Bacterial Growth (WJEC GCSE Biology)

Bacterial Growth

Bacterial colonies

• When a bacteria reproduces, the cell divides to produce an exact copy of itself

• This means that when creating a culture, each bacterium will divide to form a visible colony 

  • A colony appears on an agar plate as a shaded region

• Counting the number of colonies will indicate the number of bacteria present in the original sample

• However, inaccuracies can occur when using this method to count bacteria as bacteria can clump together during the culturing process

  • This would result in fewer bacterial colonies forming than expected

Bacterial colonies image

Image courtesy of Wikimedia Commons public domain

Each small circle represents a colony of bacteria. Each colony originated from one bacterium that divided many times to make a colony

Bacteria and food storage

• Bacterial growth is dependent on the environment in which they are cultured

• A major environmental condition, which is important to consider, is the temperature

• The effect of temperature can be summarised as follows:

  • The rate of bacterial cell division is faster at warmer temperatures

  • However, if the temperature exceeds the optimum, then the cell division slows as enzymes are denatured

    • Eventually, the bacteria will die

• This concept is important in the consideration of food storage

• Food can be kept in cooler conditions (well below optimum) to reduce the speed at which food turns bad

  • Refrigeration will slow down bacterial growth to increase the shelf life of food

  • Freezing will stop bacterial growth completely until it is defrosted again

    • Food can be stored for many months if frozen

The effect of temperature on bacterial growth diagram

Many more colonies will develop on an agar plate which is stored at 25 °C compared to 5 °C