Gram Negative Rod Agar
Country: United States
Date: April 2007
I have two students who are doing a science progect on
bacteria in water. One of the petri dishes is a gram negative rod
dish. What exactly is gram negative rod? Does this type of dish
just show the bacteria in color? Any information you could provide
on Gram negative Rods would be useful.
First, some background: The "gram stain" was invented in the late 1800's,
long before we knew much of anything about organisms and how they work. It's
basically a dye that only affects certain kinds of bacteria. The dye, which
is actually one of two dyes: either "crystal violet" or "methylene blue",
colors the cell wall of the bacteria. The bacteria whose cell walls strongly
attach to the dye are called "gram positive" and the bacteria whose cell
walls do not take up the dye are called "gram negative". Usually, a second,
lighter dye (fuschin, pink colored) is added so that you can see the
gram-negative bacteria. So after the full staining procedure is completed,
you will have purple "gram positive" and pink "gram negative" cells. It's a
quick and easy way to try to identify the bacteria you have in a sample.
The reason some cells react with the dye more strongly than others has to do
with the chemistry of their cell walls. Some bacterial cell walls are thick,
and some are thin. The kinds of molecules in the walls can be very
different. Imagine you're painting a wall -- normal housepaint will stick
well to a primed drywall surface, but it won't stick well to a shiny metal
surface. So, by analogy, some bacteria are like the metal wall, and some are
like the drywall. And therefore, some bacteria are dyed more strongly than
In addition to classifying bacteria by their gram stain, bacteria can also
be classified by their shape. This is easy to see in a microscope, so it's a
convenient way to divide bacteria. Some common shapes are rods, spheres
(cocci), and spiral (spirochete).
Gram negative rods are of particular interest because Escherichia coli (E.
coli for short) is a gram negative rod. E. coli are all over -- some are
beneficial -- they live in our intestines -- while others can cause disease.
E. coli has been studied a TON by scientists, so we know more about it than
virtually any other bacterium.
There is a ton of information about E. coli, gram staining, and bacterial
shapes online. A few minutes on Google, and you can have info coming out
your ears. There's a ton more details I skipped over, but I hope this is
enough of a primer to get you started.
Hope this helps,
The gram stain is a differential stain; in other words it has two dyes
and stains bacteria with different cell walls differently. Gram positive
bacteria have thicker cell walls and stain purplish blue and gram
negative bacteria have thinner cell walls and stain reddish pink.
If there are two bacteria on a slide and one is purple and the other
pink you can tell the difference between them. Bacteria also come in
different shapes: round (cocci), oblong (rods) and spiral (spirilli).
So pink oblong bacteria are gram negative rods. There are certain
types of agar that contain chemicals that let gram negative bacteria
grow and inhibit the growth of gram positive bacteria. This way you
can isolate the type of bacteria you are interested in.
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Update: June 2012