Rhodotorula mucilaginosa: a new potential human pathogen found in the ciliate Paramecium bursaria

Ciliates often form symbiotic associations with other microorganisms, both prokaryotic and eukaryotic. We are now starting to rediscover the symbiotic systems recorded before molecular analysis became available. Here, we provide a morphological and molecular characterization of a symbiotic association between the ciliate Paramecium tritobursaria and the yeast Rhodotorula mucilaginosa (syn. Rhodotorula rubra) isolated from a natural population. This symbiotic system demonstrates certain similarities with the symbiotic system formed by P. bursaria and its conventional endosymbionts, the zoochlorellae. Experimental infections of the endosymbiont-free P. tritobursaria and Paramecium deuterobursaria cell lines with R. mucilaginosa demonstrated that the yeast infectivity is concentration-dependent, with ciliates digesting part of the yeast cells. The endosymbiotic yeast may serve as a food reserve, providing starvation stress tolerance to the host. Since R. mucilaginosa is currently regarded as a pathogen causing opportunistic infections in immunocompromised humans, our finding gives further support to the vision that ciliates can harbor potential human pathogens and can be a vector for their dissemination.