis, and management of free-living amoeba infections.

Discovery of repurposing drug candidates for the treatment of diseases caused by pathogenic free-living amoebae

Diseases caused by pathogenic free-living amoebae include primary amoebic meningoencephalitis (Naegleria fowleri), granulomatous amoebic encephalitis (Acanthamoeba spp.), Acanthamoeba keratitis, and Balamuthia amoebic encephalitis (Balamuthia mandrillaris). Each of these are difficult to treat and have high morbidity and mortality rates due to lack of effective therapeutics. Since repurposing drugs is an ideal strategy for orphan diseases, we conducted a high throughput phenotypic screen of 12,000 compounds from the Calibr ReFRAME library. We discovered a total of 58 potent inhibitors (IC50 <1 μM) against N. fowleri (n = 19), A. castellanii (n = 12), and B. mandrillaris (n = 27) plus an additional 90 micromolar inhibitors. Of these, 113 inhibitors have never been reported to have activity against Naegleria, Acanthamoeba or Balamuthia. Rapid onset of action is important for new anti-amoeba drugs and we identified 19 compounds that inhibit N. fowleri in vitro within 24 hours (halofuginone, NVP-HSP990, fumagillin, bardoxolone, belaronib, and BPH-942, solithromycin, nitracrine, quisinostat, pabinostat, pracinostat, dacinostat, fimepinostat, sanguinarium, radicicol, acriflavine, REP3132, BC-3205 and PF-4287881). These compounds inhibit N. fowleri in vitro faster than any of the drugs currently used for chemotherapy. The results of these studies demonstrate the utility of phenotypic screens for discovery of new drugs for pathogenic free-living amoebae, including Acanthamoeba for the first time. Given that many of the repurposed drugs have known mechanisms of action, these compounds can be used to validate new targets for structure-based drug design.

Discovery of Anti-Amoebic Inhibitors from Screening the MMV Pandemic Response Box on Balamuthia mandrillaris, Naegleria fowleri, and Acanthamoeba castellanii

Pathogenic free-living amoebae, Balamuthia mandrillaris, Naegleria fowleri, and several Acanthamoeba species are the etiological agents of severe brain diseases, with case mortality rates > 90%. A number of constraints including misdiagnosis and partially effective treatments lead to these high fatality rates. The unmet medical need is for rapidly acting, highly potent new drugs to reduce these alarming mortality rates. Herein, we report the discovery of new drugs as potential anti-amoebic agents. We used the CellTiter-Glo 2.0 high-throughput screening methods to screen the Medicines for Malaria Ventures (MMV) Pandemic Response Box in a search for new active chemical scaffolds. Initially, we screened the library as a single-point assay at 10 and 1 µM. From these data, we reconfirmed hits by conducting quantitative dose-response assays and identified 12 hits against B. mandrillaris, 29 against N. fowleri, and 14 against A. castellanii ranging from nanomolar to low micromolar potency. We further describe 11 novel molecules with activity against B. mandrillaris, 22 against N. fowleri, and 9 against A. castellanii. These structures serve as a starting point for medicinal chemistry studies and demonstrate the utility of phenotypic screening for drug discovery to treat diseases caused by free-living amoebae.

Gold-Conjugated Curcumin as a Novel Therapeutic Agent against Brain-Eating Amoebae

Balamuthia mandrillaris and Naegleria fowleri are free-living amoebae that cause infection of the central nervous system, granulomatous amoebic encephalitis (GAE) and primary amoebic meningoencephalitis (PAM), respectively. The fact that mortality rates for cases of GAE and PAM are more than 95% indicates the need for new therapeutic agents against those amoebae. Considering that curcumin exhibits a wide range of biological properties and has shown efficacy against Acanthamoeba castellanii, we evaluated the amoebicidal properties of curcumin against N. fowleri and B. mandrillaris. Curcumin showed significant amoebicidal activities with an AC₅₀ of 172 and 74 μM against B. mandrillaris and N. fowleri, respectively. Moreover, these compounds were also conjugated with gold nanoparticles to further increase their amoebicidal activities. After conjugation with gold nanoparticles, amoebicidal activities of the drugs were increased by up to 56 and 37% against B. mandrillaris and N. fowleri, respectively. These findings are remarkable and suggest that clinically available curcumin and our gold-conjugated curcumin nanoparticles hold promise in the improved treatment of fatal infections caused by brain-eating amoebae and should serve as a model in the rationale development of therapeutic interventions against other infections.

