Culturing Microorganisms

• Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate
• Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics
• The mean division time is the average amount of time that it takes for one bacterial cell to divide into two
• $Area_{circle} =$$\pi r^2$
• When doing an investigation using a culture of microorganisms contamination by unwanted microorganisms will affect the results or could potentially result in the growth of pathogens
• Petri dishes and culture mediums need to be sterilised before use (by heating at a high temperature) to kill any unwanted microorganisms
• Inoculating loops used to transfer microorganisms to the culture medium must be sterilised by passing them through a flame to kill unwanted microorganisms
• The lid of the Petri dish should be secured with adhesive tape and stored upside down to stop drops of condensation falling onto the agar plate
• In school laboratories, cultures should generally be incubated at 25°C because at temperatures above this harmful pathogens are likely to grow
• Investigating antibiotics practical

  • Place paper discs with different types or concentrations of antibiotics on an agar plate that has an even covering of bacteria
  • The antibiotic will kill any non-resistant strains of the bacteria and leave an inhibition zone
  • One paper disk will not be soaked in antibiotic but instead sterilised water to act as a control
  • Leave the plate for 48 hours at 25 °C
• The inhibition zone is the area where bacteria have died
• The greater the area of the inhibition zone the more effective the antibiotic is