When the World Health Organization (WHO) issued its “Guidelines for the Evaluation of Probiotics in Food,” it identified additional criteria for probiotics, starting with the requirement that probiotic bacteria display documented benefits not merely at the genus or species levels, but at the level of the strain. Strain specificity is a particularly important consideration for probiotics; different strains of even the same species of bacteria may have different probiotic functions.

The scientific rationale that effects must be considered strain-specific is based mostly on in vitro and animal data where strain differences are evident. Attributes such as acid tolerance, sensitivity to therapeutic antibiotics, bile resistance, lactase activity, hydrogen-peroxide production, growth on prebiotics, genetic accessibility, production of antimicrobial compounds and stability in product have all been tested for a variety of strains in vitro.^1,2,3,4,5,6 Among tested strains, differences are clear. Frequently such testing compares strains of different species, but less commonly comparison of multiple strains of the same species has been conducted in in vitro tests.

In animal models, differences in responses evoked in tests of immune function are apparent. When one strain of each of Lactobacillus salivarius Ls-33 and Lactobacillus rhamnosus were tested via oral administration in a mouse model of colitis, researchers observed significant reduction in inflammation. However, one strain each of Lactobacillus acidophilus, Lactococcus lactis and Streptococcus gordonii showed no improvement.⁷ The importance of testing specific strains for effects is further emphasised in a study that documented that a strain of L. paracasei isolated from an endocarditis patient actually worsened colitis in an animal model of severe inflammation.⁸

It is possible to visualize differences among strains of the same species at the DNA level as well. The chart below illustrates such differences among several strains of L. crispatus. Such results are typical among strains of lactobacillus species. Interestingly, findings with commercial bifidobacterium strains suggest more genetic similarity among strains of the same species.

In one recent assessment, researchers found that among 39 independent isolates of B. animalis subspecies lactis strains from commercial products, only four different types were identified, based on pulsed field gel electrophoresis (PFGE).⁹ This finding may reflect a de facto greater similarity among strains of the same bifidobacterium species, that commercial products contain the same strains, or that differences are not evident from this type of chromosomal analysis. Interestingly, large strain-specific differences in immunopotential were observed among different strains of the same bifidobacterium species.¹⁰

Head-to-head comparisons of different strains in human studies are rare. For example, B. lactis BB-12 was compared to L. reuteri SD2112 and to a placebo in a study determining the impact of supplementing infant formula with one of these strains at identical doses on incidence, symptoms and absences due to intestinal or respiratory infections in infants in day-care centers.¹¹ Both strains showed statistically significant improvements over the placebo control; however, the L. reuteri group outperformed both BB-12 and the placebo control, with significant decreases in number of days with fever, clinic visits, child-care absences and antibiotic prescriptions. The rate and duration of respiratory illnesses did not differ significantly among groups.

In another clinical study, B. infantis 35624 was compared to L. salivarius UCC4331 for its ability to reduce symptoms of irritable bowel syndrome.¹² B. infantis 35624 improved symptoms, whereas L. salivarius UCC4331 did not.
What these studies do not tell us is if two strains of the same species would have performed equivalently. However, it cannot be presumed that they will.