Thauera sp. Sel9 a polyhydroxyalkanoates-storing bacterial strain as a potential ingredient for aquaculture feed: the safety assessment

Background: Bacteria are commonly employed as ingredients in food and feed. The use of polyhydroxyalkanoates (PHA)-producing cells showed prebiotic properties such as the protection of fish from pathogens. Thauera sp. Sel9 can grow and accumulate PHAs using bio-based Volatile Fatty Acids from agro-industrial waste. Anyhow, the use of biological agents deliberately added in the feed requires a priori safety assessment. Objective: The aim of this study was the taxonomic identification and the safety characterization of Thauera sp. Sel9 to propose its inclusion in the QPS list (Qualified Presumption of Safety) of the European Food Safety Authority (EFSA) and allow its market authorization. Methods: Taxonomic analysis was performed by 16S rDNA gene analysis, in-silico DNA-DNA hybridization (DDH) and Average Nucleotide Identity (ANI) investigation. Production of hemolytic compounds and biogenic amines was evaluated. The presence of toxic molecules in the growth-media was investigated against different reference strains. The antibiotic resistance was evaluated by antibiogram (using Enterobacteriaceae cut-offs) and genomic analysis (CARD and ResFinder databases). Results: The taxonomic analysis showed a 98.7% similarity with Thauera butanivorans NBRC 103042T. Type strains belonging to Thauera species showed a DDH and ANI values below 70% and 95%, respectively. Sel9 was thus described for fatty acid composition (45.31% of C16:1, 27.54% of C16.0 and 14.65% of C18:1), growth temperature (12-38°C) and NaCl resistance (< 4%). Regarding safety-related traits, Sel9 was not able to produce hemolytic compounds, biogenic amines, and toxic compounds towards reference strains. The antibiogram evidenced sensitivity to ampicillin, ciprofloxacin, colistin, streptomycin and tetracycline. Sel9 exceeds the cut-offs for gentamicin and kanamycin, however the genomic analysis did not retrieve putative antibiotic resistance genes flanked by mobile elements. Conclusion: The taxonomic analysis showed the Thauera sp. Sel9 belongs to a new species. Safety investigation evidenced the possible use of Thauera sp. Sel9 as ingredient for aquaculture feed.