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The Gram Stain: Its Persistence and Its Quirks

What is more emblematic of our science than the Gram stain? Since its invention 130 years ago, it has been in frequent and continuous use. It conveniently places most bacteria into one of two groups, the Gram-positives or the Gram-negatives. Gram staining is cheap, effective, quick, and relatively easy to interpret. Its most useful application is in the clinical setting. When examining a smear of, say, pus from an abscess, this stain often allows to include for consideration roughly half the clinically relevant bacterial species while excluding the others. Or consider a patient with meningitis. Here, speed is of the essence because treatment must be initiated right away. A Gram stain of the…

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Articles of Interest

Categorising bacteria in purple and pink

Gram staining colours the bacteria either purple, in which case they are referred to as "Gram positive," or pink which are known as "Gram negative". Although Christian Gram didn't realise it at the time, this different response to the staining technique is due to a fundamental and important change in the structure of bacteria. Gram positive cells have large bulky cell walls made out of a substance called peptidoglycan. In contract Gram negative bacteria have two thin cell membranes with a thin peptidoglycan layer between them.

Peptidoglycan - The bacterial wonder wall

So how did Gram’s stain work? Because of the peptidoglycan layer. The thickened peptidoglycan layer in Gram positive cells allows them to retain the stain (hence remaining ‘stain positive’ or ‘Gram positive) where as the thin layer seen in Gram negative cells cannot prevent the stain from leeching out (hence stain and Gram negative). Of course Gram himself didn’t know this but his stain was a success and it was 1884 so give him a break.