Abstract.A new species of amphizoic amoeba, Nuclearia pattersoni sp. n., isolated from gills of Rutilus rutilus L. is described. It is characterised by elongate flattened trophozoites of irregular shape. The longer dimension of their bodies is 13.2 (11.0-15.7) µm. Filopodia radiating mostly from the poles are 2 to 2.5 times longer than the body. The diameter of less frequently observed spherical trophozoites is 8.2-10.8 µm; their filopodia radiate to all directions. Cyst-like stages have shorter pseudopodia that arise from one pole only. The surface of locomotive forms from agar plate cultures has a thin amorphous glycocalyx, while most cells are covered by two layers of extracellular matrix. Mitochondria have flattened cristae, dictyosomes are located in the perinuclear zone. A conspicuous ultrastructural feature of the morphologically similar N. simplex, perinuclear striated band, is not present. Light microscopic and ultrastructural data are completed with the sequence of SSU rRNA gene and phylogenetic analysis including sequences of related taxa. The bacterial endosymbiont found in N. pattersoni type strain RR2G2 is assigned to the genus Rickettsia.

In the voluminous literature on pathogenic free-living amoebae, special attention has been paid to human pathogens, among others to strains of the genus Acanthamoeba Volkonsky, 1931. As a result of previous extensive studies on the occurrence of amphizoic amoebae in the organs of freshwater fishes (Dyková I., Lom J.Acanthamoeba strains were isolated from organs of asymptomatic individuals (Dyková I., Lom J., Schroeder-Diedrich J.M., Booton G.C., Byers T.J. 1999: J. Parasitol. 85: 1106-1113).The relationship revealed by molecular techniques between fish strains of the genus Acanthamoeba and strains that are commonly isolated from human infections, especially Acanthamoeba keratitis, strengthened the effort to clarify two important questions. First, what kind of reaction do the Acanthamoeba strains elicit in fish hosts and second, how fish in normal or debilitated condition cope with the permanent presence of amoebae in the environment.Six of ten Acanthamoeba clones (3668/IV, 4422/II, 4436/III, 4337/II, 4800/V, 4465/I) characterised by Dyková et al. (1999, op. cit.) were selected for assessment of their pathogenicity. Since in preliminary experiments these clones induced only minor histopathological changes, immunosuppressive agents were applied in order to increase the susceptibility of fish to infection. Amoebae were cultured and harvested as previously described by Dyková et al. (1999, op. cit.). Immediately before application, the culture medium was washed off amoebae with phosphate buffered saline and cells were concentrated by centrifugation (1000 rpm/10 min).Laboratory-bred common carp Cyprinus carpio L., tilapia Oreochromis niloticus niloticus (L.) and goldfish Carassius auratus auratus (L.) (5-8 cm in size) were used in the study. In addition, hybrids of isogenic homozygous line (HIHL) of common carp obtained from the Veterinary School, Hannover were used. Prior to the experiments, samples of brain, kidney, spleen and liver tissue of thirty control fish (ten of each species) were tested for the occurrence of amoebae by isolation attempts and histological examination. In total, 253 experimental fish were screened. To test the pathogenicity of Acanthamoeba strains on homoiothermic organisms we used three-month-old laboratory-bred NMRI mice. Assessment of pathogenicity was based on the presence of amoeba trophozoites and cysts in organs and on histopathological changes induced in experimental animals.

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