Growing microbes

About ‘grow’ and ‘reproduce’

Growing microbes practical

B Growing microbes

We can grow or culture microbes in the laboratory using agar.

Agar is a jelly made from seaweed. We dissolve chemicals in the agar to provide food for the microbes. It is then called nutrient agar.

The nutrient agar is sterilised before use so we know that any microbes that grow came from the test sample and not from the agar mixture. The sterile mixture is poured into sterile Petri dishes, covered and left to set.

A Petri dish filled with nutrient agar is called an agar plate. Microbes are usually grown on agar plates.

Growing microbes

About ‘grow’ and ‘reproduce’

Scientists have listed the 7 characteristics of living things using the mnemonic MRS GREN, which includes the terms ‘growth’ and ‘reproduction’.

In everyday life ‘grow’ means ‘get bigger’, but when a person or plant gets bigger it is because that organism gets more cells. So ‘grow’ really means ‘to increase the number of cells’.

‘Reproduction’ means ‘produce more organisms’. For multicellular organisms such as humans, ‘grow’ and ‘reproduce’ mean different things, but when talking about single-celled organisms like bacteria the terms ‘grow’ and ‘reproduce’ both mean ‘to increase the number of cells’.

Growing microbes practical

You will be given some Petri dishes containing sterile nutrient agar, and some sterile cotton buds.

Do not touch the cotton part of the bud, or the agar.

Collect samples of microbes by rolling the cotton tip of your cotton bud over everyday surfaces.

Use a different cotton tip for each surface.

Transfer the microbes to the agar plate by rolling the cotton tip over the jelly.

Press just hard enough to make a good contact with the jelly: not hard enough to break the jelly.

Keep the lid on the plate as much as possible to protect the plate from airborne microbes.

Put the lid back on and label the underneath (jelly) side with your name and where the sample(s) were taken.

Seal the plate with sellotope.

Turn it upside down.

You will incubate the plates in an incubator at 25 °C.

This temperature reduces the risk of growing types of microbes harmful to humans.

Incubate at least one unopened plate alongside your inoculated plates.

If microbes grow on the control plate, it will prove that the agar wasn’t sterile.

Incubate all plates UPSIDE DOWN.

This plate was incubated without turning it upside down.

Notice how condensation has formed on the upper surface.

Some of the water droplets are larger than the bacterial colonies on the plate.

If a large water drop lands on a growing colony it will spread out the bacteria and spoil the plate.

As expected, our control plate has no microbe colonies growing on it, which indicates that the nutrient agar was sterile.

Bacterial colonies look like shiny or greasy dots – like small drops of paint.

Fungal colonies look fuzzy.

Notice how the different types of bacteria on this plate have formed colonies of different colours.

This plate shows fungal colonies at the top and at least two different types of bacteria at the bottom.

Viruses do not grow on agar plates.

It is important to keep the plates sealed up after incubation.

Each of these bacterial colonies contains several million cells. If a fragment from the colony broke free (perhaps because the plates were dropped), you could breathe in a particle containing several thousand bacteria. While your immune system can easily cope with the small numbers of bacteria you pick up on your skin or transfer to your sandwiches, it may be overcome by a mass attack coming directly into your lungs.

When you have finished with the plates they should be sterilised before disposal.

Growing microbes

Microbes are grown or cultured on nutrient agar in a Petri dish. We call this an agar plate.

A cotton bud is rubbed on a surface to collect microbes, then rolled over sterile agar. Next the plate is sealed and placed upside down in an incubator for several days.

Large groups of microbes called colonies can be seen with the naked eye.

Bacterial colonies look like shiny drops of paint. Different kinds of bacteria may be different colours.

Fungal colonies often look fuzzy, like cotton wool.

Viruses do not grow on agar plates. 6B 1 Culturing microbes; 6B 2 Growing microbes

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