Integrating genomic evidence for an updated taxonomy of the bacterial genus Spiribacter

The genus Spiribacter encompasses halophilic bacteria widely distributed in hypersaline environments worldwide. Despite their ecological significance, initially isolating Spiribacter species under laboratory settings was challenging due to the lack of knowledge of their growth and cultivation requirements. However, with improved understanding of their ecological niche and metabolic pathways, additional species of Spiribacter have been successfully isolated and identified from diverse locations around the globe. Enriched media with sodium pyruvate as carbon source facilitated the isolation of twelve new strains closely related to the genus Spiribacter from hypersaline environments in Spain. Genome sequencing and analysis of these new strains and previously described Spiribacter species provided insights into their genomic features and phylogenomic relationships, supporting the delineation of three distinct new species within this genus, designated as Spiribacter insolitus sp. nov., Spiribacter onubensis sp. nov., and Spiribacter pallidus sp. nov. In Spiribacter species, streamlined genomes enhance survival in hypersaline environments by reducing non-essential genes and optimizing resource utilization. Key genes involved in osmoprotectant mechanisms, including those for the metabolism of myo-inositol, hydroxyproline, and L-proline, were identified and numerous transporters were noted, ensuring efficient nutrient acquisition and osmotic balance. Notably, these new species, along with other Spiribacter strains, exhibit metabolic diversity in utilizing inorganic sulfur compounds, including thiosulfate and tetrathionate, for energy production and adaptation to hypersaline environments. The presence of thiosulfate dehydrogenase (TsdA) genes suggests their capability to oxidize thiosulfate to tetrathionate, potentially influencing both aerobic and anaerobic respiration. Furthermore, the prevalence of the sqr gene indicates a role for sulfide oxidation in Spiribacter metabolism, underlining their metabolic versatility in saline habitats. These adaptations allow Spiribacter to thrive in nutrient-limited, high-salinity habitats. Moreover, genome mining analysis and physiological disparities observed in the already described species Spiribacter halobius raise significant challenges to its classification within the genus Spiribacter.