Definition
Antimicrobial resistance is the ability of microbes, such as bacteria and fungi, to survive the drugs meant to kill them. When a microbe is resistant, the drug is given but the microbe keeps multiplying, so the medicine stops working.
Only the microbes that survive the drug live on
When a wound gets infected, doctors often use an antimicrobial drug (a medicine that kills bacteria or stops them from multiplying). But the invading bacteria are not one identical clump with the same genes.
As bacteria multiply, small random mutations pop up naturally. Most ordinary bacteria die when the drug hits them. But a few mutant bacteria happen to have a defense against the drug, and they hang on.
With all their competitors gone, the survivors start multiplying fast. They take over the food and the space, and they quickly build a new population.
If this repeats, the bacterial crowd fills up with microbes that have the ability to withstand the drug. In a twist, the drug acts like a filter. It picks out the resistant germs and helps them spread.
Your body isn't getting used to the drug. The microbe is changing.
You may hear people say, "My body got used to the medicine, so it stopped working." But the one that becomes resistant isn't the human body. It's the invading microbe.
Antimicrobial drugs don't target human cells. They attack things like a bacterium's cell wall or its protein-making machinery. So when resistance shows up, it doesn't mean your body changed. It means the target bacteria gained a way to shrug off the drug's attack.
Some bacteria run pumps that push the drug back out of the cell. Others slightly bend the shape of the target protein the drug needs to latch onto, so the drug can't attach.
Bacteria can even hand these defense genes over to neighboring bacteria, sometimes of a different kind, and spread the skill around. This spread of genetic change inside microbe populations is what resistance really is.
A bit more precisely: selective pressure and what scientists wrestle with
To be a bit more precise, antimicrobial resistance is the result of natural selection. The drug creates a harsh environment, and certain microbes survive in it. The drug acts as a strong selective pressure on the bacterial population and gives resistant bacteria room to grow.
Researchers have studied how drug exposure conditions shape this process. If the drug concentration is too low, less-sensitive bacteria survive and can step by step build stronger defenses.
On the other hand, if exposure drags on for too long, even the helpful microbes nearby are under selective pressure for a long time. Between the risk of leftover bacteria and the length of exposure, the paths a microbe can take to survive get very complicated.
This article covers only the biological mechanics of how microbial populations survive and adapt in a drug-filled environment. Working out how drug-exposed microbes survive is the heart of the science here.
Common misconceptions
When you take antibiotics, the body of the person who took them is what becomes resistant.
Resistance isn't your body getting used to a drug. It's a change on the microbe's side. The target bacteria gain the ability to withstand the drug through genetic mutations or adaptation.
If you take antibiotics for a cold or the flu, they will kill the virus causing it.
Antibiotics are drugs that attack bacteria. They don't work on colds or the flu, which are caused by viruses, and they can only put needless selective pressure on the bacteria in your body.
Where you meet it
Staphylococcus (staph) bacteria were once easily wiped out by penicillin. Some evolved to carry an enzyme that breaks penicillin down, which made the old drug useless against them.
Multidrug-resistant bacteria, which survive attacks from several different drugs, are sometimes found in the environment and in hospitals.
In one sentence
Antimicrobial resistance is an evolutionary process. A few microbes survive the drug and multiply, until the whole population is able to resist it.