How Do Antibiotics Work?

Antibiotics are chemical compounds — originally found in nature, now often made in laboratories — that are toxic to bacteria but largely harmless to human cells. They work by exploiting the differences between bacterial biology and human biology.

Bacteria have structures and processes that our cells simply do not have: different types of cell walls, different protein-building machinery, different ways of copying their DNA. Antibiotics target those specific bacterial features. They are precision weapons aimed at things the bacteria need and we do not.

One crucial point: antibiotics do not work on viruses. Viruses and bacteria are entirely different kinds of organisms. A virus does not have a cell wall to break down or bacterial protein machinery to disrupt. Taking antibiotics for a cold or flu does nothing to treat the illness.

Different antibiotics use different strategies. Some are bactericidal — they actively kill bacteria by attacking the cell wall, which human cells do not have. Without its cell wall, the bacterium cannot hold its shape and bursts. Penicillin and amoxicillin work this way.

Others are bacteriostatic — they do not kill bacteria directly but stop them from reproducing. They interfere with the mechanisms bacteria use to make proteins or copy their DNA. Without the ability to multiply, the bacterial population stalls while your immune system destroys what is already there.

Your doctor's choice of antibiotic depends on what type of bacteria is causing the infection, where in the body it is, and which antibiotics are likely to work against that particular strain.

Once you swallow an antibiotic, it moves into your digestive system and is absorbed into the bloodstream — usually within an hour or two. From there, it travels around the body and starts reaching the tissues where the bacteria are active.

The antibiotic does not navigate on its own — it circulates broadly. This is why some antibiotics are better suited to certain locations. An antibiotic that concentrates well in urine is more useful for a UTI than one that does not reach that tissue effectively.

As the antibiotic reaches the bacteria, it begins attacking them while your immune system continues to fight alongside it. The two work in parallel — the antibiotic weakening or killing bacteria, your immune defences clearing the remnants. When you stop taking an antibiotic, levels in your body drop, which is why completing the full course matters.

You might take your first dose and feel noticeably better within a day. What has usually happened is that the bacteria have stopped multiplying — which immediately reduces the pace of damage — and the inflammation in your body is beginning to settle.

But feeling better and being free of infection are not the same thing. Surviving bacteria may still be present, just not reproducing at the rate they were. Stop taking the antibiotic early and those bacteria can rebound — and the ones that survive are more likely to be resistant to that medicine.

How quickly you feel improvement also depends on where the infection is. Skin infections can respond within a day or two. Deeper infections in the lungs, sinuses, or ears take longer because it takes time for the antibiotic to reach those tissues in effective concentrations.

This depends on several things at once: the type of infection, the specific antibiotic, the dose, and the person's own immune response. There is no single answer that applies to all situations.

For many straightforward infections — a UTI, strep throat, a simple skin infection — people often notice some improvement within 24 to 48 hours. For others — a chest infection, sinus infection, or ear infection — symptoms can take several days to ease noticeably even when the antibiotic is working as it should.

Anyone who is not improving as expected, or who gets worse during a course of antibiotics, should speak to a healthcare professional rather than waiting and hoping.

Antibiotics are active in your body as long as they maintain a therapeutic level in your blood and tissues. The dosing schedule — twice a day, three times a day, or once a day — is designed to keep levels high enough to remain effective throughout.

The course length — typically five to ten days depending on the infection — is set to ensure bacteria are fully suppressed long enough for your immune system to finish the job.

Once you stop, levels drop. The bacteria are no longer held back. This is why completing the full course as prescribed matters, even if you feel completely better before it ends. Stopping early is one of the main drivers of antibiotic resistance.

In September 1928, Alexander Fleming returned to his laboratory at St Mary's Hospital in London after a summer holiday. He had left bacterial cultures in petri dishes before he left. When he came back, one had been contaminated by mould.

Most scientists would have thrown it away. Fleming looked closer. The bacteria around the mould were dead. A clear ring of empty space surrounded it, as though whatever the mould was producing had refused to let anything live nearby.

The mould was Penicillium notatum. The substance it produced became penicillin — the world's first antibiotic. Within two decades, it was being manufactured at industrial scale and saving lives on a staggering scale. Fleming did not set out to change medicine that day. He came back from holiday and looked carefully at something that had apparently gone wrong. That habit of curiosity is the reason antibiotics exist.