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Bacterial infections are treated with antibiotics. Antibiotic resistance occurs when an infection responds poorly to an antibiotic that once could treat it successfully. It’s the bacteria that have become resistant to the antibiotic, not the patient. This happens because the bacteria acquire a mutation – a change in their DNA – that gives them a new protein as a tool to fight the antibiotic. There are many mechanisms by which this tool may work. It can:

– prevent the antibiotic from entering the bacterial cell;

– pump the antibiotic out of the cell;

– destroy the antibiotic by enzymatic reaction;

– modify the antibiotic’s target so it no longer binds to the drug;

– or give the cell a way to bypass the antibiotic’s target, making the drug irrelevant.

Mutations in bacterial genome occur all the time, spontaneously, but only the ones that confer a certain advantage would persist to the next generation. Let’s consider a situation when a new mutation emerges and makes the bacteria resistant to a certain antibiotic. In the absence of the antibiotic, such mutation offers no advantage, and because mutations usually come with slower growth, the mutation would be diluted and eventually disappear in the following generations. On the other hand, in the presence of the antibiotic, only bacteria that carry such mutation would survive and they would soon take over the whole bacterial population. The use of antibiotic is therefore the factor that drives the selection of antibiotic-resistant organisms.

Mutations that confer antibiotic resistance can be transmitted not only vertically, from parent cells to offspring, but also horizontally, from one bacterial cell to another, using mobile genetic elements such as plasmids or bacteriophages. This means a bacterial strain can share their antibiotic resistance with other bacterial strains and even with distantly related bacterial species. Horizontal transfer is a major mechanism underlying the spread of antibiotic resistance among bacterial species. These antibiotic-resistant bacteria can infect humans, animals and spread between them through food and the environment.

Antibiotic resistance is one of the biggest global health concerns. Infections by antibiotic-resistant bacteria are much harder, sometimes impossible, to treat. Some bacteria, called superbugs, are resistant to most of the common antibiotics, and are especially difficult to kill. Treatments for infections caused by such bacteria are costly and toxic to the patients.

While the emergence of antibiotic resistance is inevitable, the process is greatly accelerated by misuse and overuse of antibiotics. To help control spread of antibiotic resistance, antibiotics must be taken correctly, only when prescribed by a healthcare professional, who should do so only when antibiotics are needed, according to current guidelines. Antibiotics should not be used to promote growth or prevent diseases in healthy animals. Measures that help prevent infections also help reduce antibiotic overuse.