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  • Alex Cloherty

Multifunctional microbe manure

When Hippocrates said, “Let food be thy medicine and medicine be thy food” he was on to something. But one thing that Hippocrates didn’t know was the importance of microbes in this connection between your diet and your health.

We all know by now that what you eat has a big effect on your health over your lifetime. As this review manuscript (which is where I found all the information I used for today's article), puts it, “Diet is central to human health and the etiology of noncommunicable chronic diseases that have reached epidemic proportions”. In other words, diet has a profound impact on your likelihood of developing now-common, non-infectious diseases spanning from inflammatory bowel disease and colitis all the way to cancer and heart disease. And, it turns out that your gut microbiome - i.e. the microbes living in your intestines - are major players in this diet-health connection.

How so, you ask? Well, one major reason is that sometimes it isn’t the human cells in your body that digest your food. It’s the bacterial cells living within you that do the digestion for you.

Let’s take a look at dietary fibres as an example of this. Dietary fibres, which are found mostly in plant foods, are by definition sugary structures that we humans cannot digest - but our gut microbes can. One particular subset of these dietary fibers that scientists call “microbiota-accessible carbohydrates” (MACs) are especially tasty for our bugs. These fibres can be considered ‘prebiotics’, or microbe food that the bacteria living within us can eat up and break down.

After our gut bacteria are done eating up these MACs, just like us, they do a sort of poop, releasing the leftover materials that they didn't digest themselves. And it just so happens that this bacteria poop is an excellent fertiliser.

These leftover, partially digested bits and bobs that our gut bacteria poop out can provide food for their bacterial neighbours - including the more testy of their neighbours, who might otherwise start to cause some problems in our intestine. It's actually pretty common for us humans to host bacteria in our intestines that usually don’t cause any problems, but given the right (or, wrong) conditions, can make us sick.

To understand how this can be, you need to think about our intestines the way that microbiologists do. First, you have to notice that our whole digestive system is one long tube - running all the way from our mouths to our butts. And lining that tube, is tissue that functions sort of like our skin. Just like skin, this tissue that lines our digestive tracts is a great barrier: a wall between the potentially dangerous outside world and the true insides of our bodies.

That’s why microbiologists consider anything that is still inside this tube that is our digestive system, as still being on the ‘outside’ of our bodies. You can see how this works in the cross-section of a piece of the large intestine, in the image to the right. The bacteria are swirling about in the tube of tissue, but not actually invading into the walls of the intestine. And our body is very happy to have them there.

So, it’s totally safe for there to be a lot of bacteria inside our intestines, as long as they stay in place within the intestinal tube. Although we often consider these bacteria as living inside us, they are more truly living next to us, or on us. And if everything goes well, the bacteria in our guts don’t ever march into our true insides - they stay out of the actual tissues of our bodies.

And this is where we get back to the importance of MACs.

Remember that bacterial poop that is left behind after our gut bacteria digest MACs? And how it can act as tasty grub for their bacterial neighbours? Well those bacterial neighbours, if they were hungry enough, might actually start to invade truly into our bodies in search of food. To them, the walls of our intestines look like a juicy steak, and it only starts to look better and better if there’s no freely available bacterial faeces floating around them to gobble up. Yum yum…

As you might have guessed, if the bacteria in our gut start eating us from the outside in, that is a pretty big problem. Our intestinal cells and our immune system will fight back against the invading, hungry bugs, but even if the human team wins the fight the battle can still cause a lot of inflammation in the intestines. Over time, this type of inflammation is thought to be a potential cause of some chronic diseases like inflammatory bowel disease.

Nonetheless, when given the choice, bacteria will go for the easy grub - i.e. the excrement of their MAC-eating neighbours. This complicated process is one of the reasons why eating lots of plants, which contain lots of fibres like MACs, can reduce inflammation and immune activity in the guts of mice and men.

This is of course only part of the story. For example, the ‘poop’ that bacteria excrete after digesting MACs can have lots of other downstream effects directly on our own bodies, too. This multifunctional manure can accomplish all sorts of things, from altering our hormone production and insulin sensitivity, all the way to helping our gut be a better barrier, which even further reduces the likelihood of invasion by the more nasty bugs in our guts. And of course, plants are not the only things that humans eat. Nowadays, there’s more and more research coming out about the impact of highly processed foods on us and our gut microbiota. But that’s a story for another time.

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