Biochemical properties and DNA-DNA reassociation studies of Lactobacillus acidophilus strains isolated from humans and animals indicate that these include six genomospecies. Two new species can be differentiated from the established species of the genus Lactobacillus: L. gallinarum sp. nov. (type strain, ATCC 33199) and L. johnsonii sp. nov. (type strain, ATCC 33200). Furthermore, it was clarified that L. acidophilus group A3 (Johnson et al. 1980) is synonymous with L. amylovorus.

Eleven strains of a hitherto undescribed Gram-positive, catalase-negative, facultatively anaerobic rod-shaped bacterium from human sources and medical care products were characterized by phenotypic and molecular taxonomic methods. The phenotypic properties of the bacterium were consistent with its assignment to the genus Lactobacillus but it was readily distinguished from all currently described species of this genus by its biochemical characteristics and by SDS-PAGE analysis of its cellular proteins. Comparative 16S rRNA gene sequence analysis demonstrated that the unknown bacterium was a member of rRNA group I Lactobacillus which includes Lactobacillus delbrueckii, the type species of the genus, and close relatives. Lactobacillus gasseri and Lactobacillus johnsonii were the nearest phylogenetic relatives of the unknown bacterium, but 16S rRNA sequence divergence values of > 4% clearly showed that it represents a distinct species. Based on both phylogenetic and phenotypic evidence, it is proposed that the unknown bacterium should be classified in the genus Lactobacillus, as Lactobacillus iners sp. nov. The type strain of Lactobacillus iners is CCUG 28746T.

To obtain porcine isolates related to Lactobacillus amylovorus, we screened strains from piglet intestine grown on Lactobacillus-specific MRS agar for hybridization to a fluorescent 16S rRNA-targeted DNA probe. Six strains were isolated and further characterized by phenotypic and molecular taxonomic methods. The isolates were Gram-positive, catalase-negative, facultatively anaerobic rods. They had similar phenotypic characteristics and displayed genomic DNA-DNA relatedness values of >78 % to each other, indicating that they belong to a single species. Comparative 16S rRNA gene sequence analysis demonstrated that the novel isolates were members of Lactobacillus rRNA group I, which includes Lactobacillus delbrueckii, the type species of the genus. Based on 16S rRNA gene sequence similarity, Lactobacillus kitasatonis (99 %), Lactobacillus crispatus (98 %) and Lactobacillus amylovorus (97 %) were the nearest relatives of the novel isolates, but their DNA-DNA relatedness was found to be lower than 49 %. One of the isolates, strain OTU171-001T, was further characterized using physiological and biochemical tests. Together, the results enabled genotypic and phenotypic differentiation of strain OTU171-001T from the other species that showed 16S rRNA gene sequence similarity values greater than 97 %. Strain OTU171-001T merits species status and the name Lactobacillus sobrius sp. nov. is proposed. The type strain is OTU171-001T (= DSM 16698T = NCCB 100067T).

Lactobacillus sobrius sp. nov., which was recently isolated from the intestine of weaning piglets, has potential probiotic properties. To follow the fate of L. sobrius strain 001T in dietary interventions, a novel and strain-specific quantitative detection procedure was developed. This procedure was based on the isolation of specific genomic fragments from the type strain by representational difference analysis and their detection by real-time PCR. The described strain-specific quantification approach may be used in studies aimed at tracking bacterial strains added to specific environments.

While studying the taxonomy of six lactic acid bacterium isolates from Finnish porcine intestine and faeces, the taxonomic positions of Lactobacillus sobrius type strain DSM 16698T and strain AD5 based on comparative 16S rRNA sequence analysis were found to be controversial, as they showed high similarity to Lactobacillus amylovorus strains. Therefore, the taxonomy of these species was addressed in a polyphasic taxonomy study that included, in addition to re-evaluating the 16S rRNA gene sequence and DNA-DNA reassociation results, multilocus sequence analysis (MLSA) of the housekeeping genes encoding the phenylalanyl-tRNA synthase alpha subunit (pheS) and RNA polymerase alpha subunit (rpoA) as well as numerical analysis of HindIII and EcoRI ribotypes. 16S rRNA gene sequence analysis demonstrated a very high similarity between the L. sobrius and L. amylovorus type and reference strains and representative Finnish porcine isolates (99.6-99.9 %). The MLSA data showed the close phylogenetic relationship of these strains; pheS and rpoA gene sequence similarities were 98.5-100 % and 99.6-99.8 %, respectively. Numerical analyses of HindIII/EcoRI ribotypes placed these strains in a single cluster by both enzymes. Finally, the DNA-DNA reassociation experiments revealed high reassociation levels (higher than 79 %) between the strains. These results indicate that DSM 16698T, AD5 and the related porcine lactobacilli strains from Finland constitute a single species, Lactobacillus amylovorus, and that the name Lactobacillus sobrius should be considered as a later synonym of Lactobacillus amylovorus.