S4.4.5 Chapter 5
Antibiotics and medicines
Why did the health worker say that antibiotics will not help my cold?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
- Explain why antibiotics kill bacteria but not viruses
- Explain how misuse of antibiotics leads to resistance
- Describe how new medicines are tested in trials
A medicine is a substance used to treat, cure or prevent a disease, or to ease its symptoms. This chapter looks at three kinds of medicine and how a new one is tested. A health worker decides which medicine a person needs, and how it is used.
A painkiller eases pain. It does not kill pathogens. Medicines for a cold or a fever ease symptoms in the same way, while the white blood cells destroy the pathogen. The body, not the painkiller, defeats the infection.
An antibiotic is a medicine that kills bacteria or stops them growing. The first one was found in a mould, and antibiotics have saved very many lives. Today they treat infections that bacteria cause, such as tuberculosis and some chest and skin infections.
How does an antibiotic work? Bacteria have parts that human cells do not have, such as a cell wall. Some antibiotics stop the cell wall from forming, so the bacterium bursts. Others stop the bacterium making its proteins. They harm the bacteria and do little harm to human cells, though side effects can happen.
A virus is different. It has no cell wall and makes no proteins of its own. It lives inside the cells of the body and uses them to multiply. An antibiotic has nothing to attack. A medicine that harmed a virus inside a cell would risk harming the cell too.
So antibiotics do not work on viruses, for example the viruses that cause colds, influenza and measles. For some viruses, such as HIV, there are other medicines made for those viruses. A health worker decides when they are needed.
Antibiotics have a problem. Over time some bacteria stop being killed by them. This is antibiotic resistance. It is natural selection, which you met in the evolution book, working on bacteria.
Draw it first
How resistance spreads
1 In a large group of bacteria, a few are resistant by chance, because of a mutation in their DNA. The antibiotic kills the others.
2 The resistant bacteria are left with plenty of food and room. They divide.
3 All of their offspring are resistant. The group is now mostly resistant, and the antibiotic no longer works.
Resistance is in the bacteria, not in the person. A person does not become resistant. A person can be infected by bacteria that are already resistant, and then the usual antibiotic will not help.
Some uses of antibiotics make resistance spread faster. Taking them for a cold or influenza does nothing against a virus, but it gives resistant bacteria in the body a chance. Stopping a course early can leave some bacteria alive, and these are more likely to be resistant. Sharing antibiotics or using old ones is also wrong, because it may be the wrong medicine. A health worker says how long to take an antibiotic, and the course should last that long.
Resistant bacteria are a serious problem. Some infections, including some forms of tuberculosis, are now hard to treat. New antibiotics are slow and hard to find. So antibiotics are used only when needed, and as directed.
Try it
A bean model of resistance
You need: 20 dry beans, 18 of one kind and 2 of another, a cup, a sheet of paper and a pencil, a spare handful of both kinds of bean.
Be safe: Keep beans out of mouths, and away from small children. Do not eat the beans you used.
- Put the 18 beans of one kind and the 2 of the other kind into the cup. The 2 stand for resistant bacteria. Write the numbers as round 1.
- The antibiotic arrives. Take out all 18 beans of the first kind. Write what is left as round 2.
- The survivors divide. For each bean in the cup, add one more bean of the same kind from your spare handful. Write the numbers as round 3.
- Let them divide once more, and write the numbers as round 4.
What you will see: The resistant beans are the only ones left after the antibiotic. After they divide, there are more of them each time. The whole group now carries resistance. Real bacteria divide much faster than you can add beans.
Now we turn to new medicines. Before a new medicine is used, scientists must show that it works and that it is safe. They test it in steps, called a trial. The picture shows them.
First, the medicine is tested in a laboratory, on cells. This shows whether it works against the disease, and whether it harms cells. Then, in most places, the medicine must be tested on animals before any person takes it. Rules aim to limit harm and to use as few animals as possible.
Next, a few healthy volunteers take small amounts. This checks that the medicine is safe for people. Everyone who takes part has been told what the test involves, and chooses to take part.
Last, the medicine is tested on patients who have the disease. They are put into two groups by chance. One group gets the medicine. The other gets a placebo, which looks the same but has no medicine in it. When a good treatment already exists, the second group gets that treatment instead of a placebo, so that nobody goes without care.
Why a placebo group? Some people in any group get better, because many illnesses get better on their own. And people who expect to get better may feel better. Only a comparison with the placebo group shows how much the medicine itself helps.
The trial is also double blind. Neither the patients nor the doctors who check them know who is in which group until the end. This stops hopes and guesses from changing what people report or notice. Many patients are needed, so that chance does not decide the result.
The bar chart shows a made up result for a trial with 100 patients in each group. It shows how many patients in each group got better.
Worked example
Use the bar chart. How many more patients got better in the medicine group than in the placebo group?
- Read the bars. Medicine group: 72. Placebo group: 40.
- Subtract: 72 − 40 = 32.
Answer: 32 patients
Worked example
In another trial, 60 of the 80 patients in the medicine group got better. In the placebo group, 48 of the 120 patients got better. By how many percentage points is the share in the medicine group higher?
- The groups are not the same size, so use percentages. Medicine group: 60 ÷ 80 × 100 = 75.
