Culturing Microorganisms.

• You can grow bacteria in a lab

  • Bacteria is cultured in a 'culture medium' which contains the carbohydrates, minerals, proteins and vitamins needed to grow.
  • The culture medium used can be a nutrient broth solution or solid agar jelly.
  • Bacteria grown on agar 'plates' will form visible colonies on the jelly, or will spread out to give an even covering of bacteria.
  • At school, cultures of microorganisms are not kept above 25 degrees celsius as harmful pathogens are likely to grow above this temperature.
  • In industry, cultures are incubated at higher temperatures so they grow faster.
• • To make an agar plate, hot agar jelly is poured into shallow round plastic dishes called Petri dishes.
  • When the jelly's cooled and set, inoculating loops (wire loops) can be used to transfer microorganisms to the culture medium. Alternatively, a sterile dropping pipette and spreader can be used to get an even covering of bacteria.
  • The microorganisms then multiply.
• You need to use uncontaminated cultures

  • Unwanted microorganisms will affect your results.
  • To avoid this the Petri dishes & culture medium must be sterilised before use to kill any unwanted microorganisms
  • the inoculating loop should be sterilised first by passing it through a hot flame.
  • After transferring the bacteria, the lid of the Petri dish should be lightly taped on- to stop microorganisms from the air getting in.
  • The Petri dish should be stored upside down to stop drops of condensation falling onto the agar surface.
• Inhibition zone

  Clear area left where the bacteria has died
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  • The more effective the antibiotic, the larger the inhibition zone will be
• Control

  Sterile water soaked paper disc
• Comparing the control to the other discs shows that it is the bacteria having the effect
• To investigate the effect of antibiotics on bacterial growth, place paper discs soaked in different types or concentrations of antibiotics on an agar jelly plate with even bacterial coverage.

  Leave space between the discs.

  Antibiotics will diffuse into the agar jelly killing non-resistant bacteria.

  This will leave a clear inhibition zone.

  Include a control disc soaked with sterile water.

  Incubate for 48 hours at 25°C.

  Larger inhibition zones indicate more effective antibiotics.
• Resistant bacteria will not be killed by antibiotics
• You can compare the effectiveness of different antibiotics or antiseptics on bacteria by looking at the relative sizes of the inhibition zones. The larger the inhibition zone, the more effective the antibiotic.
• You can see if an antibiotic is more effective by eye if there is a large difference in size of the inhibition zones. But it is more accurate to find out the area of the inhibition zones. To do this find the radius of the zone and use the equation area= Pi x r^2.