gram stain gram stain in microbiology and bacteriology, 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. the name comes from the danish bacteriologist hans christian gram, who developed the technique in 1884. a gram stain of mixed staphylococcus aureus (s. aureus atcc 25923, gram-positive cocci, in purple) and escherichia coli (e. coli atcc 11775, gram-negative bacilli, in red), the most common gram stain reference bacteria 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, 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 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. uses gram stain of candida albicans from a vaginal swab. the small oval chlamydospores are 2–4 µm in diameter. 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. 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. medical