Gram stain
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] The name comes from the Danish bacteriologist Hans Christian Gram, who developed the technique in 1884.[2]
Gram staining differentiates bacteria by the chemical and physical properties of their cell walls. Gram-positive cells have a thick layer of peptidoglycan in the cell wall that retains the primary stain, crystal violet. Gram-negative cells have a thinner peptidoglycan layer that allows the crystal violet to wash out on addition of ethanol. They are stained pink or red by the counterstain,[3] commonly safranin or fuchsine. Lugol's iodine solution is always added after addition of crystal violet to strengthen the bonds of the stain with the cell membrane.
Gram staining is almost always the first step in the identification of a bacterial group. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique. This gives rise to gram-variable and gram-indeterminate groups.
History[edit]
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 in 1884. Gram devised his technique not for the purpose of distinguishing one type of bacterium from another but to make bacteria more visible in stained sections of lung tissue.[4] Gram noticed that some bacterial cells possessed noticeable resistance to decolorization. Based on these observations, Gram developed the initial gram staining procedure, initially making use of Ehrlich's aniline-gentian violet, Lugol's iodine, absolute alcohol for decolorization, and Bismarck brown for counterstain.[5] He published his method in 1884, and included in his short report the observation that the typhus bacillus did not retain the stain.[6] Gram did not initially make the distinction between Gram-negative and Gram-positive bacteria using his procedure.[5]
Orthographic note[edit]
The term Gram staining is derived from the surname of Hans Christian Gram; the eponym (Gram) is therefore capitalized but not the common noun (stain) as is usual for scientific terms.[32] The initial letters of gram-positive and gram-negative, which are eponymous adjectives, can be either capital G or lowercase g, depending on what style guide (if any) governs the document being written. Lowercase style is used by the US Centers for Disease Control and Prevention and other style regimens such as the AMA style.[33] Dictionaries may use lowercase,[34][35] uppercase,[36][37][38][39] or both.[40][41] Uppercase Gram-positive or Gram-negative usage is also common in many scientific journal articles and publications.[41][42][43] When articles are submitted to journals, each journal may or may not apply house style to the postprint version. Preprint versions contain whichever style the author happened to use. Even style regimens that use lowercase for the adjectives gram-positive and gram-negative still typically use capital for Gram stain.