aqa science © nelson thornes ltd 2006 1 b3 3 summary microbiology photo: matt meadows, peter arnold...
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AQA Science © Nelson Thornes Ltd 2006 1
B3 3 Summary
Microbiology
Photo: Matt Meadows, Peter Arnold Inc.
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AQA Science © Nelson Thornes Ltd 2006 2
What do microbes need to grow?
• Food.
• Often – but not always – oxygen.
• Warmth.
If bacteria have everything they need they grow and divide very fast indeed.
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How do you grow bacteria in the lab?
• Microorganisms are grown on nutrient agar.
• The agar is heated to around 120°C to kill off any unwanted microbes already present.
• Then a number of steps are followed ...
Photo: Digital Stock 7 (NT)
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1 Sterilise the inoculating loop, used to transfer microorganisms to the agar, by heating it to red-hot in the flame ofa Bunsen and then leaving it to cool.
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2 Dip the sterilised loop in a suspension of bacteria.
3 Use it to make zig-zag streaks across the surface of the agar.
4 Tilt the lid on your Petri dish to keep out unwanted microbes and close the lid quickly to avoid contamination.
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5 Secure the lid of your Petri dish with short pieces of tape to prevent microorganisms from the air contaminating your culture – or microbes from your culture escaping. Do not seal all the way around the edge.
6 Incubate the culture at no more than 25°C for several days.
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How does yeast help us to make bread?
• Aerobic respiration in yeast (with oxygen) produces carbon dioxide and water.
• Carbon dioxide makes the bread rise.
Photos: Stewart Miller
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What part does yeast play in making alcoholic drinks?
• When yeast respire anaerobically (without oxygen), they break down sugar to produce carbon dioxide and ethanol.
• In beer-making – yeast is added to sugar from germinating barley.
• In wine-making – natural yeast on grapes acts on the sugars in the grape juice.
Photo: Ringwood Brewery
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How are bacteria used to make food?
• Yoghurt – bacteria undergo lactic fermentation. They break down lactose, (milk sugar) to produce lactic acid. This clots the milk and thickens the yoghurt.
• Cheese – different bacteria, same lactic fermentation as in yoghurt-making, but much more lactic acid, produces the solid curds used to make cheese. Photo: Photodisc 67 (NT)
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How do we culture microbes on a large scale?
Photo: SPL Maximillian Stock
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How do microbes kill microbes?
• The fungus Penicillium makes a chemical that kills bacteria.
• The chemical can be extracted and purified to make penicillin, a medicine that is safe for people to use and cures many bacterial diseases.
• Penicillin is produced in large fermenters.
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What is biogas?• Mainly methane.• Produced when bacteria
break down animal waste products or plant material in anaerobic conditions.
• Biogas generators are used around the world to produce cheap fuel.
• Different types of generators are needed for different conditions.
Photo: SPL David Hall
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How can microbes make fuel for your car?
• When yeast respire anaerobically, they produce ethanol.
• Ethanol can be used as a fuel for cars, either pure or mixed with petrol.
• The action of yeast on sugar cane or sugar from maize produces ethanol for cars.
• The main problem is growing enough plants to produce the ethanol.