What causes the mutant form of disease-causing bacteria to become predominant?

Answer 1

The forces that affect bacteria population are competition, mutation and selection, just like any other population affected by the theory of evolution. In a large population of bacteria, some individuals will be mutated to have resistance to antibiotic drugs (or any other drug). This is a disadvantage to the individual as long as there is no antibiotic drug in the surroundings. In this case there is mutation but no selection. When an antibiotic drug is given to a patient, selection is being done in the evolutionary aspect of the bacteria. Only the drug-resistant mutants survive, since the selection does not affect them. All the non-mutant bacteria die; the mutants have no competition, and can flourish undisturbed.