What is the gut microbiome?

What lives inside our guts? And are we more microbe than human?
02 September 2022

Interview with 

Ben Mullish, Imperial College London




Julia Ravey is taking a look into what we know about our gut microbes and their role in disease by talking to Ben Mullish from Imperial College London, whilst also hoping to find out a little bit more about her own own inhabitants…

Julia - So I've been sent a health kit box, to better understand my own body pretty much. And here, there is the blood test that I have to do to look at the way my body handles processing fat. There's a glucose monitor, which I'm gonna wear to better understand how my body can deal with blood sugars. And then, there is the piece de resistance, a small tube, and a swab to collect a bit of stool sample mm-hmm yeah, we're going there because what our stool can reveal to us is a bit about the microbes that live in our gut. There has been a flurry of research recently that's linked the bugs that live inside us to our health and also disease. So I'm really keen to understand what bugs are living in my gut and what can that tell me about my health. And then also, just as a wider question, how much can the bugs that live inside us actually impact our overall health? To learn a little bit more about what is termed 'The Gut Microbiome', I contacted Ben Mullish from Imperial College, London, and he gave me the inside knowledge.

Ben - We probably have about a hundred trillion bacteria alone within our large intestine, within our colon. And in all, what we think is that actually bacteria outnumber us, that they are probably about 1.3 bacteria in us for every one human cell. So in other words, you might say that only about 35% of us are actually human with the rest of us actually being microbial.

Julia - So we're essentially carriers for the microbes?

Ben - Yeah, exactly. That's right. There's a sort of two-way relationship in one way, we provide a nice home for them. We keep the heating on, we provide nice temperature for these bugs, but they provide lots of functions for us. Roles they play in contributing to digestion of foods or chemicals they make, that we now recognize more than ever are contributing to a large number of different aspects to our health.

Julia - And how is this population established in an individual and what can impact what ends up in say, for example, our gut microbiome?

Ben - When a baby is born or the point of being born, it probably has very few members of its microbiome, but as the baby comes into the world, it's instantly exposed to a huge number of different bacteria. As babies are delivered through the delivery canal and exposure to microbes all around us in the world, instantly, or within a very short period of time, babies start to establish their own microbiome. And what we think is that in really early life, that undergoes a huge amount of changes before we really develop our fully flourished, final stable microbiome. But actually, as you say, what we also know is there are many different aspects that we are exposed to in life that might impact upon our microbiome and make changes to it. So one particular obvious example is antibiotics. We think age is important, it changes throughout life. Your diet is very important whereby we know that fiber and other prebiotic elements, so are things that are actually nutritional sources for our bacteria, or ways that can impact upon our gut microbiome and might have downstream effects. Whether a child is delivered a normal delivery or caesarean section, whether that child is breastfed or bottle fed. So there are different factors throughout the whole course of life that we are all exposed to that have pretty marked effects on what a microbiome is and what a microbiome is able to do.

Julia - How does the population of one person's microbes differ to the next person?

Ben - Each person looks different just as like, you know, we're all recognizable in some way, because we've all got faces, arms, bodies that look the same, but we all have very distinctive characteristics no one else has. So our microbiome is a bit like that as well. Whereby there is some overlap in terms of the sort of broad patterns but everyone has a very distinctive fingerprint, everyone has their own particular patterns. However, where it starts to get a bit more complicated is when you actually start to look at what the microbiome does. So though we might look at the composition and everyone looks very, very distinctive, there seems to be a sort of key number of functions of the microbiome that are preserved among healthy people that we can see.

Julia - The stomach, and the gut in general, is one of the most important entry points really to the body and so it's really important, I guess, in protecting us from nasty pathogens, viruses, et cetera. How does the microbiome contribute to our immune response?

Ben - This is something that's been a huge area of research over the past few years about actually how our microbiome might tailor and affect our immune responses throughout life. Lots of the work that we've been involved in at the moment has looked at people with hematological blood conditions and particularly people who have blood cancers like leukemia and people who have been unwell enough to require a stem cell transplants treatment for that. And we've found over the past few years, you can pretty accurately predict how well someone is going to respond to a stem cell transplant, or whether they're gonna have complications or might need further treatment actually based on the gut microbiome before they have the stem cell transplant and specifically what we call microbiome diversity. In other words, the degree of different mixture of bacteria and other microorganisms in their guts.

Julia - The gut also has this reputation of being called the second brain. Why do you think it's got this reputation?

Ben - I think lots of us have recognized for a very long time that how our gut works and how our brain and nervous system works are pretty closely connected. All of us have known that feeling, you know, before we've had an exam, for instance, when you feel churning in your stomach and a lot of focus on that has actually been nerves going from our brain or our spinal cord directly into our guts. But what we've started to recognize is that actually part of this communication between our guts and our brain is also related to our gut microbiome. For instance, if you look at the gut microbes of old mice who are not able to perform some neurological tasks so well, and you replace the microbes in the gut with those from younger mice, you can actually start to see changes in the structure and function of their brain. Again, just speaking from a clinical perspective, a lot of work that I do is working with people with advanced liver disease. And we've recognized for some time about a complication of advanced liver disease called hepatic encephalopathy, which is a pretty horrible confusional state. And we've recognized for a long time that the treatment for this is medications that impact upon the gut microbiota. So we give people lactulose, which is a sort of sugary medication that we don't absorb very well, but impacts upon the gut microbiota. And we also give them Rifaximin, which is a antibiotic that we don't absorb from the gut, so it stays in the gut and acts directly there. And they're effective medications and not perfect treatments, but they're effective medications.

Julia - Is it that the microbes are essentially talking to each other and releasing different messages and chemicals that are allowing them to chat, and then that is what stimulates the nerves that are sat around the gut and the brain is then getting signals from that?

Ben - Exactly right. So I guess the key question is if we think the gut microbiome is talking to our brain in our nervous system, what are the sort of messengers for doing that? And we think some of the key aspects of that are what we call metabolites - the chemicals that are either produced by the bacteria themselves - or the chemicals that are produced by us, but which the bacteria in our gut or elsewhere can actually impact upon change the composition on. And what we know is that these metabolites, these chemicals can transition from our guts up into our bloodstream. And in some cases, all the way to our brain and impact directly there upon aspects of how our brain functions?


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