Acanthamoeba induces cell-cycle arrest in host cells

A Narrative Review of Acanthamoeba Isolates in Malaysia: Challenges in Infection Management and Natural Therapeutic Advancements

Acanthamoeba, a free-living amoeba (FLA) found in diverse ecosystems, poses significant health risks globally, particularly in Malaysia. It causes severe infectious diseases, e.g., Acanthamoeba keratitis (AK), primarily affecting individuals who wear contact lenses, along with granulomatous amoebic encephalitis (GAE), a rare but often life-threatening condition among immunocompromised individuals. AK has become increasingly prevalent in Malaysia and is linked to widespread environmental contamination and improper contact lens hygiene. Recent studies highlight Acanthamoeba's capacity to serve as a "Trojan horse" for amoeba-resistant bacteria (ARBs), contributing to hospital-associated infections (HAIs). These symbiotic relationships and the resilience of Acanthamoeba cysts make treatment challenging. Current diagnostic methods in Malaysia rely on microscopy and culture, though molecular procedures like polymerase chain reaction (PCR) are employed for more precise detection. Treatment options remain limited due to the amoeba's cyst resistance to conventional therapies. However, recent advancements in natural therapeutics, including using plant extracts such as betulinic acid from Pericampylus glaucus and chlorogenic acid from Lonicera japonica, have shown promising in vitro results. Additionally, nanotechnology applications, mainly using gold and silver nanoparticles to enhance drug efficacy, are emerging as potential solutions. Further, in vivo studies and clinical trials must validate these findings. This review highlights the requirement for continuous research, public health strategies, and interdisciplinary collaboration to address the growing threat of Acanthamoeba infections in Malaysia while exploring the country's rich biodiversity for innovative therapeutic solutions.

Sublethal Exposure to Plasma-Activated Water Influences the Morphological Characteristics, Phagocytic Ability, and Virulence of Acanthamoeba castellanii

PURPOSE

This study aimed to examine the ultrastructure, cytotoxicity, phagocytosis, and antioxidant responses of Acanthamoeba castellanii trophozoites exposed to sublethal plasma-activated water.

METHODS

Trophozoites were exposed to a sublethal treatment of PAW and compared to untreated viable trophozoites via adhesion assays on macrophage monolayers, osmo- and thermotolerance tests. Bacterial uptake was assessed in treated cells to evaluate their phagocytic characteristics. Oxidative stress biomarkers and antioxidant activities were compared in treated and untreated trophozoites. Finally, the expression of the mannose-binding protein (MBP), cysteine protease 3 (CP3), and serine endopeptidase (SEP) genes was determined in cells.

RESULTS

In PAW-treated trophozoites, cytopathic effects were more extensive and resulted in the detachment of macrophage monolayers. Treated trophozoites could not grow at high temperatures (43 °C). Moreover, they showed osmotolerance to 0.5 M D-mannitol but not to 1 M. Results demonstrated a higher bacterial uptake rate by PAW-treated trophozoites than untreated cells. Activities of superoxide dismutase and catalase and catalase were significantly greater in the treated trophozoites, and the glutathione and glutathione/glutathione disulfide were significantly lower in the PAW-treated cells. Exposure to PAW also significantly increased the malondialdehyde level and total antioxidant capacity. Treatment with PAW led to significantly higher expression of virulent genes like MBP, CP3, and SEP.

CONCLUSION

PAW is a double-edged sword against A. castellanii. PAW is an effective antiamoebic agent in proper usage, whereas its sublethal exposure may reduce its effectiveness and increase amoebas' pathogenicity. An agent's adequate concentration and exposure time are essential to achieve optimum results.

