When culturing bacteria, other contaminating bacteria will compete for nutrients in the broth/agar. Some bacteria could also be harmful,so would complicate the results of experiments when testing the efficiency of antibiotics or other anti-microbial compounds.

Aseptic technique to prepare an uncontaminated bacterial culture:

• Petri dishes and culture media must be sterilised before use
• inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame
• the lid of the Petri dish should be secured with adhesive tape and stored upside down
• in school laboratories, cultures should generally be incubated at 25°C.

Testing Antibiotics:

1. Place paper discs soaked in different types of antibiotics in an agar plate with an even covering of bacteria
2. Use one control paper disc soaked in sterile water
3. Incubate at 25 degrees
4. Antibiotic resistant bacteria will continue to grow while the rest die creating an inhibition zone
5. Measure the diameter or area of the inhibition zone or count the bacteria (Dilute with some water so that the bacteria spread out, making it easier to count)

🧠 Exam-style questions

• How do bacteria divide? (1)
• Why are bacteria grown at 25 degrees at school but 37 degrees in a laboratory? (2)
• 2 ways bacteria can be grown? (2)
• 2 ways we can reduce bacterial growth in food storage? (2)
• 2 ways bacteria can be grown quicker for industrial use? (2)