Antibiotics and Drug Testing

Antibiotics are drugs which specifically kill bacteria. The discovery of antibiotics revolutionised medicine and saved millions of life. You need to know how to compare the effectiveness of different antibiotics and how the overuse of antibiotics can result in antibiotic resistance.

Antibiotics

Antibiotics are substances which prevent the growth of bacteria. There are two types of antibiotics - bacteriocidal antibiotics which kill bacterial cells and bacteriostatic antibiotics which inhibit the growth of bacteria. They work by either damaging the cell wall or preventing protein synthesis:

1. Antibiotics can prevent bonds forming between the murein (peptidoglycan) molecules which make up the cell wall. This weakens the cell wall and prevents bacterial cells from growing properly. If water enters the bacterial cell by osmosis, the weakened cell wall cannot withstand the increased hydrostatic pressure and can burst (lysis), killing the bacteria.

2. Antibiotic molecules bind to bacterial ribosomes and prevent them from carrying out translation. This prevents the synthesis of enzymes (which are proteins), which catalyse important metabolic reactions. If these metabolic reactions do not take place, the bacterial cell cannot grow and reproduce properly.

This explains why antibiotics have no effect on eukaryotic cells (such as human cells) since eukaryotic cells do not have a cell wall and their ribosomes are a different size (which means that antibiotics cannot bind to them). Similarly, antibiotics have no effect on viruses because they do not possess either a cell wall or ribosomes. This is why it would be pointless prescribing antibiotics for a viral infection, such as measles or the flu.

Preparing plant extracts for antimicrobial testing

Some plants, including garlic, mint and turmeric, contain natural antibiotic molecules and can inhibit the growth of microorganisms. There’s loads of other plants in the world which haven’t been tested and may also have antimicrobial properties - this is one of the reasons why maintaining plant biodiversity is so important. These compounds can be extracted and used to manufacture antibiotic drugs.

In order to test a plant for antimicrobial properties, you will first have to prepare a plant extract using the following method:

1. Dry out the plant by leaving in the sun until the water has evaporated out then grind using a pestle and mortar.

2. Soak the ground plant tissue in ethanol - this will extract the antimicrobial substances which are soluble in ethanol.

3. Filter the solution to remove the pieces of plant tissue. Keep the solution containing the dissolved plant extract.

Testing antibiotics

The efficiency of different antibiotics (or plant extracts containing antimicrobial substances) can be tested by soaking pieces of filter paper in the antibiotic solutions and placing on a Petri dish containing bacteria. The stronger the antibiotic, the more bacteria are killed. Regions of the Petri dish where bacteria have been killed will appear as a clear zone - the size of the clear zone is proportional to the strength of the antibiotic. The experiment is carried out by carrying out the steps below:

Aseptic Techniques

When carrying out experiments involving bacteria, it is important to carry out aseptic (sterile) techniques. This is because bacteria are everywhere - all over your hands, in your breath and covering the lab surface - and you don’t want cross-contamination to interfere with your results. Aseptic techniques include the following:

• Use of sterile equipment - this can be done using a machine called an autoclave which looks like a dishwasher but uses steam and high pressure to sterilise the equipment.

• Disinfect the lab bench and work surfaces.

• Wear gloves.

• Place a Bunsen burner near to your work space when transferring bacteria from one container to another. The heat from the flame draws any microorganisms in the air away from the work space because hot air rises.

• Close windows and doors to prevent draughts blowing microbes towards you.

Hospital Acquired Infections (HAIs)

HAIs are infections which are transmitted in hospital. They spread as a result of poor hygiene practices and can spread easily in hospitals because patients have weakened immune systems. The spread of HAIs can be prevented by good hygiene practices, such as hand washing and sterilising surgical equipment. Infections caused by antibiotic-resistant bacteria such as MRSA are particularly dangerous forms of HAIs because they cannot be treated by antibiotics. Hospitals put measures in place to prevent the development of antibiotic resistance, including:

1. The rotated use of different antibiotics

2. Taking the full course of antibiotics

3. Avoiding overuse of antibiotics (e.g. for minor infections or for viral diseases).

Historical Drug Testing

Back in the eighteenth century there wasn’t such thing as clinical trials and placebos - doctors had the power to give their patients whatever ‘cure’ they thought might work and hoped for the best. The classic example is that of William Withering, a doctor who in 1785 heard that one of his patients who had swollen tissues recovered after drinking a herbal remedy. He found out that the remedy contained foxglove extract (Digitalis) - the problem was that too little digitalis would be an ineffective treatment whereas too much would be fatal since foxgloves are poisonous. Not letting it worry him too much, he prepared a digitalis soup which a concentration he hoped would work and gave it to his patients. He varied the concentration he gave to each patient and used a crude method of trial and error to work out which was the optimum concentration.

Modern Drug Testing

Nowadays, drug testing is much more rigorous and (usually) safer than Withering’s deadly soup. Before a drug is available to the public, it must first be tested for toxicity on animals before undergoing three phases of testing.

• Phase 1 - the drug is tested on a small group of healthy volunteers. This is to check for side effects and to establish a safe dosage.

• Phase 2 - the drug is given to a larger group of people who have the disease that the drug is designed to target. The purpose of this phase is to check whether the drug actually works to reduce symptoms. Phase 2 trials will usually involve a placebo - this is an inactive substance which resembles the drug and is used to rule out any psychological benefits patients may have from believing they are receiving treatment. Phase 2 trials are often double-blind studies, which is when neither the patients nor the doctors know who is receiving the treatment and who is receiving the placebo, since the doctor’s expectations may influence the results and make the data biased.

• Phase 3 - the drug is given to hundreds or thousands of patients in order to compare the drug to existing treatments. The patients are randomly split into two groups where one group receives the new drug and the other receives the best treatment currently available. Phase 3 trials can also involve a double-blind design where neither patients or the doctors know which patients are receiving the new treatment.

There are instances where it is unethical to use a placebo in a clinical trial. For example, if you were conducting a phase 2 trial of an anti-cancer drug, it would be unethical to give cancer patients an inert substance. In these cases, existing cancer treatments will be given to the control group.

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