The tendency to generalise about 'probiotic' effects is widespread. Underlying such generalisations is the erroneous assumption that the body of research on specific probiotic strains can be applied to any product marketed as a probiotic. Mary Ellen Sanders, PhD, explains why not just any probiotic will do for your next product launch
Probiotics are live microbes that, when administered in adequate amounts, confer a health benefit on the host.1 There is mounting awareness in the United States about probiotics, spurred in part by a growing number of high-profile probiotic-containing products, such as Dannon's successful Activa yoghurt. The potential of this product category seems evident, especially when considering the success of such products in Europe and Asia. One report estimated that more than 100 companies in the United States market probiotic supplements and approximately two million US adults use them.2
Furthermore, there are many people who stand to benefit from efficacious probiotic products. Studies documenting effects on a variety of gastrointestinal disorders including irritable bowel syndrome (IBS), vaginal infections and immune enhancement resulting in getting sick less often are compelling reasons for including probiotics as part of a healthy diet.3,4,5
When considering this product area, a few guidelines can help provide context for understanding the complexities of the science and use of probiotics.
Although a consensus scientific definition has been advanced, no legal definition of the term 'probiotic' exists. The term unfortunately can be (and is) used on products that do not meet the minimum criteria that the probiotic be alive, delivered in adequate dose (through the end of shelf life); and shown to be efficacious in controlled human studies.6 The 'truthful and not misleading' FDA standard for content and support of structure/function label claims on products is not, in practice, enforced by the FDA. Therefore, it is incumbent on the industry to maintain integrity in formulation and labelling of these products so that consumers can be confident in this product category.
There is little scientific evidence that in order to qualify as probiotics, these beneficial bacteria must demonstrate specific physiological attributes such as be of human origin, adhere to intestinal cells or produce bacteriocins. Although many have used these criteria as a basis for selection of strains 'appropriate' for use as probiotics, no studies have compared isogenic strains (ie, strains that are identical genetically except with altered capacity for one specific attribute) with and without these traits in humans to determine their significance. Until such studies are conducted, it is more productive to focus on proof of their ability to improve human health, regardless of mechanism. Notably, one research group did conduct a study of colitis in mice comparing physiological effects of isogenic strains of L. crispatus which differed only in their ability to auto-aggregate (or clump).7 They found that the aggregating strain, but not the non-aggregating mutant or heat-killed aggregating strain, reduced severity of colitis.
Another attribute that is sometimes questioned is the requirement that probiotics be alive. It is true that some research shows beneficial effects from cells killed by heat or radiation.8 By definition, however, these substances do not qualify as bona fide probiotics. Studies have demonstrated superior activity of live compared to killed probiotics, in in vitro and in human studies.9,10,11 Even though not all studies show an advantage to viability;12,13 probiotics by definition must be administered alive.
There is one particularly important consideration for probiotics: strain-dependent effects. Just as different breeds of dogs have attributes that are distinguishing, so too 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.14,15,16,17,18,19 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.20 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.21
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.
Strain-specifics: human data
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.24 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.25B. 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.
The implications of the strain-specificity of effects are:
- Documentation of health effects must be conducted on the specific strain being sold.
- Review articles that discuss the many studies done on specific strains are not sufficient evidence to support health effects of an untested strain.
- Studies that document efficacy of specific strains at a specific dose are not sufficient evidence to support health effects at a lower dose (see 'Formulators' sidebar, below).
- The role of carrier in delivering functional benefits is not well understood (see 'R&D' sidebar, below).
This issue is complicated by the fact that the mechanisms that lead to specific health effects are often not known. When these are better understood, it may be possible to predict functionality in vivo. Certainly there are some physiological characteristics that are present in essentially all strains of given species. Such physiological similarities contribute to their grouping into the same species. For example: reuterin production by L. reuteri, although levels produced vary by strain;26 high in vivo lactase activity in strains of S. thermophilus;27 lactate production by all lactobacilli and acetate production by bifidobacteria. However, how such physiological or metabolic characteristics are expressed in vivo among different strains drives the need to confirm functionality in the target host.
Taken from a different perspective, the current body of published literature suggests that similar effects are observed for a variety of different strains. Effects on diarrhoeal illnesses, enhanced immune responses and improvement of symptoms of lactose intolerance are associated with more than one strain. Therefore, multiple strains of the same species may in fact have functional traits in common. But they may not. So again, studies on the specific strains are still needed.
The tendency to generalise about 'probiotic' effects is widespread. Underlying such generalisations is the erroneous assumption that the body of research on specific probiotic strains can be applied to any product marketed as a probiotic. Although it is cumbersome to always need to define what strains at what doses are known to lead to health effects, it is essential to do so to prevent misrepresentation of a product.
In addition, commercial products that are marketed with no specific human studies documenting effects should not be marketed as probiotics, but perhaps as 'potentially beneficial cultures.' The term 'probiotic' should be used only for products composed of microbes that are alive, delivered in adequate dose (through the end of shelf life), and shown to be efficacious in controlled human studies (see box, below). As mechanisms leading to probiotic effects are better understood, perhaps extrapolation of results from certain strains to strains possessing the pertinent biological and physiological traits will be possible. Certainly, genomic sequencing efforts will grease the wheels for such advances.28
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21. Daniel C, et al. Selecting lactic acid bacteria for their safety and functionality by use of a mouse colitis model. Appl Environ Microbiol 2006 Sep;72(9):5799-805.
22. Masco L, et al. Culture-dependent and culture-independent qualitative analysis of probiotic products claimed to contain bifidobacteria. Int J Food Microbiol. 2005 Jul 15;102(2):221-30.
23. Pot B. Personal communication.
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27. Guarner F, et al. Should yoghurt cultures be considered probiotic? Br J Nutr. 2005 Jun;93(6):783-6.
28. Klaenhammer TR, et al. Genomic features of lactic acid bacteria effecting bioprocessing and health. FEMS Microbiol Rev. 2005 Aug;29(3):393-409.)