AQA Biology 4.1.1.6
Culturing microorganisms
Bacteria double, again and again. Set up a plate safely, measure the clear zones around antibiotic discs, then work out how many bacteria there will be.
Triple Science · Foundation tier
Start here
Two slices of bread.
Two slices from the same loaf. One goes in a bag on the kitchen worktop, the other in a bag in the fridge. A week later, you check them.
If you had to guess, would mould and bacteria grow faster somewhere warm, or somewhere cold?
Binary fission
One becomes two, every division.
Bacteria multiply by simple cell division, called binary fission, as often as once every 20 minutes if they have enough nutrients and a suitable temperature. They can be grown in a nutrient broth solution or as colonies on an agar gel plate. Each division doubles the number. Press Divide and watch the count.
0 min: 1
One bacterium at the start.
Aseptic technique
Keep the culture pure, and keep yourself safe.
Uncontaminated cultures are needed to investigate disinfectants and antibiotics. Tap each step for the reason.
Biology required practical 2
Which antibiotic works best?
Investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition. Spread bacteria evenly over the agar. Add paper discs soaked in each antibiotic, and a control disc soaked in sterile water. Use the same disc size, concentration and incubation time for a fair test. Incubate at 25 °C. Tap a disc to measure its clear zone.
Tap a disc.
Equation · area of a clear zone
You measure the diameter, but the equation needs the radius. Halve it first, as its own line:
r = diameter ÷ 2
Keep every length in mm, so the area comes out in mm2: cm × 10 gives mm. π is a fixed number, about 3.14. Tap the quantity you want to find.
Equation
Area of a circle
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
A = π r²
- A
- area
- mm²
- π
- pi, about 3.14
- r
- radius
- mm
Two steps · how many bacteria after a time?
Step 1 finds how many times the bacteria divide, using the triangle below. Time and mean division time must be in the same unit: hours × 60 gives minutes. Step 2 doubles the starting number once for each division, one line per division, just as you did with the Divide button.
Equation
Number of divisions
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
number of divisions = time ÷ mean division time
- t
- time
- min
- n
- number of divisions
- no unit
- d
- mean division time
- min
bacteria = starting number × 2 for each division
Think again · body temperature?
“Incubate at 37 °C: that’s body temperature, so they’ll grow best.”
Your call
ChooseWhy are school cultures incubated at no more than 25 °C?
Sort
Do it, or don’t?
Place each action.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Explain why
- Each step, then what it prevents.
- Calculate
- Halve, then πr². Or divide, then double.
- Compare
- Zone sizes, in a fair test, against the control.
Key fact
Bacteria multiply by binary fission, as often as every 20 minutes. The bigger the clear zone around a disc, the more effective the antibiotic or antiseptic.
Key note · AQA 4.1.1.6
Culturing microorganisms
- Bacteria multiply by binary fission, as often as once every 20 minutes with enough nutrients and a suitable temperature.
- Grow them in nutrient broth or as colonies on an agar gel plate.
- Aseptic technique: autoclave dishes and media; flame the loop; lift the lid slightly; tape the lid with a few strips; store upside down; incubate at 25 °C in school.
- Required practical 2: the bigger the clear zone, the more effective the antibiotic or antiseptic. Use a control disc.
- Area of a clear zone: halve the diameter, then A = πr².
- Bacteria after a time: number of divisions = time ÷ mean division time, then double once for each division.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure when to halve the diameter?
Zone sizes on the plate are round figures for practice. The plate and bacteria drawings are schematic.