Yes, heat kills germs, so if you heat something to a sufficient temperature in a microwave - typically around 165 degrees Fahrenheit -- any bacteria on it will die, too.
Problem is, microwaves don't always heat things evenly. This is especially true when you're reheating something. Microwave-ready meals are usually designed in a way to heat evenly.
When heating anything else in a microwave, though, part of the food might get more than hot enough to kill off bacteria, and other parts might be cold.
So, while you're right that microwaves sterilize, you can't count on the microwave getting all of the germs to a sufficient temperature. Probably more than fine for a day-old slice of pizza, but not so much for your home dentistry kit.
I believe there is also some speculation that the microwave's radiation kills germs, too. But as far as I know, the only thing that's proven 100% is that heat kills germs. This has been established ever since Louis Pasteur discovered that heating milk is an effective method for preventing the spread of illness. Pasteur also lent his efforts to wine, not just milk. He was French, after all.
Posted 2960 day ago