Name: Kathy
Status: student
Grade: 12+
Location: Outside U.S.
Country: USA
Date: Summer 2012
Question:
Why is it that some bacteria don't react with stain/dye?
Replies:
Kathy,
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.
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