361 | Bonnie Bassler on How Bacteria Talk and Work Together

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**Sean Carroll** (1:32)
Hello everyone and welcome to the Mindscape Podcast. I'm your host, Sean Carroll. As I am recording this, I just this week, finally sent in the manuscript to my publisher for book three of the biggest ideas in the universe, called Complexity and Emergence. I do apologize to everyone who's been waiting for this book. I'm not exactly sure when it's going to come out. I'll let you know when I do. It was supposed to come out around now, but that didn't happen since obviously I was very late getting in the actual book. But it is going to come out. That is the good news. So it's on my mind, all of these issues of complexity and emergence. I've been writing about them quite a bit.
The last chapter, the culmination of the book is on complexity. A lot of that is about self-organization in different kinds of systems, whether you have a bunch of little things, whether they're cells or ants or birds or human beings or whatever, which have their individual motivations and dynamics and ways of thinking about what they do, and they come together to form something a little bit unanticipated. Of course, anyone who thinks about emergence and complexity knows that it's very difficult to define exactly what you mean by unanticipated in those contexts. But more is different is the motto that is usually thrown around here, coined by physicist Philip Anderson. The idea that as a system, whether it's an ant colony or sand in a sand pile, you can get behavior at the collective level that you would not have thought of just by thinking about the individual units that are coming together.
So I felt like in writing this, it's a topic that I know quite a bit about, but not nearly as much as one could, because it's a sprawling, gigantic topic. And I felt that very, very strongly while writing about it. Like the things that I did say in that chapter, which again is the culmination of the book, I loved. I liked everything I said. But you had to make choices about what to exclude, what kind of arbitrary classifications to invent, to make this sprawling landscape of possibilities more coherent, etc. We'll see how that goes over. I'm thinking about this and saying this right now because today's podcast is a classic example of self-organization and one that I was not able to include in the book, even though I had really thought about it. The units in question are bacteria. So just about the simplest living organisms that you can imagine.
Nevertheless, they have enough complexity to come together in interesting ways and do things as a collective that you wouldn't have guessed. They were going to do as individual cells. And the mechanism for them doing this is something called quorum sensing. Quorum sensing was discovered back in the 1970s by Woody Hastings and other people. But the world's expert in quorum sensing is Bonnie Bassler, who is our guest today. Bonnie was really the one starting in the 1990s who explained, discovered, and then explained to the rest of us how quorum sensing works. So the idea is that you have bacteria, and every individual bacterium does its thing in its kind of interesting ways. But then when you get enough of them in a region and high enough density of bacteria, they are sending signals so they realize, oh my goodness, there's a bunch of us grouped together here. We are now more powerful. We can do more things than we were otherwise able to do just as individual cells on our lonesome. So it turns out this is not only cool and interesting to people who care about complexity and emergence for purely intellectual reasons, it's also super relevant to biology, including the biology of human beings. As we will discuss in the podcast, there's a lot of bacteria that you carry with you, probably at least as many if not more bacterial cells in your body as there are your cells in the sense that your cells have your DNA in them, and there's a symbiotic relationship. Those bacteria are doing really, really important things for you just as you're providing a home for them. So the dynamics of the bacteria working together and also how they fight off threats and how they symbiotically interact with other subsystems of your body are crucially important. It's a really fun topic in just about every possible way.