Natural selection acts on existing genetic variation in a population. If there is a mutation that makes the bacteria more fit, then it is more likely to survive and divide to pass its mutation on to daughter cells. Eventually, this variation will become more common in the population. This is natural selection in action.
Examples of mutations that could make the bacteria more fit are if it has antibiotic resistance, the ability to use a new substance as a food source, or is more able to grow at a range of different temperatures.
If no variation exists, then there is no genetic reason one bacterium is fitter than other, so no natural selection will take place.
