Mind & Body

There's No Such Thing as Good Bacteria and Bad Bacteria

When it comes to bacteria, you might as well be talking about the witches of Oz: Are you a good microbe or a bad microbe? Good bacteria is the stuff that's in our yogurt and kombucha; it's said to do vague things like "promote gut health" and "aid in digestion." Bad bacteria is the stuff that's on toilets and kitchen sponges and airport check-in kiosks; they're the bugs that can make you sick. The way people talk about them, you'd think there must be a list somewhere defining the bacteria that are beneficial and the bacteria that are harmful. But there isn't, and there never will be. That's because there's no such thing as "good" or "bad" bacteria. What they do for you depends on context.

A Microbe by Any Other Name

There are all sorts of examples of bacteria doing favors for other animals: bioluminescent bacteria illuminate the "lure" of the anglerfish and help the Hawaiian bobtail squid see in the dark; another type of bacteria helps the aphid get more nutrition from its food. The microbe Xenorhabdus nematophilus does double-duty: It's toxic to insects but not to nematode worms, which helps the nematode invade its victims. The same kinds of things are going on in your body. Bacteria living inside of you, known as your microbiome, help you digest food, fight infection, and even protect against cancer. They can also cause infection.

In science, "good" and "bad" bacteria fall under titles like "mutualist" when both the bacteria and host benefit from their presence, "commensal" when only the bacteria benefit but don't cause harm, and "parasite" when their presence actively causes harm. But that's a black-and-white categorization for a very complex phenomenon. Helicobacter pylori is one example of a bacterium that protects you against cancer — cancer of the esophagus, anyway. But the exact same strain is also responsible for causing ulcers and stomach cancer. Is H. pylori good bacteria or bad bacteria? What about the bacteria that live in your gut — are they still good when they enter your bloodstream and cause sepsis?

"In reality, bacteria exist along a continuum of lifestyles, between 'bad' parasites and 'good' mutualists," Ed Yong writes in "I Contain Multitudes: The Microbes Within Us and a Grander View of Life." "...All of this means that labels like mutualist, commensal, pathogen, or parasite don't quite work as badges of fixed identity. These terms are more like states of being, like hungry or awake or alive, or behaviours like cooperating or fighting."

Like a Teardrop in the Ocean

But what about the stuff in probiotics? Surely that's good for you. In fact, people have been consuming live bacteria to benefit their health since as far back as the late 1800s, when biologist Elie Metchnikoff drank sour milk to extend his life in the belief that it was the reason so many Bulgarian peasants lived to 100. Unfortunately, scientists Christian Herter and Arthur Isaac Kendall later discovered that the bacterium he championed probably wouldn't have survived the trip to his gut.

Researchers have worked hard to find modern formulations that survive that journey, but there are still many problems with probiotics today, Yeong writes. "Even the most concentrated probiotics contain just a few hundred billion bacteria per sachet. That sounds like a lot but the gut already holds at least a hundredfold more." What's more, the bacteria in these probiotics aren't even important denizens of the human gut. They're chosen more because they're easy to produce than for any particular health benefit.

While probiotics have shown promise for certain people, such as those with autoimmune disorders, those taking antibiotics, and preterm infants, there's little evidence that they do anything for people who are already healthy. "There is no evidence to suggest that people with normal gastrointestinal tracts can benefit from taking probiotics," Matthew Ciorba, a gastroenterologist at Washington University in St. Louis, told Scientific American. "If you're not in any distress, I would not recommend them."

In fact, they might even do harm, as Augusta University researchers reported this month when they discovered that 22 patients complaining of brain fog and bloating had systems flooded by toxic chemicals produced by the gut microbes they ingested with their probiotics.

That's not to say they're never beneficial. Eating yogurt or downing probiotic capsules is a good idea when you're on a course of antibiotics since they can wipe out your beneficial microbes in their quest to vanquish the bad bacteria. But it's important to remember that bacteria, like human beings and Keanu Reeves movies, aren't just good or bad — they exist along a rich spectrum.

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For a fascinating dive into the world of the microbiome, check out Ed Yong's "I Contain Multitudes: The Microbes Within Us and a Grander View of Life." The audiobook is free with a trial of Audible. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

The "Second Brain" in Your Gut

Written by Ashley Hamer August 24, 2018

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