The Numbers Myth

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The Myth:
"The more the strains, the merrier."

Research says:
It depends on the condition. The most researched probiotic for diarrhoea is Saccharomyces boulardii - this is usually researched as a single strain.


Due to the already diverse nature of the gut microflora, you might assume that a probiotic containing many different strains is your best option. However, existing research suggests that the opposite is true.

The research - in-depth

Clinical trials testing either single- or two-strain formulas have demonstrated excellent health benefits. For instance, S. boulardii is the most researched probiotic for diarrhoea in adults1, and it is a single strain. Furthermore, B. lactis BB-12® is the most documented of all bifidobacteria strains, and clinical trials show it is particularly helpful for symptoms of constipation2. Again, this is typically researched as a single strain!

Sometimes pairs of strains appear to work well together if they have similar properties. The two strain combination of L. rhamnosus Rosell-11 and L. acidophilus Rosell-52 has been shown in multiple clinical trials to reduce antibiotic-associated diarrhoea3. Similarly, L. reuteri RC-14® and L. rhamnosus GR-1® are usually used together and have been shown clinically to have a significant role in the management of thrush4, cystitis5 and bacterial vaginosis6.

Single-strain versus multi-strain – which is better?

We can see from the above research therefore that, contrary to popular belief, having more strains in a probiotic does not necessarily guarantee a better outcome. The efficacy of broad-spectrum multi-strain probiotics is something which requires further research, as their effects can depend on other factors, such as the health conditions you're trying to support or the age of the person.

Even if you are not trying to support specific health conditions and are just looking to provide general support for the microbiome, it may not be necessary to use multi-strain supplements.  A good quality probiotic strain should work in synergy with our body by creating a favourable environment for the indigenous bacteria to flourish7. So although a probiotic supplement might not contain many different bacteria, if it is performing well it will naturally enhance the diversity of bacteria in the body. But if you wish to target a particular health priority, then many people look for supplements containing strains which have been researched for their specific condition.

What is strain specificity and how does it work?

Strain specificity involves choosing probiotic strains because of their particular mechanism of action in the body. Research is showing us that certain strains can support specific conditions, as well as encouraging other beneficial strains to flourish in our gut gardens. So a good probiotic will not only top our levels of friendly bacteria, but encourage other species and strains to proliferate. This in turn helps us to foster and maintain a healthy gut.

Further research

One study8 that demonstrates this  effect involved just two strains used in supporting the health of women’s intimate flora. Women with abnormal vaginal flora containing a high number of pathogenic (undesirable) bacteria are at a higher risk of developing symptomatic infections in the vagina or bladder.

In a study involving 64 healthy women, with no occurrences of genitourinary infections over the preceding 12 months, participants were randomly given oral capsules of Lactobacillus rhamnosus GR-1® and Lactobacillus fermentum RC-14® , or a placebo, once daily for 60 days. They were analysed to determine alterations in pathogenic load and overall changes in vaginal bacteria profiles.

Results from the study showed an overall increase in the lactobacilli and a decrease in yeast counts compared with the placebo. By day 28, lactobacilli populations were detected in more women in the lactobacilli-treated group than in the placebo group, and levels of pathogenic E. coli, a common cause of vaginal infections, were decreased.

Key takeaways from this study

1. The two-strain probiotic increased levels of friendly lactobacilli and decreased levels of potentially pathogenic yeasts, and bacterial strains such as E. coli, within a period of 4 weeks, thus improving the overall diversity of the microbiome.

2. These two strains demonstrate a therapeutic action in their ability to maintain a normal vaginal microbiome, and reduce bacterial vaginosis and candidiasis.

3. Strain specificity is key here, because strains within the same species may have different properties. For example,  Lactobacillus rhamnosus GR-1®  typically prefers to live in the vaginal microbiome, whereas L. rhamnosus GG® prefers to live in the gut and does not appear to colonise the vagina.

In summary

Using a multistrain probiotic may not be the best way of supporting your gut health. Strain specificity provides a more targeted and therapeutic approach to improving gut diversity and integrity. By selecting well researched probiotics with few or individual strains, you can encourage other species of good bacteria to flourish. Adding more strains to the mix does not necessarily provide better results for all diseases, and more research is needed to explore the interaction between strains. Microorganisms are so delicate that it’s quite a challenge to ensure they are compatible alongside each other – if a supplement contains a large number of strains, survival becomes an even bigger concern10. This is why using a supplement with fewer strains, which are known to work well in combination, and which have been researched for the health conditions you want to support, can provide a more personalised approach to improving your microbiome.

Look out for...

The most researched probiotic for your health concern – even if it contains fewer strains.

Further reading

Health professionals can read more on B. lactis BB-12® and What is Saccharomyces boulardii?

This myth has been busted by Megan Crowch, BSc (Hons) Physiology, Herbal Medicine Diploma (IRH practicing member); and Katie Wheaton, Dip NT, mBANT CNHC.


  1. McFarland (2010) Systematic review and meta-analysis of Saccharomyces boulardii in adult patients. World J Gastroenterol; 16, 18: 2202-22
  2. Eskesen et al. (2015) Effect of the probiotic strain Bifidobacterium animalis subsp. lactis, BB-12®, on defecation frequency in healthy subjects with low defecation frequency and abdominal discomfort: a randomised, double-blind, placebo-controlled, parallel-group trial. Br J Nutr; 114, 10: 1638-46.
  3. Foster et al. (2011) A comprehensive post-market review of studies on a probiotic product containing Lactobacillus helveticus R0052 and Lactobacillus rhamnosus R0011. Benef Microbes; 2, 4: 319-34.
  4. Martinez et al. (2009) Improved treatment of vulvovaginal candidiasis with fluconazole plus probiotic Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14. Lett Appl Microbiol; 48, 3: 269-74.
  5. Beerepoot et al. (2012) Lactobacilli vs antibiotics to prevent urinary tract infections: a randomized, double-blind, noninferiority trial in postmenopausal women. Arch Intern Med; 172, 9: 704-12.
  6. Anukam et al. (2006) Augmentation of antimicrobial metronidazole therapy of bacterial vaginosis with oral probiotic Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14: randomized, doubleblind, placebo controlled trial. Microbes Infect; 8, 6: 1450-4.
  7. Studies have demonstrated that even when taking specific strains in a probiotic supplement the general levels of Bifidobacteria & Lactobacilli in the gut can increase e.g. Laake et al. (1999) Influence of fermented milk on clinical state, fecal bacterial counts and biochemical characteristics in patients with ileal- pouch- anal-anastomosis. Microbial Ecology in Health and Disease, 11: 211-217.
  8. Gregor, R et al (2003) Oral use of Lactobacillus rhamnosus GR-1 and L. fermentum RC-14 significantly alters vaginal flora: randomized, placebo-controlled trial in 64 healthy women. FEMS Immunology and Medical Microbiology 35. pp. 131-134.
  9. Kontiokari, T et al, (2001) Randomised trial of cranberry-lingonberry juice and Lactobacillus GG drink for the prevention of urinary tract infections in women. Br. Med. J. 522; pp. 1-4.
  10. Training event from Chr. Hansen representative on latest research development (2014) Andover, UK.

You might also be interested in: 

The Research Myth 
The Antibiotic Myth 
The Strain Myth 
The Survival Myth 
The Food Myth 
The Cure-All Myth 
The Billions Myth 
The Fridge Myth