Journal of Eukaryotic Microbiology

AbstractOur knowledge of ciliate endosymbiont diversity greatly expanded over the past decades due to the development of characterization methods for uncultivable bacteria. Chlamydia‐like bacteria have been described as symbionts of free‐living amoebae and other phylogenetically diverse eukaryotic hosts. In the present work, a systematic survey of the bacterial diversity associated with the ciliate Euplotes octocarinatus strain Zam5b‐1 was performed, using metagenomic screening as well as classical full‐cycle rRNA approach, and a novel chlamydial symbiont was characterized. The metagenomic screening revealed 16S rRNA gene sequences from Polynucleobacter necessarius, three previously reported accessory symbionts, and a novel chlamydia‐like bacterium. Following the full‐cycle rRNA approach, we obtained the full‐length 16S rRNA gene sequence of this chlamydia‐like bacterium and developed probes for diagnostic fluorescence in situ hybridizations. The phylogenetic analysis of the 16S rRNA gene sequences unambiguously places the new bacterium in the family Rhabdochlamydiaceae. This is the first report of chlamydia‐like bacterium being found in Euplotes. Based on the obtained data, the bacterium is proposed as a new candidate genus and species: “Candidatus Euplotechlamydia quinta.”

ABSTRACT. 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.