0610 · 15.1Drugs

Drugs, bacteria and viruses

0:00Imagine two people feel unwell, and both ask for antibiotics. One has a bacterial infection, while the other has influenza, usually called flu. Antibiotics may help the first person, but they will not treat the second person's infection. Why does the same medicine work against one cause of illness but not the other, and why can using it unnecessarily make future infections harder to treat? By the end of this video, you will understand what a drug is, how antibiotics treat bacterial infections, and why they do not affect viruses.

0:30We will also follow what happens when some bacteria survive an antibiotic. That will explain why using antibiotics only when essential helps preserve their effectiveness. First, remember that your body depends on chemical reactions. These reactions release energy, build substances and help your cells function. A substance that changes these reactions can change how your body works. The syllabus definition is precise. A drug is any substance taken into the body that modifies or affects chemical reactions in the body.

1:00That definition is much broader than the everyday idea of an illegal drug. Medicines can be drugs too, because they affect chemical reactions in the body. The word drug describes what a substance does, not whether it is helpful, harmful, legal or illegal. In this topic, we are focusing on one important group of medicines called antibiotics. We need just one distinction before discussing antibiotics. Bacteria are cells, with structures and chemical processes that allow them to live and reproduce.

1:28Viruses are not cells, and they reproduce using the machinery inside living host cells. This difference matters because a treatment that attacks a bacterial structure or process will not necessarily have anything equivalent to attack in a virus.

How antibiotics and resistance work

1:42Antibiotics are used for the treatment of bacterial infections. They kill bacteria, and some antibiotics work by stopping bacteria from growing and reproducing. Different antibiotics act on different bacterial structures or processes. As one useful example, some prevent bacteria from building their cell walls properly, which can cause susceptible bacteria to die. You do not need to memorise a catalogue of antibiotic mechanisms here. The important idea is that antibiotics interfere with features of bacteria.

2:11Viruses do not have bacterial cell walls or the same independent chemical machinery. Antibiotics therefore do not affect viruses, so they do not work against colds or flu. Notice that treating an infection is different from relieving its symptoms. A medicine might reduce a fever or ease pain without killing the organism causing the illness. Feeling a little better after taking a medicine does not prove that the infection itself has been treated.

2:35The treatment must match the cause, rather than simply match how unpleasant the symptoms feel. Now comes the problem of resistance. Some bacteria are resistant to particular antibiotics, meaning those antibiotics do not kill them or stop their growth effectively. Other bacteria are susceptible, meaning the antibiotic does affect them. Resistance reduces an antibiotic's effectiveness because the resistant bacteria can remain alive even when the antibiotic is used. How can resistance arise in the first place?

3:03A chance mutation, which is a change in genetic material, can make a bacterium resistant to an antibiotic. The mutation does not happen because the bacterium wants protection or tries to adapt. It arises by chance, and the antibiotic then changes which bacteria are most likely to survive. Think of the antibiotic as a filter, rather than a teacher. It does not teach bacteria how to become resistant. Instead, it removes susceptible bacteria while resistant bacteria survive.

3:31Those survivors can reproduce and pass on the resistance, so resistant bacteria can become a larger proportion of the population.

Selection of resistant bacteria

3:39Let us follow that sequence carefully. Before treatment, a bacterial population may contain mostly susceptible bacteria and a few resistant bacteria. When the antibiotic is used, susceptible bacteria die, but resistant bacteria survive. The survivors then reproduce, producing more resistant bacteria, so the population becomes harder to treat with that antibiotic. This explains why using antibiotics only when essential can limit the development of resistant bacteria. Every unnecessary use creates another opportunity for susceptible bacteria to be killed while resistant ones survive and reproduce.

4:13Avoiding unnecessary use reduces these opportunities for selection. It does not guarantee that resistance will never arise, but it helps limit its development and spread. An important example is methicillin resistant Staphylococcus aureus, often shortened to M R S A, a bacterium resistant to certain antibiotics. Do not interpret that name as meaning that every antibiotic is ineffective against it. The key point is that resistance makes an infection harder to treat because an antibiotic that would normally work may no longer be effective.

4:45Using antibiotics only when essential helps limit the development of resistant strains like this one. Here is a worked example using an imagined population, not experimental results. Suppose there are ten bacteria, with nine susceptible to an antibiotic and one resistant. Before treatment, only one out of ten bacteria is resistant. Now suppose the antibiotic kills all nine susceptible bacteria while the resistant bacterium survives. Immediately afterwards, the total number of bacteria is smaller, but every surviving bacterium is resistant.

5:17The survivor can reproduce, and its offspring can inherit the resistance. So killing most of the original bacteria does not necessarily mean that the infection has been successfully eliminated. We must consider which bacteria survived, not just how many died. This is also why you should take antibiotics exactly as prescribed and complete the prescribed course unless your healthcare professional changes the instructions. Feeling better does not necessarily mean that the bacteria causing the infection have all been eliminated.

5:45Stopping treatment on your own can leave surviving bacteria able to reproduce. Taking leftover antibiotics or someone else's prescription is not a suitable way to decide whether treatment is essential.

Explaining antibiotic resistance

5:56Watch out for three common misunderstandings. First, it is the bacteria that become resistant, not the person taking the medicine. Second, bacteria do not deliberately change because they need to survive an antibiotic. Third, an antibiotic is not a general treatment for every infection, because it does not affect viruses. Now try an exam-style question. Explain how using antibiotics only when essential can limit the development of resistant bacteria. The command word explain means you need to connect the cause to the outcome, rather than simply say that unnecessary use is bad.

6:30Start with the difference between susceptible and resistant bacteria, then follow survival and reproduction. Finish by showing how reducing antibiotic use reduces this selection. A clear answer would be this. Resistance can arise through a chance mutation in a bacterium. Antibiotics kill susceptible bacteria, while resistant bacteria survive. Resistant bacteria reproduce and pass on their resistance to their offspring. Using antibiotics only when essential reduces opportunities for resistant bacteria to be selected and reproduce, limiting the development of resistant strains.

7:05Let us put the whole topic together. A drug is any substance taken into the body that modifies or affects chemical reactions in the body. Antibiotics treat bacterial infections by killing bacteria or stopping their growth, but they do not affect viruses. Some bacteria are resistant, which reduces antibiotic effectiveness, and using antibiotics only when essential helps limit the selection and reproduction of those resistant bacteria.