Sponges

"To kill germs in kitchen sponges, microwave them at full power
for two minutes. This reduces microbes by more than 99 percent.
Sponges are a common source of microbes, including Salmonella,
especially when they remain wet. Air-drying sponges does not
significantly reduce germs, and though putting them in the dishwasher
cleans them, it doesn't sufficiently decontaminate them. To guard
against fire, make sure sponges contain no metal and are wet before
you microwave them. Be careful when you remove them, since they
will be very hot."

-University of California, Berkeley Wellness Letter July 2012