Triple Science Only - Culturing Microorganisms
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of cell structure, prokaryotic cells and cell division.
What type of cell are bacteria?
Prokaryotic cells
State 3 structures found in bacteria that won't be found in an animal cell?
Cell wall, loop of DNA, plasmids
What type of cell division do bacteria use to reproduce?
Binary fission
Topic Explainer Video
Check out this @JoeDoesBiology video that explains culturing microorganisms, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Bacterial Growth
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Bacteria multiply by binary fission - a form of simple cell division.
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Under ideal conditions, some bacteria can divide every 20 minutes.
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The number of cells doubles after each division (2 → 4 → 8 → 16 etc)
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For example: Starting with 1 bacterium, after 1 hour (60/20 = 3 divisions), you would have 8 cells.
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The population size of a bacteria can be calculated using the equation
Population =
E.g. If a bacterium divides every 45 minutes, how many bacteria will there be after 2 days? Give your answer to 2 significant figures and in standard form.
Step 1 - Calculate the number of divisions.
2 days = 2 x 24 x 60 = 2,880 minutes
2,880 / 45 = 64 divisions
Step 2 - Calculate the population size
Population =
1.8 x
Culturing microorganisms
Microorganisms such as bacteria are cultured for a range of scientific, medical, and industrial purposes. Growing them under controlled laboratory conditions allows scientists to study their behaviour, test substances that affect them, and develop new products or treatments.
Bacteria can be grown in two main types of culture media:
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Nutrient broth solution – a liquid medium containing carbohydrates, minerals, proteins, and other nutrients required for bacterial growth.
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Agar gel plates – a solid growth medium made by adding agar (a jelly-like substance) to nutrient broth. This allows bacteria to grow in visible colonies, each colony originating from a single bacterium.
Aseptic Technique – Preparing an Uncontaminated Culture
When growing microorganisms in a school laboratory, it is essential to prevent contamination by unwanted microbes from the air or surfaces. This is done using aseptic techniques, which ensure that only the desired bacteria are cultured.
Key aseptic procedures include:
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Sterilising equipment and media - All Petri dishes, pipettes, and culture media must be sterilised before use, typically using an autoclave (which uses steam under pressure). This destroys any pre-existing microorganisms that could contaminate the culture.
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Sterilising inoculating loops - The inoculating loop used to transfer microorganisms should be sterilised by passing it through the blue flame of a Bunsen burner until it glows red hot. It must then be left to cool before use.
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Working next to a bunsen burner - The heat from the bunsen burner creates a convection current to move air, and any microorganisms in the air, away from the work area.
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Opening the petri dish - The petri dish lid should be opened for the minimum amount of time possible to prevent any microorganisms from the air contaminating the agar.
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Securing the Petri dish - The Petri dish lid should be secured with adhesive tape at two or three points. This prevents microorganisms from the air entering or escaping, but does not completely seal the dish (to avoid anaerobic conditions that might allow harmful pathogens to grow).
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Incubation conditions - In school laboratories, cultures are incubated at a maximum temperature of 25°C, as higher temperatures could encourage the growth of dangerous pathogens that are harmful to humans.
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Maintaining hygiene - Hands and benches should be disinfected before and after the experiment to reduce contamination risk.
Steps to prepare a sterile culture of microorganisms.
Key Terms
- Aseptic technique - Methods used to prevent contamination of cultures and the environment.
- Binary fission - Simple cell division used by bacteria to reproduce.
No answer provided.
Exam Tips
- Be able to link the aseptic technique to the explanation of how it prevents contamination.
- Culturing microorganisms could be a way to investigate how certain substances affect the growth of bacteria, such as antibiotics.
No answer provided.
Practice Question
Describe how to prepare an uncontaminated culture of a microorganism in a school laboratory. (4 marks)
Any 4 from:
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Disinfect work surface and wash hands with soap
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Work next to a bunsen burner
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Sterilise petri dish and culture medium before use
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Sterilise inoculating loop by passing it through a flame
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Dip the loop into the bacterial suspension and transfer it onto agar using a zig-zag streak
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Replace and secure the lid with tape to prevent contamination
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Incubate at 25°C
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!