Microwaves can not penetrate more than 2 cm’s (1-inch) into food. So microwaving becomes dependant on the size, shape and nature (liquids/solids) of the food you are reheating.
By default does the microwave kill bacteria?
No. Not in the way you would expect it to.
Surprisingly, most people think that a microwave cooks from the inside out. This is simply not the case.
A microwave actually cooks from the outside in. Meaning that the surface cooks before the core of the food does.
This results in cold spots where bacteria can survive the heating process. This is most evident when attempting to use the defrost function of the microwave oven. The surface can become cooked, while the inside core remains frozen.
However, the heating process, like any other oven, can easily kill bacteria if used correctly. This means fully frosting foods and cooking these for the correct amount of time.
If we look at how the microwave works, then NO.
The walls of the microwave oven do not get heated to the point that can kill bacteria. This means that you need to keep your microwave clean. Any food spillages, especially blood spillage from meats should be cleaned and disinfected immediately. Essentially following the clean-as-you-go procedure.
Studies have shown that both E.coli and Salmonella can survive inside the microwave. This study claimed that microwaves can actually be microbial hotspots.
Not only is the inside of the microwave risky, but also pose a risk in public/general areas. Especially in offices and retail stores, the handle for the microwave ovens have been shown to be cleaned less frequently than expected. Creating a high chance of cross-contamination.