Since the metal wall is conductive, the standing wave pattern inside a typical microwave oven has a magnitude that goes to zero at the conductor surface. The heating power also goes to zero at the wall serface. Additionally, anything in thermal contact with the metal wall will remain close to the temperature of the wall even if there is a heat flux. Bacteria may well be cooked on the carousel plate, but there will be regions within the cooking cavity where bacteria will not experience high enough temperatures for sanitization.