According to a recent paper from the university of Edinburgh, the ground of Mars contains a lot of perchlorate, which combined with the high level of ultraviolet radiation, makes a very bacteria hostile environment.
Let's imagine that we are going to colonize Mars. Bacteria which would go out of our spaceships would die because of the environment. Let's now suppose that our spaceship is a sterile environment, that the water is treated against bacteria, and the food is only dry food contained in sterile aluminium cans (no bacteria in it).
It would be reasonable to say that the only possible bacteria in such environment would come from the human body of our astronauts.
Now let's suppose that our astronauts would clean themselves a few times per day with betadyne, and would be under heavy dose of wide-spectrum antibiotics.
How likely a human could survive in such a bactericide environment?
EDIT: I am aware that good bacteria play an important part in our diggestive and immune system, but could we live even without these? The same situation could also be imagined on earth in a sterile bunker.