by Ilia Gelfat
figures by Nicholas Lue

Whether you are perusing the aisles of your local grocery store or scrolling through news articles, there’s a term that has been hard to miss in the past few years – probiotics. You might know it has something to do with bacteria helping digestion, but this broad view of the concept might leave you fuzzy on some of the specifics. Who are probiotics intended for? Do they actually do anything? And what exactly are they? If you have been pondering these questions, you are in good company – they have been the subject of much research and debate within the scientific community.

On definitions and discoveries

To understand probiotics, we must first be aware of a fact that may seem surprising: our bodies are home to trillions of microbes – tiny living entities, such as bacteria and fungi, too small to see with the naked eye. This collection of microbes that inhabit our bodies, from the surface of our skin to the depths of our digestive tracts, is called the human microbiome. Thanks to discoveries made over the past few decades, it is now clear that our microscopic companions are not merely passive hitchhikers, but instead have significant roles in digestion and protection from various diseases.

In light of these findings, researchers began to form a theory: since our microbiome is so intimately linked to our health, perhaps consuming the right kind of microbes could result in some positive, healthful effects. The initial candidates for such microbes were the bacteria found in fermented foods like yogurt, since consuming these foods had long been speculated to promote intestinal health. The term “probiotics” has emerged as a name for these hypothetically beneficial bacteria. In the past decade or so, as the idea became more widespread, the probiotic label was added to an ever-increasing number of products – pills, pet food, lotions, and even mattresses. Consumers became a bit confused, and rightfully so, as bombastic claims outpaced the science (Figure 1).

Figure 1. What are probiotics? The definition of probiotics was unclear at first, with many products, from yogurt to mattresses, being labelled as containing these healthful microbes.

Various authorities subsequently stepped in to help us all make sense of the mess. In 2001, the World Health Organization (WHO) and the Food and Agriculture Organization of the United Nations (FAO) came up with a definition of probiotics: “live microorganisms which, when administered in adequate amounts, confer a health benefit on the host.” Put simply: if it has living microbes in it and promotes your health in some way, you can call it a probiotic.

This definition has since been widely accepted and has helped guide policy makers and companies alike. Regulations vary from country to country, but most probiotic products contain bacterial strains commonly used in the food industry, which have a good track record of safety. In the US, probiotics manufacturers generally avoid making any assertions regarding treatment or mitigation of disease, as this would subject them to the stringent regulatory requirements of a drug. Instead, probiotics are regulated as foods and dietary supplements, which allows companies to claim their products benefit the digestion and nutrition of healthy individuals without carrying out experiments to verify these statements. Testing such claims, as well as exploring potential medical applications of probiotics, is left to the scientific community.

So now that we know what it is – does it actually work?

Unfortunately, testing the health benefits of probiotics is not so simple. Scientists have made a wide range of hypotheses regarding the potential effects of probiotics, researching their utility in ameliorating anything from allergies to cancer. To further complicate matters, there are several varieties of probiotic bacteria, spanning multiple species and numerous types within each species (also known as strains), with some products containing a mix of strains. Currently, there are over a dozen bacterial strains (and various combinations thereof) in commercially available probiotics. Given this, what if the various types of bacteria in these products do completely different things, or behave differently when mixed in with others? If so, is there even a meaning to the term “probiotic,” which lumps all of them into one category?

On October 23rd, 2013, a panel of experts convened to answer these questions. The panel was mostly comprised of doctors and researchers specializing in microbiology, intestinal health, family medicine, and food science, including some of the original members of the FAO/WHO 2001 committee. After deliberating and considering the vast body of research carried out in the intervening years, the experts reached their conclusions – the idea of probiotics is valid and is largely supported by experimental evidence.

One example of such supporting evidence comes from a 2012 review by the non-profit organization Cochrane, which examined whether probiotics were an effective treatment for persistent diarrhea in children. Looking at results from four separate studies, with a total number of 464 patients, the researchers concluded that probiotics reduced the duration of disease by about 4 days on average. Similar reviews also suggested probiotics could reduce the risk of a type of severe gut inflammation called necrotizing colitis in preterm infants, and of Clostridium difficile-associated diarrhea. The National Institute of Health (NIH) has also published some useful information and guidelines about probiotics (though there is no substitute for consulting your doctor).

Furthermore, despite the multitude of strains and formulations, the panel agreed that the term “probiotics” is a reasonable way to describe this diverse group of microbes. There seems to be a common core of health benefits associated with consuming these bacteria, which includes making it harder for harmful microbes to take root and restoring the normal microbial community after disease. Other effects exist, but they are not as ubiquitous and depend more on the kind of bacteria in question (Figure 2).

Figure 2. Benefits of probiotics. Shown here are examples of some of the common benefits of probiotics, as well as those associated with a specific strain, as described by the 2013 International Scientific Association for Probiotics and Prebiotics panel.

A shadow of doubt

While it seems like the big picture is largely agreed upon, it is important to remember that there are many details left to elucidate. The world is full of people with different genetics, lifestyles, and microbiomes, and it will be important to determine whether probiotics can be a safe and effective tool to ameliorate disease in each individual case. Some recent studies have been casting doubt on the ability of probiotics to accomplish this task.

For example, in a study published earlier this year, scientists at the Weizmann Institute in Israel examined the intestinal microbes of volunteers treated with antibiotics. After killing off some of the native microbiome with antibiotic treatment, some of the subjects were given a probiotic pill containing 11 common microbial strains, while others were given nothing. A third group was given an autologous fecal microbiome transplant (aFMT) – essentially a dose of the microbes from their own poop, collected before taking any antibiotics. The group that did not receive anything saw their microbial community restored to its initial composition after 21 days, and the aFMT group recovered after as little as 24 hours. Surprisingly, those treated with probiotics took the longest to return to their original state – some patients took as long as five months.

So how should we think about these findings? Should we reach for some supplements the next time our stomach rumbles, or perhaps avoid probiotics altogether? It is important to keep in mind that all studies have their limitations. In this case, the trials were quite small – 21 subjects in total – which means they are not necessarily representative of the general population. Furthermore, the study consisted of healthy adult volunteers. It is quite possible that the results would have been different in other populations – children, the elderly, or people suffering from a specific intestinal disorder. Perhaps an alternate probiotic formulation could have changed the outcomes as well. Even though it might leave us more confused than before, studies such as this are extremely important – they expose flaws in our current theories and prompt us to improve and refine them. Overall, better understanding will emerge over time, with the accumulation of knowledge gleaned from many experiments and observations.


As we have seen, there is a lot to talk about when it comes to probiotics. They are new enough to get us interested, promising enough to get us excited, and complex enough to make us confused. In short – they are yet another emerging trend in health and science. But we will continue to hone our understanding of how probiotics work, and in the end, science will be able to tell us when a probiotic is a life-changing medical treatment and when it’s just overpriced yogurt.

Ilia Gelfat is a 3rd year Bioengineering PhD student in the John A. Paulson School of Engineering and Applied Sciences at Harvard University.

Nicholas Lue is a graduate student in the Chemical Biology PhD program at Harvard University. You can find him on Twitter as @nicklue8.

For more information:

  • For more background on probiotics and their usage, see these NIH guidelines.
  • Check out this recent SITN post on the surprising ability of one probiotic to prevent Staph infection.
  • To learn about another interesting aspect of our microbiome, check out this SITN post on bacteria-killing viruses.
  • For more on the effects of the gut microbiome on the immune system and cancer treatment, see this SITN article.

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