Encystation of Acanthamoeba castellanii: Dye uptake for assessment by flow cytometry and confocal laser scanning microscopy

Effect of Nitric Oxide on Acanthamoeba castellanii

Purpose

Acanthamoeba keratitis is a well-known intractable corneal infectious disease. We investigated the anti-Acanthamoeba effect of exogenous nitric oxide (NO).

Methods

Acanthamoeba castellanii was axenically cultured and exposed to various concentrations of NO donors, such as sodium nitrite, sodium nitroprusside (SNP), and NO-releasing silica nanoparticles (coated in branched polyethylene imine, size:100 nm), for 1 to 7 days (sodium nitrite and SNP: 0, 0.1, 1, 10, 100, and 1000 μM; silica nanoparticles: 0, 6.25, 12.5, 25, 50, and 100 μg/mL). Human corneal epithelial cells (HCECs) were cultured and exposed to sodium nitrite, SNP (0, 0.1, 1, 10, 100, and 1000 μM), and silica nanoparticles for 1, 2, and 3 days.

Results

Sodium nitrite and SNP showed a dose-dependent inhibitory effect on A. castellanii viability. A more prominent inhibitory effect was observed with SNP (less than 10% of organisms survived at 7-day culture with 1000 μM) compared with sodium nitrite. However, more cytotoxicity on HCEC was observed with SNP. NO-releasing silica nanoparticles were successfully internalized into the amoebic cytoplasm and accumulated in large vacuoles. Although blank silica nanoparticles had no inhibitory effect on A. castellanii viability, NO-releasing silica nanoparticles showed a dose-dependent amoebicidal effect. Furthermore, no cystic transformation of A. castellanii was observed under a phase contrast microscope or transmission electron microscope after exogenous NO treatment.

Conclusions

Our results demonstrated the anti-Acanthamoeba effect of exogenous NO. This finding suggests that NO-releasing drug platforms, including nano-carriers, can be a promising therapeutic strategy for Acanthamoeba keratitis.

Flow cytometric characterization of encystation in Entamoeba invadens

Entamoeba histolytica causes dysentery and liver abscess mostly in countries that lack proper sanitation. Infection is acquired by ingestion of the cyst form in contaminated food or water. E. histolytica does not encyst in vitro; thus, E. invadens, a reptilian parasite that encysts in vitro, has been used as a surrogate. Cysts are small and possess chitin-rich walls. These are characteristics that may be exploited by flow cytometry. We stained encysting E. invadens cells with a fluorescent chitin stain, and analyzed fluorescence and forward scatter by flow cytometry. We demonstrate that flow cytometry can be used to track differentiation, reveal unique cell populations, and evaluate encystation inhibitors.

Encystment in Acanthamoeba castellanii: a review

Differentiation of Acanthamoeba castellanii trophozoites involves massive turnover of cellular components and remodelling of organelle structure and function so as to produce a cryptobiotic cell, resistant to desiccation, heat, freezing, and chemical treatments. This review presents a summary of a decade of research on the most studied aspects of the biochemistry of this process, with emphasis on problems of biocide and drug resistances, putative new targets, molecular and cell biology of the process of encystment, and the characteristics of the encysted state. As well as the intrinsic pathogenicity of the organism towards the cornea, and the ability of related species to invade the human brain, its propensity for harbouring and transmitting pathogenic bacteria and viruses is considerable and leads to increasing concerns. The long-term survival and resistance of cysts to drugs and biocides adds another layer of complexity to the problem of their elimination.

War on terror cells: killing the host that harbours 'superbugs' is an infection control strategy in our fight against infectious diseases

It is proposed that targeting the environmental host that harbour 'superbugs' is an effective strategy in our fight against infectious diseases.

Effects of multipurpose solutions on the viability and encystment of acanthamoeba determined by flow cytometry

PURPOSE

To evaluate simultaneously the effects of multipurpose contact lens care solution (MPS) on the viability and encystment of Acanthamoeba using flow cytometry.

