Friendly bacteria: the microbiome
Once food has passed through the mouth, it travels down the oesophagus and then the stomach where food is further digested by acid and enzymes; it’s then released slowly into the small and then the large intestines, which together form a 7 metre-long tube where nutrients and then water get absorbed into the bloodstream. But that’s not all the intestines do, because in recent years we’ve realised that they also provide a home for an additional previously unrecognised organ that weighs more than your brain and it also plays a huge role in keeping you healthy. Guessed what it is yet? Well that organ is your microbiome, the collection of microorganisms that live inside you. Chris Smith spoke to Kings’ College London scientist Tim Spector, who studies the microbiome.
Tim - It’s a community of microbes that inhabit our bodies and 99% of them are actually in our lower intestine, the colon. There are a 100 trillion of them, most of them are bacteria. As well as those, there are five times as many viruses, there are also fungi, and other little parasites as well. Together these 100 trillion microbes weigh several kilograms and one way of looking at them is not as a disparate group of bugs but, actually, as a newly discovered organ in our bodies because they weigh pretty much the same as a large spleen or your brain. And we’re just discovering how many different things they can do and how they help us survive.
Chris - How did they get there in the first place Tim?
Tim - The way they inhabit us starts as birth because we are pretty much born sterile and we get our first microbes through the birth canal. These microbes get down into the infant’s guts and start proliferating, which allows you then to go on and develop normally and have a normal immune system and a defence system.
Chris - And this is a community of lots of different types of bacteria which we pick up from the environment, some of which suit us, some of which don’t, some of which therefore thrive, and some of which don’t. So we end up a bit like a meadow with lots of different flowers and grasses in it and that’s our microbiome?
Tim - Yes, I think the garden analogy is exactly that and, not forgetting just the meadow on top, it’s also the soil underneath, the mixture of fungi and microbes. A good, healthy microbiome community is one that is extremely diverse, has lots of different species all working together so that no one group takes over. Every little grain of nutrient is used and every byproduct of every microbe goes to another microbe, and so nothing’s left to waste.
These guys are constantly producing things for the human body we didn’t think were possible. Because, as well as having the same number of cells as the average human, they have two or three hundred more genes than we do that not only help us digest complex carbohydrates, which we couldn’t otherwise eat or get nourishment from, they are also key messengers for our immune systems. They also produce a third of our vitamins, and brain chemicals so they can influence our mood and even our appetite.
Chris - Now you said that they get there by the birth process, but not everyone’s born the same way, and not everyone’s breast fed. So what are the influences of not having a normal vaginal delivery or not being breast fed? Does that change the spectrum of bugs?
Tim - We’re only just finding out the real issues here of this epidemic of cesarean sections, which currently is about 23% of the UK, but many parts of the world it’s over 50%. There were thought to be no downsides, but we do know that the microbes of the first three years of a baby born by cesarean section are very different to that provided by the normal birthing process. It turns out that this isn’t particularly healthy for them and there’s a slightly increased rate of obesity and allergies in children born to cesarean sections. It’s something we didn’t really think about at all before, we just thought it was a routine operation and would have no consequences.
Chris - Now that’s babies having the wrong bugs from birth, but what about adults? Are there any examples of adults where the spectrum of bugs in the gut goes off kilter and that’s linked to adult diseases?
Tim - If you take a group of people with a diseases or disorder and compare them with healthy controls, there’s one consistent finding in all of these studies is that the diseased people have less diversity of microbes.
Chris - Is that because they’re taking drugs for their disease or they’ve got their disease and that’s influencing the environment these bugs are asked to thrive in, or did the bugs change and that caused the disease?
Tim - Well, you can’t tell just from the human studies because it could be one or other, or it could be both. We did this in a study where we found some microbes associated with being thin, and we took those microbes from our twins and we put them into germ-free mice. We found that with those skinny microbes, you could overfeed these mice and keep them still skinny. So, it turns out that if you look at the majority of diseases, some are undoubtedly caused by having the disease, but it’s also the case that those same individuals lack protective microbes that when you give those protective microbes to other animals you can prevent them from getting the disease.
So it’s probably a complex mixture of being unhealthy makes you have an environment which stimulates more bad microbes to be there but it’s also, initially, the lack of good microbes which seems to be predominant. I think that’s really telling here because we’ve been, in the past, focusing on microbes as the enemy - use of domestos and other sprays saying you must get rid of microbes and you mustn't get infected and it turns out that possibly the reverse is true. The more microbes we have, the more we might be protected against all kinds of diseases.
Chris - I suspect you get asked this a lot, especially at dinner parties where people will turn to you and go “so should kids be eating more dirt”? Would you say yes then?
Tim - If I had to give a short answer it would be yes. There is pretty good evidence that playing in a healthy garden, getting dirt on you, playing with animals, being exposed to many of the rural microbes that we’ve lost, is generally good for you.
Next week I’m going to Tanzania to the Hadza hunter-gatherers, and these people live like we used to live. They eat everything that’s around them and the concept of dirt doesn’t really appear, and they have 40% more species than healthy people in the West. Many of which we’ve just eliminated through our obsession with hygiene, overuse of antibiotics, and our move to cities which only attracts certain types of microbes.
So I think it is crucial that we re-look at this whole idea, particularly in the early life in kids, by moving back from this over-sterile world and realise that in these, what we call more primitive environments, they really don’t have allergies or autoimmune diseases which are reaching epidemic levels just in the last 30 years in most western countries.