Bacteria reproduce by dividing into two, and these new individuals grow so quickly that they are ready to divide again in about half an hour. Hence, in ten hours, under the most favorable conditions, a single bacterium can produce over a million bacteria. That is one reason for it being so difficult to ensure any object is completely free from any kind of living organisms. In addition, some forms of bacteria have a waxy envelope outside their cell wall and are thus more difficult to kill.