Chemotaxis and selective interactions of Trichomonas vaginalis with the vaginal bacteria

Abstract Trichomonas vaginalis is an extracellular parasite that inhabits the human genital tract, yet little is known about how it senses and responds to the complex vaginal microbial ecosystem. Here, we show that T. vaginalis exhibits chemotactic behavior on semisolid surfaces, forming multicellular assemblies that coordinate collective migration. Parasite colonies display both positive and negative chemotactic responses, indicating the ability to detect and react to diffusible signals. Different parasite strains display marked mutual avoidance between neighboring colonies, highlighting specific recognition mechanisms. Furthermore, we show that T. vaginalis is strongly attracted to acidic environments, revealing a niche-adapted pH taxis. Given that vaginal bacteria critically shape local pH, we examined parasite responses to representative members of the vaginal microbiota. T. vaginalis exhibited preferential chemotactic migration toward Lactobacillus gasseri , a hallmark species of eubiotic community state types (CSTs), over Gardnerella vaginalis , which is associated with dysbiotic CST-IV communities, while showing no detectable attraction to Escherichia coli . This selective migration correlated with a robust chemotactic response to lactic acid, a major metabolite produced by lactobacilli. Additionally, when the parasite is co-cultured with the equal number of L. gasseri and G. vaginalis , T. vaginalis exhibits a clear preferential binding to L. gasseri , as demonstrated by flow cytometry and fluorescent microscopy. We show that co-culture of T. vaginalis with either L. gasseri or G. vaginalis results in enhanced parasite growth only in the presence of L. gasseri . Collectively, these findings reveal pH taxis; bacteria-directed migration and preferential association with Lactobacillus as previously underappreciated behavioral traits of T. vaginalis . Such behaviors may destabilize protective microbial communities and drive the transition toward a CST-IV–type dysbiotic state which is frequently associated with trichomoniasis.