METHODS

Viability and encystment rate were evaluated using Acanthamoeba castellanii (ATCC 50514 and ATCC 50370) and three clinical strains of Acanthamoeba spp. isolated from patients with Acanthamoeba keratitis. Acanthamoeba trophozoites (1.0 × 10(5) cells/mL) were exposed to four kinds of commercially available MPSs for 24 hours. After dispensing the cell suspension into two portions, one portion was stained with 0.004% Congo Red (CR), a fluorescence dye to stain the inner cell wall of cysts, and the other portion was stained with a mixture of Congo Red and 3% sarkosyl (CRS), a detergent to lyse the trophozoites and pseudocysts. Flow cytometric analysis of the treated portions was then carried out on an EPICS ALTRA flow cytometer. The encystment rate and disinfecting efficacies (percentage of rounded trophozoites, "pseudocyst") were calculated by the rates of CR-stained, CR-nonstained, and CRS-stained populations, respectively. Ultrastructural features of resistant (mature or immature) cysts and pseudocysts were observed by transmission electron microscopy.

RESULTS

Resistant cysts and rounded trophozoites (pseudocysts) were stained with CR, whereas native (unrounded) trophozoites were not. Resistant cysts were also stained with CRS unlike pseudocysts. Three clinical isolates showed higher resistance and higher encystment rates than two ATCC strains when treated with encystment-positive control solution. Disinfecting efficacy of each MPS was not directly related to each encystment rate. Transmission electron microscopy observations showed basic differences in the ultrastructure of pseudocysts produced by MPSs and resistant cysts.

CONCLUSIONS

These results suggest that viability and encystment of Acanthamoeba are independent phenomena, and therefore disinfecting efficacy of MPS and encystment rates of Acanthamoeba should be evaluated, respectively. Thus, it is important to evaluate simultaneously the disinfecting efficacies and encystment rates of newly developed premarket MPS using the authors' novel flow cytometric methods.

Toxicity detection of sodium nitrite, borax and aluminum potassium sulfate using electrochemical method

Based on the inhibition effect on the respiratory chain activity of microorganisms by toxicants, an electrochemical method has been developed to measure the current variation of a mediator in the presence of microorganisms contacted with a toxicant. Microelectrode arrays were adopted in this study, which can accelerate the mass transfer rate of an analyte to the electrode and also increase the total current signal, resulting in an improvement in detection sensitivity. We selected Escherichia coli as the testee and the standard glucose-glutamic acid as an exogenous material. Under oxygen restriction, the experiments in the presence of toxicant were performed at optimum conditions (solution pH 7.0, 37 degrees C and reaction for 3 hr). The resulting solution was then separated from the suspended microorganisms and was measured by an electrochemical method, using ferricyanide as a mediator. The current signal obtained represents the reoxidation of ferrocyanide, which was transformed to inhibiting efficiency, IC50, as a quantitative measure of toxicity. The IC50 values measured were 410, 570 and 830 mg/L for sodium nitrite, borax and aluminum potassium sulfate, respectively. The results show that the toxicity sequence for these three food additives is consistent with the value reported by other methods. Furthermore, the order of damage degree to the microorganism was also observed to be: sodium nitrite > borax > aluminum potassium sulfate > blank, according to the atomic force microscopy images of E. coli after being incubated for 3 hr with the toxic compound in buffer solutions. The electrochemical method is expected to be a sensitive and simple alternative to toxicity screening for chemical food additives.

The effect of in vitro growth conditions on the resistance of Acanthamoeba cysts

Despite increasing concerns of direct pathogenicity and/or their role as hosts for other microorganisms there are currently no standard methods for the inactivation of amoebae that belong to the genus Acanthamoeba. Methods used to grow amoebae and produce cysts for these tests may be important as they can dramatically modify cyst susceptibility. We compared resistance of cysts produced from trophozoites grown in peptone-yeast extract-glucose broth or by feeding on HEp-2 cells and then encysted in Neff's medium. We observed that trophozoites grown using HEp-2 cells as a nutrient source produce cysts that are significantly more resistant to SDS and to most biocides tested, including heat. Increased resistance is likely due to a higher proportion of mature cysts presenting thicker cell walls as demonstrated using transmission electron microscopy. This was confirmed by calcofluor white staining demonstrating higher cellulose content in cysts produced from trophozoites grown using HEp-2 cells as a feeding source. These results demonstrate that not only methods used to produce cysts from trophozoites are critical, but that methods used to grow trophozoites before encystment should also be chosen carefully. This should be taken into account for the development of protocols to evaluate biocides and antimicrobials against amoebal cysts.

