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Rouch Science Carriacou: Antibacterial cream challenge

Updated Sunday, 27th January 2008

Explore the science behind the team's challenge to make an antibacterial cream and prove that it works.

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Time to get thinking.

An antiseptic is a substance that destroys, or stops the growth of, germs on living tissue. They must be strong enough to fight germs but mild enough not to irritate sensitive tissues. Antiseptics should not be confused with disinfectants and antibiotics.

An antiseptic cream that causes the death of bacteria is called a bactericidal cream or antibacterial cream and is useful for preventing cuts, scratches and insect bites from becoming infected.

anti-bacterial cream on the surface of the skin

What is the best way to approach this challenge?

First we need to know what we are fighting.

Bacteria are single celled organisms and can be divided into three groups according to their shape. To the naked eye they may look identical but under a microscope they look very different. They can only be seen under a microscope because they measure just 1 - 5 microns (µm) across.

magnified look at types of bacteria

bacteria dividing Bacteria usually increase in number by dividing down the middle and splitting into two. Before dividing, the bacteria must make enough protein and DNA for two complete progeny cells. When these cells mature, they will in turn divide so that the number of bacteria doubles each generation.

As bacteria can divide as often as once every 15 minutes, one single bacterium can lead to a vast population after 24 hours. The following animation shows how a single bacterium will divide:

Bacteria are very diverse. They can cause skin irritations, stomach upsets, and in the worst cases, fatal diseases. Not all bacteria are harmful though and many help humans, for example, in the production of food and medicine. We also need bacteria so that our bodies work properly.

harmful bacteria helpful bacteria

How can we grow bacteria?

We need a suitable surface and container to grow bacteria. Petri dishes are used in biology laboratories but on the island, bottle top lids were a perfect substitute.

As well as a stable physical environment, bacteria need nutrients on which to feed. Traditionally, scientists have used types of jelly. The most common are agar and gelatin.

Agar is made by boiling seaweed in fresh water, straining and reducing it to a thick syrup. When it cools it forms a jelly-like substance.

Is agar alone sufficient for growing bacteria?

Bacteria need vitamins, proteins and minerals not found in agar and so we add a nutrient broth, which is also referred to as 'media' by microbiologists, to the Petri dishes.

What else do bacteria need?

Because we want to make a cream that destroys bacteria living in or on humans, we need to provide the bacteria with an environment which is broadly similar to that found in or on our bodies.

Our mouths are a good home for bacteria. They tend to be humid and at a temperature of about 37oC, so it's logical to grow bacteria at the same temperature. If it gets too hot (say over 40oC) the bacteria will die but if it gets too cold the bacteria simply won't grow.

bacterial growth chart

We are nearly ready to collect our bacteria. We have agar on which bacteria can grow, suitable nutrients and a bug-friendly environment but there is one last step before catching the bugs themselves: all equipment that could be in direct contact with the bacteria or the media has to be sterilised.

The best methods of sterilisation are autoclaving and baking in an oven. Metal equipment such as tweezers, wire loops and spoons are sterilised before use by dipping them into 70% ethanol (or powerful rum!) and setting them alight.

oven for sterilisation

Finally, it's time to catch some bacteria.

Where do we get the bacteria from?

Bacteria can be found both in and on our bodies but one of the most accessible and populated areas is the mouth. By wiping sterile swabs around the inside of the mouth, placing the saliva into a small bottle of enrichment media (the nutrient broth), and incubating them at body temperature for an hour, the colony will begin to grow.

Just a small drop of this enrichment culture spread on the surface of some nutrient agar in a Petri dish will produce colonies of bacteria if they are kept in an insulated box.

What do you think you would see if you had the ideal concentration of bacteria? Hover your mouse over the dishes below to find out.

Wrong - There is nothing growing on this plate. Try again Wrong. This agar jelly has just turned a funny colour.

Wrong. Hairs indicate a growth of fungi.

Correct! Colonies of bacteria look like little dots - and smell pretty bad.

Now that we have grown the bacteria, how can they be destroyed?

