Antibiotics are compounds that can kill or interfere with the growth of microorganisms such as bacteria or viruses. Because antibiotics are often used unnecessarily or incorrectly, microorganisms have been able to adapt and survive. This is called antibiotic resistance. One way this happens is genetic mutations, which are random changes in genetic material that can be passed on from one virus or bacterium to successive generations. For example, penicillin is one of the oldest and most commonly used antibiotics. Some bacteria have developed an enzyme or protein that changes the structure of penicillin and prevents it from killing bacteria.