The ubiquitous distribution of small free-living amoebae in nature, and the ability of certain species to survive and grow especially in freshwater, suggesting a role in human and animal disease (Ma et al., 1990). The small free-living amoebae, Acanthamoeba spp., Naegleria spp. and Balamuthia mandrillaris, are distributed worldwide, and thus provide a potential source of meningoencephalitis or keratoconjunctivitis infection for humans and other mammals.

Based on differences in size as well as on the morphologic characters of the cysts, in 1977 Pussard and Pons recognized three groups within the genus Acanthamoeba. However, species differentiation based on morphology alone may not always be correct. Thus, many attempts have been made to include other features, such as isoenzyme profiles and restriction fragment length polymorphisms (RFLP) of mitochondrial DNA (mtDNA), in the classification of Acanthamoeba (Bogler et al., 1983; De Jonckheere, 1983). The classification of Acanthamoeba spp. at the subgenus level has been attempted by riboprinting, e.g., by RFLP analysis of the 18S small subunit ribosomal RNA (srRNA) gene by polymerase chain reaction (PCR) (Chung et al, 1998). Kong and Chung (2002) described a riboprinting scheme for the identification of unknown Acanthamoeba Korean isolates at the species level by using the PCR-RFLP of 18S srRNA using four kinds of restriction enzymes. In addition, the interstrain polymorphisms of the isoenzyme profiles and the RFLP patterns of mtDNA were observed in seven strains of Acanthamoeba polyphaga and four strains of Acanthamoeba castellanii (Kong et al., 1995).

This paper described the identification of the Acanthamoeba Korean isolate YM-4 at the species level based on morphologic characteristics, biochemical and molecular genetic criteria, by reviewing previous reports and adding some new figures and experimental data for 18S srDNA RFLP. Moreover, we propose a new species, Acanthamoeba sohi (Korean isolate YM-4), isolated from the gills of a freshwater fish collected in a fish market in Seoul, Korea, as presented in the 36th annual meeting of The Korean Society for Parasitology in 1994.

Acanthamoeba spp. is an amphizoic amoebae, which belongs to the genus Acanthamoeba Volkonsky 1931, emend Page (1967), and was incorporated into the family Acanthamoebidae by Sawyer and Griffin (1975). Pathogenic free-living amoebae belong to the genera Acanthamoeba and Naegleria. Group taxonomy, including pathogenic free-living amoebae, was clarified and updated at the generic level by P.C. Page (1967), who published a taxonomic key (Page, 1976). The genus Naegleria Alexeieff 1912, emend Calkins 1913, belongs to the family Vahlkampfiidae. The genus Acanthamoeba Volkonsky 1931, emend Page (1967), was included in the family Acanthamoebidae Sawyer and Griffin (1975). The genus Acanthamoeba was established in 1931, and until recently considerable confusion existed in terms of its taxonomic status. In 1966, Pussard stated that spindle shape is not a satisfactory feature for intergeneric differentiation but considered the characteristic morphology of the cyst to be a decisive character at the generic level and recognized the genus Acanthamoeba. In 1967, Page considered the presence of acanthopodia and the structure of the cyst to be sufficiently distinctive and concluded that the generic designations of Acanthamoeba and Hartmannella were justified. In 1975, Visvesvara and Balamuth identified clearly demonstrable differences not only in the trophozoite and cyst stages of Acanthamoeba and Hartmannella, but also in their nutritional requirements and serologic responses. Sawyer and Griffin created the family Acanthamoebidae in 1975, and Page created the suborder Acanthopodina under the order Amoebida. However, Page separated Acanthamoeba and Hartmannella at the ordinal level by creating a new order, Acanthopodida, which includes the family Acanthamoebidae and placed Hartmannella in the family Hartmannellidae, order Euamoebida.

As a result of experiments and reviews based on morphological characteristics, pathogenicity to mice, in vitro cytotoxicity, isoenzyme patterns and RFLP analysis of mtDNA and 18S srRNA, Acanthamoeba YM-4 was found to differ significantly from A. culbertsoni, A. polyphaga, and A. royreba and other reference Acanthamoeba spp. Thus, the Acanthamoeba Korean isolate YM-4 should be identified as a new species, assigned as Acanthamoeba sohi. The Korean isolate YM-4 was proposed as new species, Acanthamoeba sohi, at the 36th annual meeting of The Korean society for Parasitology in 1994. This new species is named in honor of Dr. Chin-Thack Soh, a Emeritus Professor of Yonsei University College of Medicine, Seoul, Korea.