Why is it important to define a probiotic bacteria on strain level?

Bacteria are classified into genus, species and strains. Probiotic strains of the same species have different characteristics, and may therefore have different health effects. For example, clinical studies with the two strains, Lactobacillus reuteri ATCC PTA 6475 and Lactobacillus reuteri DSM 17938, have shown different effects in clinical studies. That is why a probiotic product must be defined on strain level.