Bacteria have colonised the planet more thoroughly than any other class of organism. They live 11km deep in the Pacific Ocean, and at altitudes of 40km, near the top of the stratosphere. There are bacteria that live in solid rock, metabolising radioactive waste, and even some that survive in boiling water. The incredibly dry Atacama Desert in Chile was once thought to be lifeless, but in 2006 scientists found bacteria there that take advantage of the minute amounts of moisture absorbed from the air by salty rocks.
In our own environment, we can reduce the number of bacteria but not eliminate them entirely. One species has even evolved to survive the rigorous sterilisation process used in NASA clean rooms. In fact, probably the only place on Earth that we can be confident will always be entirely bacteria-free is the lava crater of an actively erupting volcano.