Final answer:
The overuse and incorrect use of antibiotics has resulted in the natural selection of resistant forms of bacteria, leading to an increase in antibiotic resistance.
Step-by-step explanation:
One of the main reasons for resistant bacteria is the overuse and incorrect use of antibiotics, such as not completing a full course of prescribed antibiotics. The incorrect use of an antibiotic results in the natural selection of resistant forms of bacteria. The antibiotic kills most of the infecting bacteria, and therefore only the resistant forms remain. These resistant forms reproduce, resulting in an increase in the proportion of resistant forms over non-resistant ones.