Bacterial staining - basic principle

Simple staining - positive and negative Refers to when only one dye is used to observe the morphology of bacteria Some bacteria are hard to stain; acid dyes such as nigrosin are repeled by negative charge of the bacteria; the background gets the darker color of the dye; the bacteria appear lighter Bacteria pick the dye and get the color; the background appears lighter

Gram + and - Refers to using two dyes to differentiate between bacteria, especially for diagnosis Gram negative bacteria have a thinner cell wall and double lipoprotein membranes (outer and inner); cells pick up crystal violet-iodine to get purple color; alcohol denatures cell membrane and the thin cell wall allows the cells to lose the purple stain during decolorization with alcohol; cells now pick up the red color of the counter stain (safrinin) Gram positive bacteria have a thick cell wall composed of peptidoglycan; cells pick up the dye (crystal violet) and mordant (iodine) to give cells purple color; thick cell wall does not allow the cells to lose crystal violet-iodine complex when decolorized with alcohol; cells are unable to pick up red counterstain (safrinin)

Acidfast and non-acidfast Refers to using two dyes to differentiate between bacteria, especially for diagnosis Non acid-fast bacteria pick up the red dye (carbolfuchsin) to give the cells red color; cells are decolorized with alcohol; cells pick up blue counterstain (methylene blue) giving them a blue color Acid-fast bacteria pick up the dye (carbolfuchsin) to give cells red color; cells are not decolorized with alcohol; cells are unable to pick up blue counterstain (methylene blue)