Exposure to sublethal concentrations of chlorine enhances the cytotoxicity of Acanthamoeba castellanii

Free-living amoebae belonging to the genus Acanthamoeba are the causative agents of infections in humans and animals. Many studies are being conducted to find effective compounds against amoebae, but their sublethal concentration effects on surviving amoebae seem to have been overlooked. Chlorine is a common disinfection agent commonly added to public water facilities and supplies. In this study, the cytopathic and phagocytic properties of Acanthamoeba castellanii trophozoites following exposure to sublethal concentrations of chlorine were examined. Two hours of exposure to 5 ppm hypochlorite calcium was considered the sublethal concentration for A. castellanii trophozoites. To compare the pathogenic potential of treated and untreated Acanthamoeba trophozoites, cytotoxicity, adhesion assays in RAW 264.7 macrophages, osmo, and thermotolerance tests were carried out. Bacterial uptake was assessed in treated cells to evaluate their phagocytic characteristics. Oxidative stress biomarkers and antioxidant activities were compared in treated and untreated trophozoites. Finally, the mRNA expression of the mannose-binding protein (MBP), cysteine protease 3 (CP3), and serine endopeptidase (SEP) genes was determined in cells. In all the experiments, untreated trophozoites were considered the control. In comparison to untreated trophozoites, in chlorine-treated trophozoites, cytopathic effects were more extensive and resulted in the detachment of macrophage monolayers. Treated trophozoites could not grow at high temperatures (43 °C). Besides, they showed osmotolerance to 0.5 M D-mannitol but not to 1 M. Results demonstrated a higher bacterial uptake rate by chlorine-treated trophozoites than untreated cells. The treated and untreated cells had significantly different glutathione and glutathione/glutathione disulfide ratios. Antioxidant enzyme activities, total antioxidant capacity, and malondialdehyde levels were increased significantly in chlorine-treated cells. Quantifying mRNA expression in chlorine-treated trophozoites revealed that virulence genes were upregulated. Chlorine can form resistance and virulent amoebae if it is not used at a proper concentration and exposure time. Identification of stress responses, their mechanisms in Acanthamoeba, and their relation to amoeba virulence would give us a better perception of their pathophysiology.

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.

New strategies in the treatment of diseases caused by Acanthamoeba based on nanoparticles: a systematic review

BACKGROUND

Acanthamoeba is one of the opportunistic parasites with a global prevalence. Currently, due to the side effects and the emergence of drug resistance to this parasite, much research has been performed on the use of nano-drugs to treat Acanthamoeba-caused diseases. Therefore, this systematic review study aims to evaluate new strategies for treating diseases caused by Acanthamoeba based on nanoparticles (NPs).

METHODS

We designed a systematic review based on the articles published in English between 2000 and 2022. Our search strategy was based on syntax and specific tags for each database, including ScienceDirect, PubMed, Scopus, Ovid, and Cochrane. From the articles, those that had inclusion criteria were selected, and their data were extracted and analyzed.

RESULTS

In this study, 26 studies were selected. Metallic nanoparticles were mostly used against the Acanthamoeba species (80.7%). 19.2% of the studies used polymeric nanoparticles, and 3.8% used emulsion nanoparticles. Most studies (96.1%) were performed in vitro, and only one study (3.8%) was carried out in vivo. Silver NPs were the most used metallic nanoparticles in the studies. The best effect of the anti-Acanthamoeba compound was observed for green synthesized nanoparticles based on stabilization by plant gums, loaded with citrus fruits flavonoids hesperidin (HDN) and naringin (NRG) with a 100% growth inhibition at a concentration of 50 μg/mL.

CONCLUSION

This study showed that chlorhexidine and other plant metabolites loaded with silver and gold nanoparticles increase the anti-Acanthambae activity of these nanoparticles. However, green synthesized nanoparticles based on stabilization by plant gums, loaded with citrus fruits flavonoids hesperidin (HDN) and naringin (NRG), showed the best anti-Acanthambae effect. Nevertheless, further studies should be performed to determine their safety for human use.

