IGCSE Biology / Using microorganisms
Practical: aseptic technique revision notes
Edexcel International GCSE Biology (4BI1), chapter 21: Using microorganisms, specification point 5.8. Free lesson.
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Aseptic technique prevents unwanted microorganisms from contaminating a culture, helping you grow only the organism you intend to study. It also makes antibiotic comparisons fair and reduces the risk of growing harmful pathogens.
How to prepare a culture without contamination
Each step blocks a route by which unwanted microorganisms could reach the culture. Sterile means that microorganisms have been killed or removed.
Why the plate is incubated at 25 °C
At 25 °C, the organism you added should grow in the pattern you spread it. Any other growth is contamination. School cultures are incubated at 25 °C rather than 37 °C, because human pathogens grow best at body temperature. The lower temperature reduces the risk of growing harmful pathogens.
How to compare antibiotics
To compare antibiotics, bacteria must cover the agar evenly. This gives each antibiotic a fair test because the starting bacterial growth is uniform. A clear zone is an area around a disc where no bacteria have grown.
Measure the clear zone and tape the lid safely
Diameter is a useful measurement, but it does not show the full area affected. The area is proportional to the square of the radius, so doubling the diameter makes the clear-zone area four times as large. Compare areas, not just diameters, when making a proper comparison. Tape the lid in two or three places to hold it on while still letting air in. Sealing it all the way round creates anaerobic conditions, meaning there is no oxygen. Some of the most dangerous pathogens are anaerobes, so a completely sealed plate can encourage their growth. Remember: sterilise everything, flame the loop…