In 2018, Nouioui et al. transferred Bifidobacterium gallinarum and Bifidobacterium saeculare to Bifidobacterium pullorum as B. pullorum subsp. gallinarum and B. pullorum subsp. saeculare on the basis of digital DNA-DNA hybridization (dDDH) values. These two new subspecies were validated in the same year. However, we found that the genome (GenBank/ENA/DDBJ accession number JGZJ01000000) of B. pullorum used by Nouioui et al. in the dDDH analysis cannot represent B. pullorum. So, the taxonomic relationship between B. gallinarum, B. saeculare and B. pullorum should be re-examined. B. pullorum DSM 20433T had 88.7-89.0 % average nucleotide identity (ANI) values and 37.5-38.0 % dDDH values to the type strains of B. gallinarum and B. saeculare, respectively, less than the threshold for species demarcation, confirming that B. pullorum represents a different species from B. gallinarum and B. saeculare. The ANI values and dDDH values between the type strains of B. gallinarum and B. saeculare were 96.7-96.9 % and 73.0-73.3 %, respectively, greater than the threshold for species demarcation, confirming that they represent the same species. Relatively low dDDH values (less than the 79-80 % threshold for subspecies demarcation) between the type strains of B. gallinarum and B. saeculare indicated that B. saeculare can be considered as a subspecies of B. gallinarum. On the basis of the results presented here, (i) B. gallinarum and B. saeculare should not be transferred to B. pullorum; (ii) we propose B. saeculare Biavati et al. 1992 as a later heterotypic synonym of B. gallinarum Watabe et al. 1983 and as a new subspecies of B. gallinarum, for which the name B. gallinarum subsp. saeculare subsp. nov. is proposed.
Keywords: Bifidobacterium; genome; reclassification.