What is Gram staining?
[[File:Gram stain 01.jpg|thumb|A Gram stain of mixed ''[[Staphylococcus aureus]]'' gram-positive cocci (in purple) and ''[[Escherichia coli]]'' gram-negative bacilli (in red)]] '''Gram staining''' (or '''Gram's method''') is a way of classifying [[bacteria]] into two large groups: [[gram-positive]] and [[gram-negative]]. The name comes from its inventor, Hans Christian Gram. Gram's method stains bacteria according to the chemical and physical properties of their [[cell wall]]s. First, a violet dye is put on the bacteria. This dye stains [[peptidoglycan]], a thick layer that is only found in gram-positive bacteria.<ref name=Bergey_1994>{{cite book | last = Bergey | first = David H. | display-authors = etal | title = Bergey's manual of determinative bacteriology | edition = 9th | publisher = Lippincott Williams & Wilkins | year = 1994 | isbn = 0-683-00603-7 }}</ref> After the first stain, another stain (usually [[safranin]] or [[fuchsin]]) gives all gram-negative bacteria a red or pink colour. The Gram stain is almost always the first step in the identification of a bacterial organism. However, not all bacteria can be classified by this technique. Bacteria for which the method does not work are called 'gram-variable' or 'gram-indeterminate'. Gram developed the technique working with another scientist, Carl Friedländer in a hospital in [[Berlin]]. However, Gram first used the test to make bacteria in the [[lung]]s easier to see.<ref>{{Cite journal |last=Austrian |first=R. |year=1960 |title= The Gram stain and the etiology of lobar pneumonia, an historical note |journal=Bacteriol. Rev. |volume=24 |issue=3 |pages=261–265 |doi=10.1128/br.24.3.261-265.1960 |pmid=13685217 |pmc=441053}}</ref> He published his finished method in 1884.<ref name=Gram_1884>{{Cite journal |last = Gram H.C. |title = Über die isolierte Färbung der Schizomyceten in Schnitt- und Trockenpräparaten |journal = Fortschritte der Medizin |language=German |year = 1884 |volume = 2 |pages = 185–189}}.<br>English translation in: {{Cite book |last=Brock