What are Probiotics? (An Ode to the Microbiome)

by David on March 11, 2013

You’ve probably seen commercials with Jamie Lee Curtis extolling the virtues of some yogurt or another, obliquely referencing the joys of regular bowel movements – often the phrase “balance” is thrown around. (If you haven’t, then just accept that such commercials exist.) But it is never entirely clear what’s being “balanced,” nor is it evident what these “probiotics” are, that are apparently vital for your intestinal well-being. According to the World Health Organization (WHO), probiotics are “live microorganisms which when administered in adequate amounts confer a health benefit to the host” – essentially, they are “good” bacteria that you eat.  Why are you eating bacteria? It all has to do with your microbiome.

The last thing I want to think about when I’m eating yogurt is Mrs. Curtis’ regularity. (source: http://bit.ly/YVqbn2)

Of Microbiomes and Men

Although we tend to think of our bodies as, well, ours, the reality is that the human shapes that we think of as our own are covered – inside and out – with microorganisms. In fact, the number of microbes living on and in us outnumbers our own cells by ten to one! However, what human cells lack in number they make up for  in mass, and all of those stowaways only account for about three pounds of our body weights. This collection of microbial critters is called the microbiome, and it has become a hot topic in the last decade or so as researchers unravel the human health consequences of being covered in microbes.

After that last paragraph, you might wish to lie down in a vat of Purell. Engaging in such an activity is discouraged.

We often think of bacteria as our enemies, since we generally only notice them when they make us sick, but this couldn’t be further from the truth. For one thing, the vast majority of bacteria are harmless to humans, and many have important roles in the environment – rhizobia, for example, pull nitrogen out of the air and convert it to a form that plants can use, acting like a natural fertilizer. For another, the microbes that compose our microbiomes help us out in a (rapidly expanding) number of ways, from the digestion of food, to the regulation of our immune systems, and even protecting us from the bacteria that can make us sick. The more we learn about  our microbiomes, the more we understand that these little fellows aren’t just along for the ride, they are actively engaging with our bodies for the sake of mutual benefit – we give them food and somewhere to stay, they help keep the place up.

We are basically like bacteria condos

So what’s going on with probiotics? Well as I said before, the number of friendly or ambivalent bacteria far outnumbers the bad, and there are several thousand species living in the average healthy human gut, forming an extremely complex ecological web. When we take antibiotics, or go to a new place and consume food or drink containing foreign microbes, we change the balance of the intestinal ecosystem: some species languor while others flourish, and this alters the way that the microbiome interacts with our bodies. Sometimes this leaves us vulnerable to intestinal pathogens (as is sometimes the case with antibiotics), while other times it means that our ability to digest food is affected. The idea behind probiotics is to increase the numbers of the beneficial microbes (generally by eating them, although there are other ways that have gotten a lot of press lately) to help crowd out the pathogenic species – real estate in the intestine is tough to come by, and if a pathogen can’t find space, it may simply pass through.

Of course, we are only beginning to understand the complex relationships at work in the microbiome, so it’s tough to say which bacterial species are doing what, or how we might manipulate them for our benefit. At present, however, there is substantial evidence that probiotics are gaining notoriety for their efficacy in treating a variety of gastrointestinal ailments, from C. difficile disease, to ulcers, to IBS. And there is mounting evidence that the components of the microbiome are involved with obesity and chronic inflammatory diseases – it could even affect the aging process!

Yogurt is a fermented milk product, as is cheese; ergo, I will eat cheese until I live forever. Do not attempt to convince me otherwise.

As we gain a greater appreciation of the microbial communities living within us, we will no doubt change the way that we think about the treatment of diseases – and who knows? In the future, maybe we’ll treat all kinds of diseases with some kind of super yogurt? I just hope it’s got fruit on the bottom.


Dan Kegel March 11, 2013 at 9:56 am

Great post as far as it goes, but how could you leave out fecal transplants?


Lola March 11, 2013 at 11:16 am

He didn’t. He alludes and links to the “other ways” to get the beneficial microbes.

Thanks David! You know I love stool transplant talk.

David March 11, 2013 at 4:25 pm

Ah, well spotted, Lola!

Yeah, Dan, I would have been remiss not to at least reference fecal transplants, but I didn’t want to focus on them too much, since they’ve gotten so much media attention lately. It is a pretty interesting (and ancient!) technology.

Angela March 12, 2013 at 12:58 pm

Thank you for the post! Could you say a bit more about how yoghurts and other everyday probiotic products perform their advertised tasks (or not)? E.g. for maintaining or re-building gut bacterial ecologies?
Also found Dan’s link to fecal transplants fascinating. Found an appropriately graphic article in Slate magazine on it…
I once got some kind of hospital superbug ones that affected my entire digestive system. It seemed to be resistant to antibiotics, so I tried drinking lots of fennel tea and eating probiotic yoghurt, and it went away. Not sure which one did the trick, or whether it was just time…

David March 14, 2013 at 11:01 pm

Good question, Angela! My general sense of the available literature on commercial products – which is largely funded by purveyors of said products – is that there are indeed measurable benefits to bowel movement regularity and general gastrointestinal (GI) health, but such benefits are somewhat short-lived and minor. So you would have to be using the products consistently and in appropriate quantity to have any kind of sustaining effect.

Thing of it is, the probiotic products on the market really only offer a few species of gut bacteria, which, even if they are beneficial, only compose a small fraction of the thousands which form the gut microbiome. The microbiome is just as complicated as any ecosystem on earth, with different groups of producers, consumers, and predators, all forming a dense web of interactions, and once it is established, it is very difficult to alter. Also, environmental factors such as, what you eat, what you drink, and even what chemicals you absorb through your skin, will affect the environmental (i.e. your GI) conditions that these bacteria enter – thus affecting their ability to join the ecosystem at all. All of this is to say, it is difficult to intentionally make meaningful and lasting effects on the gut microbiome, based on what we know about it.

There is a growing body of literature propounding the efficacy of using probiotic approaches to combat GI health, so I wouldn’t be surprised if your yoghurt and fennel regimen helped the process along!

Thanks for the comment!

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