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Petri dish showing the effect of Penicillin on bacteria, United Kingdom, 1944

How do you experiment with bacteria? It's not an easy task. First you have to grow them, a process known as growing a culture. Louis Pasteur, the French pioneer of bacteriology, first observed germs in liquids such as beer and milk, and later grew them in nutrient-rich broth. Scientists used household objects to grow germs in the 1800s. Joseph Lister used wine glasses, while Pasteur favoured empty bottles. Generations of students have grown cultures in the bottom of their unwashed tea and coffee mugs. These are not ideal conditions for experimentation. But how could scientists improve on these floating cultures? Robert Koch came up with a different method – adding gelatine or agar and growing the germs on these solid layers. He used glass plates or the side of a flat bottle, but his assistant Julius Robert Petri improved the method with an invention he patented? in 1887. Can you guess what it was? Yes, the Petri dish – a shallow dish specifically designed for growing cultures. Did it revolutionize experiments with bacteria? The dish was easier to handle, and the even layer of nutrient meant all the germs on the surface got the same amount of food. Scientists could now watch and measure the growth of the culture. And better yet, they could observe the germs’ reactions to all kinds of substances. The anti-bacterial effects of penicillin, for example, were discovered in a Petri dish, when bacteria did not grow near the penicillin.

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Glossary: penicillin

The first antibiotic drug to treat infections which is made from the mould penicillium. Its discovery is attributed to Alexander Fleming in 1928.

Glossary: sterilisation

To make an object free of live bacteria or other micro-organisms. Usually achieved by heat or chemical means.

Glossary: bacteria

Micro-organisms which can cause disease but have an important role in global ecology.

Glossary: antibiotic

A substance that is used to treat infections.

Glossary: bacterial culture

A population of bacterial micro-organisms grown in a laboratory environment. Usually developed in liquid or solid state.

Glossary: Petri dish

A shallow dish used in science to grow micro-organisms. A Petri dish is circular, transparent and has a lid.