Culturing Microorganisms Lesson | GCSE Biology Edexcel Higher Triple

Culturing Microorganisms

This lesson covers:

The techniques used to produce cultures of microorganisms
How aseptic techniques are used to prevent contamination of microorganism cultures
How cultures of microorganisms are used to investigate the action of disinfectants and antibiotics

Culturing bacteria

In order to study bacteria, we often need to grow them in a laboratory setting. This is called 'culturing' bacteria.

The cultured bacteria can be used to investigate the action of disinfectants and antibiotics.

Conditions needed for bacterial growth

Under optimal conditions, bacteria can multiply via binary fission as often as once every 20 minutes.

Factors affecting the speed of bacterial growth:

Temperature - most bacteria grow fastest in warm environments.
Nutrient availability - bacteria need a good supply of nutrients in order to grow rapidly.
Moisture - most bacteria grow fastest in moist conditions.
Oxygen - different types of bacteria either need the presence or absence of oxygen for growth.

Nutrient broth & agar plates

Bacteria need a mixture of nutrients in order to reproduce, including carbohydrates (for energy), nitrogen compounds (for protein synthesis), vitamins, and minerals.

Nutrient broth and agar are two nutrient-rich substances used to grow bacteria.

(you will often see these substances referred to as 'mediums' or 'media')

Illustration of a flask containing nutrient broth used for growing bacteria.

Nutrient broth - a liquid medium (like water)

Illustration of an agar plate used for growing bacteria.

2Agar - a gel medium (like firm jelly)

(image shows an 'agar plate' - a plastic dish containing a thin layer of agar)

Bacteria grow on agar plates as colonies

Bacterial colony growing on an agar plate.

An individual bacterium is invisible to the naked eye, but will multiply rapidly when placed in a growth medium.

The resulting group of bacterial cells is called a bacterial colony. Each colony is visible to the naked eye, containing millions of bacteria.

Preventing contamination

It is extremely important that when we grow bacteria, we prevent contamination (the presence of unwanted microorganisms).

To prevent contamination, scientists use aseptic techniques.

Aseptic techniques

Aseptic techniques are a set of procedures used to make sure a culture medium is not contaminated when attempting to culture microorganisms.

Examples of aseptic techniques

Cleaning surfaces with disinfectant - e.g. cleaning the work area before and after use with alcohol.
Washing hands with antiseptic soap and warm water before handling microorganisms.
Sterilising all instruments, solutions, and mediums before they are used. Usually sterilisation involves heating objects to a temperature at which all contaminating microorganisms are destroyed.
Creating a sterile field using a Bunsen burner - a sterile field is a sterilised area created by the updraft of the flame. Biologists can try to work within the sterile field, and minimise the time that cultures and growth media are open to the environment.
Growing bacteria in incubators set to a maximum of 25°C. This prevents the growth of harmful pathogens which prefer body temperature (37°C).

Growing bacterial colonies

Inoculation refers to the process of transferring bacteria from a broth to an agar plate.

Illustration showing an inoculating loop being sterilised over a Bunsen burner flame.

1The inoculating loop is sterilised before use by being placed in a Bunsen burner flame until it is red hot.

2The inoculating loop is then held in the air in the sterile field area, and allowed to cool.

Illustration showing the process of inoculation with a Bunsen burner and a flask containing broth.

3The lid from the bacterial culture bottle is removed.

4The neck of the bottle is placed in the Bunsen burner flame. This moves air out of the bottle and prevents other microorganisms from entering the bottle and contaminating the bacterial solution.

Illustration of inoculation process transferring bacteria from broth to agar plate using an inoculating loop.

5The sterilised inoculating loop is dipped into the flask containing the bacterial culture.

Illustration showing the inoculation process with a flask, broth, and a Bunsen burner.

6The neck of the bottle is flamed again and the cap is replaced.

Illustration of bacterial inoculation on an agar plate using an inoculation loop.

7The Petri dish lid is lifted slightly.

8Zig-zag streaks are made gently and carefully with the inoculating loop across the agar.

9The Petri dish lid is quickly replaced.

Illustration showing the inoculation process of transferring bacteria from a broth to an agar plate.

10The lid of the Petri dish is secured to prevent contamination.

11The Petri dish is then stored upside down in an incubator.

12In schools and colleges, the maximum temperature in the incubator is 25°C, to prevent the growth of harmful bacteria.

Illustration of an inoculation loop being sterilised in a Bunsen burner flame.

13The inoculating loop is sterilised again and all work surfaces are disinfected.

Investigating the effect of antiseptics and antibiotics on bacterial growth

Agar plates are commonly used to investigate the effect of antiseptics or antibiotics on bacterial growth.

By taking filter paper discs, soaking them in different antiseptic or antibiotic solutions, and then placing them on an agar plate, scientists can find out how good the different solutions are at killing bacteria.

Zones of inhibition

Bacteria that are not affected by the substance will grow - bacterial growth will be visible close to the paper discs.
Bacteria that are affected by the substance will die - leaving a clear area surrounding the paper discs.

This clear area is called the inhibition zone, and it can be calculated using the equation:

area = π r²

A larger zone of inhibition indicates the substance is more effective at killing the bacteria.

Agar plate showing zones of inhibition around antibiotic discs A B and C

We can look at the zones of inhibition and assess the effectiveness of each antibiotic at killing the bacteria:

Antibiotic A is not effective - there is no zone of inhibition.
Antibiotic B is somewhat effective - there is a small zone of inhibition.
Antibiotic C is very effective - there is a large zone of inhibition.

When bacteria are grown on agar plates, each visible 'group' of bacteria is referred to as a:

Colony

Camp

Collection

Why is the Petri dish lid only partially lifted?

To prevent the entry of microorganisms from the air

To allow microorganisms in

To stop heat escaping

To allow heat in

Illustration of a bacteria cell showing internal structures and flagella.

Which of these conditions are needed for bacteria to multiply?

Suitable temperature

Lack of nutrients

Presence of nutrients

Moisture

Freezing temperature

Bacteria can be grown in a...

Test tube with water

Nutrient broth

Agar plate

Empty Petri dish

The agar gel provides bacteria with the n they require to grow.

utrients

What is the term for the procedures used to prevent contamination when culturing microorganisms?

Inoculation

Safe culturing

Decontamination

Aseptic techniques

A student placed an antibiotic disc on an agar plate containing bacteria. There was no clear area around the antibiotic disc.

What does this observation tell you about the effectiveness of the antibiotic?

The antibiotic is effective

The antibiotic is ineffective