Crystal violet, CV. In the aqueous solution created, CV exists as its cation CV+. CV+ ions penetrate the cell wall of the bacteria. CV+ binds to negative moieties of the peptidoglycan cell wall. Iodine is added. CV and I- complex and crystallise.

Decolorization, the variable step, strips off the outer membrane of gram negative, but not gram positive bacteria. As the cell wall in gram negative becomes permeable, CVI diffuses out an colour is lost. <br /> Gram positive retains colour from CVI stain.

Safarin is a positively charged counterstain. Binds colourless membrane of gram negative.

Results in pink gram negative and purple gram positive.

Gram staining allows for the differentiation of gram positive and gram negative bacteria.

• Ehrlich's magic bullet.
• Salvarsan brought to market in 1910.
• The systematic selection of compounds, informed by the specific causative agent, still used at present to bring new antibiotics today.

Fleming's serendipitous discovery of penicillin.

• In addition, the Egyptians noted that the yearly river Nile mud gave rise to many frogs so they concluded that this mud spontaneously generated frogs.