Culturing microorganisms

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

1. 50 bacteria, divide every 30 minutes, for 3 hours

Big answers go in standard form (Higher): 100 × 2²⁰ = 104 857 600 = 1.05 × 10⁸.

Working backwards: a culture that grows from 30 to 960 cells has multiplied by 960 ÷ 30 = 32 = 2⁵, so it doubled 5 times. If that took 150 minutes, the mean division time is 150 ÷ 5 = 30 minutes.

Bacteria are grown in a nutrient broth or on agar in a Petri dish.

Aseptic technique is the set of methods that keeps unwanted microorganisms out of a culture:

• 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 (microorganisms that cause disease) that like body temperature are unlikely to grow

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. Repeating each test with more discs lets you spot anomalies (results that do not fit the pattern) and calculate a mean, so the results are more reliable. A disc that works best on agar has not been tested inside a person, so it is not yet proven to be the best treatment.

area of clear zone = π r² (r = diameter ÷ 2)

1. Clear zone 14 mm across: r = 7 mm
2. π × 7² = π × 49 = 153.9... = 154 mm² (3 s.f.)

To compare two discs, work out both areas and subtract: zones 20 mm and 12 mm across have areas π × 10² = 314 mm² and π × 6² = 113 mm², a difference of 201 mm².

Halve the diameter first. Squaring the diameter makes the area four times too big.