Gram stain

Gram staining is a method of staining bacteria to add colouration and facilitate visualisation under a microscope. 

 

Method

1. Gram-positive organisms can take up the first stain and retain it during discolourisation.
2. Gram-negative organisms can take up the first stain, but lose it during discolourisation.
3. There is a wash with a counterstain and this adds a second colour to the Gram-negative organisms.
4. This produces two different colours, which allows a microbiologist to differentiate Gram-positive from Gram-negative species.

 

Limitations

• Acid-fast bacteria are resistant to staining and then resistant to discolouration. They must be identified with a Ziehl–Neelsen stain. 

 

Origin

• Danish bacteriologist Hans Christian Gram invented this technique.