Francisella novicida can utilize Paramecium bursaria as its potential host

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.

Effects of the Symbiotic Chlorella variabilis on the Host Ciliate Paramecium bursaria Phenotypes

Paramecium bursaria, a ciliated protist, forms a symbiotic relationship with the green alga Chlorella variabilis. This endosymbiotic association is a model system for studying the establishment of secondary symbiosis and interactions between the symbiont and its host organisms. Symbiotic algae reside in specialized compartments called perialgal vacuoles (PVs) within the host cytoplasm, which protect them from digestion by host lysosomal fusion. The relationship between P. bursaria and symbiotic Chlorella spp. is characterized by mutualism, in which both organisms benefit from this association. Furthermore, symbiotic algae also influence their host phenotypes, and algae-free P. bursaria can be obtained through various methods and reassociated with symbiotic algae, making it a valuable tool for studying secondary endosymbiosis. Recent advancements in genomic and transcriptomic studies on both hosts and symbionts have further enhanced the utility of this model system. This review summarizes the infection process of the symbiotic alga C. variabilis and its effects on the algal infection on number of host trichocysts, mitochondria, cytoplasmic crystals, total protein amount, stress responses, photoaccumulation, and circadian rhythms of the host P. bursaria.

Exploring the landscape of symbiotic diversity and distribution in unicellular ciliated protists

BACKGROUND

The eukaryotic-bacterial symbiotic system plays an important role in various physiological, developmental, and evolutionary processes. However, our current understanding is largely limited to multicellular eukaryotes without adequate consideration of diverse unicellular protists, including ciliates.

RESULTS

To investigate the bacterial profiles associated with unicellular organisms, we collected 246 ciliate samples spanning the entire Ciliophora phylum and conducted single-cell based metagenome sequencing. This effort has yielded the most extensive collection of bacteria linked to unicellular protists to date. From this dataset, we identified 883 bacterial species capable of cohabiting with ciliates, unveiling the genomes of 116 novel bacterial cohabitants along with 7 novel archaeal cohabitants. Highlighting the intimate relationship between ciliates and their cohabitants, our study unveiled that over 90% of ciliates coexist with bacteria, with individual hosts fostering symbiotic relationships with multiple bacteria concurrently, resulting in the observation of seven distinct symbiotic patterns among bacteria. Our exploration of symbiotic mechanisms revealed the impact of host digestion on the intracellular diversity of cohabitants. Additionally, we identified the presence of eukaryotic-like proteins in bacteria as a potential contributing factor to their resistance against host digestion, thereby expanding their potential host range.

CONCLUSIONS

As the first large-scale analysis of prokaryotic associations with ciliate protists, this study provides a valuable resource for future research on eukaryotic-bacterial symbioses. Video Abstract.

Comparative Analyses of the Symbiotic Associations of the Host Paramecium bursaria with Free-Living and Native Symbiotic Species of Chlorella

Each symbiotic Chlorella variabilis associated with the ciliate Paramecium bursaria is enclosed in a symbiosome called the perialgal vacuole. Various potential symbionts, such as bacteria, yeasts, other algae, and free-living Chlorella spp., can infect P. bursaria. However, the detailed infection process of each of them in algae-free P. bursaria is unknown. Here, we aimed to elucidate the difference of the infection process between the free-living C. sorokiniana strain NIES-2169 and native symbiotic C. variabilis strain 1N. We investigated the fate of ingested algae using algae-free P. bursaria exposed separately to three types of algal inocula: NIES-2169 only, 1N only, or a mixture of NIES-2169 and 1N. We found that (1) only one algal species, preferably the native one, was retained in host cells, indicating a type of host compatibility and (2) the algal localization style beneath the host cell cortex varied between different Chlorella spp. showing various levels of host compatibilities, which was prospectively attributable to the difference in the formation of the perialgal vacuole membrane.

Role of host ciliate Paramecium bursaria mitochondria and trichocysts for symbiotic Chlorella variabilis attachment beneath the host cell cortex

Symbiotic Chlorella variabilis is encased in the perialgal vacuole (PV) membrane of ciliate Paramecium bursaria. The PV membrane is stably anchored below the host cell cortex by adhesion to host mitochondria. Host trichocysts, which are defensive organelles against predators, are present in the mitochondria and PV membrane vicinity. The mechanism by which PV attaches beneath the host cell cortex remains unknown. When P. bursaria is centrifuged at high speed, the symbiotic algae are displaced from the host cell cortex and concentrate at the posterior end. When centrifugation is stopped, the dislocated algae reattach beneath the host cell cortex with fast cytoplasmic streaming. The densities of mitochondria and trichocysts before and after centrifugation were compared using indirect immunofluorescence microscopy with monoclonal antibodies. Almost all trichocysts were shed by high-speed centrifugation, but dislocated algae could reattach even in the absence of trichocysts. In contrast, host mitochondria were unaffected in localization and number, and the dislocated algae also reattached. These findings suggest trichocysts are unnecessary for algal relocalization and that mitochondria are colocalized with the algae. However, many mitochondria were also present in the cell's anterior region without symbiotic algae. Therefore, not all areas with mitochondria contained algae, but there was an algal localization bias within the host cell.

Establishment of an unfed strain of Paramecium bursaria and analysis of associated bacterial communities controlling its proliferation

The ciliate Paramecium bursaria harbors several hundred symbiotic algae in its cell and is widely used as an experimental model for studying symbiosis between eukaryotic cells. Currently, various types of bacteria and eukaryotic microorganisms are used as food for culturing P. bursaria; thus, the cultivation conditions are not uniform among researchers. To unify cultivation conditions, we established cloned, unfed strains that can be cultured using only sterile medium without exogenous food. The proliferation of these unfed strains was suppressed in the presence of antibiotics, suggesting that bacteria are required for the proliferation of the unfed strains. Indeed, several kinds of bacteria, such as Burkholderiales, Rhizobiales, Rhodospirillales, and Sphingomonadales, which are able to fix atmospheric nitrogen and/or degrade chemical pollutants, were detected in the unfed strains. The genetic background of the individually cloned, unfed strains were the same, but the proliferation curves of the individual P. bursaria strains were very diverse. Therefore, we selected multiple actively and poorly proliferating individual strains and compared the bacterial composition among the individual strains using 16S rDNA sequencing. The results showed that the bacterial composition among actively proliferating P. bursaria strains was highly homologous but different to poorly proliferating strains. Using unfed strains, the cultivation conditions applied in different laboratories can be unified, and symbiosis research on P. bursaria will make great progress.