Culturing microorganisms
Binary fission
Bacteria reproduce by binary fission: one cell splits into two.
With enough nutrients and a suitable temperature this can happen every 20 minutes or so: the mean division time.
number of bacteria = starting number × 2ⁿ (n = number of divisions)
n = time ÷ mean division time
- 50 bacteria, divide every 30 minutes, for 3 hours
- 3 hours = 180 minutes; 180 ÷ 30 = 6 divisions
- 50 × 2⁶ = 50 × 64 = 3200 bacteria
Big answers go in standard form (Higher): 100 × 2²⁰ = 104 857 600 = 1.05 × 10⁸.
Aseptic technique
Bacteria are grown in a nutrient broth or on agar in a Petri dish.
Every step keeps unwanted microorganisms out:
- sterilise dishes and agar before use; pass the inoculating loop through a flame
- lift the lid only a little when adding bacteria
- tape the lid on, but not all the way round, so oxygen gets in and harmful anaerobic bacteria do not grow
- store the dish upside down so condensation does not drip on the agar
- in school, incubate at 25 °C at most, so pathogens that like body temperature are unlikely to grow
Required practical: clear zones
Spread bacteria over agar, add paper discs soaked in different antibiotics or antiseptics (and one in sterile water as a control), incubate, then measure the clear zone around each disc.
A bigger clear zone means more bacteria killed.
area of clear zone = π r² (r = diameter ÷ 2)
- Clear zone 14 mm across: r = 7 mm
- π × 7² = π × 49 = 153.9... = 154 mm² (3 s.f.)
Halve the diameter first. Squaring the diameter makes the area four times too big.
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