Detection and subtype distribution of Blastocystis sp. in aquatic biofilm from the soto stream, Buenos Aires, Argentina.

Blastocystis is one of the most prevalent gastrointestinal parasites in humans. This protist has been detected in water from various sources, with the highest prevalence in wastewater. In a previous study, we demonstrated that Blastocystis can adhere to and maintain viability within aquatic biofilms in wastewater. Therefore, the objective of this study was to evaluate wastewater contamination by viable Blastocystis at four sites along an urban stream (Buenos Aires, Argentina) using aquatic biofilms. Two sampling strategies were employed: retrieval from fallen and submerged plant leaves (natural surfaces) and from submerged glass slides (artificial surfaces) at three time points. Biofilms were cultured in Jones' medium to assess parasite viability, and Blastocystis subtypes were identified by PCR and sequencing of the SSU rRNA gene. Contamination levels at each site were evaluated 15 days after the glass slides were immersed, and the results were expressed as cysts per milligram of biofilm. Biofilms containing viable parasites were more frequently detected at sites with higher population density. The loads ranged from 1 to 61 cysts per milligram of biofilm, with remarkably higher values at the sites with most significant human impact. Molecular analyses revealed the presence of subtypes ST1 (allele 4), ST4 (allele 42), and ST8 (allele 21 and allele 95), with ST1 and ST8 being the most prevalent across sampling periods. The two sampling strategies proved useful, although their applicability depends on the specific objective of the study. This study provides the first evidence supporting the use of aquatic biofilms as an effective tool for monitoring viable Blastocystis in wastewater and for assessing spatial and temporal contamination patterns.