Did you know that over half the cells in your body are not actually human? You and I contain masses of bacteria, fungi, viruses and archaea. They live in every part of us - on our skin, in our hair, in our mouths, in our bloodstream, in our lungs, and most especially in our digestive tract.
For instance, how many non-human cells are there in our bodies? One rather complicated article I read estimates the number is within the range of 3.8-1013 - that is, somewhere between 38 and 100 trillion cells. They estimate that the ratio of non-human to human cells in our bodies is somewhere around 1.3:1 in the 'average man', 2.2:1 in the 'average woman', and 2.3:1 in a young infant, with other variations for people of different ages and body sizes.
These numbers are unfeasibly large to get our heads around, and also have a huge range of error. The numbers could be quite a bit bigger or smaller, and most likely vary from person to person and from week to week in any individual.
Another way to look at this is to think about how many species there are. One estimate suggests that each person has somewhere around 900-1,000 species living in and on them. But this is one person. The species vary from person to person, and from body part to body part. Our hands, for instance, are sites of massive diversity. Your right hand has about 150 microbial species on it. So does your left hand, but only about 17% of these species will live on both hands, and only about 13% on someone else's hand. Your gut has about 160 different species and once again these differ between people. The human genome has approximately 20,000 pieces of encoded genetic instruction, but the microbes that live in and on you have somewhere between 2 and 20 million.
That's enough big numbers, right? These tell us two things - we are intimate with a lot of microbes, and we don't know much about them - not even something as apparently simple as how many types of them we host. Each of us is a diverse and rich ecosystem, or even a series of ecosystems - our gut, our bloodstream, our lungs, our skin, each present very different environments and are home to different microbes. Our stomach is home to a different set of microbes to our colon. Our armpits have a different set to our scalp.
Considering that we have only a fairly hazy idea of what all these little beings are, it's not surprising that we have an even hazier one of what they do. We know that they're important. They are, in the main, not parasites that harm us, and many if not all of them are more than simply passengers. For instance, we know our gut bacteria and fungi are essential to digestion. If they weren't there we would likely starve. We also know that the various microbes all around our body play an important immune function, helping our own immune systems to keep out harmful invaders.
This opens up possibilities - can human microbes be used to treat disease? Can we improve our health by eating bacteria? Following this path has led to a lot of dead ends. 100 years ago Elie Metchnikoff surmised that Bulgarian peasants were so long-lived because they drank sour milk, and drank it every day for the rest of his life. In the 1930s Minoru Shirota went looking for helpful bacteria and ended up creating Yakult yogurt, incorporating the much marketed Shirota strain of gut microbe. He and his company became rich but in actual fact they don't seem to provide much health benefit beyond the fact the yoghurt is nutritious anyway. The bacteria in Yakult, and in sour milk, just pass through.
On the other hand, there have been some successes. One example is the disease Clostridium difficile, a bacterial gut infection that is highly resistant to antibiotics and leaves sufferers with uncontrollable diarrhoea, living in adult nappies and turning skeletal with weight loss. If antibiotics don't work (and they often don't) then it can be treated by what is called a faecal microbiota transplant - you take some faeces from a healthy donor, blend it and insert it into the sick person's gut either via a colonoscopy or by a tube inserted through the nose or mouth. This resets the person's own microbiota and enables them to fight off the infection. It's not a perfect cure and the disease can return, but it's more effective than anything else.
There's a lot of uncertainty about this stuff because no-one is really sure how it works. You can't do a randomised control trial because there's no way of knowing exactly what the composition is of each faecal sample you inject. And there's lots of other illnesses where similar approaches have been tried and failed. A big part of the problem is that the whole thing is so complicated. You can't extract a person's microbiome and examine it piece by piece, and you can't rid a person of all their microbes and add them one by one to see what happens. This means that a lot of what we think we know is based on doing the same thing to lab mice bred in completely biota-free environments. It's kind of analogous because mice are similar to humans in many ways, but not exactly the same and in any case if you introduce something to a biota-free mouse and it has a certain effect, there is no guarantee it will do the same in a human who is already teeming with trillions of microbes. I feel kind of sorry for the mice.
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All this is way above my pay grade. If the best microbiologists know so little there's no chance I could know anything much at all. What fascinates me is the fact that the world is like this. We habitually think of ourselves as individuals. We carry a genetic inheritance from our parents and grandparents and pass it on to our children and grandchildren. The the genes make different combinations so everyone is differently but ultimately we know what we are dealing with. But actually this is not how it is at all.
When we are born, we get out genes from our parents, but we also get a set of microbes. These aren't passed on to us in the uterus, which is a sterile environment, but on the way out, especially if we have a vaginal delivery, and in our first days and weeks of life as we are cuddled, fed and have our nappy changed by various parents, family members and friends. We pick up more when we cuddle the dog or cat, dig in the garden or crawl around on the carpet. The little creatures we pick up can harm us but more often they are our helpers and friends, coming along fir the ride and doing things that we can't do so well using our own genetic equipment.
It's tempting to think of ourselves as their hosts, as providing them with a home and a place of safety, but that is a very anthropocentric way of looking at the world. Microbes have existed for over 3 billion years, while Homo sapiens only appeared around 300,000 years ago. We are, in a sense, built out of microbes. Eukaryotic cells, which are the building blocks of all multicellular life, incorporate parts of archaea and bacteria. All complex life, animal or plant, exists in symbiosis with a collection of micro-organisms. There doesn't seem to be any living being of more than a few cells that can live without some kind of bacteria or fungus working alongside it.
So you could say, in the misleading language of intention that biologists use when writing for people like me, that we brought the bacteria on board to help us survive, but it would be more accurate to see that they built us to provide them with a place to live. If you like at the whole story of life, it's not about us, it's about them. From our own point of view, we are the lords of the universe, conscious beings who create glorious works of art and architecture, explore the stars and ponder the meaning of life.
From the point of view of a microbe, if a microbe has such a thing, we are a handy place to live, a source of food and a way to get from place to place. We are, perhaps, an example of their marvellous creativity. We might like to flatter ourselves that we are their greatest creation but I doubt it. That would be whales.
The 19th century naturalist JBS Haldane was once asked what he had learned about God from his study of nature. He replied that God has an amazing fondness for beetles. Learn a little about beetles and you will see why this is so. If Haldane had known about microbes he would, perhaps, have sung a different tune. Compared to mammals, and even insects, microbes are hugely diverse and prolific, endlessly adaptable and, viewed under a powerful enough microscope (sure God has such a microscope) extraordinarily complex and beautiful. If we were able to ask God what was God's greatest creation, I suspect this would be the answer.
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