The E. coli is one of the most common
bacteria, and can be found abundantly in the
human intestine (about 0.1% of all the bacteria in the intestine, based on
Western diet). In each such
bacterium there are only about 5,000
genes. This is the reason it is the first
organism to have its
genome mapped.
Under favourable conditions, a population of E. coli can double in number every 12.5 minutes. So if you start with one, in two hours you will have about 1000, and in four hours about a million. This is faster than the rate of human reproduction.
A bacteria can have a 1 in 200 chance of having a mutation in its genes. This is the reason that bacteria can adapt so quickly to changing environment (in your face proof of evolution). In four hours, one successful mutation has become one million.
This fast adaptation is one of the reasons for bacterias' success as an organism.