Why do evolutionists think that resistant bacteria is proof of evolution?

It would be better phrased to say that *increased* resistance to stronger antibiotics is evidence for evolution. When an antibiotic is introduced, it is usually done so because the effects are dramatic in suppressing bacterial populations. Therefore, an increasing incidence of resistant bacteria goes to show that the selection pressure from antibiotics induced a need for a resistance factor in order for the bacteria to survive and reproduce. Bacteria without the resistance factor simply die off. Successive generations will have increased resistance. In turn, we will need to synthesise newer, more effective means of killing bacteria. The mantra is : Variation -> Selection -> Reproduction