Using quantitative reverse transcriptase PCR and cell culture plaque assays to determine resistance of Toxoplasma gondii oocysts to chemical sanitizers

Toxoplasma gondii oocysts are highly resistant to many chemical sanitizers. Methods used to determine oocyst infectivity have relied primarily on mouse, chicken, and feline bioassays. Although considered gold standards, they only provide a qualitative assessment of oocyst viability. In this study, two alternative approaches were developed to quantitate viable T. gondii oocysts following treatment with several common sanitizers. The first is a quantitative reverse transcriptase real-time PCR (RT-qPCR) assay targeting the ACT1 and SporoSAG genes to enumerate viable T. gondii oocysts. RT-qPCR CT values between Wescodyne®, acidified ethanol, or heat treated oocysts were not significantly different as compared with untreated controls. By contrast, treatment with formalin or Clorox® resulted in a 2-log10 reduction in CT values. An in vitro T. gondii oocyst plaque assay (TOP-assay) was also developed to measure oocyst viability. This assay used a combination of bead milling and bile digestion, followed by culturing the excysted sporozoites in a confluent fibroblast cell monolayer. Results showed that no significant reduction in sporozoite viability was detected in acidified ethanol or Wescodyne® treated oocysts while at least a 2-log10 reduction in plaques formed was observed with Clorox® treated oocysts. Moreover, formalin or heat treatment of oocysts resulted in at least a 5-log10 reduction in plaques formed. This study demonstrates that an mRNA-based PCR viability assay targeting the ACT1 or SporoSAG genes is a relatively rapid technique compared to in vitro and in vivo assays. In addition, the TOP-assay proved very effective and sensitive at quantifying oocyst viability when compared with animal bioassays.