How Antibiotics Fight Bacteria

Before the twentieth century, a simple cut or a common infection could be a death sentence. Diseases caused by bacteria spread through communities with little to stop them, and doctors could often do no more than keep a patient comfortable and hope the body would win the fight on its own. The arrival of antibiotics changed medicine profoundly. These are substances that kill bacteria or stop them from multiplying, and they made it possible to cure infections that had once been fatal. Operations, childbirth and injuries all became far less dangerous once the threat of bacterial infection could be controlled.

Antibiotics work specifically against bacteria, and this point is often misunderstood. Bacteria are single-celled living organisms, quite different in structure from the human cells around them and different again from viruses. Because antibiotics target features that are unique to bacteria, they can attack the invaders without harming the patient's own cells. It is precisely these differences that antibiotics exploit. For the same reason, antibiotics have no effect on illnesses caused by viruses, such as the common cold or influenza. Taking an antibiotic for a viral infection brings no benefit and, as we shall see, can cause real harm.

There are several ways in which an antibiotic can defeat a bacterium. Many bacteria are surrounded by a tough outer wall that holds the cell together and protects it. Some antibiotics prevent bacteria from building or repairing this wall, so that the growing cell effectively bursts and dies. Others interfere with the internal machinery that bacteria use to make the proteins they need, or disrupt their ability to copy their genetic material. Whatever the precise mechanism, the aim is either to kill the bacteria outright or to weaken them enough that the body's own immune system can finish the task.

No single antibiotic works against every kind of bacterium. Some drugs, described as broad-spectrum, act against a wide range of bacteria, while others are narrow-spectrum and effective only against particular types. Ideally a doctor identifies the bacterium responsible for an infection and chooses a drug known to work against it. In practice this is not always possible, and treatment sometimes has to begin before the exact cause is known. Choosing well matters, because using a broad-spectrum drug when a narrow one would do can have unwanted consequences.

One of those consequences is harm to the helpful bacteria that live in and on the human body. Not all bacteria are enemies; the gut in particular is home to vast numbers of them that aid digestion and help keep harmful species in check. A powerful antibiotic cannot easily tell friend from foe and may kill these beneficial bacteria along with the ones causing disease. This is why a course of antibiotics sometimes leaves a person with digestive upset, and why such drugs should not be taken carelessly.

The most serious problem facing antibiotics today is resistance. Bacteria multiply rapidly, and each time they divide there is a small chance of a change, or mutation, in their genetic material. Occasionally such a change happens to make a bacterium less vulnerable to a particular antibiotic. When that drug is used, the vulnerable bacteria die but the resistant ones survive and go on to reproduce, passing their resistance to the next generation. Over time, and with repeated exposure to the drug, a whole population of bacteria can become resistant, and the antibiotic that once cured the infection no longer works.

The way antibiotics are used has a powerful influence on how quickly resistance spreads. Every use of an antibiotic gives bacteria another opportunity to develop resistance, so using these drugs when they are not needed, such as for viral infections, only hastens the problem. Stopping a course of treatment too early can be equally unwise, because it may leave behind the hardier bacteria that were slowest to be killed. For these reasons doctors now stress that antibiotics should be taken only when genuinely necessary and exactly as prescribed.

Resistance cannot be avoided altogether, because it arises from the natural process of evolution, but it can be slowed. Using existing antibiotics carefully, developing new ones and improving the way infections are prevented in the first place are all part of the response. The stakes are high, for a world in which antibiotics no longer worked would resemble, in some respects, the dangerous era that existed before they were discovered. Protecting these medicines is therefore not only a task for doctors and scientists but a responsibility shared by everyone who uses them.