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  • Alex Cloherty

Sharing spit

If you’ve been keeping up with Microbial Mondays articles, you’re already pretty well versed in the fact that there are a lot of bacteria living on and in our bodies. But how do those bacteria get there in the first place? Well, it turns out that, at least in terms of the intestines, this has been a somewhat difficult question to answer. Scientists have known for a while already that a lot of bacteria that go on to live in our intestines are acquired at birth - our mothers don’t only pass on their DNA to us, but also their bugs. But the story of our gut bacteria doesn’t end there - not even close. The population of bacteria living in our guts changes over time. Eventually, that population of bacteria becomes so personal that some scientists have been able to use “microbial fingerprinting” to ID the exact human that a group of gut microbes came from. In other words, over time, the population of bacteria living in your gut grows to look quite different from the population of bacteria that you inherited from your mother.

You might think, well, sure, I eat different stuff than my Mum. And I live in a different city than her, too. Surely I expose myself to different bugs than she does, from eating an excess of avocado toast, or biting my nails after touching the door of the metro. So of course my bugs are different!

But the catch is, there are actually not many gut microbes that can survive for very long after they end up outside of human bodies. That’s because most of the bacteria that live in our guts, unlike us, do not like oxygen. This makes them perfectly suited for living within our intestines, where there isn’t much of that particular gas (hehehe). But when those bugs exit our guts and are exposed to air, for them it’s as if they’ve suddenly arrived on a different planet with a strange and dangerous atmosphere.

Now, some of our intestinal bacteria can pull a range of tricks that together amount to something like throwing on a space suit, and help them survive the sudden abundance of oxygen. For instance, the lactic acid bacteria found in fermented foods like sauerkraut or kombucha make pretty good ‘astronauts’ in this sense. But, not all of our gut bacteria can whip up a space suit out of thin air.

So, if the population of microbes in our guts changes over time, but passively picking up bugs from the environment can’t account for all of the influx of new bugs… Where are those new bacteria coming from?

Well, just this month a new scientific study gave us what may be the most detailed answer to that question yet.

Using a beautiful marriage of biology and computer science, the authors of this study analysed more than thirty different data sets across a variety of countries in the Americas, Africa, Europe, and Asia to try to tease apart exactly where our gut microbes come from.

By examining the amount of shared bacterial strains between people with differing levels of contact - for instance living together, versus living in the same village, versus living on opposite sides of the world - the researchers were able to show that there was a “social distance-based gradient” in regards to how many types of bacteria that two given people have in common. In other words, you likely have a much more similar intestinal bacterial ‘fingerprint’ to your child or your spouse, versus your boss or your neighbour from down the street. The study also confirmed the trend towards the bacterial populations in the intestines of children becoming less and less similar to those of their mothers, with age - presumably because they come more and more into contact with other people who they can source their bugs from. But overall, in the words of the authors, “cohabitation drives transmission” of bacteria: no matter where in the world we live, humans share a lot of bugs with the people they live with, whether that be children, spouses, or housemates. And beyond the home, the more contact you have with somebody, the more likely you are to share some bugs with them.

There were other more detailed findings within the study as well - for instance, they showed that specific types of bacteria are better than others at jumping from human to human - but what stuck with me most after reading the manuscript was the beautifully detailed picking-apart of an, at first, seemingly simple question.

It may well have occurred to you before reading this article that you’d pass some bacteria back and forth with the people with whom you’re most likely to end up sharing spit. In other words, you could have already formed the hypothesis that you and your child share a lot of bugs with each other. In my opinion, this study is a great example of how scientists would tackle such a hypothesis: through a detailed, systematic examination of every possibility, which continues until one explanation seems to present itself as the most likely based on the data you’ve collected. While some people might say the devil is in the details, as a scientist I think the details are simply where you find the closest approximation of the truth. Just as it takes a thousand brush strokes to paint a masterpiece, it can take thousands of individual pieces of data to paint the clearest possible picture of where our gut microbes come from. ______________

The main source for today's article can be found here, and the image was created with

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