Bacterial growth, plant medicines and drug testing — Edexcel International A Level Biology

Revision notes

4 short notes, in the order of the specification. Each one in short:

1. Conditions required for bacterial growth

   Bacteria grow and divide quickly when they have a supply of nutrients, a suitable temperature, the correct oxygen level, a suitable pH and enough water. These conditions are provided in the laboratory using sterile nutrient broth or agar. Bacteria reproduce by binary fission, so a population can multiply rapidly when conditions are right.

2. Antimicrobial and therapeutic substances from plants

   Many plants make chemicals that protect them against microorganisms and herbivores, and some of these have antimicrobial or other therapeutic effects in humans. Examples include allicin from garlic, salicin from willow bark, and digitalis from foxglove. Plants are therefore a source of potential new medicines, which is one reason to conserve biodiversity.

3. Core practical 9: antimicrobial properties of plants and aseptic technique

   Practical

   Antimicrobial properties of plants are investigated by spreading bacteria on agar, adding discs soaked in plant extracts and incubating. A clear zone of inhibition around a disc shows that the extract prevents growth. Aseptic technique prevents contamination of the culture and protects the worker, and controls show that the effect is due to the extract.

4. The development of drug testing

   Early drug testing, such as William Withering's work on digitalis soup, relied on observing patients without proper controls. Modern testing uses laboratory and animal tests, then three phases of clinical trials, with placebos, random allocation and double-blind design. These reduce bias and the placebo effect, and protect the safety of patients.