In the recent years, AK incidents have been increasingly recognized worldwide. Available anti-amoebic therapies are not fully effective and results in high cytotoxicity to the human eye. The main key predisposing factor for AK is contact lens use. Effective contact lens disinfection is the best approach to minimize the number of AK incidences. In this study, we tested multipurpose contact lens disinfecting systems containing different active ingredients but characterized by similar mode of action, resulting in cell membrane perturbation (Table 1) The obtained results confirmed lack of amoebicidal activity of all tested multipurpose contact lens solutions against Acanthamoeba strain. Our results are in accordance with other publications and showed that disinfecting capabilities of market available contact lens solutions are insufficient [11],[12],[28]-[31].
Fast development of nanotechnology showed significant anti-microbial potential of the nanoparticles, especially silver nanoparticles (AgNPs) [14],[32],[33]. Specific mechanism of action of AgNPs is still not entirely understood, however recent studies conducted on bacteria, shed more light on this process. We know that nanoparticles cause damages leading to disruption of membrane permeability and alteration of the respiratory functions of the cell. This process eventually leads to disruption of the cell integrity. After crossing the cell membrane, nanoparticles can penetrate inside the cell and interact with DNA, RNA and proteins altering both transcription and translation processes. Presence of nanoparticles in the cell matrix raises the oxidative stress. The intracellular damages and disruption of enzymatic pathways are done by the free radicals. Altogether, nanoparticles cause cytotoxic effects and finally lead to the cell death. Cytotoxicity of AgNPs depends on their physico-chemical properties such as size and density. Typically, smaller nanoparticles have relatively increased stability and enhanced anti-microbial activity. Similarly, higher concentrations of nanoparticles show increased anti-microbial activity. However, this property is strictly correlated to the tested microbial species and type of nanoparticles used. Shape of the nanoparticles has not been proved to be crucial factor influencing the anti-microbial activity. Some authors showed that truncated triangular or similar geometries such as hexagonal and octahedral shape of the AgNPs are more effective against bacteria while other authors reported that shape of AgNPs does not have any influence on their activity [34]-[36]. Recent publications showed, that nanoparticles can prolong the ocular retention of some topical drugs, thus enabling treatment of eye diseases using reduced drug dosages [37],[38]. It was confirmed, that nanoparticles coated on the contact lenses caused significant reduction in microbial colonization on the contact lens surface [39]. Contact lenses impregnated with AgNPs, after 6 h of incubation, did not exhibit desirable anti-bacterial activity against Staphylococcus aureus while demonstrated excellent anti-bacterial effects against Pseudomonas aeruginosa [40]. Silver-impregnated lens cases showed lower proportion of microbial contamination compared to the control cases. Most microorganisms isolated from silver-impregnated cases were members of the normal skin flora [41].
There are just few studies that examined nanoparticles influence against Acanthamoeba spp. Cobalt nanoparticles have been studied for their anti-amoebic potential and confirmed that hexagonal microflakes showed the most promising anti-Acanthamoeba effects compared to nanoflakes and granular cobalt nanoparticles. Apart from the concentration and size, also composition and morphology of the tested noncompounds determined their anti-amoebic activity [42],[43]. AgNPs are well absorbed by the Acanthamoeba trophozoites and integrate in the cell matrix. The nanoparticles decrease trophozoites viability and alter their metabolic activity on the dose dependent manner [44]. In our previous studies we confirmed, that AgNPs conjugated with contact lens solutions showed dose dependent enhanced anti-amoebic activity [45]. Recently published studies confirmed enhanced anti-microbial effect of silver and gold (AuNPs) nanoparticles conjugated with commonly used drugs like chlorhexidine, fluconazole or amphotericin B as well as with some disinfectants [25],[46]. Guanabenz, a drug already approved for hypertension that crosses the blood − brain barrier, conjugated with AuNPs and AgNPs showed significant anti-amoebic activity against both A.castellanii and Neagleria fowleri. Significant reduction in the host cell cytopathogenicity, especially for silver nanoconjugates, was revealed and associated with negligible cytotoxicity against human cells [47].
In the 21st century, eco-friendly and cost-effective bio-nanotechnology techniques are used to prepare anti-microbial active conjugates as potential candidates to eradicate infections and reduce microbial contaminations of a healthcare equipment including contact lenses. The integration and conjugation of bioactive agents into nanomaterials was tested mainly for their anti-bacterial activities. Green synthesis of AgNPs, AuNPs and platinum (PtNPs) nanoparticles showed enhanced anti-bacterial activity after combining with different classes of antibiotics [48]. Biosynthesis of AgNPs with the plant extract of Salvia spinosa resulted in increased bactericidal activity against Gram-positive and Gram-negative bacteria [49]. Novel conjugates using biogenic AgNPs from Convolvulus arvensi extract and chitosan showed anti-microbial, anti-biofilm, and anti-cancer potentialities [50]. Extract of Oscillatoria limnetica conjugated with silver nanoparticles exhibited strong anti-bacterial activity against multidrug-resistant bacteria as well as cytotoxic effects against both human breast cell line and human colon cancer cell line [51]. Synthesis of silver chloride nanoparticles (AgCl-NPs), using walnut green husk extract as well as silver nanoparticles with Peganum harmala L leaf extract resulted in significant inhibitory effects against Escherichia coli and S.aureus clinical isolates [52],[53]. Bio-nanotechnology has been not studied on protozoan species extensively. There are just a few published studies focusing on the influence of nanoparticles conjugated with plants extracts on amoebae. Studies performed on the Jatropha curcas, Jatropha gossypifolia and Euphorbia milii extracts combined with nanoparticles exhibited that such combination caused significant reduction of the Acanthamoeba trophozoites with low cytotoxic effect to human cells [23]. In our previous studies we confirmed that tannic acid-modified silver nanoparticles showed increased anti-amoebic activity and less cytotoxicity to human cells in comparison to the pure silver nanoparticles [22]. In this study we revealed that tannic acid-modified silver nanoparticles conjugated with contact lens solutions exhibited even better anti-amoebic activity in relation to the cytotoxicity than in results obtained in our previous studies where we tested pure silver nanoparticles conjugates [45]. We conclude that differences in the anti-amoebic activity of the tested conjugates may be mainly driven by the anti-amoebic activity of the pure contact lens solutions. Nanoparticles in the tested concentration seem to enhance the already existing anti-amoebic potential of the selected contact lens solution.