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Go to CuriosityStream.com/microcosmos to start streaming thousands of documentaries and for a special offer that helps support our show. Our world is made up of invisible highways.

They intersect and divert and congest, taking organisms on short trips and long voyages alike in a flurry of activity. Now, we may not be able to see all of those highways laid out, the way commuters might see long stretches of concrete stacked on each other. But we can start to see their outlines in the lives that are traced upon those roads.

The microcosmos is no exception. In fact, it is where all those journeys begin. When Antonie van Leeuwenhoek made his first observations of microbes, he was captivated by those primitive paths, even though he didn’t know what he was observing.

He didn’t even know what creatures he was observing. He wrote, “…the motion of most of these animalcules in the water was so swift, and so various upwards, downwards and round about that ’twas wonderful to see.” Leeuwenhoek hired a draughtsman to help draw what he observed. And sharing his employer’s delight, the draughtsman reportedly exclaimed, “Oh, that one could ever depict so wonderful a motion!” He probably did it in Dutch but you get the idea.

The turn of the 18th century was not a particularly great time for video. But fortunately, times have changed, and now we have ways to depict so wonderful a motion. And that means we can share things like this, a band of bacteria—or really more like a cylinder of thousands of bacteria—that have gathered for some unseeable reason that must be important to them.

We think—though we can’t quite be sure—that the culprit is oxygen. James, our master of microscopes, kept the slides in a humidity chamber that was low in oxygen—an environment that was well-suited for these bacteria. But when he took the slide out to check it under the microscope, oxygen began to seep in from the sides.

And while many organisms love oxygen, these bacteria do not. And with their habitat being slowly encroached upon by the gas, the bacteria gathered in the middle of the slide where the oxygen is lower. If you take a quick glance, the band of bacteria seems pretty orderly.

But it just takes a few more seconds to see that it is made up of chaos, bacteria that are swimming and wiggling around in what seems like a completely undirected fashion. Except, of course, it is directed. They’re navigating inside an invisible road shaped by oxygen.

In 1882, the scientist Theodor Wilhelm Engelmann used a microscope designed by Carl Zeiss himself to watch algae as they photosynthesized. But it wasn’t just the algae he was interested in. Engelmann also watched as oxygen-loving bacteria congregated to the areas where algae lay, seeking out the oxygen byproduct of their neighbor’s photosynthetic activity.

Engelmann’s work provided descriptions of bacteria changing their behavior around invisible chemicals. But it would take a contemporary of his, and then several decades of waiting, for scientists to figure out the mechanisms underlying those changes. His contemporary was named Wilhelm Pfeffer, and he designed an experiment using a thin capillary tube filled with chemical solutions to study how bacteria gathered first to the tube, and then inside of it.

After Pfeffer the field becomes quieter until the middle of the 20th century, when a scientist named Julius Adler began searching for his next research project, the one he would pursue after studying butterflies in college and rat liver mitochondria in grad school. It was 1957, and he just happened to be going through a library filled with old scientific journals, where he found Pfeffer’s publications. What he read would end up inspiring decades of his work, as well as the work of his students, and colleagues to decipher the chemical and biological language that bacteria were using to navigate the world around them.

He established, for instance, that bacteria are able to detect molecules around them without actually consuming those molecules, using receptors at their surface to process and respond. One student, Mel DePamphilis, used an electron microscope to capture images of the bacteria’s motor, called the flagella. Later, he would say, “I was so excited I could not take photos but kept thinking, ‘My God, I’m the first person to see this.’” A tiny peek behind the curtain, there’s a note on the script here, and James, our Master of Microscopes has made a comment, and he says, “Aha!

This happens to me all the time!” What a joyous feeling that must be, but also look, all of us together are seeing that thing that Mel DePamphilis first saw all those decades ago and it is no less remarkable for having been seen by others. Anyway, over time, what became apparent is that the flagella doesn’t just move bacteria, because swimming in the microcosmos is kind of difficult when you’re small. The water is thick, like honey.

But what the bacteria’s combination of flagella and receptors do is let the organism move randomly…but also purposefully. The systems can vary among different bacteria—they do, after all, have different needs from their environments but so much of what we know comes from studies of E. coli. And in E. coli, the movement of the flagella dictates whether the organism will tumble around or propel forward.

Now, why would an organism want to tumble? What does randomly turning about do? Well, for you and I, tumbling is probably, at best, not useful, at worst, disorienting, but for bacteria, it’s a way to get reoriented, allowing them to point in different directions until they find the right way forward.

And to determine what the right way is, they measure gradients, looking for changes in the amount of those chemicals they either want to get closer to or further away from. When those changes point to something good, the bacteria propels onward. And when they point to something bad, it tumbles.

They can do this incredibly fast, capable of speeds that are several hundred body lengths per second. It’s like sprinting home in the dark, guided only by the familiar changes in the streets around you but it is enough to have helped bacteria survive this world and its many, many challenges for billions of years. Of course, it also means that other organisms have inherited those mechanisms too, like these ciliates drawn as they are to their bacterial prey through an invisible connection, whether or not they knew that’s what drew them there in the first place.

Because even when the roads are unseen, and even when we may not know where we’re going, we can surely figure out what to do when we get there. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. This episode was brought to you by CuriosityStream, it’s a subscription streaming service that offers thousands of documentaries and non­fiction TV shows from some of the world's best filmmakers, including award winning exclusives & originals.

They cover topics like history, nature, science, food, technology, travel, and more! There, you can watch the documentaries like “Never Home Alone” where they explore the unseen wildlife that lives in all of the nooks and crannies of your home and take a microscopic journey through your food, cabinets, and shower heads. You can stream CuriosityStream’s library, including their collections of curated programs handpicked by their experts, to any device for viewing anytime, anywhere, and if you go to curiositystream.com/microcosmos and use the code “Microcosmos” to sign up, it will only cost you $14.99 for an entire year!

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If you would like to see more from our Master of Microscopes James Weiss, you can check out Jam and Germs on Instagram, or you can see him in Emily Graslie’s new video where the two of them discuss microscopy and Emily even painted a painting inspired by some of the tardigrades you have seen on this very channel. And you can go check that out at our link in the description. And if you want to see more from us, hey, there’s always a subscribe button somewhere nearby.