Strains of the same probiotic species can be different, which has been demonstrated both in vitro and in animals, although similar data in humans are rare. Thus, clinical results from one study are applicable only to the strain or strains being evaluated in that study. Given this, however, different strains may have the same effects, and similar immune effects have been documented for different strains [14]. As research continues, certain species- or genus-specific attributes are likely to be found. Identification of genes or gene systems may make it feasible to predict in vivo function, and documentation of expression of these genes may become adequate substantiation for in vivo function. Functional genomics may greatly aid this research, which is progressing at a rapid pace [15]. Currently, however, research specific to the strain or strain combinations should be used to substantiate claims of physiological benefits. Furthermore, the genus, species, and strain designations should be specified for all probiotic strains in a product.
Dose levels of probiotics should be based on levels found to be efficacious in human studies. One dose level cannot be assumed to be effective for all strains. For example, the efficacy of Bifidobacterium infantis 35264 has been documented at 10 8 cfu/day [16], whereas the recommended dose of VSL#3 (VSL Pharmaceuticals) is (1.8×1012) cfu/day, a 4-log cycle difference [17].
The impact of product format on probiotic function has yet to be explored in depth. The common quality-control parameter of colony-forming units per gram may not be the only parameter indicative of the efficacy of the final product. Other factors, such as probiotic growth during product manufacture, enteric coating, preservation technology, metabolic state of the probiotic, and the presence of other functional ingredients in the final product, may play a role in the effectiveness of a product. More research is needed to understand how much influence such factors have on in vivo efficacy.