Because bacteria are almost impossible to see without the aid of a microscope, it is sometimes hard to believe that there are significant differences between them. But bacteria are very, very different. There are literally tens of thousands of different types of bacteria, maybe more. As such, scientists have classifications for them. One of the big classifications that is used in medicine is gram positive and gram negative.
In order to explain the distinction between gram positive and gram negative microbes, I first have to explain what gram staining is. Bacteria are single-celled organisms, often found in large groups. The wall of their cells have numerous proteins and other structures. These structures serve a wide range of functions.
Some bacteria have a cell what that contains a structure known as a peptidoglycan. When the a special stain is applied to these cells, the stain attaches to the peptidoglycan and turns the cell a dark purple. The stain is called, most appropriately, Gram stain. It was named after the scientist who invented it.
A bacterium that does not have this particular chemical in its cell wall, or does not have much, will stain a light pink on exposure to Gram stain.
Doctors use Gram staining as the first way to differentiate one bacteria from another in a clinical setting. It’s vitally important to know what bacteria is infecting a person so that you know what you’re dealing with, and can pick the right treatments to cure the infection.
Gram positive bacteria will respond to a very different set of antibiotics than a Gram negative bacteria will. There are specific chemical reasons for this – many antibiotics attack the cell wall of a bacteria. Since the Gram stain tells you a little about the structure of the cell wall, it can tell you if the bacteria will be killed by a given antibiotic.
Of course, nature is rarely simple enough to have just two categories for all bacteria. There are many bacteria that are partially Gram positive, or where the Gram stain is indeterminate. Of course, even this can be of help to a doctor when trying to identify the bacteria, as we know which bacteria fall in to these categories as well. So even with these partial Gram staining bacteria, the test can still be useful.
There are many tests that can follow a Gram stain when trying to determine what bacteria you may be dealing with. The Gram stain is almost always the first test, and is quick and inexpensive to run.