Horn, Matthias

Abstract Chlamydiae are ubiquitous intracellular bacteria and infect a wide diversity of eukaryotes, including mammals. However, chlamydiae have never been reported to infect photosynthetic organisms. Here, we describe a novel chlamydial genus and species, Candidatus Algichlamydia australiensis, capable of infecting the photosynthetic dinoflagellate Cladocopium sp. (originally isolated from a scleractinian coral). Algichlamydia australiensis was confirmed to be intracellular by fluorescence in situ hybridization and confocal laser scanning microscopy and temporally stable at the population level by monitoring its relative abundance across four weeks of host growth. Using a combination of short- and long-read sequencing, we recovered a high-quality (completeness 91.73% and contamination 0.27%) metagenome-assembled genome of A. australiensis. Phylogenetic analyses show that this chlamydial taxon represents a new genus and species within the Simkaniaceae family. Algichlamydia australiensis possesses all the hallmark genes for chlamydiae–host interactions, including a complete type III secretion system. In addition, a type IV secretion system is encoded on a plasmid and has previously been observed for only three other chlamydial species. Twenty orthologous groups of genes are unique to A. australiensis, one of which is structurally similar to a protein known from Cyanobacteria and Archaeplastida involved in thylakoid biogenesis and maintenance, hinting at potential chlamydiae interactions with the chloroplasts of Cladocopium cells. Our study shows that chlamydiae infect dinoflagellate symbionts of cnidarians, the first photosynthetic organism reported to harbor chlamydiae, thereby expanding the breadth of chlamydial hosts and providing a new contribution to the discussion around the role of chlamydiae in the establishment of the primary plastid.

AbstractChlamydiae like Chlamydia trachomatis and Chlamydia psittaci are well-known human and animal pathogens. Yet, the chlamydiae are a much larger group of evolutionary ancient obligate intracellular bacteria that includes predominantly symbionts of protists and diverse animals. This makes them ideal model organisms to study evolutionary transitions from symbionts in microbial eukaryotes to pathogens of humans. To this end, comparative genome analysis has served as an important tool. Genome sequence data for many chlamydial lineages are, however, still lacking, hampering our understanding of their evolutionary history. Here, we determined the first high-quality draft genome sequence of the fish pathogen “Candidatus Clavichlamydia salmonicola”, representing a separate genus within the human and animal pathogenic Chlamydiaceae. The “Ca. Clavichlamydia salmonicola” genome harbors genes that so far have been exclusively found in Chlamydia species suggesting that basic mechanisms important for the interaction with chordate hosts have evolved stepwise in the history of chlamydiae. Thus, the genome sequence of “Ca. Clavichlamydia salmonicola” allows to constrain candidate genes to further understand the evolution of chlamydial virulence mechanisms required to infect mammals.

AbstractFree-living amoebae are well known for their role in controlling microbial community composition through grazing, but some groups, namely Acanthamoeba species, also frequently serve as hosts for bacterial symbionts. Here we report the first identification of a bacterial symbiont in the testate amoeba Cochliopodium. The amoeba was isolated from a cooling tower water sample and identified as C. minus. Fluorescence in situ hybridization and transmission electron microscopy revealed intracellular symbionts located in vacuoles. 16S rRNA-based phylogenetic analysis identified the endosymbiont as member of a monophyletic group within the family Coxiellaceae (Gammaprotebacteria; Legionellales), only moderately related to known amoeba symbionts. We propose to tentatively classify these bacteria as ‘Candidatus Cochliophilus cryoturris’. Our findings add both, a novel group of amoeba and a novel group of symbionts, to the growing list of bacteria-amoeba relationships.

Summary Virtually all aphids maintain an obligate mutualistic symbiosis with bacteria from the Buchnera genus, which produce essential nutrients for their aphid hosts. Most aphids from the Lachninae subfamily have been consistently found to house additional endosymbionts, mainly Serratia symbiotica . This apparent dependence on secondary endosymbionts was proposed to have been triggered by the loss of the riboflavin biosynthetic capability by Buchnera in the Lachninae last common ancestor. However, an integral large‐scale analysis of secondary endosymbionts in the Lachninae is still missing, hampering the interpretation of the evolutionary and genomic analyses of these endosymbionts. Here, we analysed the endosymbionts of selected representatives from seven different Lachninae genera and nineteen species, spanning four tribes, both by FISH (exploring the symbionts’ morphology and tissue tropism) and 16S rRNA gene sequencing. We demonstrate that all analysed aphids possess dual symbiotic systems, and while most harbour S. symbiotica , some have undergone symbiont replacement by other phylogenetically‐distinct bacterial taxa. We found that these secondary associates display contrasting cell shapes and tissue tropism, and some appear to be lineage‐specific. We propose a scenario for symbiont establishment in the Lachninae, followed by changes in the symbiont's tissue tropism and symbiont replacement events, thereby highlighting the extraordinary versatility of host‐symbiont interactions.

