🧪 Nutrient broth
A nutrient broth is a liquid solution containing the nutrients bacteria need. The bacteria grow all through the liquid, which becomes cloudy.
Cell Structure » Culturing Microorganisms
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.1.1.6 (biology only)
- How bacteria multiply and how to grow them in a nutrient broth or on an agar plate
- How to prepare an uncontaminated culture using aseptic technique, and why each step matters
- How to calculate the number of bacteria after a given time
- How to calculate the cross-sectional area of a colony
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseHow bacteria multiply
Bacteria reproduce by binary fission. This is simple cell division: one bacterial cell splits into two identical cells. Those two cells each split again, and so on.
If bacteria have enough nutrients and a suitable temperature, they can divide as often as once every 20 minutes. That is very fast. A single bacterium could become a huge number of bacteria in just one day.
This is why a small number of bacteria in food or in a wound can quickly become a problem. It is also why scientists can grow large numbers of bacteria in the lab in a short time.
Key terms:
- Binary fission: simple cell division in which one bacterial cell splits into two.
- Mean division time: the average time it takes for a bacterium to divide into two.
- Culture: a population of microorganisms grown on purpose, in a lab, in or on a food supply.
- Colony: a visible group of bacteria on an agar plate, all grown from one original cell.
Growing bacteria
To grow bacteria you must give them what they need. There are two common ways to do this.
🔵 Agar plate
Agar is a jelly-like gel that contains nutrients. It is poured into a flat dish called a Petri dish. Bacteria grow on the surface as separate colonies you can see and count.
Cultures of microorganisms must be uncontaminated when they are used to investigate the action of disinfectants and antibiotics. You will carry out that investigation in the next lesson, Required Practical: Antibiotics and Bacterial Growth.
Preparing an uncontaminated culture
A culture is contaminated if unwanted microorganisms get into it, for example from the air, your skin or the bench. The unwanted microbes can grow too and spoil the results. Some could also be harmful to people.
Aseptic technique is the set of steps that stops this happening. This is how you prepare a culture:
- Sterilise the Petri dish and the agar before use. This kills any microorganisms already on them.
- Sterilise the inoculating loop by passing it through a flame. The inoculating loop is a thin wire loop used to transfer bacteria onto the agar. Let it cool before use.
- Dip the loop in the bacteria and spread them over the agar. Open the lid of the dish as little as possible, and only for a short time.
- Put the lid back on straight away and secure it with adhesive tape.
- Turn the dish upside down and store it.
- Incubate the dish at 25°C. To incubate means to keep something at a set temperature so that it can grow.
Key terms:
- Aseptic technique: methods used to stop unwanted microorganisms contaminating a culture.
- Sterilise: to kill all microorganisms on something.
Why each step is needed
Examiners ask you to explain why, so learn the reason for each step.
🔥 Sterilising dishes, agar and loops
Petri dishes and the culture medium must be sterilised before use so that no other microorganisms are already there to grow and contaminate the culture. The loop is passed through a flame to kill any microorganisms on it, so that only the bacteria you want are transferred.
🔒 Taping the lid
The lid is secured with adhesive tape to stop microorganisms from the air getting in, and to stop the bacteria you are growing getting out.
⇅ Upside down
The dish is stored upside down so that drops of condensation collect in the lid and do not fall onto the agar. If drops fell on the agar they could spread the bacteria and ruin the colonies.
🌡 25°C in school
In school laboratories, cultures are generally incubated at 25°C. At this temperature, harmful bacteria that could infect humans are less likely to grow. Higher temperatures would grow them faster, which is not safe in a school.
Common mistakes
Do not say the loop is flamed "to kill the bacteria on the agar". It is flamed so that it does not bring in unwanted microorganisms. Always say why a step is done, not only what is done.
Calculating the number of bacteria
If you know the mean division time, you can work out how many bacteria there will be after a given time. The number doubles each time the bacteria divide.
- Work out how many divisions happen: total time ÷ mean division time.
- Double the starting number once for each division.
Worked example 1
One bacterium has a mean division time of 20 minutes. How many bacteria are there after 1 hour?
1 hour = 60 minutes. Number of divisions = 60 ÷ 20 = 3.
1 → 2 → 4 → 8.
Answer: 8 bacteria.
Worked example 2
A culture starts with 50 bacteria. The mean division time is 20 minutes. How many bacteria are there after 2 hours?
2 hours = 120 minutes. Number of divisions = 120 ÷ 20 = 6.
The number doubles 6 times: 26 = 64.
50 × 64 = 3200 bacteria.
Higher tier only
Higher tier students must be able to give large answers in standard form, as you did in earlier lessons.
A culture starts with 100 bacteria. The mean division time is 20 minutes. How many bacteria after 5 hours?
5 hours = 300 minutes. Divisions = 300 ÷ 20 = 15.
215 = 32 768.
100 × 32 768 = 3 276 800.
In standard form: 3.2768 × 106.
Calculating the area of a colony
The size of a colony can be measured by working out the area it covers on the agar. A colony is roughly a circle, so you use the area of a circle:
area = πr2
Here r is the radius, which is half the diameter. The area is a cross-sectional area, meaning the area you see when you look down on the colony from above. The same calculation works for a clear circular area around a colony.
Worked example 3
A colony has a diameter of 8 mm. Calculate its cross-sectional area. Use π = 3.14.
Radius = 8 ÷ 2 = 4 mm.
Area = πr2 = 3.14 × 4 × 4 = 3.14 × 16 = 50.24 mm2.
Common mistakes
The most common mistake is putting the diameter into the formula instead of the radius. Always halve the diameter first. Also remember that area is in squared units, such as mm2 or cm2.
Exam-style question
A student grew a culture of bacteria on an agar plate in a school laboratory.
(a) Give one way the student could stop the culture becoming contaminated by microorganisms from the air. [1 mark]
(b) Explain why the Petri dish was stored upside down. [2 marks]
(c) The culture was incubated at 25°C. Explain why the temperature was not higher. [2 marks]
(d) One bacterium divides every 20 minutes. Calculate the number of bacteria after 2 hours. [2 marks]
Model answer
(a) Secure the lid of the Petri dish with adhesive tape. (1)
(b) So that drops of condensation collect in the lid (1) and do not fall onto the agar, where they could spread the bacteria or contaminate the culture. (1)
(c) At a higher temperature, harmful bacteria that could infect humans (1) would be more likely to grow, which would be unsafe in a school. (1)
(d) 2 hours = 120 minutes, and 120 ÷ 20 = 6 divisions. (1) 26 = 64 bacteria. (1)
Exam tip
In part (d), show the number of divisions first. Even if you slip on the last step, you can still earn a mark for the working.