What is a Peptidoglycan?

Peptidoglycan is a type of polymer found in the cell walls of bacteria. This polymer is responsible for the phenomenon known as gram staining, in which certain types of bacteria acquire a rich purple color when exposed to crystal violet dye, and it serves a number of important biological functions for the organisms it encases. In addition to being visually very interesting, the gram staining process can provide important information about the structure of the bacteria under examination, and it can be used as a key identifying tool.

This polymer, also known as murein, is made from crosslinked chains of sugars and amino acids. It forms a rigid matrix which contributes to the integrity of the bacterium. While peptidoglycan cell walls will not determine the shape of an organism, they will help it hold its shape, ensuring that it is not compromised. In addition, they provide protection from external forces which could threaten the bacteria, such as antibiotic drugs.

In gram positive bacteria, the cell wall is made from a very thick layer of peptidoglycan which will hold the color of a gram stain clearly. Gram negative bacteria have a thinner layer of peptidoglycan, encased in a layer of lipids, and they will flush pink when exposed to crystal violet. By looking at the color of a bacterium, a scientist can learn about the structure of its cell wall, which can provide a clue into identity or into compounds which could be used to eliminate the bacterium.

In addition to providing basic information about cell wall structure, a gram stain can also be used to visualize the structure of a bacterium under a microscope. A peptidoglycan-rich organism will be highlighted with the dye, creating a very detailed, crisp, high contrast image which can be used to gather information about what the organism looks like inside. This information can be used to learn more about how bacteria work, and to gather additional clues to the identity of an organism under examination.

A similar compound called pseudopeptidoglycan or pseudomurein is similar in chemical structure to peptidoglycan, but not identical. Pseudopeptidoglycan can be found in addition to peptidoglycan in the cell walls of some bacteria. This polymer resists specialized enzymes known as lysozymes, which are designed to break down the cell wall so that a bacterium will die. Bacteria which can resist lysozymes have a better chance of withstanding the assault of an active immune system, which will allow the organisms to spread.