DiscussionThe anaerobic protozoan parasite Trichomonas vaginalis is the most common non-viral sexually transmitted disease, and the closely related species Trichomonas gallinae is an avian parasite of ecological and economic importance. T. vaginalis shares a strong clinical association with the independent sexually transmitted pathogen Mycoplasma hominis, and the uncultured bacterium Mycoplasma girerdii, with the latter association being an order of magnitude stronger. M. hominis has been shown to profoundly influence T. vaginalis growth, energy production and virulence-associated mechanisms. We aimed to investigate the molecular basis of symbiosis between T. vaginalis and Mycoplasma spp. through transcriptomic profiling. The results showed major shifts in parasite gene expression in response to the bacterium, including genes associated with pathogenesis and energy generation, providing a solid framework for further experimental characterisation. In addition, we will present evidence for a new undescribed Mycoplasma sp., related to M. girerdii, and associated with T. gallinae in the upper digestive tract of domestic pigeons. This new finding suggests a long-term association between Trichomonas and Mycoplasma spp. which has been conserved across evolutionary time and the host species barrier.