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Growth of Bacteria
Reproduction of bacteria
Bacteria reproduce by splitting into two- the growth of bacteria doubles at regular intervals.
Therefore their growth is exponential.
As a result, if a harmful bacterium gets into the body, an infection can develop really fast-
before your immune system has a chance to respond.
The work of Louis Pasteur
Until the 19th century, people use to believe diseases spontaneously appeared from
Louis Pasteur argued that there are microbes in the air which cause disease and
decomposition. He carried out experiments to prove his theory:
Pasteur heated broth in two flasks and left them open.
One of the flasks had a curved neck so the bacteria in the air would settle in the loop
and not get through to the broth.
This broth stayed fresh, but the other broth didn't- showing that it was microbes and
not air causing it to go off.
Pasteur then invented the process of pasteurisation- heating something to around 70 C
and then rapidly cooling it. This kills off most harmful microbes, so the product shouldn't
make you ill.
All processes that reduce contamination by microbes are called aseptic techniques.
Pasteurisation is used to treat raw milk, to kill harmful bacteria-making it safer to drink.
Pasteurisation is preferable to sterilisation (UHT). Sterilisation kills off everything, making
milk taste funny and destroying some of the vitamins.
You can use Resazurin dye to investigate microorganism growth
Resazurin is a dye that's sensitive to oxygen. When there's a lot of oxygen present, it's
blue. As the concentration of oxygen decreases, it changes to lilac, then mauve then pink
Microorganisms use up oxygen when they respire. The more microorganisms there are the
more oxygen is used up.
You can use resazurin to monitor the growth of microorganisms- the more microorganisms
there are, the greater the colour change will be.
This means you can look at different conditions (like temperature or pH) affect the growth of