Ah Antimicrobial Investigation

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A Science and Plants for Schools – Student Project Starter Investigating Antimicrobials

This is a project starter, suitable for Advanced Higher biology investigations or A-level extended projects. Don’t forget to credit this resource in your bibliography by including the title, the website, the web address and the date you accessed it.

Background information

One often sees claims that a wide range of products, both synthetic and natural, are antibacterial, antifungal or both. Disc diffusion tests provide a simple and reliable method of testing antibacterial or anti-yeast properties.

Sterile discs of filter paper can be impregnated with standard quantities of the substance to be tested. They are then placed on a freshly spread lawn plate of the test organism. After incubation, the effectiveness of the substance is determined by measuring the area of inhibition of growth around the disc.

For materials which are not liquid, extracts or infusions can be prepared. The effects of preparation method of the material could be explored. Emulsions or creams may need to be mixed with water, another useful point for consideration.

A number of variables that influence the diffusion of the antimicrobial substance may affect the size of areas of inhibition of growth. For example: • depth of the agar
• concentration of agar in the medium
• age of the plates (they may have dried out)
• pH of the medium
• temperature and other relevant conditions of incubation • size of inoculum
• spread of inoculum
• rate of growth of inoculum

These are useful discussion points in the planning and evaluation of project work. Wells

Another way of applying the test material to a plate is by placing it in wells made in the agar plate.

Wells can be made in agar plates by using the wide end of a blunted sterile Pasteur pipette, inserting it and twisting it slightly to remove the plug of agar. Alternatively, cork borers sterilised with alcohol may be used. A mounted needle or a pair of forceps, sterilised by flaming in alcohol, may be required to remove the agar.

The zone of inhibition can be traced on to an acetate grid with felt pen and measured against squared or graph paper to determine its size.

Antimicrobials incorporated in agar

The above techniques are not suitable for examining the effects of antimicrobials on moulds. However, the agar plug technique for making wells can be used to provide a standard size inoculum of a mould which is inoculated on to agar containing the test substance. Some substances may not be suitable for incorporation into agar e.g. if they are too oily.

The inoculum should be taken from a non-sporing region of a 3 – 4 day old mould colony. Note: If the culture is old or producing large numbers of spores, it should not be opened.

After incubation, the size of the colony can be measured, either by measuring the diameter or determining the area from a squared acetate grid.

All of the above techniques provide good quantitative results which can be graphed and analysed.

Technical information

A student who does a project to investigate antimicrobial activity is likely to need the following equipment and materials

Equipment
• Broth cultures of bacteria or yeasts
• Sterile nutrient or yeast glucose agar plates
• Sterile spreader
• Sterile Pasteur pipettes
• Forceps
• Alcohol
• Sterile filter discs
• Marker pen
• Antimicrobials e.g. disinfectants, antibacterial handwash or other products, T-tree oil, garlic, cinnamon

Preparation of Materials

All work involving micro-organisms including preparation of sterile materials and disposal should be carried out with reference to the following publications by SSERC:

• Microbiology and Biotechnology: A Code of Practice for Safe Working with Micro-organisms. • Microbiology and Biotechnology: Microbiology Techniques Cards •...
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