Negatively charged dyes are repelled by the negative charges on the cell wall of bacteria and hence the background is colored but not the cell. Used to demonstrate capsulated microbes. A fluorescent substance emits light of one color when light of a different color shines on it. Used in medical microbiology and immunology laboratories. The sections are stained with heavy metals such as Osmium tetraoxide, Uranyl acetate or phosphotungstic acid which scatter electrons.
Gold is used as a stain for scattering electrons.
It is essential to study pure cultures of microorganisms. A pure culture is a culture consisting of only one type of microorganism. This pure culture multiplies and cell numbers increase. A pure culture is obtained by the principle of cloning. A clone is a collection of cells derived from one cell.