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Table 1 Criteria that may be acceptable* for combining different probiotic strains into the same ‘class of intervention’ for a specific outcome

From: A review of the systematic review process and its applicability for use in evaluating evidence for health claims on probiotic foods in the European Union

Rationale Probiotic formulation defining the intervention class Example Comment
Common identity Single strain of one specific genus and specie L. acidophilus strain A All studies include the same strain; may include studies conducted in different food matrices
Common identity Single defined blend of multiple strains (strains A + B + C) L. acidophilus strain A and L. casei strain B and B. lactis strain C Strains must be maintained in equivalent relative doses in all studies
Common taxonomy Studies used different strains of same species or subspecies? L. acidophilus strain A, L. acidophilus strain B, or L. acidophilus strain C Different studies using different strains included
Common structural or secreted product (e.g., beta-glycosidases, exopolysaccharides) Different strains from a defined taxonomic group that may include different species or genera All strains of L. bulgaricus and S. thermophilus Different studies using different strains included
Common mechanism of action known to be necessary and sufficient for the effect (e.g., production of a specific bacteriocin or range of bacteriocins known to be active against a specific pathogen, or induction of immune mechanisms needed for the effect) Different strains from a defined taxonomic group that may include different species or genera L. salivarius strain A Different studies using different strains included
L. salivarius strain B
Common physiological effect previously proven by at least one human study of quality Different strains from a defined taxonomic group that may include different species or genera All strains Different studies using different strains included
  1. *To the extent that commonalities cannot be defined for different probiotic strains, studies on them should not be pooled into a meta-analysis. For example, probiotics such as Saccharomyces boulardii, Bacillus coagulans and Bifidobacterium species likely are too different biologically and mechanistically from each other to form one class of intervention. Additionally, even if taxonomically similar, if different strains have different mechanisms responsible for an effect, it may be inappropriate to pool studies. For example, a meta-analysis on anti-infective actions of probiotics likely should not pool data from a Lactobacillus rhamnosus strain where effects are known to be due exclusively through immune enhancing activity in the small intestine and a Lactobacillus salivarius strain where effects are known to be due primarily through bacteriocin production in the colon.
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