GMU:Introduction to Microscopy/Denise Nicoau: Difference between revisions

In their natural state, most of the cells and microorganisms that we observe under the microscope lack colour and contrast. This makes it difficult, if not impossible, to detect important cellular structures and their distinguishing characteristics without artificially treating specimens. Staining is almost always applied to colour certain features of a specimen before examining it under a light microscope. Stains, or dyes, contain salts made up of a positive ion and a negative ion.

Dyes are selected for staining based on the chemical properties of the dye and the specimen being observed, which determine how the dye will interact with the specimen. Dyes can be ''basic'', ''acidic'' or a combination of the two. Acidic dyes carry a negative charge, so they bind to positively charged cells structure. On the other hand basic dyes carry a positive charge, so they bind to negatively charged cells structure.

In most cases, it is preferable to use a positive stain, a dye that will be absorbed by the cells or organisms being observed, adding colour to objects of interest to make them stand out against the background. However, there are scenarios in which it is advantageous to use a negative stain, which is absorbed by the background but not by the cells or organisms in the specimen. Negative staining produces an outline or silhouette of the organisms against a colourful background.

* capsule staining.