Culturing Organisms

Bacterial Reproduction

Bacteria multiply by a type of simple cell division in which the bacterial cells split in two. This process is known as binary fission.

Diagram showing the process of binary fission

1. Under optimal conditions (the availability of nutrients and suitable temperature), the bacterial cell prepares for division.

2. The bacterial cell replicates the genetic material inside it, so it increases in size.

3. Each circular piece of DNA moves to opposite ends.

4. The cytoplasm divides

5. Now the bacterium cell becomes two daughter cells.

Some species of bacteria can replicate in as little as 20 minutes, such as e.coli. So the number of bacteria can increase very rapidly. Two ways bacteria can be cultured are:

  • In a nutrient broth (or culture medium)
  • On an agar plate

As they both contain all the essential nutrients that bacteria need to live and grow successfully.


Under optimal conditions, a bacterial cell divides every 20 minutes. Calculate the number of bacteria present after 8 hours.

To calculate the number of bacteria present, we use the equation 2^{n}, with n being the number of divisions.

Now we need to figure out how many minutes are in 8 hours. To do this, we multiply 8 by 60.

8 × 60 = 480 minutes

Then we divide the number of minutes by 20 because the bacteria divide every 20 minutes.

480 / 20 = 24

So, 24 rounds of division took place. Now let’s put this into the equation:


So, the number of bacteria present after 8 hours is 16,777,216

Uncontaminated Cultures

It is important to sterilise solutions and equipment to kill the bacteria already on them. Otherwise, they will grow and contaminate the culture that you are analysing.

An uncontaminated culture of microorganisms can be grown using sterilised petri dishes and agar jelly. First, we sterilise the inoculating loop before use and fix the lid of the Petri dish to prevent unwanted microorganisms from getting in. Then we pour hot agar jelly into a sterilised petri-dish and leave it to set and cool.

  • Heating the culture medium and petri dish to a high temperature kills microorganisms that can contaminate and disrupt your experiment.

Uncontaminated cultures of microorganisms are used to investigate the effects of disinfectants and antibiotics. Bacterial cultures should be incubated at a maximum temperature of 25°C in schools and colleges to reduce the likelihood of pathogens growing that might be harmful to humans.

Aseptic techniques

Aseptic techniques are used to prevent the contamination of specific microorganisms. For example, bacteria and fungi, which we work with in a lab. These techniques are also used to prevent contamination of the room and people working with the microorganism.

Let’s look at a few aseptic techniques that are important when growing uncontaminated cultures of bacteria.

Do not incubate bacterial cultures above 25°C Warmer temperatures encourage the growth of bacteria. By incubating bacterial cultures at 25°C, we are encouraging the growth of the culture. However, we are reducing the likelihood of harmful bacteria growing.

Not incubating bacterial cultures at human body temperature (or above 30°C) reduces the risk of growing pathogens that are harmful to humans.

  • The lid of the petri dish must be secured with tape and the dish stored upside down – This prevents condensation droplets from falling onto the surface of the agar jelly, which can disrupt or compromise the culture.
  • Sterilise inoculation loops before and after use by passing them through a flame or dipping them in alcohol Kills microorganisms on the inoculation loop, which prevents this form of contamination.
  • Use an autoclave to sterilise your Petri-dishes and culture media Sterilising your culture media, Petri dishes and utensils prevents contamination from microorganisms that you don’t want in your experiment