- Placebo group: 48 ÷ 120 × 100 = 40.
- Subtract: 75 − 40 = 35 percentage points.
Answer: 35 percentage points
A wrong idea many people have
If I take antibiotics a lot, my body becomes resistant to them.
Why it feels right: A medicine can seem to work less well each time, and people say that the body gets used to it.
The test that breaks it: The cells of your body do not change when you take an antibiotic. Resistance is a feature of bacteria. A mutation in a bacterium makes it resistant, and its offspring inherit the mutation.
What is true: The bacteria become resistant, not the person. Using antibiotics only when needed keeps them working for everyone.
Key words
- medicine
- a substance used to treat, cure or prevent a disease, or to ease its symptoms.
- painkiller
- a medicine that eases pain. It does not kill pathogens.
- antibiotic
- a medicine that kills bacteria or stops them growing. It does not work on viruses.
- antibiotic resistance
- bacteria that an antibiotic can no longer kill.
- placebo
- a treatment that looks the same as a medicine but has no medicine in it.
- double blind
- a trial in which neither the patients nor the doctors know who gets the medicine.
Back to your guess
The answer is C: A cold is caused by a virus, and antibiotics kill bacteria, not viruses.
Antibiotics work by harming bacteria. A virus is not a bacterium, and it multiplies inside the cells of the body, so antibiotics have no effect on it. A cold is caused by a virus.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Why will an antibiotic not help a person with a cold?
See the answer
A cold is caused by a virus, and antibiotics do not work on viruses.
A virus has no cell wall and makes no proteins of its own, so an antibiotic has nothing to attack. It multiplies inside body cells.
- If you chose A: Mildness is not the reason. A cold is caused by a virus, and antibiotics do not work on viruses.
- If you chose C: A cold is caused by a virus, not a fungus.
- If you chose D: Viruses are usually much smaller than bacteria. An antibiotic has nothing to attack in a virus, because it has no cell wall or protein making of its own.
What does an antibiotic do?
See the answer
It kills bacteria or stops them growing.
Antibiotics harm parts of bacteria that human cells do not have, such as the cell wall.
- If you chose A: Antibiotics work on bacteria only.
- If you chose B: Antibodies are made by white blood cells. A vaccine helps the body make them. Go to S4.4.3.
What is antibiotic resistance?
See the answer
Bacteria that an antibiotic can no longer kill.
A mutation can make some bacteria resistant. They survive the antibiotic and multiply, and so the group is soon mostly resistant.
- If you chose A: The person does not become resistant. The bacteria do.
- If you chose B: Resistance is about bacteria that survive the antibiotic, not harm to the person.
- If you chose C: Antibiotics do not work on viruses at all. Resistance is about bacteria.
Look at the picture. What percentage of the bacteria in the first panel are resistant?
See the answer
12.5%
The first panel shows 16 bacteria, and 2 of them are hatched, which means resistant. 2 ÷ 16 × 100 = 12.5.
Which of these helps resistance spread?
See the answer
Using an antibiotic for a cold.
A cold is caused by a virus, so the antibiotic does no good. It still gives resistant bacteria in the body a chance to multiply.
- If you chose A: Finishing the course kills the infection. It is the right thing to do.
- If you chose B: That is the right way to use them. It keeps antibiotics working.
Why does a medicine trial have a placebo group?
See the answer
To compare, so that any extra improvement can be put down to the medicine.
Some patients get better anyway. Only a comparison with a group that has a placebo shows what the medicine itself does.
- If you chose B: Safety is checked earlier, on volunteers. The placebo group is there for a fair comparison.
- If you chose C: A placebo does not make anyone better or worse by being a medicine. It is there for comparison.
What does double blind mean in a medicine trial?
See the answer
Neither the patients nor the doctors know who is getting the medicine.
If nobody knows who gets the medicine, hopes and guesses cannot change what people report or notice.
- If you chose A: Blind here means not knowing, not not seeing.
- If you chose B: Double blind is about who knows, not about how many medicines there are.
- If you chose C: Double blind is not about repeating the trial.
From an earlier chapter
Which kind of medicine is given to prevent a disease before the pathogen comes?
See the answer
A vaccine
A vaccine trains the body, so that the response to the real pathogen is fast. Antibiotics treat a bacterial infection that has already started.
- If you chose A: An antibiotic treats a bacterial infection that is already there.
- If you chose C: A painkiller eases pain. It does not prevent a disease. Go to S4.4.4.
In a trial, 64 of 80 patients who took a medicine got better. What percentage got better?
See the answer
80%
64 ÷ 80 × 100 = 80.
In a trial, 100 patients took a medicine and 70 got better. 100 patients took a placebo and 45 got better. By how many percentage points is the share who got better higher in the medicine group?
See the answer
25 percentage points
Medicine group: 70 ÷ 100 × 100 = 70. Placebo group: 45 ÷ 100 × 100 = 45. 70 − 45 = 25.
Keep it
Copy this into your exercise book.
Antibiotics kill bacteria, not viruses. Misuse lets resistant bacteria survive and multiply. New medicines are tested on cells, usually on animals, then volunteers, then patients, with a placebo group and double blind.
Draw: Two boxes of bacteria, one before and one after an antibiotic, with the resistant ones hatched.