A Comparative Genomic Approach to Determine the Virulence Factors and Horizontal Gene Transfer Events of Clinical Acanthamoeba Isolates

Acanthamoeba species are among the most ubiquitous protists that are widespread in soil and water and act as both a replicative niche and vectors for dispersal. They are the most important human intracellular pathogens, causing Acanthamoeba keratitis (AK) and severely damaging the human cornea. The sympatric lifestyle within the host and amoeba-resisting microorganisms (ARMs) promotes horizontal gene transfer (HGT). However, the genomic diversity of only A. castellanii and A. polyphaga has been widely studied, and the pathogenic mechanisms remain unknown. Thus, we examined 7 clinically pathogenic strains by comparative genomic, phylogenetic, and rhizome gene mosaicism analyses to explore amoeba-symbiont interactions that possibly contribute to pathogenesis. Genetic characterization and phylogenetic analysis showed differences in functional characteristics between the "open" state of T3 and T4 isolates, which may contribute to the differences in virulence and pathogenicity. Through comparative genomic analysis, we identified potential genes related to virulence, such as metalloprotease, laminin-binding protein, and HSP, that were specific to the genus Acanthamoeba. Then, analysis of putative sequence trafficking between Acanthamoeba and Pandoraviruses or Acanthamoeba castellanii medusaviruses provided the best hits with viral genes; among bacteria, Pseudomonas had the most significant numbers. The most parsimonious evolutionary scenarios were between Acanthamoeba and endosymbionts; nevertheless, in most cases, the scenarios are more complex. In addition, the differences in exchanged genes were limited to the same family. In brief, this study provided extensive data to suggest the existence of HGT between Acanthamoeba and ARMs, explaining the occurrence of diseases and challenging Darwin's concept of eukaryotic evolution. IMPORTANCE Acanthamoeba has the ability to cause serious blinding keratitis. Although the prevalence of this phenomenon has increased in recent years, our knowledge of the underlying opportunistic pathogenic mechanism maybe remains incomplete. In this study, we highlighted the importance of Pseudomonas in the pathogenesis pathway using comprehensive a whole genomics approach of clinical isolates. The horizontal gene transfer events help to explain how endosymbionts contribute Acanthamoeba to act as an opportunistic pathogen. Our study opens up several potential avenues for future research on the differences in pathogenicity and interactions among clinical strains.

Acanthamoeba Keratitis: Current Status and Urgent Research Priorities

Background:

First discovered in the early 1970s, Acanthamoeba keratitis has remained a major eye infection and presents a significant threat to the public health, especially in developing countries. The aim is to present a timely review of our current understanding of the advances made in this field in a comprehensible manner and includes novel concepts and provides clear directions for immediate research priorities.

Methods:

We undertook a search of bibliographic databases for peer-reviewed research literature and also summarized our published results in this field.

Results:

The present review focuses on novel diagnostic and therapeutic strategies in details which can provide access to management and treatment of Acanthamoeba keratitis. This coupled with the recently available genome sequence information together with high throughput genomics technology and innovative approaches should stimulate interest in the rational design of preventative and therapeutic measures. Current treatment of Acanthamoeba keratitis is problematic and often leads to infection recurrence. Better understanding of diagnosis, pathogenesis, pathophysiology and therapeutic regimens, would lead to novel strategies in treatment and prophylaxis.

An update on Acanthamoeba keratitis: diagnosis, pathogenesis and treatment

Free-living amoebae of the genus Acanthamoeba are causal agents of a severe sight-threatening infection of the cornea known as Acanthamoeba keratitis. Moreover, the number of reported cases worldwide is increasing year after year, mostly in contact lens wearers, although cases have also been reported in non-contact lens wearers. Interestingly, Acanthamoeba keratitis has remained significant, despite our advances in antimicrobial chemotherapy and supportive care. In part, this is due to an incomplete understanding of the pathogenesis and pathophysiology of the disease, diagnostic delays and problems associated with chemotherapeutic interventions. In view of the devastating nature of this disease, here we present our current understanding of Acanthamoeba keratitis and molecular mechanisms associated with the disease, as well as virulence traits of Acanthamoeba that may be potential targets for improved diagnosis, therapeutic interventions and/or for the development of preventative measures. Novel molecular approaches such as proteomics, RNAi and a consensus in the diagnostic approaches for a suspected case of Acanthamoeba keratitis are proposed and reviewed based on data which have been compiled after years of working on this amoebic organism using many different techniques and listening to many experts in this field at conferences, workshops and international meetings. Altogether, this review may serve as the milestone for developing an effective solution for the prevention, control and treatment of Acanthamoeba infections.

