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Name: Marci
Status: Educator
Grade:  4-5
Location: WY
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 others.

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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