Antibiotics and Antiseptics
In this lesson you will learn about antibiotics, why they only work against bacteria, the critical problem of antibiotic resistance, and the difference between antibiotics, antiseptics, and disinfectants. This is one of the most frequently examined topics in GCSE Biology.
What Are Antibiotics?
Antibiotics are chemicals that kill bacteria or stop them from reproducing inside the body. They are used to treat bacterial infections.
How Antibiotics Work
Antibiotics target structures and processes found in bacterial cells but not in human cells:
| Target | How the antibiotic works | Example |
|---|
| Cell wall synthesis | Prevents bacteria from building or maintaining their cell walls → the cell bursts (lysis) | Penicillin |
| Protein synthesis | Blocks bacterial ribosomes from making proteins → bacteria cannot grow or reproduce | Tetracycline, erythromycin |
| DNA replication | Prevents bacteria from copying their DNA → cannot reproduce | Ciprofloxacin |
| Metabolic pathways | Interferes with essential chemical reactions in the bacterium | Trimethoprim |
Bactericidal vs Bacteriostatic
| Type | Action |
|---|
| Bactericidal | Kills bacteria directly (e.g. penicillin) |
| Bacteriostatic | Stops bacteria reproducing, giving the immune system time to destroy them (e.g. tetracycline) |
Why Antibiotics Do NOT Work Against Viruses
This is one of the most important facts in this topic:
Antibiotics cannot kill or stop viruses because:
- Viruses are not cells — they are tiny particles with genetic material inside a protein coat
- Viruses do not have cell walls — so antibiotics that target cell walls (like penicillin) have nothing to act on
- Viruses do not have their own ribosomes or metabolic pathways — so antibiotics that target these have nothing to block
- Viruses replicate inside host cells — any drug that targeted the virus's replication machinery would also damage the host cell
This is why doctors should never prescribe antibiotics for viral infections such as colds, flu, or COVID-19. Taking antibiotics for a viral infection:
- Will not help the patient recover
- May cause side effects
- Contributes to the development of antibiotic resistance
Exam tip: "Antibiotics do not work on viruses" is one of the most tested statements in GCSE Biology. Make sure you can explain WHY — viruses do not have the cellular structures (cell walls, ribosomes, metabolic pathways) that antibiotics target, and viruses replicate inside host cells.
The Discovery of Antibiotics
Alexander Fleming and Penicillin
- In 1928, Alexander Fleming noticed that a mould (Penicillium notatum) had contaminated one of his bacterial culture plates
- The bacteria around the mould had been killed — the mould was producing a substance that destroyed bacteria
- Fleming named this substance penicillin
- Howard Florey and Ernst Boris Chain later developed penicillin into a usable drug in the 1940s
- Penicillin was the first antibiotic and revolutionised medicine
- Fleming, Florey, and Chain shared the Nobel Prize in Physiology or Medicine in 1945
Antibiotic Resistance
Antibiotic resistance is one of the greatest threats to global health. It occurs when bacteria evolve to survive exposure to antibiotics that previously killed them.
How Resistance Develops
- Within any population of bacteria, there is natural genetic variation due to mutations
- Some bacteria may have random mutations that give them resistance to a particular antibiotic (e.g. a mutation that changes the shape of the protein targeted by the antibiotic)
- When the antibiotic is used, it kills the non-resistant bacteria but the resistant bacteria survive
- The resistant bacteria reproduce rapidly (by binary fission), passing the resistance gene to their offspring
- The entire population becomes resistant to the antibiotic — this is natural selection
- The resistant bacteria can also transfer resistance genes to other bacteria through horizontal gene transfer (e.g. via plasmids)
Key Concept: Natural Selection in Action
This is a perfect example of evolution by natural selection:
- Variation exists in the population (some bacteria have resistance mutations)
- There is a selection pressure (the antibiotic kills non-resistant bacteria)
- Resistant bacteria are better adapted to survive
- They survive and reproduce, passing resistance genes to offspring
- Over time, the resistant trait becomes more common in the population
Exam tip: Antibiotic resistance is a very common 6-mark question topic. Make sure you can describe the process step by step using the language of natural selection: mutation → variation → selection pressure → survival of the fittest → reproduction → resistant population. The antibiotic does NOT cause the mutation — the mutation arises randomly BEFORE the antibiotic is applied.
MRSA — A Resistant Superbug
MRSA stands for Methicillin-Resistant Staphylococcus aureus.
- Staphylococcus aureus is a common bacterium found on the skin and in the nose
- Some strains have evolved resistance to methicillin and many other antibiotics
- MRSA is particularly dangerous in hospitals where patients may have weakened immune systems or open wounds
- MRSA infections can cause serious skin infections, wound infections, pneumonia, and bloodstream infections (septicaemia)
- Treatment requires the use of last-resort antibiotics such as vancomycin, but even resistance to these is emerging
Reducing Antibiotic Resistance