Topic Summaries

• Bacteria multiply by cell division (binary fission) as frequently as once every 20 minutes, but only if they have enough nutrients and a suitable temperature.
  • Bacteria can be grown in a nutrient broth solution.
  • Bacteria can be grown as colonies on an agar gel plate.
• Uncontaminated cultures of microorganisms are used to investigate the action of disinfectants and antibiotics. They are prepared using aseptic technique:
  • Petri dishes and culture media must be sterilised before use to prevent growing foreign microorganisms.
  • Inoculating loops must be sterilised by passing them through a flame before being used to transfer microorganisms to the petri dish.
  • The Petri dish should be stored upside down with the lid secured with adhesive tape to reduce the contamination risk from airborne microorganisms and stop condensation from pooling.
  • Cultures should be incubated at 25 °C to encourage growth of the microorganisms without growing contaminating microorganisms from humans (which grow best at 37 °C).

Effect of three antibiotics (A, B and C) on bacterial growth:
A = No effect
B = Inhibits growth weakly
C = Inhibits growth strongly