No one likes freeloaders. The idea of people who just sit back with their feet up while others do all the hard work is something we are pretty much hardwired to despise. So what about your microbiome? In the previous issue of this blog we explored the concept of the gut microbiome and the trillions upon trillions of bacterial cells who call our gastrointestinal tract home. They are sitting there in every one of our bellies waiting to be fed … but what do these trillions of little guys do for us in return?
…Well, quite a bit actually! Technically speaking it is possible to live without a microbiome, although there are many reasons why this is not a healthy (or particularly pleasant) way to live. Let’s start with how these friendly bacteria help protect us from their less affable cousins. Nature, as they say, abhors a vacuum. Whether this is physical vacuum such as the one generated by your hoover, or the more conceptual power variety caused by overly liberal use of medieval weaponry on Game of Thrones, something will always get sucked in to fill it. The same is true in biological systems. If your gut microflora were to instantaneously disappear it would leave a vacuum of its own: in this case a nice warm environment with a never-ending supply of food. Something would try to move into this prime real estate pretty quickly, and chances are they would not be the nicest of neighbours to the host i.e. you. By having an ecosystem full of friendly bacteria already in place, pathogenic bacteria (the bad guys) find it much harder to gain a foothold in our intestines and to make us sick. The gut microflora also helps protect us from pathogenic bacteria by essentially keeping the immune system on its toes. Our immune cells are mostly able to recognise our microbiome as being made up of friendly bacteria - but their presence causes it to keep its guard up, just in case. It does this, among other ways, by developing the gut lining into a complex, multi-layered protection system, and also by secreting proteins called immunoglobulins that are able to target pathogenic bacteria if any appear.
Another way that a healthy microbiome helps is with our digestion and metabolism. Although our gastrointestinal tract does a very good job of breaking down and absorbing much of the food we eat, there are some things that we just can’t tackle alone. This is where the good guys in our microbiome come in. Anything we are unable to process will end up in the large intestine, where the majority of our gut microflora resides. Once there, the friendly bacteria get to work breaking down these resistant foods, and in turn release molecules that we can absorb and extract more energy from. In one study involving mice without gut microflora, it was found that they needed to consume an additional 30% of their standard calorie intake just to compensate for the loss of this microbial-assisted digestion step. These molecules are collectively known as microbial secondary metabolites, and the most interesting group of these are called short-chain fatty acids (SCFAs). Not only can these be used by our cells to produce energy, but they have also been shown to act like hormones to regulate a wide variety of metabolic process. So far links have been found to how we control the sugar and lipid levels in our blood, the way our appetite is regulated, and once again, how our immune system functions. There is also considerable evidence to link the gut microbiome to our moods and emotions through the gut-brain-axis. These are topics that I will come on to cover in subsequent blogs.
But for now, yes, we do provide the trillions of little guys in our microbiome with somewhere to live and something to eat, but on balance it’s safe to say that they do a huge amount for us in return. So much so that I slightly wonder whether there are a pair of bacterial cells sitting in my colon right now, complaining about their free-loading human host…