Acanthamoeba castellanii: proteins involved in actin dynamics, glycolysis, and proteolysis are regulated during encystation

Acanthamoeba castellanii is a pathogenic free-living amoeba. Cyst forms are particularly important in their pathogenicity, as they are more resistant to treatments and might protect pathogenic intracellular bacteria. However, encystation is poorly understood at the molecular level and global changes at the protein level have not been completely described. In this study, we performed two-dimensional gel electrophoresis to compare protein expression in trophozoite and cyst forms. Four proteins, specifically expressed in trophozoites, and four proteins, specifically expressed in cysts, were identified. Two proteins, enolase and fructose bisphosphate aldolase, are involved in the glycolytic pathway. Three proteins are likely actin-binding proteins, which is consistent with the dramatic morphological modifications of the cells during encystation. One protein belongs to the serine protease family and has been already linked to encystation in A. castellanii. In conclusion, this study found that the proteins whose expression was modified during encystation were likely involved in actin dynamics, glycolysis, and proteolysis.

Amoebicidal activities of alexidine against 3 pathogenic strains of acanthamoeba

OBJECTIVES

Effective pharmacotherapy for Acanthamoeba keratitis has been hampered because of the marked resistance of various stains to a variety of antimicrobial agents. In view of the fact that topical Brolene (propamidine isethionate) and neosporin are currently considered to be the first-line medical treatment of choice in Europe, we sought to determine whether Alexidine is equally effective, because the latter drug is more readily available in the United States.

METHODS

Trophozoites and cysts from 3 pathogenic corneal isolates (A. castellanii, A. polyphaga, and A. rhysodes) were incubated in peptone-yeast extract-glucose medium containing different concentrations of Alexidine for 24 hr. The number of trophozoites was counted by hemocytometer. The cysts were plated in to nonnutrient agar plates precoated with Escherichia coli and observed for viability or excystment over a period of 2 weeks. The capacity of different concentrations of Alexidine to induce cytolysis of corneal epithelial cells was tested in vitro. Chinese hamster corneas were treated with 5 microL of Alexidine topically, every hour; 6 times a day and the corneas were stained with fluorescein to asses the epithelial defects in vivo.

RESULTS

Alexidine was effective in killing the trophozoites at a concentration of 10 microg/mL. However, a higher concentration of Alexidine (100 microg/mL) is required to kill Acanthamoeba cysts and the cytotoxic activities of Alexidine are comparable with chlorhexidine. We have also demonstrated that both Alexidine and chlorhexidine at 100 microg/mL induced significant cytopathic effect on the corneal epithelial cells in vitro. In vivo results indicate that Alexidine at a concentration of 100 microg/mL is less toxic than chlorhexidine when applied topically to the Chinese hamster cornea.

CONCLUSIONS

Our study has identified Alexidine as a novel anti-Acanthamoeba drug and suggests that Alexidine may be an effective therapeutic option because of its potency and low toxicity to the corneal tissues when applied topically in vivo.

Photosensitized inactivation of Acanthamoeba palestinensis in the cystic stage

AIMS

To develop alternative approaches for medical and environmental control of pathogenic Acanthamoeba spp. by means of photodynamic treatment with a tetracationic Zn(II)-phthalocyanine (RLP068).

METHODS AND RESULTS

Incubation of cyst cultures with RLP068 for 1 h caused an accumulation of readily detectable concentrations of the phthalocyanine, even at doses as low as 0.5 micromol l(-1). RLP068 exhibited no dark toxicity towards cysts up to 5 micromol l(-1) concentration. A decrease of c. 50% in cyst survival in comparison with controls was measured upon incubation of the cysts with 0.5 micromol l(-1) RLP068, followed by exposure to light (600-700 nm) for 20 min at a fluence rate of 50 mW cm(-2) (60 J cm(-2)). After incubation with 3 and 5 micromol l(-1) RLP068 and irradiation, the cysts lost their excystment ability as early as day 5 and up to day 10, and were clearly damaged when observed under an interference contrast microscope.

