Creationism does not explain how bacteria become resistant over time to antibiotics. Starting a lesson on Natural Selection with Darwin and working your way toward genetic mutations that cause bacteria to adapt to toxins, is backwards.
Our understanding of genetics makes it more practical today to start with the tiny, generational steps in evolution that occur with each cell division and the tiny, almost unnoticeable mutations that occur in DNA with every new generation of an organism.
It's important to understand the impact of environmental conditions on the adaptation and survival of an organism. When a bacteria is exposed to toxins, most will die off, but there will be a small number that, through DNA mutation, survive in the new environment. This raises questions about whether the adaptation to the toxin was intelligent, or random chance.
The statistics involved in calculating how many bacteria survive an attack by antibiotics suggests that adaptive mutations occur not because of the presence of antibiotics, but despite it. In other words, there will always be rare mutations of offspring in any species. Some will be capable of surviving a drastic change in the environment and some will not be capable of surviving in a normal environment.
Biology throws a wide net of genetic mutations when it comes to reproduction of a species.Nature governs which members survive and which members thrive with superior features. The superior members of a species then dominate the inferior members which whither and die. This applies just as much to weeds overcrowding a vegetable garden as it does to human poverty overcrowding the avarice of a civilization's elite, depending on where your social values reside.
The genetic mutations are so small with each generation that it's hard to identify an intermediary fossil between two drastically different versions of the same species.