Moumen, Bouziane

Abstract Ba.be.lo'ta. N.L. fem. n. Babela , a Candidatus genus name; L. neut. pl. n. suff. ‐ ota , ending to denote a phylum; N.L. neut. pl. n. Babelota , the Candidatus Babela phylum. Candidatus Babelota is a phylum of strictly intracellular bacteria, currently divided into two class‐level groups and at least five orders based on 16S rRNA sequence analyses. However, all known biological isolates to date belong to the same order of the Babelales, within the Babelia class. Presently, four isolates representing three families have been described and maintained in laboratory cultures through the cultivation of their hosts, which are known to be amoeboid and flagellated protists. Genomic analysis of Ca . Babelota consistently reveals a profound reduction in metabolic capabilities, suggesting a phylum‐wide, conserved parasitic lifestyle. To date, all lineages covered by genomic data indicate that parasitism is a defining trait of this phylum. Ca . Babelota exhibits a cosmopolitan environmental distribution, with sequences detected across a spectrum of habitats including soil, freshwater, and saline or hypersaline water bodies.

AbstractThe order Holosporales (Alphaproteobacteria) encompasses obligate intracellular bacterial symbionts of diverse Eukaryotes. These bacteria have highly streamlined genomes and can have negative fitness effects on the host. Herein, we present a comparative analysis of the first genome sequences of ‘Ca. Hepatincola porcellionum’, a facultative symbiont occurring extracellularly in the midgut glands of terrestrial isopods. Using a combination of long-read and short-read sequencing, we obtained the complete circular genomes of two Hepatincola strains and an additional metagenome-assembled draft genome. Phylogenomic analysis validated its phylogenetic position as an early-branching family-level clade relative to all other established Holosporales families associated with protists. A 16S rRNA gene survey revealed that this new family encompasses diverse bacteria associated with both marine and terrestrial host species, which expands the host range of Holosporales bacteria from protists to several phyla of the Ecdysozoa (Arthropoda and Priapulida). Hepatincola has a highly streamlined genome with reduced metabolic and biosynthetic capacities as well as a large repertoire of transmembrane transporters. This suggests that this symbiont is rather a nutrient scavenger than a nutrient provider for the host, likely benefitting from a nutrient-rich environment to import all necessary metabolites and precursors. Hepatincola further possesses a different set of bacterial secretion systems compared to protist-associated Holosporales, suggesting different host-symbiont interactions depending on the host organism.