The life history of Lactobacillus acidophilus as a probiotic: a tale of revisionary taxonomy, misidentification and commercial success.

Lactobacillus acidophilus is a commercially significant bacterial probiotic, originally isolated from the human gastrointestinal tract and designated Bacillus acidophilus in 1900. Throughout the development of methods to identify and characterise bacteria, L. acidophilus has undergone multiple taxonomic revisions and is now the type species of a phylogenetic subgroup in the highly diverse and heterogeneous Lactobacillus genus. As a result of the limitations of differentiating phenotypically similar species by morphological and biochemical means and revisionary nature of Lactobacillus taxonomy, the characterisation of L. acidophilus has struggled with misidentification and misrepresentation. In contrast, due to its global use as a probiotic supplement in functional foods, L. acidophilus sensu stricto is now one of the most well-characterised Lactobacillus species. Here, we establish the provenance of L. acidophilus strains, unpicking historical and current misidentifications of L. acidophilus, and reviewing the probiotic, genomic and physiological characteristics of this important Lactobacillus species.