Some pretty crazy things can change hands at auction houses: Think decades-old slices of cake or the world’s (arguably) most intense cat painting. But a recent auction makes those finds look mild. Someone just paid $46,250 for some vintage mold preserved between two pieces of glass. It’s not just any mold, though. Rather, the scientific sample is part of the very culture that helped Alexander Fleming discover the first antibiotic.

In 1928 Fleming, a British bacteriologist, came back from vacation to realize that Petri dishes on which he had cultured some Staphylococcus aureus had been contaminated by mold that came in through an open window. At first, it seemed like an annoyance—until Fleming realized that in some spots, the Penicillium notatum mold that had taken hold appeared to have killed off some of the staphylococci.

On further investigation, Fleming realized he might have something big on his hands. As Howard Markel writes for the PBS NewsHour, a pathologist named Howard Florey and colleagues then tested its antibacterial properties and developed a way to mass a drug called penicillin. (Fleming, Florey and a colleague named Ernst Chain all won the Nobel Prize in Physiology or Medicine for their discovery in 1945).

The finding was revolutionary. Not only was penicillin the world’s first antibiotic, but it immediately changed the way the world worked. At the time, it was considered nothing short of a medical miracle for its ability to fight once insurmountable diseases like staph and meningitis, and it kicked off the antibiotic era. The drug even affected World War II, reducing sepsis, a potentially deadly infection, in soldiers who landed in Europe during and after D-Day.

These days, an estimated 70 billion doses of antibiotics are taken every year—and though dangerous antibiotic resistance is on the rise, the fact that humans can now fight bacterial infections will continue to drive world health in the years to come.

No wonder, then, that the mold itself tempted a buyer with deep pockets. It was sold as part of a science and technology auction at Bonhams in New York. Buyers also snatched up a handwritten letter from Charles Darwin for $93,750, a Darwin-signed copy of On the Origin of Species for $125,000, and an Enigma machine used by the German Navy during World War II for $463,500.

Apparently, part of the original mold culture made its way into the hands of Fleming’s housekeeper, who then gave it to their neighbors. As Bonham’s writes, the neighbor scared off would-be burglars from Fleming’s home at one point, so he gave them the mold as a souvenir. Who knows—maybe the buyer purchased the mold as a present for a science lover. In any case, Fleming’s accidental mold is the medical gift that keeps on giving.