You store it in a ziplock bag. Moldy makeup sponges do happen, but it's all due to human error (that's your bad). "When you remove oxygen and light from anything that is moist, you have the opportunity for mold and bacteria to start forming," explains Rea Ann. So a damp Beautyblender stuffed in your dark, enclosed makeup bag is a bad idea. Instead, use a breathable mesh or organza bag to transport the sponge when you travel.
You tried to microwave the germs away. "The worst possible thing you can do is put it in the microwave," Rea Ann says. "I don't think it kills bacteria, it just melts everything."
You kept it too long. Even though you clean your sponge every day (right?), the Beautyblender needs to be replaced every three months. The older it gets, the more likely it will rip or tear.