An Oct4-pRb axis, controlled by MiR-335, integrates stem cell self-renewal and cell cycle control

The pluripotency of mouse embryonic stem cells (mESCs) is controlled by a network of transcription factors, mi-RNAs, and signaling pathways. Here, we present a new regulatory circuit that connects miR-335, Oct4, and the Retinoblastoma pathway to control mESC self-renewal and differentiation. Oct4 drives the expression of Nipp1 and Ccnf that inhibit the activity of the protein phosphatase 1 (PP1) complex to establish hyperphosphorylation of the retinoblastoma protein 1 (pRb) as a hallmark feature of self-renewing mESCs. The Oct4-Nipp1/Ccnf-PP1-pRb axis promoting mESC self-renewal is under control of miR-335 that regulates Oct4 and Rb expression. During mESC differentiation, miR-335 upregulation co-operates with the transcriptional repression of Oct4 to facilitate the collapse of the Oct4-Nipp1/Ccnf-PP1-pRb axis, pRb dephosphorylation, the exit from self-renewal, and the establishment of a pRb-regulated cell cycle program. Our results introduce Oct4-dependent control of the Rb pathway as novel regulatory circuit controlling mESC self-renewal and differentiation.

An insight into the ligand-receptor interactions involved in the translocation of pathogens across blood-brain barrier

Traversal of pathogen across the blood-brain barrier (BBB) is an essential step for central nervous system (CNS) invasion. Pathogen traversal can occur paracellularly, transcellularly, and/or in infected phagocytes (Trojan horse mechanism). To trigger the translocation processes, mainly through paracellular and transcellular ways, interactions between protein molecules of pathogen and BBB are inevitable. Simply, it takes two to tango: both host receptors and pathogen ligands. Underlying molecular basis of BBB translocation of various pathogens has been revealed in the last decade, and a plethora of experimental data on protein-protein interactions has been created. This review compiles these data and should give insights into the ligand-receptor interactions that occur during BBB translocation. Further, it sheds light on cell signaling events triggered in response to ligand-receptor interaction. Understanding of the molecular principles of pathogen-host interactions that are involved in traversal of the BBB should contribute to develop new vaccine and drug strategies to prevent CNS infections.

The role of proteases in the differentiation of Acanthamoeba castellanii

Proteases are significant determinants of protozoan pathogenicity and cytolysis of host cells. However, there is now growing evidence of their involvement in cellular differentiation. Acanthamoeba castellanii of the T4 genotype elaborates a number of proteases, which are inhibited by the serine protease inhibitor phenylmethylsulphonyl fluoride. Using this and other selective protease inhibitors, in tandem with siRNA primers, specific to the catalytic site of Acanthamoeba serine proteases, we demonstrate that serine protease activity is crucial for the differentiation of A. castellanii. Furthermore, both encystment and excystment of A. castellanii was found to be dependent on serine protease function.

Acanthamoeba and the blood–brain barrier: the breakthrough

Acanthamoeba granulomatous encephalitis is a rare disease that almost always proves fatal. Death occurs mainly due to neurological complications; however, the pathogenesis and pathophysiology associated with this disease remain incompletely understood. Haematogenous spread is a key step in the development of Acanthamoeba encephalitis, but it is not clear how circulating amoebae breakthrough the blood–brain barrier to gain entry into the central nervous system to produce the disease. This review of the literature describes the parasite factors and immune-mediated mechanisms involved in the blood–brain barrier dysfunction leading to neuropathogenesis.

