Friday is the 167th birthday of Julius Richard Petri, inventor of the famous Petri dish.
In 1876, bacteriologist Robert Koch identified the bacteria that causes anthrax; it was the first time anyone had ever traced a disease back to the microbe that caused it. That was a great discovery, and it’s shaped the course of microbiology and medicine ever since; 150 years later, it’s easy to forget unglamorous details like the fact that in those early days, Koch grew many of his bacterial cultures on potato slices. There just wasn’t anything better. Today, he gets much of the credit for developing better (and less starchy) methods for growing bacteria, especially the little round dishes called Petri dishes. But as the name suggests, Petri dishes weren’t Koch’s invention at all — although Koch certainly deserves credit for knowing a good idea when he saw it.
Petri joined Koch’s lab at Germany’s Imperial Health Office in 1877, and shortly after that he came up with a decent substitute for potatoes: a round, shallow glass dish with a lid. It caught on quickly in Koch’s lab, where researchers cultured bacteria on various mixtures of gelatin and beef broth. The design hasn’t changed much since the late 1870s; modern dishes also come in plastic versions, and most now have rings on their lids so they’re easier to stack, but the basic shape is the same.
But all was not yet well in the world of early bacteriology, because Koch and his assistants were wrangling with the problem of how to feed their bacteria colonies, now that they had them in Petri’s neat little dishes. Beef broth provided most of the nutrients a growing colony needed, but it turned out to be nearly impossible to pick bacteria out of the resulting soup. They needed something firmer to grow on.
Gelatin — a protein found in collagen and used to make familiar jiggly snacks like Jell-O — seemed like a good way to firm things up. But many species of bacteria just ate their way through the gelatin. In other cases, it melted into goo in the heat of the incubators. Good luck isolating bacteria from that.
Angelina Hesse had been cooking up the beef broth for untold billions of bacteria since her husband, Walther Hesse, joined Koch’s lab in 1881. She suggested using agar — a compound found in the cell walls of red algae and often used to make jellies and ice cream or thicken soups — to firm up the growth medium for the bacteria. She’d learned the trick from Dutch friends, who in turn had learned it from their neighbors during a stay in Indonesia; Angelina Hesse was just the first person to bring agar into a biology lab (although today her husband and his boss still get most of the credit, which gives her something else in common with Petri).
Agar, mixed with a nutritious broth and plated up in Petri dishes, proved to be an excellent way to grow and study bacteria, and that’s how Koch eventually identified the bacterium that causes tuberculosis.