Species identification of genus Bifidobacterium based on partial HSP60 gene sequences and proposal of Bifidobacterium thermacidophilum subsp. porcinum subsp. nov.

Sequence homology of partial 60 kDa heat-shock protein (HSP60) genes was analysed for 50 Bifidobacterium strains that represent 12 Bifidobacterium species and subspecies with validly published names. Sequence similarities were 96.5-100 % within the same species, 95.5-97 % at the subspecies level and 80-96 % (mean, 88 %) at the interspecies level among the 10 Bifidobacterium species. Hence, the HSP60 gene was a more accurate tool for species identification within the genus Bifidobacterium than 16S rDNA. Two new Bifidobacterium strains isolated from piglet faeces were shown to be closely related to the thermophilic bifidobacterial group, based on 16S rDNA sequence analysis: strain P3-14(T) (=AS 1.3009(T)=LMG 21689(T)) exhibited 97.9 % similarity to Bifidobacterium boum JCM 1211(T), 97.2 % similarity to Bifidobacterium thermacidophilum AS 1.2282(T) and 97 % similarity to Bifidobacterium thermophilum JCM 1207(T). However, higher levels of DNA-DNA relatedness (83 %) and HSP60 gene sequence similarity (97 %) were determined between B. thermacidophilum AS 1.2282(T) and strain P3-14(T), indicating a closer relationship between them. The new strains differed from B. thermacidophilum AS 1.2282(T) in some phenotypic characteristics, such as growth at a lower temperature (46.5 degrees C), as well as different sugar-fermentation patterns. Hence, a novel Bifidobacterium subspecies, Bifidobacterium thermacidophilum subsp. porcinum subsp. nov., is designated.