culturing microorganisms

• bacteria are cultured (grown) in a culture medium (a nutrient broth solution or an agar jelly) that contains the nurtients they need (carbohydrates, protines, minerals, vitamins).

  the bacteria grown on agar plates will grow in either colonies, or spread out evenly.
• agar plates:

  1. hot agar jelly is poured into petri dishes,
  2. once the jelly has cooled and set, inoculating (wire) loops or a sterile dropping pipet and a spreader (for an even covering) are used to transfer microrganisms onto it.
  3. then the microorganisms multipy,
  4. in school cultures are not kept above 25 celcus to prevent harmefull pathoges from growing, however in industrial conditions they are kept at higher tempratures and will grow faster.
• testing bacterial growth:

  1. place paper disks soacked in diffrent antibiotics (and one control to make sure effect is from antibiotic alone) on an agar plate with an even covering of bacteria
  2. antibiotics will diffuse into the jelly, antibiotic resistant bacteria will continue to grow but non-resistant strains will die (the dead area is a inhibition zone)
  3. leave the plate for 48 hours at 25c
  4. the more effective the antibiotic the larger the inhibition zone
• how to prevent contamination:

  1. petri dishes and culture medium must be sterilised before use (e.g by heat) to kill any micoorganisms
  2. inoculating loops should be passes through a flame before use
  3. petri dishses lids should be taped shut to prevent microorganisms from the air getting in
  4. petri dishes should be stored upside down to stop condensation from falling on the agars surface
• compare antibiotic effectiveness by the relative sizes of the inhibition zones (larger inhibition zone, more effective), do this by fiding the area of each ($A =$$π r²$) and comparing to find the biggest, most effective.