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202 SCIPAD - BIG IDEAS - BOOK 2
LIFE OF MICROBES
The Effect of Temperature on Bacterial Growth Rate
Under ideal conditions (plentiful food, moisture and favourable temperatures) bacteria grow
rapidly. However, the enzymes involved in the life processes of bacteria are extremely sensitive
to temperature. Most bacteria grow and reproduce most rapidly in the range of temperatures
between 10 °C and 60 °C. At 10 °C reproduction does occur, but it is very slow. With every 10 °C
rise in temperature, their reproductive rate approximately doubles. Most bacteria that are harmful
to humans have an optimum temperature between 30 °C and 40 °C. At these temperatures their
reproductive rate is fastest. As the temperature rises above this optimum, the rate slows. Most
bacteria die at temperatures above 60 °C.
30
15
0 45 60 75
Temperature (°C)
Reproductive Rate
Optimum Temperature
Too Cold Too Hot
Bacterial Growth Rate at Various Temperatures
1. State the relationship between bacterial population growth rate and temperature.
2. Define the term ‘optimum temperature’:
3. State the optimum temperature for most of the bacteria that are harmful to humans.
4. Discuss why food kept in the fridge lasts longer than the same food kept in the pantry.
5. Two identical pieces of chicken have 100 bacterial cells on them. One piece is placed
in the fridge (temperature = 5 °C) and the other piece is placed on the kitchen bench
(temperature = 20 °C). In the fridge the bacterial reproductive rate is once every hour. On
the kitchen bench the reproductive rate is once every 20 minutes.
Calculate how many bacteria would be on each piece of chicken after 8 hours.
Chicken in fridge: number of bacteria after 8 hours =
Chicken on bench: number of bacteria after 8 hours =
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