Cell Biology
Culturing Microorganisms
AQA GCSE Biology
How bacteria multiply
- Bacteria multiply by binary fission. Never write mitosis in bacteria.
- The number levels out because of a shortage of nutrients or oxygen, so cells die.
- To keep the rate high: add more sugar, add more amino acids, add more oxygen, remove toxins, maintain pH, stir the culture.
- Number of divisions = total time divided by the mean division time, with both times in the same unit.
- The number of divisions is a plain number. Never write a unit on the number of divisions.
- Number of bacteria = starting number × 2number of divisions.
- Give the number of bacteria in standard form.
Aseptic technique
| Step | Reason |
|---|---|
| disinfect hands and the work surface | kills microorganisms on the hands and bench |
| sterilise the Petri dish and the agar before use, by heating or in an autoclave | kills unwanted microorganisms in the dish and the agar |
| pass the inoculating loop through a flame before use | kills microorganisms on the loop |
| only lift the lid of the Petri dish a little, and tilt the lid when placing discs on the agar | to minimise contact with air |
| secure the lid of the Petri dish with adhesive tape | stops microorganisms from the air getting in |
Work next to a Bunsen flame, and sterilise the neck of the bacteria bottle by passing it through a flame.
Incubation
| Step | Reason |
|---|---|
| incubate the Petri dish upside down | stops condensation dripping onto the agar |
| incubate at 25 °C | prevents the growth of pathogens, bacteria that are harmful to humans |
Scientists incubate at 37 °C because it is body temperature, where bacteria grow best.
Required practical 2: antiseptics and antibiotics on bacterial growth
Required practical 2
- Wipe the table with a disinfectant, and sterilise the Petri dish and the agar before use.
- Melt the agar, pour the agar into the Petri dish and allow the agar to cool and set.
- Pass the inoculating loop through a flame, work next to a Bunsen flame, and pass the neck of the bacteria bottle through a flame.
- Transfer the bacteria using the inoculating loop, lifting the lid only a little.
- Place paper discs soaked in each antibiotic or antiseptic on the agar, with a control disc soaked in water, tilting the lid to do it.
- Tape the lid and incubate upside down at 25 °C.
- Measure the radius of each clear zone and calculate the area of each clear zone.
- Repeat and calculate a mean.
- Independent variable: the type or concentration of antibiotic or antiseptic on each disc. Dependent variable: the area of the clear zone.
- The water disc is a control, to show the effect of no antibiotic.
The water disc is a control, not a control variable
The water disc is a whole extra test with no antibiotic on it, so never write the water disc is a control variable. Write that it is a control, for comparison: any clear zone round an antibiotic disc then comes from the antibiotic and not from the paper disc.
Measuring a clear zone
Method: area of a clear zone
- Measure the radius of the clear zone from the centre of the disc. Where the diameter is measured, halve it.
- Area = πr2, with π taken as 3.14.
- Give the unit as cm2 when r is in cm, and mm2 when r is in mm.
Reading the plate
- A clear zone round a disc means the bacteria are killed there.
- The most effective antibiotic has the largest zone of inhibition because it killed the most bacteria.
- The least effective antibiotic kills the fewest bacteria: it has the smallest area where no bacteria were growing.
- With no clear zone, the bacteria grow right up to the edge of the antibiotic disc: none of the bacteria have been killed.
- To show a more effective antibiotic on a drawing, draw a larger ring around its disc.
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