Resistant bacteria — AQA GCSE Combined Science

Mutations in bacterial pathogens create new strains with different characteristics. If a mutation gives resistance to an antibiotic, that strain is not killed by the antibiotic. The resistant bacteria survive, reproduce by binary fission, and pass the resistance allele to their offspring, so the proportion of resistant bacteria in the population rises. This is natural selection. The resistant strain then spreads between people because humans are not immune to the new resistant strain, and because the antibiotic no longer treats the infection effectively. For example, MRSA is a strain of Staphylococcus aureus resistant to methicillin and other antibiotics, so infections are harder to treat. Overuse of antibiotics increases selection pressure and speeds up the spread of resistance.

MRSA stands for meticillin-resistant Staphylococcus aureus, a bacterium that commonly lives harmlessly on skin but can cause serious infection if it enters wounds or the bloodstream. Resistance means an antibiotic that once killed the bacterium, or stopped its growth, no longer works against it. In MRSA this resistance arose by natural selection: random mutations in bacterial DNA produced some individuals able to survive exposure to meticillin and related antibiotics. Those survivors reproduced rapidly by binary fission, passing the resistance allele to their offspring, so the resistant population increased. Because MRSA is not affected by many commonly used antibiotics, infections are harder to treat and doctors must use other antibiotics or combinations. This explains why antibiotic resistance is a serious medical and evolutionary issue.

Antibiotic resistance develops when bacteria with random mutations survive exposure to an antibiotic and reproduce. The more often bacteria meet an antibiotic, the stronger the selection pressure favouring resistant individuals. To slow this process, doctors prescribe antibiotics only when they are needed, for example for bacterial infections rather than viral illnesses such as colds. Patients should complete the full prescribed course so that all susceptible bacteria are killed and partially resistant survivors are less likely to remain. Using the correct antibiotic for the specific infection, and avoiding unnecessary or broad use in agriculture, also reduces selection pressure. Good hygiene and infection control limit the spread of resistant strains between people. These actions do not stop mutation, but they reduce the chances that resistant bacteria survive and multiply.

Antibiotics kill bacteria by disrupting cell wall formation, protein synthesis or DNA replication, but they have no effect on viruses because viruses lack these bacterial targets and reproduce inside host cells. Prescribing antibiotics for a viral infection such as influenza or the common cold therefore gives no benefit to the patient while exposing the patient's bacteria to the drug. Any bacterium with a random mutation that lets it survive is more likely to reproduce, so the resistant allele increases in the population. Doctors reduce this selection pressure by prescribing antibiotics only when a bacterial infection is likely and the illness is serious enough to need them, for example a confirmed bacterial chest infection, rather than a mild sore throat that is probably viral.

A course of antibiotics is prescribed so that the concentration of the drug remains high enough for long enough to kill the bacteria causing the infection. If a patient stops early because they feel better, some bacteria may still be alive. These survivors are more likely to include individuals with random mutations that make them less affected by the antibiotic. They reproduce quickly by binary fission, so the resistant allele becomes more common in the population. Completing the course keeps killing susceptible bacteria and reduces the chance that resistant survivors multiply and spread. This is why patients should follow the prescribed dose, interval and duration rather than stopping when symptoms improve.

Antibiotics are medicines that kill bacteria or stop their growth. When farmers give antibiotics to livestock, often to prevent disease in crowded conditions or to promote faster growth, bacteria in those animals are exposed to the drug. Any bacterium with a random mutation that lets it survive reproduces rapidly, so resistant strains build up in the animals, in their waste and in the environment. These resistant bacteria can reach humans through food, water or direct contact, making infections harder to treat. Restricting agricultural use means giving antibiotics only when a vet confirms a bacterial infection, rather than routinely, which slows the spread of resistance and keeps these medicines effective for treating serious human infections.

Discovering and testing a new antibiotic takes many years and large amounts of money. Researchers must find a compound that kills bacteria, then test it for safety and effectiveness in laboratories, animals and human trials before it can be prescribed. Because bacteria reproduce rapidly and mutations for resistance appear constantly, new resistant strains can emerge faster than new drugs can be approved. This means relying only on new antibiotics is not a complete solution. Reducing unnecessary use, completing prescribed courses and preventing infection through hygiene and vaccination all help slow resistance, so existing antibiotics remain useful for longer.