Core Practical Garlic and Mint

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Purpose

·     To investigate the antibacterial properties of plants.

·     To develop certain experimental skills, such as working safely, producing valid reliable results, recording results and drawing valid conclusions from results.

Antibacterial chemicals

Plants are susceptible to infection by bacteria and fungi; they do everything to repel such attacks. Several plants are known to, or thought to, destroy or inhibit the growth of certain bacteria. A plant with this property is known as antibacterial.

Chemicals in their cells are toxic to bacteria or interfere with their metabolism in some other way.
You can probably guess why there is mint in toothpaste, but would garlic be better? Mint may numb our gums but is it lethal to bacteria? In this activity you will investigate whether two plants contain antibacterial chemicals and their effectiveness by looking at the growth of bacteria on agar plates.

Before you start, read through the procedure and suggest what you might expect to observe on the plates. Decide how you would take precise measurements to enable you to make valid conclusions from the data.

Safety

Methylated spirits is toxic and highly flammable and because of the latter hazard should not be used while naked flames are in use – which happens in the preparation and pouring of agar plates.

Use aseptic techniques. Do not open Petri dishes containing growing microorganisms. Only bin used Petri dishes after they have been autoclaved.

Procedure

1      Agar plates seeded with suitable bacteria need to be prepared. This may have been done for you in advance; if not, follow the instructions on the sheet Pouring agar plates (page 3). The Practical Support has a sheet on plate pouring and aseptic technique.

2      Obtain a plant extract by crushing 3 g of plant material with 10 cm3 of industrial methylated spirit and shake it from time to time for 10 minutes. The advantage of using methylated spirits instead of water is that it kills any bacteria that might otherwise contaminate the extract.

3      Pipette 0.1 cm3 of extract onto a sterile 13 mm Whatman antibiotic assay paper disc. (If these are not available, discs cut from new filter paper using a hole punch can be used.)


4      Let the paper discs dry for…

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