Abstract The bacterial family Rickettsiaceae includes a group of well-known etiological agents of many human and vertebrate diseases, including epidemic typhus-causing pathogen Rickettsia prowazekii. Owing to their medical relevance, rickettsiae have attracted a great deal of attention and their host-pathogen interactions have been thoroughly investigated. All known members display obligate intracellular lifestyles, and the best-studied genera, Rickettsia and Orientia, include species that are hosted by terrestrial arthropods. Their obligate intracellular lifestyle and host adaptation is reflected in the small size of their genomes, a general feature shared with all other families of the Rickettsiales. Yet, despite that the Rickettsiaceae and other Rickettsiales families have been extensively studied for decades, many details of the origin and evolution of their obligate host-association remain elusive. Here we report the discovery and single-cell sequencing of ‘Candidatus Arcanobacter lacustris’, a rare environmental alphaproteobacterium that was sampled from Damariscotta Lake that represents a deeply rooting sister lineage of the Rickettsiaceae. Intriguingly, phylogenomic and comparative analysis of the partial ‘Candidatus Arcanobacter lacustris’ genome revealed the presence chemotaxis genes and vertically inherited flagellar genes, a novelty in sequenced Rickettsiaceae, as well as several host-associated features. This finding suggests that the ancestor of the Rickettsiaceae might have had a facultative intracellular lifestyle. Our study underlines the efficacy of single-cell genomics for studying microbial diversity and evolution in general, and for rare microbial cells in particular.

Abstract Fri'tsche.a. N.L. fem. n. Fritschea named after Thomas R. Fritsche, an American physician and parasitologist, in honor of his contributions to our current knowledge on chlamydial diversity. Coccoid to rod‐shaped, nonmotile, obligately intracellular bacteria, up to 2.5 μm in length . Cells show a developmental cycle with morphologically distinct stages similar to those of the Chlamydiaceae . Organisms were found in whitefly and scale insects, but have not yet been obtained in cell culture. The genus is based on the description of “ Candidatus Fritschea bemisiae” and “ Candidatus Fritschea eriococci” (Everett et al., 2005; Thao et al., 2003) (Figure 1). Type species : “ Candidatus Fritschea bemisiae ” Everett, Thao, Horn, Dyszynski and Baumann 2005, 1585. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Fritschea is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Chlamydiota / Chlamydiia / Chlamydiales / Simkaniaceae / Candidatus Fritschea Candidatus Fritschea could not be recovered in GTDB ** . * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Cla.vi.chla.my'di.a. L. n. clava a knotty branch, rough stick, cudgel, club; N.L. fem. n. Chlamydia taxonomic name of a bacterial genus; N.L. fem. n. Clavichlamydia a club Chlamydia (the morphology of the bacteria includes the characteristic head‐and‐tail cells that have been described earlier from salmonid fish suffering from epitheliocystis). Pleomorphic or elongated , nonmotile , obligate intracellular bacteria , up to 2 μ m in length . Cells show a developmental cycle with morphologically distinct stages and thrive inside a host vacuole . Organisms were found within gill lesions of fish and are differentiated from all other chlamydiae by the morphology of the proposed elementary bodies , which show a characteristic head‐and‐tail form (Figure 1). Members have not yet been obtained in cell culture. Type species : “ Candidatus Clavichlamydia salmonicola ” corrig. Karlsen, Nylund, Watanabe, Helvik, Nylund and Plarre 2008. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Clavichlamydia is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Chlamydiota / Chlamydiia / Chlamydiales / Chlamydiaceae / Candidatus Clavichlamydia The genus Candidatus Clavichlamydia can also be recovered in the Genome Taxonomy Database (GTDB) as g__Clavichlamydia (version v220) ** . GTDB classification: d__Bacteria / p__Chlamydiota / c__Chlamydiia / o__Chlamydiales / f__Chlamydiaceae / g__Clavichlamydia * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776