Antibiotic resistance occurs when an antibiotic has lost its ability to effectively control or kill bacteria growth; in other words, the bacteria are " resistant" and continue to multiply in the presence of therapeutic levels of an antibiotic. Antibiotic resistance evolves naturally via natural selection through random mutation, but it could also be engineered by applying an evolutionary stress on a population. If a bacterium carries several resistance genes, it is called a super bug. Infections caused by resistant microorganism often fail to respond conventional treatment, resulting in prolonged illness , greater risk of death and higher costs.