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.    


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