The ciliate Euplotes petzi is the natural reservoir of the bacterium Francisella in the Antarctic region.

Euplotes petzi is a strict psychrophilic ciliate isolated from coastal waters of Terra Nova Bay (Antarctica). In analyzing the results from a draft genome sequencing of total E. petzi DNA preparations, we found that more than 3% of the assembled contigs had a bacterial origin. All the bacterial DNA sequences, including one containing rRNA genes, overlapped with DNA sequences of the genus Francisella, which comprises a large number of species classified as facultative intracellular γ-proteobacteria potential noxious to their hosts. Based on these findings, we undertook the isolation, characterization and genome determination of the Francisella-like bacteria – tentatively named F. adeliensis sp. nov. – hosted in the cytoplasm of E. petzi. Genome sequence comparisons provide evidence that F. adeliensis forms a new clade in the Francisella phylogenomic tree. In addition to being well separated from all the recognised pathogenic Francisella species, this clade is distinct also from F. endociliophora, symbiont in the congeneric species E. raikovi, as well as from the Allofrancisella species that are collectively known as ‘environmental francisellas’. The inclusion of F. adeliensis in the Francisella phylogenomic tree does not support the separation of this genus between ‘intracellular’ and ‘environmental’ forms.