The nooks, crannies, and general dampness of a frequently used sponge is an ideal breeding ground for a host of different microbes. To keep the infestation—and smell—at bay, many people will pop a wet sponge in the microwave for a minute. But according to a new study, even that isn’t enough to kill them all. In fact, it could make things worse.

Microbiologists in Germany put 14 different types of kitchen sponges to the test , collecting them from households in and around the city of Villingen-Schwenningen. (People voluntarily donated their sponges for science, apparently.) The researchers then cut the sponges up and sampled the top and bottom of each. They used DNA sequencing to determine which types of microbes were living on the sponge and a special type of laser-powered microscopy to see where they were breeding.