CONCLUSIONS

These data indicate the promising use of RLP068 in phototreatment of diseases caused by pathogenic amoebae and in initial disinfection of wastewaters.

SIGNIFICANCE AND IMPACT OF THE STUDY

Rapid and extensive photodamage may be induced in the highly resistant cystic stages by means of 600- to 700-nm light sources.

Ultrastructural study of encystation and excystation in Acanthamoeba castellanii

Encystation and excystation of Acanthamoeba castellanii were studied by transmission electron microscopy. The differentiation process was induced in asynchronous cultures grown axenically. Cytoplasmic vesicles containing a dense fibrous material very similar in appearance to the cyst wall were observed in trophozoites induced to encyst. When these trophozoites were incubated with calcofluor white m2r, fluorescence was observed in cytoplasmic vesicles, suggesting that the material contained in these vesicles corresponded to cyst wall precursors. Semithin cryosections of mature cysts with the same treatment showed fluorescence in the ectocyst and a less intense fluorescence in the endocyst, suggesting the presence of cellulose in both structures of the cyst wall. In mature cysts induced to excystation, small structures very similar to electron-dense granules (EDG) previously described in other amoebae were frequently observed. The EDGs were either sparsely distributed in the cytoplasm or associated with the cytoplasmic face of the plasma membrane. Many of them were located near the ostiole. In advanced phases of excystation, endocytic activity was suggested by the formation of endocytic structures and the presence of vacuoles with fibrous content similar to that of the cyst wall. Electron-dense granules in the process of dissolution were also observed in these vacuoles. Furthermore, the formation of a pseudopod suggests a displacement of the amoeba toward the ostiole.

The interaction of Acanthamoeba castellanii cysts with macrophages and neutrophils

Acanthamoeba castellanii, a free-living amoeba, causes a sight-threatening form of keratitis. Even after extensive therapies, corneal damage can be severe, often requiring corneal transplantation to restore vision. However, A. castellanii cysts are not eliminated from the conjunctiva and stroma of humans and can excyst, resulting in infection of the corneal transplant. The aim of this study was to determine whether elements of the innate immune apparatus, neutrophils and macrophages, were capable of detecting and eliminating A. castellanii cysts and to examine the mechanism by which they kill the cysts. Results show that neither innate immune cell is attracted chemotactically to intact cysts, yet both were attracted to lysed cysts. Both macrophages and neutrophils were capable of killing significant numbers of cysts, yet neutrophils were 3-fold more efficient than macrophages. Activation of macrophages with lipopolysaccharide and interferon-gamma did not increase their cytolytic ability. Conditioned medium isolated from macrophages did not lyse the cysts; however, prevention of phagocytosis by cytochalasin D inhibited 100% of macrophage-mediated killing of the cysts. Conditioned medium from neutrophils did kill significant numbers of the cysts, and this killing was blocked by quercetin, a potent inhibitor of myeloperoxidase (MPO). These results indicate that neither macrophages nor neutrophils are chemoattracted to intact cysts, yet both are capable of killing the cysts. Macrophages killed the cysts by phagocytosis, whereas neutrophils killed cysts through the secretion of MPO.

Recent advances in the treatment of Acanthamoeba keratitis

Infection of the eye caused by Acanthamoeba species constitutes a burgeoning and unsolved problem. Of individuals with Acanthamoeba keratitis, 85% wear contact lenses; abrasion of the cornea is implicated. Corneal infection often can be prevented by good lens care and hygiene. Severe Acanthamoeba keratitis often can be very difficult to treat; surgery can be less than successful and may lead to further problems. The encysted stage in the life cycle of Acanthamoeba species appears to cause the most problems; many biocides are ineffective in killing the highly resistant cysts. Combination therapy--that is, use of 2 or 3 biocides, sometimes with antibacterial antibiotics--appears to work best. Recurrence is common if treatment is stopped prematurely. Immunologic methods are being investigated as a form of prevention, and oral immunization of animals recently has been successful in the prevention of Acanthamoeba keratitis by inducing immunity before infection occurs. Immunization thus may eventually become the best approach for reduction of the incidence of amebic infection in humans.