Abstract
Trichomonas vaginalis is recognized as the most prevalent non-viral sexually transmitted parasitic infection, establishing its presence in the vaginal microbiota alongside commensal bacteria, particularly Lactobacillus and Bifidobacterium species. Notably, a reduction in the population of Bifidobacterium breve has been documented in women infected with T. vaginalis. However, the dynamics of the interaction between T. vaginalis and these beneficial bacterial species remain inadequately understood, warranting further investigation to elucidate the underlying mechanisms and potential implications for vaginal health. We investigated the effects of B. breve on the growth of T. vaginalis, including its gene expression and interactions with the host. HeLa cells were exposed to T. vaginalis that had been pretreated with or without B. breve. We assessed cytokine responses (specifically IL-6 and IL-8) and evaluated cytopathic effects. The dynamics of the co-culture were monitored microscopically. Additionally, we conducted transcriptomic and metabolomic analyses to characterize the responses of T. vaginalis to B. breve. Co-culture of T. vaginalis with B. breve for 4 h significantly increased protozoan proliferation by 1.2-fold, while bacterial abundance was reduced by 27%. HeLa cells showed no cytopathic effects or changes after treatment with B. breve, whether given before or alongside T. vaginalis. Microscopy showed a direct physical association between the two organisms. Transcriptomic profiling of T. vaginalis co-cultured with B. breve revealed an upregulation of genes related to fatty acid metabolism and DNA replication. Metabolomic analysis confirmed changes in the content of long-chain fatty acids. Additionally, there was an increase in virulence-associated genes, including adhesins. B. breve promotes the growth of T. vaginalis and modulates parasite gene expression and lipid metabolism transiently. However, it does not confer protection on host epithelial cells during infection. These findings suggest that B. breve may contribute to maintaining microbial community balance but does not directly counteract T. vaginalis. Importantly, these insights highlight the potential value of B. breve–associated microbial or metabolic signatures as indicators of ecological shifts that predispose to parasite overgrowth. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1038/s41598-026-38866-0.