The stained cells in a microscope should resemble living cells as closely as possible. Fixation is process by which the internal & external structures of cells & microorganisms are preserved & fixed. It inactivates enzymes that might disrupt cell morphology & toughens cell structures so that they don't change during staining & observation. A microorganism is usually killed & firmly attached to microscopic slide during fixation.
There are two fundamentally different types of fixation. Bacteriologists heat fix bacterial smear by gentle & an air dried film of bacteria. This adequately overall morphology but not structures within cells. Chemicals fixation must be used to protect fine cellular structures & the morphology of larger, more delicate microorganisms. Chemical fixatives penetrate cells & react with cellular components, usually proteins,lipids to render them inactive, insoluble & immobile. Common fixatives mixtures contain such components as ethanol, acetic acid, mercuric chloride, formaldehyde, & glutaraldehyde.