Transcript No Slide Title

B3 3 Summary
Microbiology
Photo: Matt Meadows, Peter Arnold Inc.
AQA Science © Nelson Thornes Ltd 2006
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B3 3 Summary
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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B3 3 Summary
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)
AQA Science © Nelson Thornes Ltd 2006
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B3 3 Summary
1 Sterilise the inoculating
loop, used to transfer
microorganisms to the
agar, by heating it to
red-hot in the flame of
a Bunsen and then
leaving it to cool.
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B3 3 Summary
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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B3 3 Summary
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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B3 3 Summary
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
AQA Science © Nelson Thornes Ltd 2006
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B3 3 Summary
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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B3 3 Summary
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.
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B3 3 Summary
How do we culture microbes on a
large scale?
Photo: SPL Maximillian Stock
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B3 3 Summary
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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B3 3 Summary
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
AQA Science © Nelson Thornes Ltd 2006
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B3 3 Summary
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.
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