AQA GCSE Biology Foundation (8461), Foundation tier · Cell biology › Cell structure
Practise Culturing microorganisms. 9 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How bacteria multiply by binary fission, how to grow an uncontaminated culture using aseptic technique, and how to calculate bacterial numbers and the areas of colonies or clear zones. It includes the microbiology required practical on antiseptics or antibiotics. This subtopic is in GCSE Biology only, not in Combined Science.
Key facts
Binary fission: one bacterial cell splits into two; as often as every 20 minutes
Number of divisions = time ÷ mean division time (same units)
Area of a colony or clear zone = πr2, where r = diameter ÷ 2
Sterilise Petri dishes and culture media before use; flame the inoculating loop
Tape the lid on; store the dish upside down
Incubate at 25 °C in school laboratories
Larger clear zone = more effective antiseptic or antibiotic
Notes
How bacteria multiply
diagram
Bacteria multiply by simple cell division called binary fission: one cell splits into two.
Binary fission: one cell becomes two, so the number doubles each time.
With enough nutrients and a suitable temperature, some bacteria divide as often as once every 20 minutes.
Each division doubles the number of bacteria: 1 → 2 → 4 → 8 → 16 and so on.
Growing a culture
Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate. These contain the nutrients bacteria need, such as carbohydrate for energy and mineral ions.
Uncontaminated cultures (containing only the microorganism being studied) are needed to investigate the effect of chemicals such as disinfectants and antibiotics.
Aseptic technique, and why
Sterilise Petri dishes and culture media before use, to kill any microorganisms already present.
Sterilise the inoculating loop by passing it through a Bunsen burner flame (then let it cool), to kill any microorganisms on it.
Secure the lid with a few strips of adhesive tape, to stop microorganisms from the air getting in. Do not seal it all the way round: oxygen must still get in, so harmful anaerobic bacteria do not grow.
Store the dish upside down, so drops of condensation do not fall onto the agar.
In school laboratories, incubate cultures at 25 °C. This is well below body temperature (37 °C), so harmful pathogens are less likely to grow.
Required practical: clear zones
diagram
Spread bacteria evenly over an agar plate. Add paper discs soaked in different antiseptics or antibiotics, and a control disc soaked in sterile water.
After incubation, a clear zone forms around a disc where the chemical has killed the bacteria or stopped them growing. The bigger the clear zone, the more effective the chemical against that bacterium.
The bigger the clear zone, the more effective the antibiotic. Here B is the most effective; C has no effect.
Area of a clear zone or colony = πr2. Measure the diameter and halve it to find the radius, r.
How to answer each type of question
Explain the steps of aseptic technique
1 to 2 marks eachGrade 5
Say what the step prevents or kills: usually 'kills microorganisms' or 'stops contamination'.
For 25 °C, compare with body temperature (37 °C) and mention pathogens.
For 2 marks, give the reason and its consequence.
Example. A student grows bacteria on an agar plate in a school laboratory. (a) Why must the agar be sterilised before it is poured into the Petri dish? (b) Why is the Petri dish stored upside down? (c) Explain why the plate is incubated at 25 °C.
Show the model answerHide the model answer
(a) To kill any microorganisms already in it, so the culture is not contaminated (1) (b) So that condensation does not drip onto the agar / colonies (1) (c) Harmful pathogens are less likely to grow (1), because 25 °C is well below body temperature (37 °C), at which pathogens grow best (1)
Analyse the results of the clear zone practical
1 to 3 marksGrade 5
The most effective chemical has the largest clear zone.
The water disc is a control: it shows the clear zones are caused by the chemicals.
No clear zone means the chemical did not kill or stop the growth of that bacterium.
Example. A student tested three antibiotics, A, B and C, on one type of bacterium. The table shows the results.
Disc
Diameter of clear zone in mm
Antibiotic A
12
Antibiotic B
20
Antibiotic C
0
Sterile water
0
(a) Which antibiotic was the most effective? Give a reason. (b) Why did the student include the disc soaked in sterile water? (c) Suggest why there was no clear zone around antibiotic C.Show the model answerHide the model answer
(a) B, because it had the largest clear zone (1) (b) As a control, to show that it is the antibiotic that stops the bacteria growing, not the disc or the liquid (1) (c) The bacteria are resistant to antibiotic C / antibiotic C does not kill this type of bacterium (1)
Don’t lose marks
Using the diameter instead of the radius in πr2, which gives an answer 4 times too big.
Working out 2 × n instead of 2n, or multiplying the time by the division time instead of dividing.
Mixing hours and minutes in the number of divisions.
Saying the loop is flamed 'to clean it'. Say it is flamed to kill microorganisms (sterilise it).
Saying 25 °C is used 'so bacteria grow faster'. It is used so harmful pathogens are less likely to grow.
More tips
Memory tricks
Powers of 2 to know: 25 = 32, 210 = 1024 (about a thousand), 220 is about a million.
Aseptic checklist: sterilise the dish and agar, flame the loop, tape the lid, store upside down, incubate at 25 °C.
Given a diameter? Halve it before you square it.
Calculator: use the xy (or ^) key for 2n.
Exam technique
Aseptic 'explain' questions need the reason, usually 'to kill microorganisms' or 'to prevent contamination'.
In population calculations, show the number of divisions as a separate step: it usually earns a mark.
When comparing antibiotics or antiseptics, name the one with the largest clear zone and quote the numbers.
In standard form the first number must be between 1 and 10: 1.92 × 104, not 19.2 × 103.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State how bacteria reproduceBacteria multiply by simple cell division called binary fission.
Grade 4
Name two ways of growing bacteriaIn a nutrient broth solution, or as colonies on an agar gel plate.
Grade 5
Describe aseptic technique for preparing a cultureSterilise the dishes, media and inoculating loop, tape the lid, store the dish upside down and incubate at 25 °C.
Required practical:Microbiology (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
At what temperature should cultures of bacteria generally be incubated in a school laboratory?
25 °C
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Bacteria can be grown in the laboratory.
(a) Bacteria multiply by simple cell division. What is the name of this type of cell division? Tick (✓) one box.[1]
Binary fission
Differentiation
Fertilisation
Meiosis
(b) Give two ways that bacteria can be grown in the laboratory.[2]
(c) Give the reason why an inoculating loop is passed through a flame before it is used.[1]
(d) At what temperature should cultures of bacteria generally be incubated in a school laboratory?[1]
Show the answer and mark scheme
(a)Answer: Binary fission
(b)
in a nutrient broth (solution)
as colonies on an agar gel plate
(c)
to kill microorganisms on the loop / to sterilise it
(d)Answer: 25 °C
25 (°C)
Question 2Medium5 marks
A student prepared an uncontaminated culture of bacteria on an agar plate using aseptic technique.
(a) Explain why the Petri dish and the agar must be sterilised before use.[1]
(b) The student secured the lid of the Petri dish with adhesive tape. Explain why.[1]
(c) The student stored the Petri dish upside down. Explain why.[1]
(d) The student incubated the culture at 25 °C and not at 37 °C. Explain why.[2]
Show the answer and mark scheme
(a)
to kill any microorganisms already present (so that only the bacteria being studied grow)
(b)
to stop microorganisms from the air getting in and contaminating the culture
(c)
so that condensation does not drip onto the agar / colonies
(d)
harmful bacteria / pathogens are less likely to grow at 25 °C