Gram staining (or Gram's method) is an empirical method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative) based on the chemical and physical properties of their cell walls. The Gram stain is almost always the first step in the identification of a bacterial organism. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique, thus forming Gram variable and Gram indeterminate groups as well.
The word Gram is always spelled with a capital, referring to Hans Christian Gram, the inventor of Gram staining.
The method is named after its inventor, the Danish scientist Hans Christian Gram (1853–1938), who developed the technique while working with Carl Friedländer in the morgue of the city hospital in Berlin. Gram devised his technique not for the purpose of distinguishing one group of bacteria from another but to enable bacteria to be seen more readily in stained sections of lung tissue. He published his method in 1884, and included in his short report the observation that the Typhus bacillus did not retain the stain.
Gram staining is a bacteriological laboratory technique used to differentiate bacterial species into two large groups (Gram-positive and Gram-negative) based on the physical properties of their cell walls. Gram staining is not used to classify archaeabacteria, since these microorganisms yield widely varying responses that do not follow their phylogenetic groups.
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