Many modern antiseptics are based on natural plant products so it might be possible to extract a bactericidal cream from plants.

Plants with antiseptic properties often tend to be aromatic and produce an essential oil that inhibits bacterial growth.

How were the potential antiseptics tested?

Extracts of Yucca, Black Sage and Garlic were dropped on the plates where a 'lawn' of bacteria were growing.

Although we're trying to prove our antibacterial cream kills bacteria, we need to have some bacteria samples that remain untreated to show that something will grow if they are left alone. Scientists call these extra samples controls. And to overcome inaccurate measuring replicates are used.

Another beauty of controls and replicates is that simple statistics can be applied. This is particularly important if the result of each sample test was minor. Maybe we would measure the size of the cleared area of each plate and calculate the mean (average) area for each treatment.

effects of antibacterials

What would you see if a successful antibacterial cream had been made?

garlic plate sage plate yucca plate control plate

The most successful antiseptic we tried was garlic and despite the humidity, heat and bat poo, we proved that a successful microbiology lab can be recreated just about anywhere.


The BBC and the Open University are not responsible for the content of external websites.

Growth and Culturing of Bacteria - from the Bacteriophage Ecology Group site

Growing Bacteria - from the Mad Sci Network site

Making Bacteria Media from Potato - from the Indiana Biolab site


The Biology of Plants by Peter H. Raven, Ray F. Evert, Susan E. Eichorn, Worth Publishers

Green Inheritance by Anthony Huxley and David Attenborough, Gaia Books 1991

Aromatherapy Workbook by Shirley Price, HarperCollins

Healing Garden: A Natural Haven for Body, Senses and Spirit by Sue Minter, Tuttle Publishing

Economic Botany: Plants in our World by Beryl Brintnall Simpson and Molly Conner-Ogorzaly, McGraw Hill College Div

The Seaweed Book: How to Find and Have Fun with Seaweed by Rose Treat, Laura Ford (Illustrator), James Sherrington (Illustrator) and Randy Duchaine, Star Bright Books

The River by Edward Hooper, Penguin (controversial, and long book about the origin of AIDS)

The Coming Plague by Laurie Garret, Penguin (classic book about killer bugs)

The Invisible Enemy by Dorothy H. Crawford, Oxford University Press, (an OK book about viruses, up-to-date but a little inaccurate)

The Hot Zone by Richard Preston, Corgi, (cool story book based on fact - very famous)

Catching cold by Pete Davies, Penguin Books (a cool book on the 1918 flu epidemic)

Flu by Gina Kolata, Pan (a scientific history of the Great Flu Epidemic of 1918)

Virus X by Frank Ryan, Swift Publishers

The Virus Hunters by Joseph B. McCormick and Susan Fischer-Hoch, Bloomsbury

The Development, Manufacture and Control of Microbiological Culture Media by Eric Bridson, Oxoid Media Company, Unipath

Trees of the Caribbean by S.A. Seddon, Caribbean Publishing

Nature of the Islands: Plants and Animals of the Eastern Caribbean by Virginia Barlow, Cruising Guide Publications

Tropical Forests and Their Crops by Nigel J.H. Smith, J.T. Williams, Donald L. Plucknett and Jennifer P. Talbot, pub Cornell University Press (Discusses general groupings of useful plants)

Botany for Gardeners: An Introduction and Guide by Brian Capon, B.T. Batsford (A good general text on how plants function)

A Field Guide to the Families and Genera of Woody Plants of Northwest South America: (Colombia, Ecuador, Peru): With Supplementary Notes) by Alwyn H. Gentry and Adrian B. Forsyth, University of Chicago Press (An excellent resource on learning to identify tropical plants in the field/forest)

Encyclopedia of Common Natural Ingredients Used in Food, Drugs, and Cosmetics by Albert Y. Leung and Steven Foster, John Wiley & Sons Inc

Fruits and Vegetables of the Caribbean by M.J.Bourne, G.W. Lennox, and S.A. Seddon, Caribbean Publishing





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