Quantification of viable Giardia cysts and Cryptosporidium oocysts in wastewater using propidium monoazide quantitative real-time PCR.

Abstract

Real-time PCR (qPCR) is a rapid tool to quantify pathogens in the aquatic environment; however, it quantifies all pathogens, including both viable and nonviable. Propidium monoazide (PMA) is a membrane-impairment dye that penetrates only membrane-damaged cells. Once inside the cell, PMA is covalently cross-linked to DNA through light photoactivation, and PCR amplification is strongly inhibited. The goal of this study was to evaluate PMA-qPCR assays for rapid quantification of viable and heat-treated Giardia cysts and Cryptosporidium oocysts in wastewater. We observed a reduction in detection of heat-treated Giardia duodenalis cysts of 83.2, 89.9, 98.2, or 97% with PMA-qPCR assays amplifying a 75 base-pair (bp) β-giardin target, 77-bp triosephosphate isomerase (tpi), 133-bp glutamate dehydrogenase (GDH), and 143-bp β-giardin gene target, respectively. Thus, the exclusion of dead cysts was more effective when qPCR assays that produced larger amplicons were used. The PMA treatment of Cryptosporidium oocysts plus/minus heat treatment abolished the fluorescent signal for dead oocysts with a PMA-qPCR assay amplifying a Cryptosporidium parvum (150-bp) oocyst wall protein (COWP) gene. The PMA-qPCR 143-bp β-giardin assay for Giardia and the PMA-qPCR 150-bp COWP assay for Cryptosporidium accurately quantified live oo(cysts), and failed to detect dead oo(cysts), when phosphate-buffered saline and tertiary effluent wastewater were spiked with concentrations of 10(3) or 10(2) dead oo(cysts), respectively. Therefore, these assays are suitable for the detection of viable parasites that are typically present in tertiary wastewater effluents at concentrations of <10(3) oo(cysts)/l and can provide rapid risk assessments of environmental water.