**2.** *Acanthamoeba* **spp. as potential agents of human diseases**

The protozoans belonging to *Acanthamoeba* spp. complete their life cycles in different outer environments as free-living organisms. Trophozoite forms are able to feed on various micro‐ organisms dwelling in soil and water (on bacteria, algae, and yeasts as well as on other protozoans) and on small organic particles. In natural, and even in man-made environments, the amoebae develop without entering human or animal organisms. However, in predisposing circumstances, the amoebae may enter the human body. According to literature [34,35], there are evidences from various regions that humans are exposed frequently to the amoebae: it has been confirmed by 50–100% of the healthy populations having specific anti- *Acanthamoeba* antibodies. The protists may permeate into the human body without pathogenic consequences. Investigations in which serological, biochemical and molecular methods were applied showed that people may be exposed both, to pathogenic as well as non-pathogenic *Acanthamoeba* strains [reviewed in 12,13,18]. In immunocompetent individuals, infections with these amoebae may be asymptomatic and/or self-limited.

Nevertheless, several amoebic strains belonging to this genus are able to enter, and colonize human organs and multiply within them, indicating pathogenic effects [4,12,26,27,36].

For this reason, these amoebae are called amphizoic amoebae, because they are able to exist in two different modes: as free-living-exozoic organisms and as endozoic parasitic organisms, within host tissues; thus, the free-living protozoans are also believed as facultative parasites.

Trophozoites and cysts of *Acanthamoeba* were detected during infections of various human cavities and tissues: in paranasal sinuses in rhino-sinusitis, in skin inflammation and skin ulceration, and in pneumonia [7,37,38]. We also found trophozoites and numerous cysts of *Acanthamoeba* among the oral cavity microbiota in 4% somatically and mentally disordered patients. The amoebae accompanied infections with other amoebae, *Entamoeba gingivalis*, the oral amoebae associated with a prolonged deterioration of the periodontium and gingiva [39].

Developmental stages of the amphizoic species may be causative agents of an systemic opportunistic disease developing in immunocompromised individuals. This is rare but almost always fatal granulomatous *Acanthamoeba* encephalitis (GAE) [6,12,18,40]. Other infection caused by the amphizoic amoebae may result in a sight-threatening *Acanthamoeba* keratitis (AK), a non-opportunistic disease occurring mainly in immunocompetent persons, mostly in contact lens wearers [1-3,7,14].
