What antibiotics are
Antibiotics are medicines that kill bacteria or stop them from multiplying. Some come from compounds made by molds or soil microbes, and others are made in laboratories. The best-known early example is penicillin, whose antibacterial effect was noticed by Alexander Fleming in 1928 and which became widely available as a medicine in the 1940s. The arrival of antibiotics is often counted among the major medical breakthroughs of the twentieth century, because infections that were once frequently deadly became treatable.
This article is general educational information. It is not medical advice, and decisions about treatment belong with a qualified health professional.
Bacteria, not viruses
Antibiotics work against bacteria, which are single-celled organisms with their own machinery for building cell walls, making proteins and copying DNA. They do not work against viruses. Viruses are far smaller, lack that machinery, and instead take over the cells of their host to reproduce. Colds, flu and many other common illnesses are caused by viruses, so an antibiotic does not help with them. Taking one anyway offers no benefit and can contribute to resistance.
What antibiotics target
Antibiotics are useful because they attack features of bacterial cells that human cells lack or have in a different form. That lets them harm the bacteria while generally sparing the person. Different classes of antibiotics typically work in one of a few ways.
- Cell walls. Many bacteria are surrounded by a rigid wall. Penicillin and related drugs interfere with building it, so the cell weakens and bursts. Human cells have no such wall.
- Protein production. Bacteria make proteins using ribosomes, which differ in structure from those in human cells. Several classes of antibiotics bind to bacterial ribosomes and block protein-making.
- DNA copying and repair. Some drugs interfere with the enzymes bacteria use to copy or coil their DNA.
- Essential chemical pathways. Others block steps in chemical reactions that bacteria need to make certain vital molecules.
Some antibiotics kill bacteria outright, while others only halt growth long enough for the body’s immune system to clear the infection. Some work against a broad range of bacteria, and others against a narrow group.
How resistance arises
Antibiotic resistance means that bacteria survive and keep growing despite a drug that would normally stop them. It is not the human body becoming resistant; it is the bacteria changing.
Bacteria reproduce quickly, and their DNA sometimes changes by chance when it is copied. Most changes do nothing, but occasionally one happens to help a bacterium survive a drug, for example by changing the drug’s target, producing an enzyme that destroys the drug, or pumping the drug out of the cell.
Bacteria can also pass resistance to one another. They can swap small circular pieces of DNA called plasmids, and this can happen even between different species. A resistance trait can therefore spread much faster than ordinary inheritance would allow.
Why it is natural selection
Resistance is a textbook case of natural selection. In a large population of bacteria, a few may be slightly less affected by a drug. When the drug is used, the susceptible bacteria die and the less susceptible survivors are left to multiply, passing on the trait. Over time the resistant type can become common. This is the same logic Charles Darwin described for living things generally, but observed within days or weeks in bacteria instead of over thousands of years. It is also one of the clearest examples of ongoing evolution that can be seen in the modern world.
Why resistance matters
When bacteria resist the usual antibiotics, infections can be harder to treat, may need stronger or more toxic drugs, and can last longer. Public health organizations treat resistance as a serious concern. Much of modern medicine, including surgery, cancer treatment and organ transplants, depends on being able to treat or prevent bacterial infections, so a decline in effective antibiotics would affect far more than the original illness.
What contributes to resistance
Resistance develops whenever antibiotics are used, but certain patterns speed it up:
- using antibiotics for illnesses they cannot treat, such as viral infections
- not following the prescribed course as directed
- using antibiotics widely in settings where they are not clearly needed, including in some farming practices
- spread of resistant bacteria in hospitals and communities when infection control is weak
Preventing infections in the first place reduces the need for antibiotics. Good hygiene helps, and so does vaccine impact: vaccines against bacterial diseases mean fewer infections to treat.
What is being done
Researchers look for new antibiotics and for other approaches to infections, but developing a new drug takes many years and is uncertain. Health systems also work on tracking resistant bacteria, improving how antibiotics are prescribed, and educating the public. The details differ between countries.
Practical takeaways
Without giving medical advice, a few general points are widely shared by health authorities: antibiotics are for bacterial infections only, they should be taken only when a clinician prescribes them, and they should be taken as directed. Individual cases differ, so questions about a specific illness or prescription are best put to a doctor or pharmacist.