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How do you grow bacteria on a plate so that each cell forms a separate, countable colony?

Describe how to culture bacteria on agar and use streaking and spreading to obtain colonies

A focused answer to the O-Level outcome on culturing bacteria. Agar plates and broth, streaking for single colonies, spread plates for counting, and incubation conditions.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

This outcome asks you to describe how bacteria are grown on agar and how streaking and spreading produce separate colonies. It builds directly on aseptic technique, and it underpins counting (from the serial-dilution topic). A good answer gives the method and the reasons behind each practical detail.

The answer

Growing media

Bacteria are grown on nutrient media containing the food they need:

  • Agar plates. Nutrient agar is a jelly in a dish; bacteria grow on its surface, forming visible colonies.
  • Nutrient broth. A liquid medium in which bacteria grow throughout, used to build up large numbers.

Streaking for single colonies

Streaking spreads bacteria thinly so that single cells end up well separated:

  • A sterile inoculating loop is dragged across the agar in a pattern, spreading the bacteria out as it goes.
  • Toward the end of the pattern, the bacteria are so spread out that single cells are deposited far apart.
  • Each isolated cell grows into a separate colony.

Spread plates for counting

To count bacteria, a spread plate is used:

  • A small, measured volume of a diluted culture is placed on the agar.
  • A sterile spreader spreads it evenly over the surface.
  • Well-separated cells each grow into a separate colony, which can be counted.

Incubation conditions

After plating, the plate is prepared and incubated:

  • Tape the lid (without sealing completely) and label the plate.
  • Incubate inverted at a suitable temperature for a day or two.

Why a colony forms from one cell

A single bacterium that lands away from others divides again and again in one spot, building a visible mound of millions of identical cells, a colony. Because the cells were spread out, each colony came from one original cell.

Examples in context

Example 1. Isolating a pure strain. Streaking a mixed sample lets a microbiologist pick a single well-separated colony, which is a pure population from one cell. This is how a pure strain is obtained for further work, such as genetic engineering.

Example 2. Counting in quality control. A factory checks cell numbers by spreading a diluted sample, incubating, and counting colonies. Combined with the dilution factor, the colony count gives the cell density in the original culture.

Try this

Q1. State why a culture is streaked across an agar plate. [1 mark]

  • Cue. To spread the bacteria out so that single cells are separated and grow into separate colonies.

Q2. Explain why agar plates are incubated upside down. [2 marks]

  • Cue. So that any condensation collects on the lid rather than dripping onto the agar, which would let colonies spread and merge.

Q3. Explain why each colony on a spread plate can be counted as coming from one cell. [2 marks]

  • Cue. The cells were spread far apart, so each separated cell divides in one spot to form a single colony of identical cells.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksDescribe how you would prepare a spread plate of bacteria and incubate it, and explain why each cell can form a separate colony.
Show worked answer →

Examiners want the spreading method, incubation, and the reason single cells give separate colonies.

Using aseptic technique, transfer a small measured volume of a diluted bacterial culture onto the surface of a sterile nutrient agar plate. Use a sterile spreader to spread the liquid evenly across the surface, then replace the lid.

Tape the lid (without sealing it completely), label the plate, and incubate it inverted at a suitable temperature for a day or two.

Each separated bacterium that lands far enough from the others divides repeatedly in one spot, producing a visible mound of millions of identical cells, a colony. Because the cells were spread out, each colony comes from a single original cell, so the colonies are separate and can be counted.

What markers reward: aseptic transfer of a measured diluted sample, even spreading with a sterile spreader, taping, labelling and incubating (inverted, suitable temperature), and the explanation that each well-separated cell divides in place to form one colony of identical cells.

Original4 marksExplain why agar plates are incubated inverted (upside down) and why the lid is taped but not sealed completely.
Show worked answer →

The answer should give the reason for inverting and for partial taping.

Plates are incubated inverted so that any water (condensation) that forms collects on the lid rather than dripping onto the agar surface. Water on the agar would let colonies spread and merge, making them harder to count and identify.

The lid is taped to hold it on and prevent accidental opening and contamination, but it is not sealed completely so that some air can still reach the bacteria. This discourages the growth of harmful microorganisms that thrive without oxygen, improving safety.

What markers reward: inverting so condensation collects on the lid and does not drip onto the agar (preventing colonies merging), and taping but not fully sealing so air can enter, holding the lid on while discouraging harmful anaerobic growth.

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