Trichomonas vaginalis pseudocysts are a quiescent cell stage characterized by a differentially regulated transcriptome and intact membranes.

Trichomonas vaginalis is the causative agent of trichomoniasis, the most common non-viral sexually transmitted infection (STI). Despite its prevalence, low levels of public knowledge and research funding and the absence of T. vaginalis screening or control programs have led to its categorization as a "neglected" STI. Unlike other STIs in the USA, prevalence increases with age, peaking among individuals in their 40s. Both a motile trophozoite stage and a non-motile pseudocyst state have been described for the parasite, although it is debated whether the latter is a quiescent stage or a degenerate form on its way to cell death. Here we characterize the T. vaginalis pseudocyst by flow cytometry, membrane integrity assays, transcriptomics, and reversion studies. Pseudocysts were induced by culturing trophozoites in acidic media or in iron depleted media, with a variety of resulting survival rates. Flow cytometry studies showed that pseudocysts have intact cell membranes and express phosphatidylserine on their cell surface. Fluorescence-activated cell sorting studies also identified distinct sub-populations of parasites, revealing the importance of using pure live pseudocyst cultures in reversion studies. Pseudocysts were transcriptionally active for several days and had consistent subsets of genes with increased expression compared to trophozoites, although decreased transcription of genes involved in metabolism. Comparative transcriptomics of pseudocysts and trophozoites of two T. vaginalis strains revealed distinct cell states. Combined, our results provide evidence that the pseudocyst cell state is a stress-induced quiescent stage of T. vaginalis that can remain viable for days, with implications for a role in persistent infections.