Acanthamoeba differentiation: a two-faced drama of Dr Jekyll and Mr Hyde

Acanthamoeba castellanii trophozoites need oxygen for normal functioning and lipids are their preferred substrate, offering new possibilities for treatment

Acanthamoebae, pathogenic free-living amoebae, can cause Granulomatous Amoebic Encephalitis (GAE) and keratitis, and for both types of infection, no adequate treatment options are available. As the metabolism of pathogens is an attractive treatment target, we set out to examine the energy metabolism of Acanthamoeba castellanii and studied the aerobic and anaerobic capacities of the trophozoites. Under anaerobic conditions, or in the presence of inhibitors of the electron-transport chain, A. castellanii trophozoites became rounded, moved sluggishly and stopped multiplying. This demonstrates that oxygen and the respiratory chain are essential for movement and replication. Furthermore, the simultaneous activities of both terminal oxidases, cytochrome c oxidase and the plant-like alternative oxidase, are essential for normal functioning and replication. The inhibition of normal function caused by the inactivity of the respiratory chain was reversible. Once respiration was made possible again, the rounded, rather inactive amoebae formed acanthopodia within 4 h and resumed moving, feeding and multiplying. Experiments with radiolabelled nutrients revealed a preference for lipids over glucose and amino acids as food. Subsequent experiments showed that adding lipids to a standard culture medium of trophozoites strongly increased the growth rate. Acanthamoeba castellanii trophozoites have a strictly aerobic energy metabolism and β-oxidation of fatty acids, the Krebs cycle, and an aerobic electron-transport chain coupled to the ATP synthase, producing most of the used ATP. The preference for lipids can be exploited, as we show that three known inhibitors of lipid oxidation strongly inhibited the growth of A. castellanii. In particular, thioridazine and perhexiline showed potent effects in low micromolar concentrations. Therefore, this study revealed a new drug target with possibly new options to treat Acanthamoeba infections.

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.

Applications of medicinal chemistry for drug discovery against Acanthamoeba infections

Acanthamoeba is a genus of free-living amoebae, pervasively found in the environment. Most of its pathogenic species are the causative agent of sight-threatening Acanthamoeba keratitis and fatal granulomatous amoebic encephalitis. Despite the advancements in the field of chemotherapy, treating Acanthamoeba infections is still challenging due to incomplete knowledge of the complicated pathophysiology. In case of infection, the treatment regimen for the patients is often ineffective due to delayed diagnosis, poor specificity, and side-effects. Besides the resistance of Acanthamoeba cysts to most of the drugs, the recurrence of infection further complicates the recovery. Thus, it is necessary to develop an effective treatment which can eradicate these rare, but serious infections. Based on various computational and in vitro studies, it has been established that the synthetic scaffolds such as heterocyclic compounds may act as potential drug leads for the development of antiamoebic drugs. In this review, we report different classes of synthetic compounds especially heterocyclic compounds which have shown promising results against Acanthamoeba. Moreover, the antiamoebic activities of synthetic compounds with their possible mode of actions against Acanthamoeba, have been summarized and discussed in this review.

Genotype distribution of Acanthamoeba in keratitis: a systematic review

Acanthamoeba spp. are among the most worldwide prevalent protozoa. It is the causative agent of a disease known as Acanthamoeba keratitis, a painful and severe sight-threatening corneal infection that can lead to blindness. In recent years, the prevalence of Acanthamoeba keratitis has rapidly increased, growing its importance to human health. This systematic review aims to assess the distribution of Acanthamoeba sp. genotypes causing keratitis around the world, considering the sample collected type and the used identification method. Most of the cases were found in Asia and Europe. Not surprisingly, the T4 genotype was the most prevalent worldwide, followed by T3, T15, T11, and T5. Furthermore, the T4 genotype contains a higher number of species. Given the differences in pathology, susceptibility to treatment, and clinical outcome between distinct genotypes, it is essential to genotype isolates from Acanthamoeba keratitis cases to help to establish a better correlation between in vitro and in vivo activities, resulting in better drug therapies and successful treatment in cases of this important ocular infection.

Acanthamoeba and its pathogenic role in granulomatous amebic encephalitis

Acanthamoeba is a free-living amoeba that is widely distributed in the environment. It is an opportunist protist, which is known to cause rare yet fatal infection of the central nervous system (CNS), granulomatous amebic encephalitis (GAE) in humans. GAE cases are increasingly been reported among immunocompromised patients, with few cases in immunocompetent hosts. Diagnosis of GAE primarily includes neuroimaging, microscopy, cerebrospinal fluid (CSF) culture, histopathology, serology and molecular techniques. Early diagnosis is vital for proper management of infected patients. Combination therapeutic approach has been tried in various GAE cases reported worldwide. We tried to present a comprehensive review, which summarizes on the epidemiology of GAE caused by Acanthamoeba along with the associated clinical symptoms, risk factors, diagnosis and treatment of GAE among infected patients.

