Microbial Ecology

, Volume 77, Issue 3, pp 748–758 | Cite as

The Hidden World of Rickettsiales Symbionts: “Candidatus Spectririckettsia obscura,” a Novel Bacterium Found in Brazilian and Indian Paramecium caudatum

  • Michele CastelliEmail author
  • Valentina Serra
  • Marcus V. X. Senra
  • Charan K. Basuri
  • Carlos A. G. Soares
  • Sergei I. Fokin
  • Letizia Modeo
  • Giulio PetroniEmail author
Host Microbe Interactions


Symbioses between bacteria and eukaryotes are widespread and may have significant impact on the evolutionary history of symbiotic partners. The order Rickettsiales is a lineage of intracellular Alphaproteobacteria characterized by an obligate association with a wide range of eukaryotic hosts, including several unicellular organisms, such as ciliates and amoebas. In this work, we characterized the Rickettsiales symbionts associated with two different genotypes of the freshwater ciliate Paramecium caudatum originated from freshwater environments in distant geographical areas. Phylogenetic analyses based on 16S rRNA gene showed that the two symbionts are closely related to each other (99.4% identity), belong to the family Rickettsiaceae, but are far-related with respect to previously characterized Rickettsiales. Consequently, they were assigned to a new species of a novel genus, namely “Candidatus Spectririckettsia obscura.” Screening on a database of short reads from 16S rRNA gene amplicon-based profiling studies confirmed that bacterial sequences related to the new symbiont are preferentially retrieved from freshwater environments, apparently with extremely scarce occurrence (< 0.1% positive samples). The present work provides new information on the still under-explored biodiversity of Rickettsiales, in particular those associated to ciliate host cells.


Protist Bacterial symbiont IMNGS Rickettsiaceae 



The authors wish to thank Aharon Oren for advice in bacterial nomenclature and species description, Simone Gabrielli for assistance in graphical artwork, and Franziska Szokoli for invaluable support and inspiration in the project.

Funding Information

This project was funded by the European Commission FP7-PEOPLE-2009-IRSES grant 247658 (project CINAR-PATHOBACTER) to GP, and by the University of Pisa grant PRA_2016_58 to GP.

Supplementary material

248_2018_1243_MOESM1_ESM.xlsx (6 kb)
ESM 1 (XLSX 6 kb)
248_2018_1243_MOESM2_ESM.xlsx (17 kb)
ESM 2 (XLSX 17 kb)


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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature  2018
Corrected publication August/2018

Authors and Affiliations

  • Michele Castelli
    • 1
    • 2
    Email author
  • Valentina Serra
    • 2
  • Marcus V. X. Senra
    • 3
    • 4
  • Charan K. Basuri
    • 5
  • Carlos A. G. Soares
    • 3
  • Sergei I. Fokin
    • 2
    • 6
  • Letizia Modeo
    • 2
  • Giulio Petroni
    • 2
    Email author
  1. 1.Romeo and Enrica Invernizzi Pediatric Research Center, Department of BiosciencesUniversity of MilanMilanItaly
  2. 2.Department of BiologyUniversity of PisaPisaItaly
  3. 3.Departamento de GenéticaUniversidade Federal do Rio de Janeiro, UFRJRio de JaneiroBrazil
  4. 4.Departamento de ZoologiaUniversidade Federal de Juiz de Fora, UFJFRio de JaneiroBrazil
  5. 5.Department of ZoologyAndhra UniversityVisakhapatnamIndia
  6. 6.Department of Invertebrate ZoologySaint Petersburg State UniversitySaint PetersburgRussia

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