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The diversity of prokaryotic symbionts in Ciliophora and other protists is fascinatingly rich; they may even include some potentially pathogenic bacteria. In this review, we summarize currently available data on biodiversity and some morphological and biological peculiarities of prokaryotic symbionts mainly within the genera Paramecium and Euplotes. Another direction of ciliate symbiology, neglected for a long time and now re-discovered, is the study of epibionts of ciliates. This promises a variety of interesting outcomes. Last, but not least, we stress the new technologies, such as next generation sequencing and the use of genomics data, which all can clarify many new aspects of relevance. For this reason, a brief overview of achievements in genomic studies on ciliate's symbionts is provided. Summing up the results of numerous scientific contributions, we systematically update current knowledge and outline the prospects as to how symbiology of Ciliophora may develop in the near future., (© 2022 The Authors. Journal of Eukaryotic Microbiology published by Wiley Periodicals LLC on behalf of International Society of Protistologists.)

The morphology and infraciliature of a new ciliate, Metopus yantaiensis n. sp., discovered in coastal soil of northern China, were investigated. It is distinguished from its congeners by a combination of the following features: nuclear apparatus situated in the preoral dome; 18-21 somatic ciliary rows, of which three extend onto the preoral dome (dome kineties); three to five distinctly elongated caudal cilia, and 21-29 adoral polykinetids. The 18S rRNA genes of this new species and two congeners, Metopus contortus and Metopus hasei, were sequenced and phylogenetically analyzed. The new species is more closely related to M. hasei and the clevelandellids than to other congeners; both the genus Metopus and the order Metopida are not monophyletic. In addition, the digestion-resistant bacteria in the cytoplasm of M. yantaiensis were identified, using a 16S rRNA gene clone library, sequencing, and fluorescence in situ hybridization. The detected intracellular bacteria are affiliated with Sphingomonadales, Rhizobiales, Rickettsiales (Alphaproteobacteria), Pseudomonas (Gammaproteobacteria), Rhodocyclales (Betaproteobacteria), Clostridiales (Firmicutes), and Flavobacteriales (Bacteroidetes)., (© 2017 The Author(s) Journal of Eukaryotic Microbiology © 2017 International Society of Protistologists.)

We measured ingestion and digestion rates of the pathogenic bacterium Campylobacter jejuni by a freshwater ciliate Colpoda sp. to determine whether Campylobacter is able to resist protist digestion. Campylobacter and the nonpathogenic bacterium Pseudomonas putida LH1 were labeled with a 5-chloromethylfluorescein diacetate, which fluoresces in intact and active cells but fades when exposed to low pH environments, such as protistan food vacuoles. Ingestion and digestion rates were measured via flow cytometry as the change in ciliate fluorescence over time, which corresponded to the quantity of intracellular bacteria. The rate of Campylobacter ingestion exceeded the digestion rate. Ciliates retained labeled Campylobacter 5 h after ingestion was stopped. In contrast, ciliates grazing upon P. putida returned to baseline fluorescence within 5 h, indicating that P. putida were completely digested. The ability of intracellular Campylobacter to remain viable after ingestion was tested by sorting individual ciliates and bacterial cells into Campylobacter-selective media. Campylobacter growth occurred in 15% (± 5 SE) of wells seeded with highly fluorescent ciliates, whereas only 4% (± 1) of wells seeded with free-living Campylobacter exhibited growth. A key advantage of this approach is that it is rapid and should be applicable to other phagocytotis studies., (© 2011 The Author(s) Journal of Eukaryotic Microbiology © 2011 International Society of Protistologists.)

We tested whether mixotrophic ciliates are more resistant to solar ultraviolet radiation (UVR) than heterotrophic ones because symbiotic algae can provide self-shading by cell matter absorption and eventually by direct UV screening from mycosporine-like amino acids (MAAs). Sensitivity of a natural assemblage to solar radiation was tested in experiments in the original lake and in a more UV transparent alpine lake after transplantation of the ciliates. In both lakes, the assemblage was exposed either to full sunlight, to photosynthetically active radiation only, or kept in the dark. In each lake, exposure was for 5 h at the surface and at the depth corresponding to the 10% attenuation depth at 320 nm. Overall, when the assemblage was exposed to surface UVR, only one out of four dominant mixotrophic ciliates, Vorticella chlorellata, was more resistant than heterotrophic species. The higher UV resistance in V. chlorellata was related to the presence of MAAs and the high percentage of ciliate volume occupied by algal symbionts. Our results indicate that effects of UVR were species-specific and depended on efficient screening of these wavelengths, but also on the depth preference of the ciliates and thus, on their previous exposure history to UVR., (© 2011 The Author(s). Journal of Eukaryotic Microbiology© 2011 International Society of Protistologists.)

Rickettsia-like organisms (RLO) are obligate, often highly fastidious, intracellular bacterial parasites associated with a variety of vertebrate and invertebrate hosts. Despite their importance as causative agents of severe mortality outbreaks in farmed aquatic species, little is known about their life cycle and their host range. The present work reports the characterization of "Candidatus Cryptoprodotis polytropus," a novel Rickettsia-like bacterium associated with the common ciliate species Pseudomicrothorax dubius by means of the "Full-Cycle rRNA Approach" and ultrastructural observations. The morphological description by in vivo and scanning electron microscopy and the 18S rRNA gene sequence of the host species is provided as well. Phylogenetic analysis based on the 16S rRNA gene supports the inclusion of "Candidatus Cryptoprodotis polytropus" within the family Rickettsiaceae (cl. Alphaproteobacteria) together with the genera Rickettsia and Orientia. Observations on natural ciliate populations account for the occasional nature of this likely parasitic association. The presence of a previously unknown RLO in ciliates sheds a new light on the possible role of protists as transient hosts, vectors or natural reservoir for some economically important pathogens.

In situ hybridization using fluorescent oligonucleotide probes complementary to unique regions of 16S rRNA molecules provides a way of identifying the food vacuole contents of bactivorous protists. Laboratory experiments with Tetrahymena showed rRNAs in food vacuoles are degraded slowly enough to permit their use as hybridization targets for such probes. A probe specific for a hypervariable region of the small subunit rRNA of an unnamed proteobacterium abundant in a local lake was then synthesized. It was used to probe the food vacuoles of the ciliates present in fixed water samples collected from the same lake. The vacuoles of several filter-feeding ciliates bound the probe, indicating that such probes can be used to identify the food vacuole contents of ciliates collected from natural samples.