Brain-eating Amoebae Infection: Challenges and Opportunities in Chemotherapy

Pathogenic free-living amoeba are known to cause a devastating infection of the central nervous system and are often referred to as "brain-eating amoebae". The mortality rate of more than 90% and free-living nature of these amoebae is a cause for concern. It is distressing that the mortality rate has remained the same over the past few decades, highlighting the lack of interest by the pharmaceutical industry. With the threat of global warming and increased outdoor activities of public, there is a need for renewed interest in identifying potential anti-amoebic compounds for successful prognosis. Here, we discuss the available chemotherapeutic options and opportunities for potential strategies in the treatment and diagnosis of these life-threatening infections.

Occurrence of free-living amoebae (Acanthamoeba, Balamuthia, Naegleria) in water samples in Peninsular Malaysia

The aim of this study was to determine the occurrence of free-living amoebae (FLA) in Peninsular Malaysia and to compare different methodologies to detect them from water samples. Water samples were collected from tap water, recreational places, water dispensers, filtered water, etc. and tested for FLA using both cultivation and polymerase chain reaction (PCR) via plating assays and centrifugation methods. Amoebae DNA was extracted using Instagene matrix and PCR was performed using genus-specific primers. Of 250 samples, 142 (56.8%) samples were positive for presence of amoebae, while 108 (43.2%) were negative. Recreational water showed higher prevalence of amoebae than tap water. PCR for the plating assays revealed the presence of Acanthamoeba in 91 (64%) samples and Naegleria in 99 (70%) of samples analysed. All samples tested were negative for B. mandrillaris. In contrast, the centrifugation method was less effective in detecting amoebae as only one sample revealed the presence of Acanthamoeba and 52 (29%) samples were positive for Naegleria. PCR assays were specific and sensitive, detecting as few as 10 cells. These findings show the vast distribution and presence of FLA in all 11 states of Peninsular Malaysia. Further studies could determine the possible presence of pathogenic species and strains of free-living amoebae in public water supplies in Malaysia.

Development of nanoparticle-assisted PCR assay in the rapid detection of brain-eating amoebae

Brain-eating amoebae (Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri) have gained increasing attention owing to their capacity to produce severe human and animal infections involving the brain. Early detection is a pre-requisite in successful prognosis. Here, we developed a nanoPCR assay for the rapid detection of brain-eating amoebae using various nanoparticles. Graphene oxide, copper and alumina nanoparticles used in this study were characterized using Raman spectroscopy measurements through excitation with a He-Ne laser, while powder X-ray diffraction patterns were taken on a PANanalytical, X'Pert HighScore diffractometer and the morphology of the materials was confirmed using high-resolution transmission electron microscopy (HRTEM). Using nanoparticle-assisted PCR, the results revealed that graphene oxide, copper oxide and alumina nanoparticles significantly enhanced PCR efficiency in the detection of pathogenic free-living amoebae using genus-specific probes. The optimal concentration of graphene oxide, copper oxide and alumina nanoparticles for Acanthamoeba spp. was determined at 0.4, 0.04 and 0.4 μg per mL respectively. For B. mandrillaris, the optimal concentration was determined at 0.4 μg per mL for graphene oxide, copper oxide and alumina nanoparticles, and for Naegleria, the optimal concentration was 0.04, 4.0 and 0.04 μg per mL respectively. Moreover, combinations of these nanoparticles proved to further enhance PCR efficiency. The addition of metal oxide nanoparticles leads to excellent surface effect, while thermal conductivity property of the nanoparticles enhances PCR productivity. These findings suggest that nanoPCR assay has tremendous potential in the clinical diagnosis of parasitic infections as well as for studying epidemiology and pathology and environmental monitoring of other microbes.

Disseminated Balamuthia mandrillaris Infection

Balamuthia mandrillaris is a rare cause of human infection, but when infections do occur, they result in high rates of morbidity and mortality. A case of disseminated Balamuthia infection is presented. Early diagnosis and initiation of recommended therapy are essential for increased chances of successful outcomes.

