Explain how antibiotics work and why they only treat bacterial infections, the aseptic techniques used to culture microorganisms, the clear-zone core practical, and the stages of developing new medicines.

What this dot point is asking

Edexcel statements 5.16 to 5.20B want you to explain how antibiotics work and why they only treat bacterial infections, describe the aseptic techniques used to culture microorganisms (5.17B Biology only), carry out the core practical investigating antiseptics or antibiotics (5.18B) including clear-zone area calculations (5.19B), and describe the stages of developing new medicines.

How antibiotics work

Because antibiotics target features unique to bacteria, they cure bacterial diseases without harming you. Viruses reproduce inside host cells and do not carry out these bacterial processes, so antibiotics have nothing to act on, which is why they do not treat viral infections such as colds and flu. Overuse of antibiotics drives the evolution of resistant bacteria (see the natural selection page), so they should only be used when needed.

Aseptic technique (Biology only)

To study microorganisms you grow (culture) them on a nutrient agar plate. Aseptic technique keeps out unwanted microbes that would contaminate the result or be dangerous:

• Sterilise the growth medium and Petri dishes, for example using an autoclave (high-pressure steam).
• Pass the inoculating loop through a flame before and after use to kill microbes on it.
• Work near a Bunsen flame, which creates an updraught that carries airborne microbes away.
• Tape the lid of the Petri dish on (without sealing it completely) and incubate it the right way up, usually at no more than $25\,^{\circ}\text{C}$ in schools, to reduce the risk of growing harmful pathogens.

The core practical: antiseptics and antibiotics

In the core practical (5.18B) you spread bacteria over an agar plate and place paper discs soaked in different antiseptics, antibiotics or plant extracts on it. After incubation, each effective substance produces a clear zone (zone of inhibition) where the bacteria have been killed and cannot grow. A larger clear zone means a more effective antimicrobial. To compare fairly, use the same bacteria, the same volume of substance and the same incubation conditions.

Calculating the area of a clear zone

where $r$ is the radius of the clear zone. Larger areas indicate stronger antimicrobial action, so this lets you rank the substances.

Developing new medicines

New medicines must be tested for safety (is it toxic?), efficacy (does it work?) and the right dose before they can be used. The process has several stages:

1. Discovery and development: a possible new drug is found or designed.
2. Preclinical testing: the drug is tested on cells, tissues and then animals to check it is safe and works.
3. Clinical testing: the drug is tested on healthy volunteers (for safety) and then on patients (for the right dose and efficacy), often using placebos and double-blind trials.

Only drugs that pass every stage are approved, which is why developing a new medicine takes many years.

Try this

Q1. State why antibiotics do not cure a cold. [1 mark]

• Cue. Colds are caused by viruses, which live inside cells and lack the bacterial processes antibiotics target, so antibiotics have no effect.

Q2. A clear zone has a radius of $6\ mm$. Calculate its area using $\pi r^{2}$. [2 marks]

• Cue. Area $= \pi \times 6^{2} = \pi \times 36 = 113.1\ mm^{2}$ (to 1 d.p.).