Oct4 targets regulatory nodes to modulate stem cell function

Stem cells are characterized by two defining features, the ability to self-renew and to differentiate into highly specialized cell types. The POU homeodomain transcription factor Oct4 (Pou5f1) is an essential mediator of the embryonic stem cell state and has been implicated in lineage specific differentiation, adult stem cell identity, and cancer. Recent description of the regulatory networks which maintain ‘ES’ have highlighted a dual role for Oct4 in the transcriptional activation of genes required to maintain self-renewal and pluripotency while concomitantly repressing genes which facilitate lineage specific differentiation. However, the molecular mechanism by which Oct4 mediates differential activation or repression at these loci to either maintain stem cell identity or facilitate the emergence of alternate transcriptional programs required for the realization of lineage remains to be elucidated. To further investigate Oct4 function, we employed gene expression profiling together with a robust statistical analysis to identify genes highly correlated to Oct4. Gene Ontology analysis to categorize overrepresented genes has led to the identification of themes which may prove essential to stem cell identity, including chromatin structure, nuclear architecture, cell cycle control, DNA repair, and apoptosis. Our experiments have identified previously unappreciated roles for Oct4 for firstly, regulating chromatin structure in a state consistent with self-renewal and pluripotency, and secondly, facilitating the expression of genes that keeps the cell poised to respond to cues that lead to differentiation. Together, these data define the mechanism by which Oct4 orchestrates cellular regulatory pathways to enforce the stem cell state and provides important insight into stem cell function and cancer.

Acanthamoeba invasion of the central nervous system

Pathogenic Acanthamoeba are known to infect the CNS, resulting in fatal granulomatous encephalitis. The mechanisms associated with the pathogenesis remain unclear; however pathophysiological complications involving the CNS most likely include induction of pro-inflammatory responses, invasion of the blood-brain barrier and the connective tissue and neuronal damage leading to brain dysfunction. The routes of entry include the olfactory neuroepithelium pathway and/or lower respiratory tract, followed by haematogenous spread. Skin lesions may provide direct entry into the bloodstream, bypassing the lower respiratory tract. For the haematogenous route, entry of amoebae into the CNS most likely occurs at the sites of the blood-brain barrier. Recent studies have identified several molecular mechanisms associated with Acanthamoeba traversal of the blood-brain barrier and targeting those may help develop therapeutic interventions and/or design preventative strategies.

Cellulose biosynthesis pathway is a potential target in the improved treatment of Acanthamoeba keratitis

Acanthamoeba is an opportunistic protozoan pathogen that can cause blinding keratitis as well as fatal granulomatous encephalitis. One of the distressing aspects in combating Acanthamoeba infections is the prolonged and problematic treatment. For example, current treatment against Acanthamoeba keratitis requires early diagnosis followed by hourly topical application of a mixture of drugs that can last up to a year. The aggressive and prolonged management is due to the ability of Acanthamoeba to rapidly adapt to harsh conditions and switch phenotypes into a resistant cyst form. One possibility of improving the treatment of Acanthamoeba infections is to inhibit the ability of these parasites to switch into the cyst form. The cyst wall is partially made of cellulose. Here, we tested whether a cellulose synthesis inhibitor, 2,6-dichlorobenzonitrile (DCB), can enhance the effects of the antiamoebic drug pentamidine isethionate (PMD). Our findings revealed that DCB can block Acanthamoeba encystment and may improve the antiamoebic effects of PMD. Using in vitro assays, the findings revealed that DCB enhanced the inhibitory effects of PMD on Acanthamoeba binding to and cytotoxicity of the host cells, suggesting the cellulose biosynthesis pathway as a novel target for the improved treatment of Acanthamoeba infections.

Blood-brain barrier genomics and proteomics: elucidating phenotype, identifying disease targets and enabling brain drug delivery

The blood-brain barrier (BBB) regulates the passage of material between the bloodstream and the brain. Recent genomic and proteomic studies of the BBB have identified some of the unique molecular characteristics of this vascular bed, and have reinforced the concept that the BBB is heavily involved in brain function. Genomic and proteomic techniques have also been used to analyze the molecular events underlying diseases that have BBB involvement, such as multiple sclerosis, Alzheimer's disease, stroke and HIV-1 infection. It is expected that a better understanding of the complex mechanisms that link the BBB to neurological disease will ultimately lead to the development of innovative treatments.