Balamuthia mandrillaris: Morphology, biology, and virulence

Balamuthia mandrillaris is a protist pathogen that can cause encephalitis with a fatality rate of >95%. This is due to our incomplete understanding of the pathogenesis and pathophysiology of B. mandrillaris encephalitis. B. mandrillaris has two stages in its life cycle, an active trophozoite stage during which it divides mitotically. However, under unfavorable conditions, the trophozoite transforms into a dormant cyst stage. A major concern during the course of therapy is that B. mandrillaris can transform into cysts. Cysts are highly resistant to physical and chemical conditions and present a problem in successful antimicrobial chemotherapy. Several lines of evidence suggest that B. mandrillaris encephalitis develops as a result of hematogenous spread, but it is unclear how circulating amoebae enter the central nervous system and cause inflammation, blood-brain barrier disruption, and neuronal injury. Recent studies have identified several parasite-host determinants for B. mandrillaris translocation of the blood-brain barrier, and host inflammatory markers that may be associated with neuronal injury. These determinants may provide important targets for the prevention and treatment of this devastating infection. Here, we present a brief overview of the current understanding of the morphology, biology, pathogenesis, and pathophysiology of B. mandrillaris encephalitis.

Killing the dead: chemotherapeutic strategies against free-living cyst-forming protists (Acanthamoeba sp. and Balamuthia mandrillaris)

The opportunist free-living protists such as Acanthamoeba spp. and Balamuthia mandrillaris have become a serious threat to human life. As most available drugs target functional aspects of pathogens, the ability of free-living protists to transform into metabolically inactive cyst forms presents a challenge in treatment. It is hoped, that the development of broad spectrum antiprotist agents acting against multiple cyst-forming protists to provide target-directed inhibition will offer a viable drug strategy in the treatment of these rare infections. Here, we present a comprehensive report on upcoming drug targets, with emphasis on cyst wall biosynthesis along with the related biochemistry of encystment pathways, as we strive to bring ourselves a step closer to being able to combat these deadly diseases.

Balamuthia mandrillaris: role of galactose in encystment and identification of potential inhibitory targets

Balamuthia mandrillaris is a causative agent of granulomatous encephalitis that almost always proves fatal. A major concern during the course of therapy is that B. mandrillaris can transform into cysts. Cysts are highly resistant to physical and chemical conditions and present a problem in successful antimicrobial chemotherapy. However, the underlying mechanisms of B. mandrillaris transformation into cysts are not known. In this study, we examined the effects of exogenous sugars on B. mandrillaris encystment. The findings revealed that free exogenous galactose, but not other sugars, enhanced parasite differentiation into cysts, and apparently a galactose-binding protein is involved in B. mandrillaris encystment. Cytoskeletal re-arrangements and phosphatidylinositol 3-kinase (PI3K)-mediated pathways are involved in B. mandrillaris encystment based on inhibitor studies. Dual functionality of galactose-binding protein in B. mandrillaris pathogenesis and encystment is discussed further.

Balamuthia mandrillaris meningoencephalitis in an immunocompromised patient

Balamuthia mandrillaris is a rare but increasingly recognized cause of amebic encephalitis, yet it remains poorly understood. The condition is almost universally fatal, and due to diagnostic difficulty, most cases are identified postmortem. The authors report a case of Balamuthia amebic encephalitis in a patient with combined variable immunodeficiency in which a rare antemortem diagnosis was made via brain biopsy. Despite broad-spectrum antimicrobial therapy, the outcome was fatal. Such presentations are challenging, and definitive diagnosis may require biopsy in consultation with a skilled neuropathologist.

The cyst wall carbohydrate composition of Balamuthia mandrillaris

Balamuthia mandrillaris is an opportunistic cyst-producing amoeba that can cause rare, but fatal, Balamuthia amoebic encephalitis (BAE). Cysts are resistant to harsh environmental conditions and many antimicrobial compounds and thus can contribute to BAE recurrence. However, little is known of cyst wall synthesis, cyst wall composition, or how encystment is induced. In this study, we examined the carbohydrate composition of the cyst wall. The major components were mannose (20.9 mol%) and glucose (79.1 mol%), with trace amounts of galactose present in the cyst wall samples analysed. The linkage analysis showed cyst wall carbohydrates with apparently linear and branching saccharides and suggested the presence of cellulose. These components may play an important protective role by creating a permeability barrier around the cyst.

Balamuthia mandrillaris resistance to hostile conditions

The resistance of Balamuthia mandrillaris to physical, chemical and radiological conditions was tested. Following treatments, viability was determined by culturing amoebae on human brain microvascular endothelial cells for up to 12 days. B. mandrillaris cysts were resistant to repeated freeze–thawing (five times), temperatures of up to 70 °C, 0.5 % SDS, 25 p.p.m. chlorine, 10 μg pentamidine isethionate ml−1 and 200 mJ UV irradiation cm−2.