Characterization and description of Faecalibacterium butyricigenerans sp. nov. and F. longum sp. nov., isolated from human faeces

Exploiting a pure culture strategy to investigate the composition of the human gut microbiota, two novel anaerobes, designated strains AF52-21^T and CM04-06^T, were isolated from faeces of two healthy Chinese donors and characterized using a polyphasic approach. The two strains were observed to be gram-negative, non-motile, and rod-shaped. Both strains grew optimally at 37 °C and pH 7.0. Phylogenetic analysis based on 16S rRNA gene sequences revealed that the two strains clustered with species of the genus Faecalibacterium and were most closely related to Faecalibacterium prausnitzii ATCC 27768^T with sequence similarity of 97.18% and 96.87%, respectively. The two isolates shared a 16S rRNA gene sequence identity of 98.69%. Draft genome sequencing was performed for strains AF52-21^T and CM04-06^T, generating genome sizes of 2.85 Mbp and 3.01 Mbp. The calculated average nucleotide identity values between the genomes of the strains AF52-21^T and CM04-06^T compared to Faecalibacterium prausnitzii ATCC 27768^T were 83.20% and 82.54%, respectively, and 90.09% when comparing AF52-21^T and CM04-06^T. Both values were below the previously proposed species threshold (95-96%), supporting their recognition as novel species in the genus Faecalibacterium. The genomic DNA G + C contents of strains AF52-21^T and CM04-06^T calculated from genome sequences were 57.77 mol% and 57.51 mol%, respectively. Based on the phenotypic, chemotaxonomic and phylogenetic characteristics, we conclude that both strains represent two new Faecalibacterium species, for which the names Faecalibacterium butyricigenerans sp. nov. (type strain AF52-21^T = CGMCC 1.5206^T = DSM 103434^T) and Faecalibacterium longum sp. nov. (type strain CM04-06^T = CGMCC 1.5208^T = DSM 103432^T) are proposed.