Crystal Violet...
In a discussion, we were talking about the history of Gram staining. Slowly talk shifted about the use of crystal violet MacConkey agar to inhibit Gram positive bacteria. But how a simple dye inhibit a group of bacteria and become a selective agent? Here comes the answer, "The mode of action put forward by Steam & Stearn (1928) that the action of crystal violet is due to the formation of an unionized complex of bacteria with dye, is supported. Gram-negative organisms, such as E. coli , have high isoelectric points and contain less acidic components than Gram-positive bacteria which usually have lower isoelectric points, so the former combine with crystal violet less readily and are more resistant to the dye. " Very happy to discuss science with youngsters... Source: http://onlinelibrary.wiley.com/doi/10.1111/j.2042-7158.1967.tb09550.x/abstract Image Courtesy: http://www.evidentcrimescene.com