Molecular Approaches on Systematic Position, Genetic Polymorphism, and Classification of Blastocystis Isolates from Humans and Animals

Blastocystis was first reported as a harmless yeast found in human stool samples in 1912. This organism has been classified into the subphylum Sporozoa and then reclassified into the subphylum Sarcodina based on the morphological and physiological characteristics. Molecular sequencing study on the SSU rRNA gene of a human Blastocystis isolate showed that Blastocystis is not monophyletic with yeasts (Saccharomyces), fungi (Neurospora), sarcodines (Naegleria, Acanthamoeba, and Dictyostelium), or sporozoans (Sarcocystis and Toxoplasma) based on the partial sequence of the gene. Phylogenetic studies inferred with the entire sequence of the SSU rRNA gene of a human and a guinea pig isolates of Blastocystis revealed that the organism is a member of the stramenopiles. Furthermore, accumulation of the molecular data of human and animal Blastocystis isolates showed not only extensive genetic diversity among Blastocystis isolates from the same hosts but also genetic similarities were observed among the isolates from different hosts. Therefore, a consensus terminology of Blastocystis sp. is recently proposed for the isolates from mammalian and avian hosts including humans and it classifies all isolates into nine subtypes. Recently, however, several additional novel subtypes have been reported based on the phylogenetic studies of the partial sequences of the SSU rRNA gene. Therefore, the genus Blastocystis may be the most genetically polymorphic organism that parasitizes a wide range of animals.