GCSE · Biology · AQA · Spec 8461

Culturing microorganisms (biology only)

Preparing an uncontaminated culture

Follow how each aseptic precaution protects the culture or the people working with it.

  2. Sterilise the loopA metal inoculating loop is heated in a flame until red hot so it is sterilised.
  3. Let the loop coolThe loop is allowed to cool before microorganisms are transferred because a hot loop could kill them.
  4. Secure the cultureThe Petri-dish lid is secured with adhesive tape as part of the aseptic precautions used to prepare an uncontaminated culture.
  5. Turn the plate upside downUpside-down storage stops condensation dripping onto the agar and spreading the colonies.

Two ways to grow bacteria

Nutrient brothvsAgar plate

Both provide conditions for growth, but the visible evidence looks different.

Focus

Culture format

Nutrient broth

A nutrient broth solution

Agar plate

Agar gel in a Petri dish

The insight

The same type of organism can be cultured either in liquid broth or on a solid agar surface.

What growth can look like

Nutrient broth

The broth can become cloudy, or turbid

Agar plate

Bacteria can grow as colonies

Measurements in this topic

Nutrient broth

Population growth can be followed through bacterial-number calculations

Agar plate

Circular colonies or clear inhibition zones can have their cross-sectional area calculated

Measure a circular colony or clear zone

cross-sectional area = πr²

A circular colony or clear inhibition zone uses the area-of-a-circle relationship.

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Calculate bacterial population growth

number of bacteria = initial number × 2^(number of divisions)

Each division doubles the population, so repeated divisions are represented by powers of 2.

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Predict, then check

Picture what happens to water droplets inside a Petri dish.

A cultured Petri dish is stored the right way up. What problem could this cause?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Grow bacteria safely, prevent contamination and measure how cultures change.

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