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Name: Kathy
Status: student
Grade: 12+
Location: Outside U.S.
Country: USA
Date: Summer 2012

Why is it that some bacteria don't react with stain/dye?


I assume you are talking about the difference between Gram positive and Gram negative bacteria, which is a distinction developed in the late 19th century by a scientists named Hans Christian Gram. It involves staining the bacteria with a dye called crystal violet; bacteria that take up the stain are called Gram positive, and those that do not are called Gram negative. (There are also in-between cases that are a bit more complicated, but let's stick with simple for the purposes of this discussion.) The reason some bacteria are stained while others are not are due to interactions between the dye molecules and the cell wall of the bacteria. Gram positive bacteria have a thicker cell wall containing a high concentration of peptidoglycans, which are negatively stained sugar polymers with amino acid side chains. The negatively charged peptidoglycans interact with the positively charged dye and show up as positive on the stain.

There are a variety of other histological stains used in biological sciences, which are markers for many different distinctions between cells and parts of cells. In general, these stains work selectively (i.e. staining one structure/cell and not another) due to the molecular structure and electrostatic properties of the dye and target molecule. If the dye interacts strongly with the target (but not other molecules), it will be a good selective stain for that target.

S. Unterman Ph.D.

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