**3. Pathogenesis in AK**

The classification of *Acanthamoeba* species /isolates changed with the advances in molecular methods. Recently, it is based on genotype associations - the 18S rRNA gene sequence [5,20,31-33]. The modern approach differs from morphology solely: the species identification is based on combination of the morphological and molecular characterization [3,5,15,20]. At present, 18 or19 genotypes are distinguished for diagnostics and for the characterization of

**Figure 1.** *Acanthamoeba* sp. - wet-mount slides from corneal scrapings; note characteristic spine-like acanthopodia in

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

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].

clinical and environmental *Acanthamoeba* isolates [12,15,18,31-33].

trophozoites and the double-walled cysts; light micrographs; scale bar = 10µm

be asymptomatic and/or self-limited.

102 Advances in Common Eye Infections

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

In the early phase of AK, there are nonspecific symptoms variable in their intensity starting with redness, photophobia, and excessive tearing. Most commonly, only one eye is involved. Active epithelial inflammations usually progress from the outermost layer of the cornea deeper, to the stroma. Symptoms of the devastating eye disease include loss of the visual acuity. In many cases of AK, excruciating eye pain occurs, in which the intensity is often incommen‐ surate with relatively small degree of corneal deterioration. There are clinical and histopatho‐ logical evidences that the severe pain is associated with *Acanthamoeba* trophozoite activeness that result in the inflammation of the corneal nerves -radial keratoneuritis [4,27]. It is also emphasized that a characteristic ring-like corneal infiltration may occur in some patients. This amoebic eye infection may develop from a few days to several months, as a severe, prolonged disease, which, without suitable treatment may lead to blindness.

A pathogenesis of *Acanthamoeba* keratitis is a multi-factorial process connected with some factors contributing directly and indirectly in production of diseases in humans [12].

Among the direct contributing factors, the following are listed: adhesion of *Acanthamoeba* trophozoites to host cells, phagocytosis to take up food particles, neuraminidase activity, and secretion of toxins. The neuraminidase enzyme, which is active at 25-30 ºC, is believed to be important in a damaging of corneal epithelial cells [12,41]. Recently, the first toxin, acantha‐ porin, with pore-forming activity, cytotoxic for human neuronal cells was isolated from *Acanthamoeba*, which activation mechanism remains under investigations [12,42].

Among the indirectly contributing factors, there are amoebic and host determinants. Morphophysiological features: the amoeboid motion and spine-like acanthopodia allow protozoans to modulate binding to biological and inert surfaces. Also, changes in the overall numbers of the amoebae as well as in the proportion of trophozoites and cysts dependent on the environ‐ mental conditions are also listed as the amoebic pathogenesis determinants. The temperature tolerance, osmotolerance and growth at different pH allow the amphizoic *Acanthamoeba* to exist in different environments and simultaneously easy adapt to different human organ and tissues, including the human cornea. Moreover, it has been shown that both, clinical and environmental *Acanthamoeba* strains/isolates vary in their among the oral cavity microbiota pathogenicity. The thermal tolerance and ability to grow at high temperature are considered to be indirect markers of the pathogenicity of *Acanthamoeba* strains [12,17,18,43-45].
