Same bacterium, two faces

Right now, inside the mucus of your gut, lives a bacterium that can either protect you or harm you.

It is called Akkermansia muciniphila, and it is already sold in capsules, on the same shelf as probiotics. However, in a study of almost 5,000 people, it was linked to a higher risk of diabetes. Keep these two pictures in mind, because at the end we will see what decides which one is true.

It lives inside the layer of mucus that covers the wall of the gut, and it feeds on the sugars of that mucus.

But first, the good side, because it is a big one. In people with excess weight, those who had more Akkermansia had better blood sugar, and they benefited more from a diet. Metformin, the most common drug for diabetes, increases Akkermansia in the gut. In people with cancer, Akkermansia was linked to a better response to immunotherapy. In experiments, it also made the mucus thicker and reversed the metabolic damage of a high-fat diet.

On the other side, the same Akkermansia is often found in higher amounts in people with multiple sclerosis and with Parkinson's disease. And in the Swedish study, those who had more Akkermansia had a higher risk of developing type 2 diabetes within five years.

However, this link was stronger in people who ate little fiber. In people who ate a lot of fiber, did Akkermansia raise or lower inflammation? It lowered it, while with little fiber, it raised it.

Why does this happen? When the diet had no fiber at all, the bacteria that feed on mucus, Akkermansia among them, increased. As a result, the mucus became thinner, and a pathogen reached the wall of the gut.

Besides the plate, the bacteria that live around it also matter. In another experiment, the same Akkermansia was placed into two different microbiomes. In one, it reduced the bacteria that make butyrate, and it worsened a model of multiple sclerosis. In the other, nothing changed. The experiments on the mucus and on the neighbours were done in mice.

Remember the two pictures from the beginning? Which one is true depends on the fiber you eat and on the bacteria that live next to it. That is why claims that it is the best probiotic there is should not be made, at least not until we have much more evidence. And before any capsule, what matters is what we put on our plate every day: legumes, vegetables, whole grains and fruit. A bacterium does good or harm depending on the environment we build for it.