“Candidatus Mystax nordicus” Aggregates with Mitochondria of Its Host, the Ciliate Paramecium nephridiatum

Aleksandr Korotaev , Konstantin Benken, Elena Sabaneyeva

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6 Scopus citations
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Extensive search for new endosymbiotic systems in ciliates occasionally reverts us to the endosymbiotic bacteria described in the pre-molecular biology era and, hence, lacking molecular characterization. A pool of these endosymbionts has been referred to as a hidden bacterial biodiversity from the past. Here, we provide a description of one of such endosymbionts, retrieved from the ciliate Paramecium nephridiatum. This curve-shaped endosymbiont (CS), which shared the host cytoplasm with recently described “Candidatus Megaira venefica”, was found in the same host and in the same geographic location as one of the formerly reported endosymbiotic bacteria and demonstrated similar morphology. Based on morphological data obtained with DIC, TEM and AFM and molecular characterization by means of sequencing 16S rRNA gene, we propose a novel genus, “Candidatus Mystax”, with a single species “Ca. Mystax nordicus”. Phylogenetic analysis placed this species in Holosporales, among Holospora-like bacteria. Contrary to all Holospora species and many other Holospora-like bacteria, such as “Candidatus Gortzia”, “Candidatus Paraholospora” or “Candidatus Hafkinia”, “Ca. Mystax nordicus” was never observed inside the host nucleus. “Ca. Mystax nordicus” lacked infectivity and killer effect. The striking peculiarity of this endosymbiont was its ability to form aggregates with the host mitochondria, which distinguishes it from Holospora and Holospora-like bacteria inhabiting paramecia
Translated title of the contribution“Candidatus Mystax nordicus” образует агрегаты с митохондриями своего хозяина, инфузории Paramecium nephridiatum
Original languageEnglish
Article number251
Number of pages16
Issue number6
StatePublished - 19 Jun 2020


  • symbiosis
  • Paramecium
  • ciliates
  • Holospora-like bacteria
  • host–parasite interactions
  • 16S rRNA gene sequencing
  • full-cycle rRNA approach
  • TEM
  • fluorescence in situ hybridization


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