5.3 Growing Bacteria in the Lab

A free GCSE Biology lesson for AQA (8461) from OBA School: binary fission, culture media, aseptic technique and calculating the number of bacteria after a given time. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how bacteria reproduce by binary fission
• Describe how bacteria are grown in a nutrient broth or on an agar plate
• Explain each step of aseptic technique used to prepare an uncontaminated culture
• Calculate the number of bacteria after a given time from the mean division time, using standard form

Key words

• Binary fission: Simple cell division in bacteria: one cell splits into two identical cells.
• Mean division time: The average time between one division and the next (the time for the population to double).
• Culture medium: A liquid (nutrient broth) or gel (agar) containing the nutrients microorganisms need to grow.
• Colony: A visible spot on agar made of millions of bacteria, all grown from one original bacterium.
• Aseptic technique: Ways of working that stop unwanted microorganisms getting into a culture.
• Sterilise: Kill all microorganisms on equipment or in a medium, e.g. in an autoclave or a flame.

How bacteria multiply

Bacteria reproduce by binary fission (simple cell division). The cell copies its DNA, grows and splits into two. With enough nutrients and a suitable temperature, this can happen as often as once every 20 minutes.

Because the number doubles each time, a few bacteria can become millions in a few hours. In real cultures growth slows when nutrients run out or waste builds up.

Growing bacteria in the lab

Bacteria are grown (cultured) in a culture medium that contains carbohydrate as an energy source, minerals, proteins and vitamins.

• Nutrient broth: a liquid. Bacteria are added and the flask is shaken or kept warm. The broth turns cloudy as they grow.
• Agar gel plate: hot sterile agar is poured into a sterile Petri dish and left to set. Bacteria are spread over it and grow into colonies.

Scientists need uncontaminated cultures (only one type of microorganism) to investigate the effect of disinfectants and antibiotics.

Aseptic technique

• Sterilise Petri dishes and culture media before use (in an autoclave).; Reason Kills unwanted microorganisms already present.
• Pass the inoculating loop through a Bunsen flame until red hot, then let it cool.; Reason Kills microorganisms on the loop. Cooling stops it killing the bacteria you transfer.
• Lift the lid only slightly and work quickly near the flame.; Reason Fewer microorganisms from the air fall in. Rising air from the flame carries them away.
• Tape the lid on with a few strips, not all the way round.; Reason Stops the lid coming off and microbes from the air getting in. Oxygen can still get in, so harmful anaerobic bacteria do not grow.
• Store the plate upside down.; Reason Condensation does not drip onto the agar and spread the colonies.
• Incubate at no more than 25 °C in school labs.; Reason Bacteria that cause human disease grow best near 37 °C, so they are less likely to grow.

Key fact: In industry, cultures may be grown at higher temperatures to get faster growth, but this happens in special, safe conditions.

Calculating the number of bacteria

First work out the number of divisions, then double that many times:

number of divisions = time ÷ mean division time

bacteria at the end = bacteria at the start × 2ⁿumber of divisioⁿs

Worked example: using standard form

One bacterium divides every 20 minutes. How many bacteria will there be after 6 hours?
6 hours = 360 minutes. Divisions = 360 ÷ 20 = 18.
Number = 1 × 2¹⁸ = 262 144 ≈ 2.6 × 10⁵.

Answer: 262 144 bacteria (2.6 × 10⁵ to 2 s.f.)

Worked example: starting with more than one

A broth contains 300 bacteria. The mean division time is 25 minutes. How many are there after 2 hours 30 minutes?
150 minutes ÷ 25 = 6 divisions. 300 × 2⁶ = 300 × 64 = 19 200.

Answer: 19 200 = 1.92 × 10⁴ bacteria

Exam tip: Always change hours to minutes before dividing by a division time given in minutes.

Common mistakes

• Multiplying by the number of divisions instead of by 2 to the power of the number of divisions.
• Forgetting to convert hours into minutes before working out the number of divisions.
• Saying the lid is taped "to seal it". It is only taped in a few places so oxygen can still get in.
• Saying the loop is flamed "to clean it". It is flamed to kill microorganisms, then cooled before use.
• Saying 25 °C is used "so bacteria grow faster". It is used so human pathogens are less likely to grow.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the name of the process by which bacteria reproduce?

Answer: binary fission

Working: Bacteria reproduce by binary fission (simple cell division): one cell splits into two.

Question 2. Why is an agar plate stored upside down? (Choose from: So the agar does not fall out, So condensation does not drip onto the agar, So oxygen can reach the bacteria, So the bacteria grow faster.)

Answer: So condensation does not drip onto the agar

Working: Condensation forms on the lid. If the plate is upside down, drops cannot fall onto the agar and spread or disturb the colonies.

Question 3. True or false? The lid of a Petri dish should be taped all the way round to seal it.

Answer: False

Working: False. The lid is taped with a few strips only, so oxygen can still get in and harmful anaerobic bacteria do not grow.

Related lessons

• Previous lesson: 5.2 Pathogens and Disease
• Next lesson: 5.4 Preventing Bacterial Growth
• In this chapter: 5.1 Health and Disease
• In this chapter: 5.5 Preventing Infections
• In this chapter: 5.6 Viral Diseases

Continue in the full course

This lesson is section 5.3 (AGCBH0503) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.