Identification of Intracellular Bacteria in the Ciliate Balantidium ctenopharyngodoni (Ciliophora, Litostomatea)

Genome assembly of a symbiotic balantidia (Balantidium ctenopharyngodoni) in fish hindgut

Balantidium ctenopharyngodoni is identified as the sole ciliate species that exclusively resides within the hindgut of grass carp with high prevalence and intensity. In this study, the successful cultivation of B. ctenopharyngodoni enabled us to collect enough cells for genome sequencing. Consequently, we acquired a high-quality genome assembly spanning 68.66 Mb, encompassing a total of 22,334 nanochromosomes. Furthermore, we predicted 29,348 protein-coding genes, and 95.5% of them was supported by the RNA-seq data. The trend of GC content in the subtelomeric regions of single-gene chromosomes was similar to other ciliates containing nanochromosomes. A large number of genes encoding carbohydrate-binding modules with affinities for starch and peptidoglycans was identified. The identification of mitochondrion-related organelles (MROs) within genome indicates its well-suited adaptation to the anaerobic conditions in the hindgut environment. In summary, our results will offer resources for understanding the genetic basis and molecular adaptations of balantidia to hindgut of herbivorous fish.

Interactions between Balantidium ctenopharyngodoni and microbiota reveal its low pathogenicity in the hindgut of grass carp

BACKGROUND

Hosts, parasites, and microbiota interact with each other, forming a complex ecosystem. Alterations to the microbial structure have been observed in various enteric parasitic infections (e.g. parasitic protists and helminths). Interestingly, some parasites are associated with healthy gut microbiota linked to the intestinal eubiosis state. So the changes in bacteria and metabolites induced by parasite infection may offer benefits to the host, including protection from other parasitesand promotion of intestinal health. The only ciliate known to inhabit the hindgut of grass carp, Balantidium ctenopharyngodoni, does not cause obvious damage to the intestinal mucosa. To date, its impact on intestinal microbiota composition remains unknown. In this study, we investigated the microbial composition in the hindgut of grass carp infected with B. ctenopharyngodoni, as well as the changes of metabolites in intestinal contents resulting from infection.

RESULTS

Colonization by B. ctenopharyngodoni was associated with an increase in bacterial diversity, a higher relative abundance of Clostridium, and a lower abundance of Enterobacteriaceae. The family Aeromonadaceae and the genus Citrobacter had significantly lower relative abundance in infected fish. Additionally, grass carp infected with B. ctenopharyngodoni exhibited a significant increase in creatine content in the hindgut. This suggested that the presence of B. ctenopharyngodoni may improve intestinal health through changes in microbiota and metabolites.

CONCLUSIONS

We found that grass carp infected with B. ctenopharyngodoni exhibit a healthy microbiota with an increased bacterial diversity. The results suggested that B. ctenopharyngodoni reshaped the composition of hindgut microbiota similarly to other protists with low pathogenicity. The shifts in the microbiota and metabolites during the colonization and proliferation of B. ctenopharyngodoni indicated that it may provide positive effects in the hindgut of grass carp.

Mitochondrial metabolism of the facultative parasite Chilodonella uncinata (Alveolata, Ciliophora)

BACKGROUND

Chilodonella uncinata is an aerobic ciliate capable of switching between being free-living and parasitic on fish fins and gills, causing tissue damage and host mortality. It is widely used as a model organism for genetic studies, but its mitochondrial metabolism has never been studied. Therefore, we aimed to describe the morphological features and metabolic characteristics of its mitochondria.

METHODS

Fluorescence staining and transmission electron microscopy (TEM) were used to observe the morphology of mitochondria. Single-cell transcriptome data of C. uncinata were annotated by the Clusters of Orthologous Genes (COG) database. Meanwhile, the metabolic pathways were constructed based on the transcriptomes. The phylogenetic analysis was also made based on the sequenced cytochrome c oxidase subunit 1 (COX1) gene.

RESULTS

Mitochondria were stained red using Mito-tracker Red staining and were stained slightly blue by DAPI dye. The cristae and double membrane structures of the mitochondria were observed by TEM. Besides, many lipid droplets were evenly distributed around the macronucleus. A total of 2594 unigenes were assigned to 23 functional classifications of COG. Mitochondrial metabolic pathways were depicted. The mitochondria contained enzymes for the complete tricarboxylic acid (TCA) cycle, fatty acid metabolism, amino acid metabolism, and cytochrome-based electron transport chain (ETC), but only partial enzymes involved in the iron-sulfur clusters (ISCs).

