Microbial Ecology

, Volume 77, Issue 4, pp 1092–1106 | Cite as

Candidatus Hafkinia simulans” gen. nov., sp. nov., a Novel Holospora-Like Bacterium from the Macronucleus of the Rare Brackish Water Ciliate Frontonia salmastra (Oligohymenophorea, Ciliophora): Multidisciplinary Characterization of the New Endosymbiont and Its Host

  • Sergei I. FokinEmail author
  • Valentina SerraEmail author
  • Filippo Ferrantini
  • Letizia Modeo
  • Giulio Petroni
Host Microbe Interactions


We characterized a novel Holospora-like bacterium (HLB) (Alphaproteobacteria, Holosporales) living in the macronucleus of the brackish water ciliate Frontonia salmastra. This bacterium was morphologically and ultrastructurally investigated, and its life cycle and infection capabilities were described. We also obtained its 16S rRNA gene sequence and performed in situ hybridization experiments with a specifically-designed probe. A new taxon, “Candidatus Hafkinia simulans”, was established for this HLB. The phylogeny of the family Holosporaceae based on 16S rRNA gene sequences was inferred, adding to the already available data both the sequence of the novel bacterium and those of other Holospora and HLB species recently characterized. Our phylogenetic analysis provided molecular support for the monophyly of HLBs and placed the new endosymbiont as the sister genus of Holospora. Additionally, the host ciliate F. salmastra, recorded in Europe for the first time, was concurrently described through a multidisciplinary study. Frontonia salmastra’s phylogenetic position in the subclass Peniculia and the genus Frontonia was assessed according to 18S rRNA gene sequencing. Comments on the biodiversity of this genus were added according to past and recent literature.


Integrative taxonomy Systematics Symbiosis Molecular phylogeny Ultrastructure Bacterial parasite Bacterial symbiont SSU rRNA Rickettsiales Rickettsia-like organisms (RLO) 



The authors are grateful to Simone Gabrielli for his patient and precious assistance in tree editing.

Funding Information

This research was supported by a grant from Italian Research Ministry (MIUR – Ministero Italiano dell’ Università e della Ricerca) to S.I. Fokin; and by the PRA_2018_63 project from University of Pisa to G. Petroni.


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Authors and Affiliations

  1. 1.Department of BiologyUniversity of PisaPisaItaly
  2. 2.Department of Invertebrate ZoologySt. Petersburg State UniversitySt. PetersburgRussia
  3. 3.St. Petersburg Branch of the S.I. Vavilov Institute of History of Science and TechnologyRussian Academy of SciencesSt. PetersburgRussia

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