Drugs
What a drug is, how antibiotics treat bacterial infections, and why resistant bacteria are a problem.
Drugs and Antibiotics
A drug is any substance taken into the body that modifies or affects chemical reactions in the body. Antibiotics are drugs used for the treatment of bacterial infections.
| Pathogen | Effect of antibiotics |
|---|---|
| Bacteria | Antibiotics kill the bacteria, so they treat bacterial infections |
| Viruses | Antibiotics do not affect viruses, so they do not treat viral infections |
Antibiotics are drugs: they are taken into the body and kill bacteria, so they are used to treat bacterial infections.
Antibiotics are used only for infections caused by bacteria. An infection caused by a virus will not be cured by antibiotics.
Is an antibiotic suitable?
| Cause of the infection | Antibiotic suitable? |
|---|---|
| Bacteria | Yes: it kills the bacteria |
| Virus | No: it does not affect viruses |
| Bacteria that are resistant to that antibiotic | Less effective: the resistant bacteria are not killed |
Worked example (choosing a treatment): A patient has an infection caused by a virus. Explain why an antibiotic is not a suitable treatment. Step 1. Antibiotics kill bacteria. Step 2. Antibiotics do not affect viruses, so they will not treat a viral infection. Answer: antibiotics do not affect viruses.
Antibiotic Resistance
Some bacteria are resistant to antibiotics. An antibiotic does not kill them, so the antibiotic becomes less effective at treating the infection.
| Type of bacteria | Effect of the antibiotic |
|---|---|
| Not resistant | Killed by the antibiotic |
| Resistant | Not killed; the infection can continue |
Worked example (effect of resistance): State the effect on an antibiotic of some bacteria being resistant to it. Step 1. Resistant bacteria are not killed by the antibiotic. Step 2. The antibiotic is therefore less effective. Answer: reduces effectiveness.
ExtendedHow resistant bacteria become common
In a population of bacteria, a few may be resistant by chance. When an antibiotic is used, the bacteria that are not resistant are killed. The resistant bacteria survive, reproduce, and pass on resistance. After some time, most of the bacteria are resistant.
Worked example (explaining resistance): Most bacteria in a population are now resistant to an antibiotic. Explain why. Step 1. The antibiotic killed the bacteria that were not resistant. Step 2. The resistant bacteria survived, reproduced and passed on resistance. Answer: resistant survive and reproduce.
Using Antibiotics Wisely
ExtendedLimiting the development of resistance
The more often antibiotics are used, the more often resistant bacteria survive and reproduce while the others are killed. Using antibiotics only when essential limits the development of resistant bacteria such as MRSA, so that the antibiotics keep working when they are really needed.
| Use of antibiotics | Effect on resistant bacteria |
|---|---|
| Used often, including when not needed | More chances for resistant bacteria to survive, reproduce and spread |
| Used only when essential | Fewer chances; development of resistance is limited |
Worked example (explaining wise use): Explain how using antibiotics only when essential can limit the development of resistant bacteria such as MRSA. Step 1. Each use of an antibiotic kills non-resistant bacteria but allows resistant ones to survive and reproduce. Step 2. Using antibiotics less often gives resistant bacteria fewer chances to survive and spread. Step 3. So resistant bacteria such as MRSA are less likely to become common. Answer: less use; fewer chances for resistance.
Worked example (antibiotics and viral infections): A patient asks for an antibiotic for a cold. Give two reasons why the doctor refuses. Step 1. A cold is caused by a virus, and antibiotics do not affect viruses. Step 2. Using an antibiotic when it is not essential gives resistant bacteria more chances to survive and reproduce. Answer: virus; limits resistance.
Exam advice
Common mistakes
Model answer
Recall checklist
- Define a drug.
- State what antibiotics are used to treat.
- State that antibiotics kill bacteria but not viruses.
- State the effect of resistant bacteria on antibiotics.
- Describe how resistant bacteria become common (Extended)
- Explain why antibiotics should be used only when essential (Extended)
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