culturing micro-organisms

• Prokaryotic cells reproduce by binary fission, we can investigate their growth by culturing them in a lab.
• Binary fission is when prokaryotic cells(bacteria), divide and reproduce. The speed of this division depends on conditions such as temperature and nutrient concentrations. Some bacteria can divide every 20 minutes but if conditions are unfavourable binary fission will stop and cells will start to die.
• The process of binary fission is as follows:

  1. The genetic material stored in the circular DNA and plasmids get replicated
  2. The cell starts to expand and the circular DNA moves to opposite poles of the cell
  3. The cytoplasm divides and cell walls form around the 22 new daughter cells. Each daughter cell will contain a copy of the circular DNA and a variable number of plasmids.
• Number of divisions =Time spent dividing/ Mean division time
• Mean division time: Average time it takes for one bacterial cell to divide once. Mean division time can be used to work out how many times a cell has divided and therefore the number of cells produced.

  Number of divisions =Time spent dividing/ Mean division time
• Potential contamination areas:

  • skin
  • air
  • soil
  • water
• Contamination is a health and safety risk due to growth of potentially dangerous bacteria, as well as ruining test result.
• Cultures of microorganisms: Used to investigate the effects of antibiotics and disinfectants, which both kill bacteria. Microorganisms can be grown in two mediums that contain everything that bacteria need to survive:

  • Agar gel plates, microorganisms can be grown as colonies on agar gel plates.
  • Nutrient broth, microorganisms can be grown within the solution. Contains carbohydrates (energy source), minerals and sometimes other chemicals.
• Aseptic techniques:

  • Flames, inoculation loops passed through flame to sterilise. All work done on yellow flame.
  • Temperature, max temp in schools is 25C. To reduce risk of harming kids and production of harmful bacteria.
  • The lid of the petri dish must be taped on and opened as little as possible to prevent contamination from microorganisms in the air.
  • The petri dish should be stored upside down to stop condensation falling on to the agar.
  • Wash your hands and work surfaces before beginning to prevent contamination.
• REQUIRED PRACTICAL:

  1. Use a sterile dropping pipette and spreader to evenly spread the bacteria.
  2. Soak paper discs in different types or concentrations of antibiotics and antiseptics, place them evenly distributed, on the agar plate with the bacterial covering. This allows the antibiotic or antiseptic to diffuse into the agar. 
  3. Place a disc that has been soaked in sterile water onto the plate as a control.
  4. Tape the lid onto the petri dish and incubate upside down for 48 hours. 
• Analysing results:

  •  Inhibition zones, clear areas surrounding some of the discs of paper which is where the bacteria have been killed.
  • The more effective the treatment is at killing bacteria, the larger the inhibition zone.
  • Increasing the concentration of the solution increases the size of the inhibition zone.
  • Some bacteria may be antibiotic-resistant and will be able to grow in the presence of the treatment so there will be no inhibition zone. 
  • No inhibition zone around the control disc (water) and therefore it is the antibiotic/antiseptic that kills the bacteria.
• 1. Use Area of a circle=πr2 (squared) to find the area of the inhibition zone.