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What is antibiotic resistance?

Published: 23 February 2021

Antibiotics were nothing short of miraculous when they were first mass produced in 1940s. Infections from surgery, childbirth, even scraping your knee went from killing millions every year, to being easily treated.

Antibiotics work by killing or stopping the growth of bacteria. But these bacteria are always changing. Every living thing, including bacteria, undergoes natural changes to its DNA. Sometimes these changes give bacteria mechanisms that protect them from antibiotics. When bacteria evolves new characteristics that mean they cannot be killed by antibiotics, the bacteria have developed antibacterial resistance.  

Every year around 700,000 people die from bacterial infections that couldn't be treated with antibiotics, and without action that number could increase to 10 million a year by 2050.

How does antibiotic resistance happen?

Bacteria are living organisms. Like all living things they make mistakes in copies of their genetic code when they reproduce. Some of these mistakes, or mutations, have no effect, while others are negative and mean those offspring struggle to survive. Very occasionally a mutation gives that bacterium an advantage, making it more likely to reproduce, and pass that good mutation on to its offspring.  

When a colony of disease-causing bacteria is exposed to an antibiotic, all bacteria should be either killed or prevented from multiplying. But if one of the bacteria in the colony has a mutation that helps it survive and it continues to reproduce, it will create an entire strain that can now no longer be treated by that antibiotic.

How do antibiotic-resistant mutations help bacteria?

Antibiotic-resistant genes can work in a variety of clever ways. Some bacteria have genes that produce special chemicals, or enzymes that act like a pair of molecular scissors that cut up an antibiotic and stop it working. Other bacteria produce enzymes that modify the shape of the antibiotic so it can’t stick where it needs to in order to attack it. 

Implanted in the cell walls of bacteria are efflux pumps. These pumps expel any unwanted or harmful chemicals from inside the bacteria. Some drug-resistant E.coli and Salmonella bacteria may have evolved to have extra efflux pumps to quickly remove any antibiotic that makes it into the bacterium cell, before it does any damage.

A diagram of how antibiotic resistance works
Bacteria can use a number of different mechanisms to fight off an antibiotic.

How do antibiotic-resistant mutations spread?

Bacteria can pass genes to one another in two different ways: vertically and horizontally.  

Bacteria reproduce through cell division, where one 'parent' cell splits and becomes two 'daughter' cells through a process called mitosis. If a 'parent' cell contains a gene for drug resistance, this gene will be passed vertically, down the family tree, on to its 'daughter' cells. As bacteria can reproduce very quickly (some divide every 20 minutes), drug resistance can spread rapidly through a colony. 

Bacteria can also pass resistance to one another horizontally. Bacteria can pass sections of DNA to their neighbours, allowing useful mutations to spread.

Antibiotic resistance in healthcare

A significant factor in the increase of antibiotic resistance is people not being given a high enough dose of antibiotics to clear an infection or not completing the full course of antibiotics. The 'weaker' bacteria will be killed, but the stronger bacteria that can survive a low dose of antibiotic are left behind. These bacteria can then multiply, creating a new infection made entirely of stronger drug-resistant bacteria.  

However, over-use of antibiotics is also just as significant a problem. If antibiotics are used when they're not needed, to prevent an infection, treat a very mild infection, or where an infection isn't caused by bacteria, drug-resistant bacteria are given an opportunity to thrive and multiply. 

NHS leaflet from 2006, informing the public not to overuse antibiotics.
Enquiries to Science Museum, London More information about NHS leaflet from 2006, informing the public not to overuse antibiotics.

Antibiotic resistance in agriculture

As agriculture has become increasingly mechanised, antibiotics have been used on a large scale by farmers as a tool to increase growth, prevent infections and reduce mortality in their livestock. 

Nearly half of all antibiotics used in the UK are given to farm animals. In the United States, where the use of antibiotics in animals is less tightly regulated, 80% of all antibiotics are used in agriculture. 

In intense farming where large herds live in very close quarters, infection control is essential as disease can spread incredibly quickly. Across the world antibiotics are administered as a preventative measure when there is believed to be a risk of infection to a herd. This exposes more bacteria to antibiotics and increases the chances of antibiotic-resistant mutations.  

Scientists have identified that agricultural use of antibiotics is one of the most significant contributors to the rise in drug-resistant bacteria. In the 1960s the antibiotic colistin, deemed too dangerous for humans, was heavily administered to pigs in China. When doctors turned to colistin as a 'last resort antibiotic', they discovered that around 4% of the world's bacteria were already resistant to it.

Header image: Clusters of methicillin-resistant Staphylococcus aureus (MRSA) bacteria © Annie CavanaghAttribution-NonCommercial 4.0 International (CC BY-NC 4.0)