Subtype distribution of Blastocystis isolates identified in a Sydney population and pathogenic potential of Blastocystis.

Blastocystis is one of the most common enteric parasites present in humans. There is still much uncertainty about the pathogenic potential of this parasite, and it was suggested that its pathogenicity could be subtype-related. This report aimed to study 98 Blastocystis isolates found in human stool specimens to identify the subtypes present and carry out phylogenetic analysis on these isolates. This study also aimed to show the relationship between subtype and symptoms. Five-hundred and thirteen stool samples were submitted to five different diagnostic techniques for the detection of Blastocystis. Polymerase chain reaction (PCR)-positive samples were then sequenced and the small subunit (SSU) rDNA sequences were aligned and submitted to phylogenetic analysis. Ninety-eight samples were positive by any of the diagnostic methods for Blastocystis and 96 were positive by PCR. There were seven different subtypes (1, 2, 3, 4, 6, 7 and 8) identified by PCR and sequencing. This is the first large-scale study to examine the occurrence of Blastocystis in Australia. This study reports the high incidence of subtype 3 (44 %) in this population and discusses the emerging idea of subtype-dependent pathogenicity.