Why do Gram-negative bacteria appear pink after Gram staining?

Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane. The alcohol removes the crystal violet, and the safranin counterstain colours them pink.

Why is Gram staining useful in medicine?

The Gram result is available within minutes and narrows down the likely bacteria. Doctors can then choose a suitable antibiotic before full identification.

How are bacteria classified by shape?

Spherical bacteria are cocci, rod-shaped bacteria are bacilli and spiral bacteria are spirilla. Shape and arrangement, such as chains or clusters, help identification.

CCEA A-Level Life and Health Sciences 0008

Experimental Techniques

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MicrobiologyDouble Award

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Microbiology

Classifying bacteria, fungi and viruses

3 things to know
A2 6

Overview

Bergey's Manual, Gram staining and why it works, cocci, bacilli and spirilla, oxygen requirements, the four fungal groups, and classifying viruses by size, nucleic acid, capsid, host and disease.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is Bergey's Manual?

Process

Describe the steps of Gram staining.

Key term

Why do Gram-positive bacteria stay purple after Gram staining?

Questions from this subtopic

Each one with the model answer underneath.

1
What is Bergey's Manual?
Model answer
Bergey's Manual is the standard reference work for the classification and identification of bacteria. Microbiologists use it to match an unknown bacterium's features to known species.
2
Describe the steps of Gram staining.
Model answer
A heat-fixed smear is stained with crystal violet and then iodine. The smear is washed with alcohol or acetone, then counterstained with safranin.
3
Why do Gram-positive bacteria stay purple after Gram staining?
Model answer
Gram-positive bacteria have a thick peptidoglycan wall that traps the crystal violet–iodine complex. The alcohol wash cannot remove it.
4
Why do Gram-negative bacteria appear pink after Gram staining?
Model answer
Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane. The alcohol removes the crystal violet, and the safranin counterstain colours them pink.
5
Why is Gram staining useful in medicine?
Model answer
The Gram result is available within minutes and narrows down the likely bacteria. Doctors can then choose a suitable antibiotic before full identification.
6
How are bacteria classified by shape?
Model answer
Spherical bacteria are cocci, rod-shaped bacteria are bacilli and spiral bacteria are spirilla. Shape and arrangement, such as chains or clusters, help identification.

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