<P> The Gram stain is almost always the first step in the preliminary 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 . This gives rise to gram - variable and gram - indeterminate groups . </P> <P> 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 . He published his method in 1884, and included in his short report the observation that the typhus bacillus did not retain the stain . </P> <P> 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 archaea, formerly archaeabacteria, since these microorganisms yield widely varying responses that do not follow their phylogenetic groups . </P> <P> The Gram stain is not an infallible tool for diagnosis, identification, or phylogeny, and it is of extremely limited use in environmental microbiology . It is used mainly to make a preliminary morphologic identification or to establish that there are significant numbers of bacteria in a clinical specimen . It cannot identify bacteria to the species level, and for most medical conditions, it should not be used as the sole method of bacterial identification . In clinical microbiology laboratories, it is used in combination with other traditional and molecular techniques to identify bacteria . Some organisms are gram - variable (meaning they may stain either negative or positive); some are not stained with either dye used in the Gram technique and are not seen . In a modern environmental or molecular microbiology lab, most identification is done using genetic sequences and other molecular techniques, which are far more specific and informative than differential staining . </P>

What is the purpose of each step in gram staining