Antibiotics and developing medicines

Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. Specific bacteria should be treated by specific antibiotics — the right one for the infection, not any one to hand.

The use of antibiotics has greatly reduced deaths from infectious bacterial diseases. However, the emergence of strains resistant to antibiotics is of great concern, which is why they are no longer prescribed for every complaint.

Antibiotics cannot kill viral pathogens. Painkillers and other medicines treat the symptoms of disease but do not kill pathogens — they make you feel better while your immune system does the work. And it is difficult to develop drugs that kill viruses without also damaging the body's tissues, because viruses reproduce inside your own cells: killing the virus tends to mean killing the cell around it.

Bacteria multiply by simple cell division, called binary fission, as often as once every 20 minutes if they have enough nutrients and a suitable temperature. They can be grown in a nutrient broth solution, or as colonies on an agar gel plate.

To investigate the action of disinfectants and antibiotics the culture must be uncontaminated, so it is prepared using aseptic technique.

You need to be able to explain each precaution, not just list it. Petri dishes and culture media are sterilised before use, to kill any unwanted microorganisms already on them. Inoculating loops are sterilised by passing them through a flame before microorganisms are transferred. The lid is held on with adhesive tape rather than sealed all the way round, so contaminants cannot get in but harmful anaerobic bacteria cannot grow either. The dish is stored upside down so that condensation cannot drip onto the agar. And in school laboratories cultures are incubated at 25 °C, because warmer temperatures encourage the growth of pathogens that are harmful to humans.

Traditionally drugs were extracted from plants and microorganisms. Three examples are named: the heart drug digitalis originates from foxgloves, the painkiller aspirin originates from willow, and penicillin was discovered by Alexander Fleming from the Penicillium mould.

Most new drugs are now synthesised by chemists in the pharmaceutical industry, though the starting point may still be a chemical extracted from a plant.

New medical drugs have to be tested and trialled before being used, to check that they are safe and effective. They are extensively tested for three things: toxicity, efficacy and dose — is it poisonous, does it work, and how much should be given.

In a double blind trial neither the patient nor the doctor knows who has the real drug and who has the placebo, which stops either of them influencing the result. And the results of testing and trials are published only after scrutiny by peer review — other scientists check the work before anyone acts on it, which is what stops a false or exaggerated claim entering medicine.

Monoclonal antibodies are produced from a single clone of cells. The antibodies are specific to one binding site on one protein antigen, and so are able to target a specific chemical or specific cells in the body.

That specificity is the whole reason they are useful. An ordinary drug goes everywhere in the body; a monoclonal antibody goes to one thing and nowhere else.

The production sequence is a favourite exam question, and it has to be given in order:

1. Mouse lymphocytes are stimulated to make a particular antibody. 2. The lymphocytes are combined with a particular kind of tumour cell, making a cell called a hybridoma cell. 3. The hybridoma cell can both divide and make the antibody — the lymphocyte contributes the antibody, the tumour cell contributes the ability to divide endlessly. 4. Single hybridoma cells are cloned to produce many identical cells, all producing the same antibody. 5. A large amount of the antibody is collected and purified.

You are not expected to recall any specific test or treatment. What you are expected to do is explain how one works when you are given the information — which always comes back to the same sentence: the antibody binds to one specific antigen, so whatever is attached to the antibody goes only where that antigen is.

Monoclonal antibodies create more side effects than expected, and they are not yet as widely used as everyone hoped when they were first developed. An evaluation question wants the advantages and the disadvantages, and then a judgement.

Advantages: they target one type of cell, so healthy cells are damaged far less than by conventional chemotherapy; they can be produced against almost any antigen; and they made accurate, cheap diagnostic tests possible.

Disadvantages: they cause more side effects than predicted, and they are expensive and difficult to attach to drugs.

There are ethical issues to consider alongside the science. Production begins by stimulating mouse lymphocytes and fusing them with tumour cells, so animals are used and some people object to that. The cost also decides who gets treated: a therapy that works but is unaffordable in most of the world raises a question about fairness that the biology alone does not answer.