Acanthamoeba: biology and increasing importance in human health

Acanthamoeba is an opportunistic protozoan that is widely distributed in the environment and is well recognized to produce serious human infections, including a blinding keratitis and a fatal encephalitis. This review presents our current understanding of the burden of Acanthamoeba infections on human health, their pathogenesis and pathophysiology, and molecular mechanisms associated with the disease, as well as virulence traits of Acanthamoeba that may be targets for therapeutic interventions and/or the development of preventative measures.

RNA interference (RNAi) for the silencing of extracellular serine proteases genes in Acanthamoeba: molecular analysis and effect on pathogenecity

Silencing of extracellular serine protease genes was undertaken by interference RNA (RNAi). Chemically synthesized, small interfering RNA (siRNA) were highly specific and efficient in silencing the catalytic domain of extracellular serine proteases of Acanthamoeba. In order to confirm the silencing phenomenon, the extracellular serine protease activities in RNAi-treated parasites were compared to non-treated parasites, using zymography profiles, Acanthamoeba-conditioned medium (ACM) protease activity, cytotoxicity assays and extracellular serine protease mRNA levels analysis. Zymography profiles showed a decrease in the extracellular protease levels in the moderate pathogenic and pathogenic strains, after treatment with siRNA. These results were supported after the ACM protease activity and CPE assays were performed in all studied isolates, showing a lower protease activity or cytotoxicity both in the pathogenic and moderate pathogenic strains treated with RNAi. These results support that extracellular serine proteases are directly involved in the pathogenesis and virulence of Acanthamoeba.

Acanthamoeba isolates belonging to T1, T2, T3, T4 but not T7 encyst in response to increased osmolarity and cysts do not bind to human corneal epithelial cells

Acanthamoeba is an opportunistic protozoan that is widely distributed in the environment and can cause human infections. The life cycle of Acanthamoeba consists of an infective trophozoite form. However under harsh environmental conditions trophozoites differentiate into a double-walled, metabolically inactive and resistant cyst form. Research in Acanthamoeba has mostly focussed on the infective trophozoite form and its pathogenic mechanisms. In this study, we used Acanthamoeba isolates belonging to T1, T2, T3, T4, T7 genotypes and studied their cysts properties. We determined that food deprivation stimulates encystment in Acanthamoeba isolates belonging to T1, T2, T3, T4 and T7 genotypes in a sodium dodecyl sulfate (SDS)-resistant manner. In addition, increase in osmolarity triggered encystment in T1, T2, T3, T4 isolates (SDS-resistant) but T7 failed to encyst (SDS-labile). Adhesion assays revealed that Acanthamoeba cysts belonging to T1, T2, T3, T4, and T7 genotypes exhibited no and/or minimal binding (<5%) to the host cells. Fluorescein-labelled lectins showed that all Acanthamoeba isolates tested exhibited binding to concanavalin A, indicating the expression of mannosyl- and/or glucosyl-residues. Role of cysts in the transmission of infection is discussed further.

Acanthamoeba castellanii induces host cell death via a phosphatidylinositol 3-kinase-dependent mechanism

Granulomatous amoebic encephalitis due to Acanthamoeba castellanii is a serious human infection with fatal consequences, but it is not clear how the circulating amoebae interact with the blood-brain barrier and transmigrate into the central nervous system. We studied the effects of an Acanthamoeba encephalitis isolate belonging to the T1 genotype on human brain microvascular endothelial cells, which constitute the blood-brain barrier. Using an apoptosis-specific enzyme-linked immunosorbent assay, we showed that Acanthamoeba induces programmed cell death in brain microvascular endothelial cells. Next, we observed that Acanthamoeba specifically activates phosphatidylinositol 3-kinase. Acanthamoeba-mediated brain endothelial cell death was abolished using LY294002, a phosphatidylinositol 3-kinase inhibitor. These results were further confirmed using brain microvascular endothelial cells expressing dominant negative forms of phosphatidylinositol 3-kinase. This is the first demonstration that Acanthamoeba-mediated brain microvascular endothelial cell death is dependent on phosphatidylinositol 3-kinase.

Mechanisms associated with Acanthamoeba castellanii (T4) phagocytosis

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