Acanthamoeba in the eye, can the parasite hide even more? Latest developments on the disease

Acanthamoeba spp. is a free living protozoan in the environment, but can cause serious diseases. Acanthamoeba keratitis (AK), a severe and painful eye infection, must be treated as soon as possible to prevent ulceration of the cornea, loss of visual acuity, and eventually blindness or enucleation. Although the disease affects principally contact lens (CLs) wearers, it is recognized nowadays as a cause of keratitis also in non-CLs wearers. Although the number of infections caused by these amoebae is low, AK is an emerging disease presenting an extended number of cases each year worldwide mostly due to the increasing use of CLs, but also to better diagnostic methods and awareness. There are two principal causes that lead to severe outcomes: misdiagnosis or late diagnosis of the causal agent, and lack of a fully effective therapy due to the existence of a highly resistant cyst stage of Acanthamoeba. Recent studies have reported different genotypes that have not been previously associated with this disease. In addition, Acanthamoeba can act as a reservoir for phylogenetically diverse microorganisms. In this regard, recently giant viruses called Pandoravirus have been found within genotypes producing keratitis. What potential risk this poses is not yet known. This review focuses on an overview of the present status and future prospects of this re-emerging pathology, including features of the parasite, epidemiology, clinical aspects, diagnosis, and treatment.

Acanthamoeba and Dictyostelium Use Different Foraging Strategies

Amoeba often use cell movement as a mechanism to find food, such as bacteria, in their environment. The chemotactic movement of the soil amoeba Dictyostelium to folate or other pterin compounds released by bacteria is a well-documented foraging mechanism. Acanthamoeba can also feed on bacteria but relatively little is known about the mechanism(s) by which this amoeba locates bacteria. Acanthamoeba movement in the presence of folate or bacteria was analyzed in above agar assays and compared to that observed for Dictyostelium. The overall mobility of Acanthamoeba was robust like that of Dictyostelium but Acanthamoeba did not display a chemotactic response to folate. In the presence of bacteria, Acanthamoeba only showed a marginal bias in directed movement whereas Dictyostelium displayed a strong chemotactic response. A comparison of genomes revealed that Acanthamoeba and Dictyostelium share some similarities in G protein signaling components but that specific G proteins used in Dictyostelium chemotactic responses were not present in current Acanthamoeba genome sequence data. The results of this study suggest that Acanthamoeba does not use chemotaxis as the primary mechanism to find bacterial food sources and that the chemotactic responses of Dictyostelium to bacteria may have co-evolved with chemotactic responses that facilitate multicellular development.

Eukaryotic cell encystation and cancer cell dormancy: is a greater devil veiled in the details of a lesser evil?

Cancer cell dormancy is the main cause of cancer recurrence and failure of therapy as dormant cells evade not only the anticancer drugs but also the host immune system. These dormant cells veil themselves from detection by imaging and/or using biomarkers, which imposes an additional problem in targeting such cells. A similar form of hibernation process known as encystation is studied in detail for pathogenic unicellular eukaryotic microorganisms. By examination using microarray gene expression profiles, immunocytochemistry tools, and siRNAs during the process of encystation, understanding the covert features of cancer cell dormancy as proposed could be possible. This knowledge can be extended to dormant cancer cells to uncover the mechanisms that underlie this ghost, yet dangerous state of human cancers. We propose a strategy to induce dormancy and exit this state by application of knowledge gained from the encystation induction and retrieval processes in pathogenic eukaryotic microorganisms. Given that early detection and characterization of dormant malignant tumor cells is important as a general strategy to monitor and prevent the development of overt metastatic disease, this homology may enable the design of therapies that could either awake the dormant cell from dormancy to make it available for therapies or prolong such a phase to make cancer appear as a chronic disease.

Cellulose degradation: a therapeutic strategy in the improved treatment of Acanthamoeba infections

Acanthamoeba is an opportunistic free-living amoeba that can cause blinding keratitis and fatal brain infection. Early diagnosis, followed by aggressive treatment is a pre-requisite in the successful treatment but even then the prognosis remains poor. A major drawback during the course of treatment is the ability of the amoeba to enclose itself within a shell (a process known as encystment), making it resistant to chemotherapeutic agents. As the cyst wall is partly made of cellulose, thus cellulose degradation offers a potential therapeutic strategy in the effective targeting of trophozoite encased within the cyst walls. Here, we present a comprehensive report on the structure of cellulose and cellulases, as well as known cellulose degradation mechanisms with an eye to target the Acanthamoeba cyst wall. The disruption of the cyst wall will make amoeba (concealed within) susceptible to chemotherapeutic agents, and at the very least inhibition of the excystment process will impede infection recurrence, as we bring these promising drug targets into focus so that they can be explored to their fullest.