The four postulates of natural selection apply to bacteria. This is best seen in antibiotic resistance. Some antibiotics are initially successful at treating a certain disease, however, over time, the evolution of resistance in bacteria may occur. Individual bacteria in a population vary in traits (genes) due to mutations allowing for natural selection. The bacteria with a genome resistant to the antibiotic will survive and if the resistance in the bacteria is heritable, the offspring will also survive. If more offspring are produced than can survive and the survival and reproduction are not random, that is, only those bacteria that developed resistance survived to reproduce, the initial non-resistant strain of bacteria will be evolving through natural selection to create a resistant strain.
On the other hand, microbial evolution can come about in other ways besides antibiotic resistance. Microbes are easily dispered and undergo horizontal gene transfer, the transferring of genetic material other than by descent. I believe HGT is another form of natural selection. As bacteria adapt to new environments, often HGT occurs creating a variety of traits in a population. If the traits are heritable, there are high rates of population growth, and the survival and reproduction of the individuals are not random but connected to the heritable variation among individuals, the individual bacteria with the most favorable variations are naturally selected. Natural selection does not require vertical gene transfer, it can occur through transformation, conjugation, or transduction as well.