The marble statues in the Medici Chapels in Florence, Italy, have accumulated a lot of stains over the centuries. Researchers had spent much of the past decade cleaning the part of the mausoleum that was created by Renaissance artist Michelangelo in the early 1500s. But a few obstinate spots remained. So last fall, as the chapel operated on reduced hours because of the Covid-19 pandemic, scientists completed a secret experiment: unleashing grime-eating bacteria on the masterpieces. They first tested eight different strains on a patch of marble behind an altar and eventually used the bacteria that worked best on more prominent locations. The nontoxic microbes fed on dirty substances such as glue, oil, and even bodily fluid stains from a corpse in one of the tombs. And it worked: The statues are now gleaming and ready for visitors to admire. “It’s very strange, especially in this time of Covid,” a tourist commented when she learned about the bacteria. “But if it works, why not?”