Schulz, Frederik

AbstractAmoebae play an important ecological role as predators in microbial communities. They also serve as niche for bacterial replication, harbor endosymbiotic bacteria and have contributed to the evolution of major human pathogens. Despite their high diversity, marine amoebae and their association with bacteria are poorly understood. Here we describe the isolation and characterization of two novel marine amoebae together with their bacterial endosymbionts, tentatively named ‘Candidatus Occultobacter vannellae’ and ‘Candidatus Nucleophilum amoebae’. While one amoeba strain is related to Vannella, a genus common in marine habitats, the other represents a novel lineage in the Amoebozoa. The endosymbionts showed only low similarity to known bacteria (85–88% 16S rRNA sequence similarity) but together with other uncultured marine bacteria form a sister clade to the Coxiellaceae. Using fluorescence in situ hybridization and transmission electron microscopy, identity and intracellular location of both symbionts were confirmed; one was replicating in host-derived vacuoles, whereas the other was located in the perinuclear space of its amoeba host. This study sheds for the first time light on a so far neglected group of protists and their bacterial symbionts. The newly isolated strains represent easily maintainable model systems and pave the way for further studies on marine associations between amoebae and bacterial symbionts.

Abstract Amoebae serve as hosts for various intracellular bacteria, including human pathogens. These microbes are able to overcome amoebal defense mechanisms and successfully establish a niche for replication, which is usually the cytoplasm. Here, we report on the discovery of a bacterial symbiont that is located inside the nucleus of its Hartmannella sp. host. This symbiont, tentatively named ‘Candidatus Nucleicultrix amoebiphila’, is only moderately related to known bacteria (∼90% 16S and 23S rRNA sequence similarity) and member of a novel clade of protist symbionts affiliated with the Rickettsiales and Rhodospirillales. Screening of 16S rRNA amplicon data sets revealed a broad distribution of these bacteria in freshwater and soil habitats. ‘Candidatus Nucleicultrix amoebiphila’ traffics within 6 h post infection to the host nucleus. Maximum infection levels are reached after 96–120 h, at which time point the nucleus is pronouncedly enlarged and filled with bacteria. Transmission of the symbionts occurs vertically upon host cell division but may also occur horizontally through host cell lysis. Although we observed no impact on the fitness of the original Hartmannella sp. host, the bacteria are rather lytic for Acanthamoeba castellanii. Intranuclear symbiosis is an exceptional phenomenon, and amoebae represent an ideal model system to further investigate evolution and underlying molecular mechanisms of these unique microbial associations.