Antibiotic resistance GCSE biology describes how bacteria evolve, through natural selection, to survive antibiotics that once killed them. Random mutations occasionally make a bacterium resistant; when antibiotics kill the non-resistant majority, the resistant survivors multiply freely, so resistance spreads through the population over successive generations of bacteria.
How does antibiotic resistance happen?
Bacteria reproduce extremely quickly, and every time a bacterium divides, there is a small chance of a random mutation occurring in its DNA. Occasionally, a mutation happens to change a protein in a way that makes the bacterium less affected by a particular antibiotic — for example, by altering the target the antibiotic normally binds to, or by producing an enzyme that breaks the antibiotic down.
Most of the time, this mutation makes no difference because the bacterium is never exposed to that antibiotic. But when the population is exposed, everything changes.
How does natural selection explain antibiotic resistance?
Antibiotic resistance is a textbook example of natural selection acting within a human lifetime, rather than over thousands of years:
- A population of bacteria contains huge numbers of individuals, a tiny fraction of which carry a resistance mutation by chance.
- An antibiotic is used, killing the non-resistant bacteria.
- The resistant bacteria survive, because the antibiotic cannot kill them.
- With competitors removed, the resistant bacteria reproduce rapidly, passing the resistance mutation to their offspring.
- Over successive generations, the proportion of resistant bacteria in the population increases.
This is natural selection: the antibiotic acts as the "selection pressure," and resistance is the characteristic that increases survival and reproduction under that pressure.
Worked example: how quickly can a resistant population grow?
Bacteria reproduce by binary fission, and under good conditions a population can double roughly every 20 minutes. This worked example shows why a small number of resistant survivors can become a large population quickly.
Question: An antibiotic kills all but 10 resistant bacteria from a much larger population. If the resistant bacteria double every 20 minutes, how many resistant bacteria are present after 4 hours?
Working:
- 4 hours = 240 minutes
- Number of 20-minute doubling periods = 240 ÷ 20 = 12
- Population after 12 doublings = 10 × 2¹²
$$10 \times 2^{12} = 10 \times 4096 = 40,960$$
- Resistant population after 4 hours ≈ 40,960 bacteria
This is why resistant infections can become established and spread so quickly once a resistant strain survives a course of antibiotics — the surviving population multiplies exponentially, unopposed by competitors that were killed off.
What is MRSA and why is it called a superbug?
MRSA (Methicillin-resistant Staphylococcus aureus) is a well-known example of an antibiotic-resistant bacterium, sometimes called a superbug because it has evolved resistance to methicillin and several related antibiotics.
Staphylococcus aureus itself is a common bacterium, often found harmlessly on human skin. MRSA strains have accumulated mutations (and, in some cases, resistance genes passed between bacteria) that make many standard antibiotics ineffective against them. Infections caused by MRSA are harder to treat, particularly in hospital settings where vulnerable patients are more at risk and the bacterium can spread between people.
How can the spread of antibiotic resistance be slowed?
Slowing resistance means reducing the selection pressure that favours resistant bacteria and limiting how far resistant strains can spread.
| Action | Why it helps |
|---|---|
| Always complete a full course of antibiotics | Stopping early leaves partially-resistant bacteria alive, which can multiply and pass on resistance |
| Avoid prescribing antibiotics for viral infections | Antibiotics do not affect viruses; unnecessary use only increases selection pressure on bacteria |
| Reduce antibiotic use in farming | Widespread agricultural use increases the bacterial population exposed to antibiotics, accelerating resistance |
| Maintain strict hygiene in hospitals | Prevents resistant strains such as MRSA spreading between patients |
| Develop new antibiotics and alternatives | Provides new selection pressures that current resistant strains have not yet adapted to |
Doctors are also encouraged to use narrow-spectrum antibiotics (targeting a specific type of bacterium) rather than broad-spectrum antibiotics wherever possible, since broad-spectrum use applies selection pressure to a wider range of bacterial species at once.
Frequently asked questions
Can antibiotics cause a bacterium to become resistant?
No — antibiotics do not cause the mutation itself. The resistance mutation arises randomly, independent of whether the antibiotic is present. What the antibiotic does is apply a selection pressure: it kills non-resistant bacteria, leaving resistant bacteria to reproduce and become a much larger proportion of the population. This distinction — mutation is random, selection is not — is a common exam question.
Why is it so difficult to develop new antibiotics faster than resistance spreads?
Developing a new antibiotic requires years of research, testing, and regulatory approval, while bacteria can develop and spread resistance within a much shorter timeframe due to their rapid reproduction rate. Each new antibiotic also eventually faces the same selection pressure once it is widely used, meaning resistance to it can, in principle, evolve too — so slowing the spread of existing resistance remains just as important as developing new drugs.
Are viruses affected by antibiotic resistance?
No. Antibiotics only work against bacteria, not viruses, because they target bacterial structures and processes (such as cell walls or protein synthesis machinery) that viruses do not have. Antibiotic resistance specifically refers to bacteria evolving to survive antibiotics; using antibiotics against viral infections such as colds or flu contributes to resistance without treating the illness.
Does antibiotic resistance ever disappear from a bacterial population?
Resistance can decline if the antibiotic is withdrawn and maintaining the resistance mechanism carries some cost to the bacterium (for example, using extra energy), giving non-resistant bacteria a slight reproductive advantage once the selection pressure is removed. In practice, resistance genes often persist at low levels within a population for a long time, which is why reducing unnecessary antibiotic use remains important even after resistant strains seem to have become less common.
For Socratic GCSE biology with Professor Darwin — tracing antibiotic resistance from population to gene to protein — visit aitutors.me.