New publication: Deianiraea, an extracellular bacterium associated with the ciliate Paramecium, suggests an alternative scenario for the evolution of Rickettsiales

The team, in a collaborative effort with researchers of Saint Petersburg State University, University of Pisa and University of Milan published an article on the journal: The ISME Journal.

Rickettsiales have always been considered a lineage of obligate intracellular Alphaproteobacteria,  encompassing important human pathogens, manipulators of host reproduction, and mutualists. Here we report the discovery of a novel member of the Rickettsiales, Deianiraea vastatrix, associated with Paramecium and displaying a unique extracellular lifestyle, including the ability to replicate outside host cells. Furthermore, the bacterium possesses a higher capability to synthesise amino acids, compared to all investigated Rickettsiales.

Considering these observations, phylogenetic and phylogenomic reconstructions, and re-evaluating the different means of interaction of Rickettsiales bacteria with eukaryotic cells, we propose an alternative scenario for the evolution of intracellularity in Rickettsiales. According to our reconstruction, the Rickettsiales ancestor would have been an extracellular and metabolically versatile bacterium, while obligate intracellularity would have evolved later, in parallel and independently, in different sub-lineages. The proposed new scenario could impact on the open debate on the lifestyle of the last common ancestor of mitochondria within Alphaproteobacteria.