Why is strain specificity so important?

Different probiotic products contain different probiotic strains. This means that one probiotic product with Lactobacillus reuteri, for example L. reuteri NCIMB 30242, is not equal to another probiotic product that contains another strain of Lactobacillus reuteri, for example L. reuteri DSM 17938. This is why bacteria must be defined and tested in clinical trials at strain level.