What measurement is taken to compare the effect of different antibiotics on a plate?

The diameter of the clear zone around each disc where no bacteria have grown.

On an agar plate spread with bacteria, what does a clear zone around an antibiotic disc show?

It shows that the substance on the disc has killed the bacteria there or stopped them growing.

Explain why a larger clear zone shows a more effective antiseptic.

Bacteria have been killed over a wider area, so the substance is more effective at that concentration.

In an investigation into the effect of antibiotics on bacteria grown on an agar plate, describe a suitable control.

A disc soaked in sterile water is placed on the same plate, because it should produce no clear zone.

Edexcel GCSE Biology 1BI0

Key Concepts in Biology

Topic overview Eukaryotic and prokaryotic cells Specialised cells and their adaptations Microscopes and magnification calculations Size, scale and units in cell biology Enzyme action and denaturation Factors affecting enzyme activity Enzymes in synthesis and digestion, and rate calculations Food tests and the energy in food Diffusion, osmosis and active transport Osmosis in potatoes and percentage change in mass

Cells and Control

Topic overview Mitosis and the cell cycle Why mitosis matters: growth, repair and cancer Growth, differentiation and percentile charts Stem cells and their use in medicine The structure and function of the brain Investigating and treating the brain Neurones, receptors and the reflex arc The structure and function of the eye Eye defects and how they are corrected

Genetics

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Natural Selection and Genetic Modification

Topic overview Darwin, Wallace and the theory of evolution Natural selection and antibiotic resistance Fossil evidence for human evolution Stone tool evidence for human evolution The pentadactyl limb and the three domains Selective breeding Tissue culture Genetic engineering and modern agriculture The stages of genetic engineering Benefits and risks of GM organisms

Health, Disease and the Development of Medicines

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Plant Structures and Their Functions

Topic overview Photosynthesis as an endothermic reaction Limiting factors in photosynthesis Light intensity and the rate of photosynthesis Root hair cells, xylem and phloem Transpiration and translocation Leaf structure for photosynthesis and gas exchange Factors affecting the rate of transpiration Plant adaptations to extreme environments Plant hormones and their commercial uses

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Health, Disease and the Development of Medicines

Testing antiseptics and antibiotics on bacterial cultures

3 things to know
Paper 1

Overview

Spreading bacteria on agar and placing antiseptic or antibiotic discs, measuring clear zones and what they show, controls, variables and sources of error, incubating plates upside down, and calculating clear zone areas and the percentage of a plate that is clear with πr².

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Give two sources of error when measuring clear zones.

Key term

Describe how bacteria are spread over an agar plate when investigating the effect of antiseptics.

Key term

Describe how different antiseptics are applied to a seeded agar plate.

Questions from this subtopic

Each one with the model answer underneath.

1
Describe how bacteria are spread over an agar plate when investigating the effect of antiseptics.
Model answer
A known volume of bacterial culture is spread evenly over the whole agar surface using a sterile spreader.
2
Describe how different antiseptics are applied to a seeded agar plate.
Model answer
Paper discs of the same size are soaked in each antiseptic and placed on the surface of the agar.
3
What measurement is taken to compare the effect of different antibiotics on a plate?
Model answer
The diameter of the clear zone around each disc where no bacteria have grown.
4
On an agar plate spread with bacteria, what does a clear zone around an antibiotic disc show?
Model answer
It shows that the substance on the disc has killed the bacteria there or stopped them growing.
5
Explain why a larger clear zone shows a more effective antiseptic.
Model answer
Bacteria have been killed over a wider area, so the substance is more effective at that concentration.
6
In an investigation into the effect of antibiotics on bacteria grown on an agar plate, describe a suitable control.
Model answer
A disc soaked in sterile water is placed on the same plate, because it should produce no clear zone.

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Key Concepts in Biology

Cells and Control

Genetics

Natural Selection and Genetic Modification

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ANitrates cause an algal bloom that blocks the light.

BWater plants die and decay.

CThe decomposers use up the dissolved oxygen.

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