For health professionals, impartially curated by OptiBac
Probiotic supplements containing more than one strain of bacteria are commonly referred to as 'multi-strain' supplements, or sometimes as 'poly-strain' or 'polybiotics'. Generally speaking, a multi-strain probiotic can be a good option as an everyday supplement to support gut health. Most high-quality, multi-strain probiotics contain around 5 or 6 different strains, but why not have 10 or 15? In theory, you would think that the more strains a supplement has, the better.

S. boulardii has been well-researched for diarrhoea

Well, not necessarily. In practice, having 10-20 different strains in one supplement carries uncertainties. When it comes to probiotics, the question of survival is an important one, and when one product contains many strains, it is difficult to establish whether all of the strains will survive until the time of consumption.
If you are considering a probiotic supplement with a large number of different strains, make sure tests have been conducted to ensure the strains all survive together at the stated billions count until expiry.

What does the research say?

Dr Lynne McFarland from the University of Seattle has over 150 peer-reviews to her name and has concluded from current data that adding more strains doesn't always lead to better outcomes for all illnesses1. But, she also acknowledges that:

'...the benefits of multi-strain mixtures may include broader range of effects and more mechanisms of action…'
There are clearly pros and cons for single- and multi-strain probiotics – the former more stable, the latter has the potential to be more effective, dependent on the strains used and condition being targeted.

When dealing with Helicobacter pylori, for example, using a single strain leads to a 0.66 day reduction in diarrhoea, whilst a two-strain mix leads to a 0.41 day reduction. In this instance, the single strain was more effective. But, importantly, 'these are both significant reductions'.

Some of the most popular probiotic strains on the market only contain one strain. These are known as 'single-strain probiotics'. Saccharomyces boulardii on its own, for example, can be a great option for supporting people with IBS-D.

In fact, a study2 published in 2010 found that when treating diarrhoea, associated with rotavirus in children, S. boulardii reduced the duration of the virus and the duration of the fever more effectively than a mixed probiotic containing both S. boulardii along with Lactobacillus acidophilus, L. rhamnosus and B. longum strains. It is worth noting, however, that the multi-strain product was more effective at preventing vomiting associated with the virus.

Specific strains for specific conditions

Many probiotic supplements are formulated to target specific health conditions3. Each strain will be selected based on scientific studies and clinical trials, to ensure quality supplements tailored to specific needs. As such, individuals must find the right probiotic for them, as opposed to taking 'any old acidophilus'. Instead of asking 'Am I getting enough strains?', consider 'Am I taking the right strains for me?'. If you are interested you can read more about strain-specificity and its importance, over in the Probiotics Learning Lab.

Targeted probiotics for the vaginal microbiome

The homeostasis of the vaginal microbiome requires specific strains of bacteria to compete against pathogenic bacteria that can lead to common vaginal infections.

Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14 are two strains that can support a healthy vaginal microbiome. A study4 published in 2003 found that the two-strain probiotic increased levels of friendly lactobacilli and decreased levels of potentially pathogenic yeast strains within a period of 4 weeks.

Probiotics can be an excellent way to re-balance the vaginal microbiome

What did they find?

In this study 64 healthy women with no history of genitourinary infections over the last 12 months 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 gut 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 were detected in more women in the lactobacilli-treated group than in the placebo group, and E. coli levels were decreased.

Another study5 looking at the influence of two single strains on results of on genitourinary dysbiosis found that vaginal flora was normalised in patients suffering from vaginal infections following probiotic intervention.

The study looked at 250 women suffering from vaginal itching, burning, and itching, of which 125 took a probiotic containing Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14 alongside medication, and 125 took the medication only. After 6 weeks, inflammation had decreased, lactobacilli numbers had increased, and pathogenic microorganisms were lower in the group treated with probiotics.

What are the take home points from this study?

The two-strain probiotic containing Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14 was effective at reducing pathogenic load, modulating host immunity, and maintaining a normal healthy vaginal flora.

The probiotic increased levels of lactobacilli bacteria which are needed to maintain a healthy vaginal microbiome, and displaced less beneficial strains, thus reducing overall dysbiosis within the urinary tract. A good probiotic supplement will encourage other beneficial species and strains to thrive and influence the microbiome as a whole rather than just topping up or dominating with one or two strains.

Using specific strains can provide a more targeted and effective approach to gut health, as the synergy between multiple strains and the broader range of effects this may provide needs more research.

It's more important to have fewer high-quality strains that are specific to your condition, than lots of different strains

The nutritionist’s view

Strain-specific probiotic therapy can certainly provide benefits for specific conditions, but as you can see there are no universal rules - for some individuals taking a single-strain probiotic like Saccharomyces boulardii works well, whilst for others a multi-strain probiotic might be better. The quality of the probiotics strains used is by far the most important factor to consider when selecting the right supplement, and looking at the available clinical trials behind the strains will help you to select the right strain (or strains) for you.

For further reading on the topic, take a look at 'The Numbers Myth', over in the Probiotics Learning Lab.

This FAQ has been answered by Joanna Scott-Lutyens, BA (hons), DipION, Nutritional Therapist; and Katie Wheaton, Dip Nutritional Therapist, mBANT CNHC.

1. Scattergood, G. (2017). 'Multi-strain versus single strain probiotics: More doesn’t necessarily mean better outcomes'. Published online. Available at:
2. Grandy, G et al. (2010) Probiotics in the treatment of acute rotavirus diarrhoea. A randomized, double-blind, controlled trial using two different probiotic preparations in Bolivian children. BMC Infect Dis, 10: 253
3. Verna, E. and Lucak, S. (2010) 'Use of probiotics in gastrointestinal disorders: what to recommend?' Therap Adv Gastroenterol, 3(5): 307-319
4. Reid, G et al (2003) ‘Oral use of Lactobacillus rhamnosus GR-1 andL. fermentum RC-14 significantly alters vaginal flora: randomized, placebo-controlled trial in 64 healthy women.’ FEMS Immunology and Medical Microbiology 35. pp. 131-134
5. Perišić Ž, et al. (2011) ‘The influence of probiotics on the cervical malignancy diagnostics quality’, Vojnosanit Pregl. Vol 68(11 ): pp. 956–960.