Allowing the students to carry out this section of the investigation provides an added layer to what would be a standard microbiology assay. It takes the students from ‘start’ to ‘finish’ in terms of extracting and testing their own antimicrobial compounds. Students should set up and perform the extraction to gain first-hand experience of extraction methods.
Plant material can be fresh (for example, a plant leaf) or dried. It needs to be crushed, using a pestle and mortar, to provide a greater surface area. The plant material should be sufficient to fill the porous cellulose thimble (in our experiments we use an average of 14 g of thyme in a 25- x 80-mm thimble).
All equipment should be provided for students to assemble. Allowing students to build the extraction apparatus may give them a greater appreciation for the process of extraction, as opposed to testing an antimicrobial compound out of a purchased bottle. The students should begin by building a rig using stands and clamps to support the extraction apparatus. Following this, the solvent (250 ml of ethanol) is added to a round bottom flask, which is attached to a Soxhlet extractor and condenser (Fig. 1) on an isomantle. The crushed plant material is loaded into the thimble, which is placed inside the Soxhlet extractor. The side arm is lagged with glass wool. The solvent is heated using the isomantle and will begin to evaporate, moving through the apparatus to the condenser. The condensate then drips into the reservoir containing the thimble. Once the level of solvent reaches the siphon it pours back into the flask and the cycle begins again. The process should run for a total of 16 hours. Once the student has set up the extraction it can be left to run without direct supervision. It is not advised to leave the equipment completely alone due to the mix of running water and an electrical appliance, so a technician or other lab user should be made aware. The equipment can be turned on and off when overnight running is not permitted, and the time split over a number of days. For good practice, a control should be added. This could be plant material that has no known antimicrobial effect (for example, a carrier oil such as sunflower oil) at the testing stage.