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Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. It may also be used to diagnose a fungal infection . [ 1 ]
In bacteriology, Gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain is used by microbiologists to place bacteria into two main categories, Gram-positive (+) and Gram ...
There is no connection between the shape of a bacterium and its color upon Gram staining; there are both gram-positive rods and gram-negative rods. MacConkey agar can be used to distinguish among gram-negative bacilli such as E. coli and salmonella. [23]
They generally stain gram-positive, but as well as Bacillus, are often described as Gram-variable, because they show an increasing number of gram-negative cells as the culture ages. [11] The normal, reproducing cells of Clostridium, called the vegetative form, are rod-shaped, which gives them their name, from the Greek κλωστήρ or spindle.
Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet during Gram staining, resulting in a purple color. Gram-negative bacteria have a thin peptidoglycan layer which does not retain the crystal violet, so when safranin is added during the process, they stain red.
A Ziehl–Neelsen stain is an acid-fast stain used to stain species of Mycobacterium tuberculosis that do not stain with the standard laboratory staining procedures such as Gram staining. This stain is performed through the use of both red coloured carbol fuchsin that stains the bacteria and a counter stain such as methylene blue .
The encapsulated, Gram-positive, coccoid bacteria have a distinctive morphology on Gram stain, lancet-shaped diplococci. They have a polysaccharide capsule that acts as a virulence factor for the organism; more than 100 different serotypes are known, and these types differ in virulence, prevalence, and extent of drug resistance.
The stain proved popular and in 1884 was used by Hans Christian Gram to stain bacteria. He credited Paul Ehrlich for the aniline-gentian violet mixture. [ 32 ] Grübler's gentian violet was probably very similar, if not identical, to Lauth's methyl violet, which had been used as a stain by Victor André Cornil in 1875.