<P> On most Gram - stained preparations, Gram - negative organisms appear red or pink because they are counterstained . Because of presence of higher lipid content, after alcohol - treatment, the porosity of the cell wall increases, hence the CVI complex (crystal violet--iodine) can pass through . Thus, the primary stain is not retained . Also, in contrast to most Gram - positive bacteria, Gram - negative bacteria have only a few layers of peptidoglycan and a secondary cell membrane made primarily of lipopolysaccharide . </P> <P> Endospore staining is used to identify the presence or absence of endospores, which make bacteria very difficult to kill . This is particularly useful for identifying endospore - forming bacterial pathogens such as Clostridium difficile . </P> <P> Ziehl - Neelsen staining is used to stain species of Mycobacterium tuberculosis that do not stain with the standard laboratory staining procedures such as Gram staining . </P> <P> The stains used are the red coloured Carbol fuchsin that stains the bacteria and a counter stain such as Methylene blue </P>

Who discovered the use of dyes to study cells