1.1.6: Culturing microorganisms (biology only)
Not started yet — this one needs some love.
Bacteria reproduce by binary fission (simple cell division) — as often as every 20 minutes given enough nutrients and a suitable temperature.
Grown in a nutrient broth solution or as colonies on an agar gel plate.
Uncontaminated (pure) cultures are needed to investigate the action of disinfectants and antibiotics.
Aseptic technique (prevents contamination):
Petri dishes and culture media are sterilised before use
inoculating loops are sterilised in a Bunsen flame before transferring microorganisms
the lid is secured with adhesive tape and the dish stored upside down (stops condensation dripping onto colonies)
incubated at a maximum of 25 °C in school labs (not 37 °C, which reduces the risk of harmful pathogens growing)
Zones of inhibition:
clear areas around an antibiotic/antiseptic disc where no bacteria grow
measure the radius or diameter of the zone
area = πr²
Population calculations: e.g. if bacteria divide every 20 min, that is 3 divisions in an hour, so the number becomes 2³ = 8 × the starting number.
Common exam mistakes
In school labs, cultures are incubated at 25°C, not 37°C — 37°C is human body temperature and would encourage growth of pathogens harmful to humans.
Inoculating loops must be flamed by passing through a Bunsen flame — 'sterilise the equipment' or 'flame the equipment' is too vague. Name the specific step and the specific item.
Know when to use each calculation: measuring the zone requires the radius or diameter; calculating its area uses πr². An exam question asking "what measurement would you take?" expects radius or diameter, not the πr² formula.
Describing the zone of inhibition as 'a circle' or 'where bacteria don't grow' is too vague — refer specifically to 'the clear area where no bacteria are growing' or 'the area where bacteria have been killed'.
Sterilising means killing all microorganisms — do not use vague terms like 'clean' or 'disinfect'.