CONCLUSIONS

Our results showed that C. uncinata possess typical mitochondria. Stored lipid droplets inside mitochondria may be the energy storage of C. uncinata that helps its transmission from a free-living to a parasitic lifestyle. These findings also have improved our knowledge of the mitochondrial metabolism of C. uncinata and increased the volume of molecular data for future studies of this facultative parasite.

PARASITES OF THE BRAZILIAN ROCK CAVY, KERODON RUPESTRIS: REVEALING THEIR HISTORY IN THE BRAZILIAN SEMIARID REGION

Using microscopy and/or immunodiagnosis, the authors analyzed 284 fecal samples from the Brazilian rock cavy, Kerodon rupestris, that were collected between 1984 and 2015 in Serra da Capivara National Park for the presence of helminths and protozoa. Fourteen morphospecies of helminth eggs of the following taxa were found: Trematoda, Nematoda, Strongylidae, Lagochilascaris sp., Strongylida, Trichuris (2 species), Oxyuridae (3 species), Ancylostomatidae (2 species), and Ascarididae (2 species), along with 3 protozoan taxa: Coccidia, Cryptosporidium sp., and Balantidium sp. During the last 30 yr, the population of K. rupestris has increased in the region as a consequence of the creation and management of the National Park, and data from this study show a concurrent increase in the diversity of intestinal parasites in this host, including new reports. Some of these species have zoonotic potential, which suggests that K. rupestris may be in contact with domestic farm animals and/or human feces. These results show the importance of integrating different diagnostic approaches for the identification of protozoa in the region and indicate that further methods need to be employed to increase recovery. This work highlights the usefulness of parasite studies in assessing the health of ecosystems, especially in protected areas, which should be considered by park managers and health agencies.

A new species of Trichodina lishuiensis n. sp. (Ciliophora: Trichodinidae) in urinary bladder of Odorrana schmackeri (Amphibia: Ranidae) from Zhejiang, China

Endoparasitic trichodinids are rather rare ciliates. In this study we describe a new species named Trichodina lishuiensis from the bladder of Odorrana schmackeri collected in Zhejiang, China, with the prevalence of 20% (9/45). We identified T. lishuiensis as a new species by morphological comparison and molecular analysis. The ciliates were observed using the dry-silver and protargol staining methods, as well as SEM (scanning electron microscopy). Trichodina lishuiensis is a small species (cell diameter 31.8-43.9 μm), with incompact denticles connection, medium-wide blades and thick rays. We also sequenced a 1712 bp-long fragment of the small subunit ribosomal RNA gene (SSU rRNA). Phylogenetic analyses showed that the new species clustered with Trichodina unionis. The route of transmission of Trichodina species in the urinary bladder remains a mystery. We hypothesize that the transmisison takes place during the amplexus, with eggs and sperm discharged from the cloaca, and that trichodinids 'accompany' the amphibian through its whole life cycle, but further studies are needed to test this hypothesis.

Taxonomy and Molecular Phylogeny of Two New Species of Prostomatean Ciliates With Establishment of Foissnerophrys gen. n. (Alveolata, Ciliophora)

Prostomatean ciliates play important roles in the flow of material and energy in aquatic microbial food webs, and thus have attracted wide attention for over a century. Their taxonomy and systematics are, however, still poorly understood because of their relatively few taxonomically informative morphological characters. In this study, two new prostomateans, Lagynus binucleatus sp. n. and Foissnerophrys alveolata gen. n., sp. n., collected from a freshwater pool and the intertidal zone of a sandy beach, respectively, in Qingdao, China, are investigated using living observation, protargol staining, and SSU rRNA gene sequencing methods. The genus Lagynus is redefined, and the new species L. binucleatus sp. n. is established based on significant morphological differences with similar forms. Furthermore, a new genus, Foissnerophrys gen. n., is established based on a combination of morphological and molecular data with F. alveaolata sp. n. the type species by monotypy. The identities of intracellular prokaryotes of these two new species are discussed based on fluorescence in situ hybridization (FISH) data and newly obtained 16S rRNA gene sequences.