How does the misuse of antibiotics affect the evolution of disease-causing bacteria?

The antibiotics will kill some bacteria but not all and the ones that are resistant to the antibiotic will survive and reproduce passing on their resistance.

One of the major influences in the development of so called 'super bugs' is the use of low level antibiotics as livestock growth promoters. This practice has been banned in Europe and Scandinavia but is still permitted in American farming due to the influence of the farming lobby. A microbiology professor I spoke to described this as a dangerously trivial use of antibiotics that could seriously threaten the effectiveness of modern medicine.