In this lesson, discover the role that the peptidoglycan layer of the bacterial cell plays in protecting bacteria. Learn what comprises the peptidoglycan layer and how the cell wall differs between gram-negative and gram-positive bacteria.
What Is Peptidoglycan?
Envision a cold rainy day.
You’re sitting in your home, gazing out the window. But despite the bad weather, you’re warm and dry thanks to the roof over your head. Bacteria have something very similar to a ‘roof over their head’ called a cell wall. The cell wall surrounds the entire bacteria, holding the cell together and offering protection.
It also maintains osmotic pressure, meaning it lets in just the right amount of water and ions that the cell needs.Similar to the roof on our home, the cell wall is rigid to help secure the shape of the bacteria. The cell wall contains a layer of peptidoglycan, a molecule naturally found only in bacteria.
The peptidoglycan layer acts as the cell wall’s backbone, offering strength to the cell wall. The peptidoglycan layer is able to allow sugars, amino acids, and other ions into the cell as needed.
What Does Peptidoglycan Look Like?
Peptidoglycan is made of chains of alternating molecules called N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). When these two molecules are covalently bonded together, it is called a glycan chain.
Like the shingles on our roof, there can be many layers of glycan chains in the peptidoglycan layer. Gram-positive bacteria can have upwards of 30 sheets of glycan chains. These glycan chains are held together by branches of four amino acids called the tetrapeptide chain.
The Bacterial Cell Wall
The most easily identifiable feature of the gram-positive bacteria is its thick peptidoglycan layer. As mentioned previously, this layer may be up to 30 sheets of glycan chains thick. Just beneath the peptidoglycan layer, like the frame beneath our roof, is the plasma membrane.
This is a lipid bilayer, or two fatty sheets, which functions to hold the cell cytoplasm together. The space between the outer membrane and the plasma membrane is filled with a gel-like fluid called periplasm.The gram-negative bacterial cell wall is more intricate than the gram-positive cell wall.
On the outermost surface of the cell, lies the outer membrane. The outer membrane is a lipid bilayer, or two fatty sheets. Just inside the outer membrane is the peptidoglycan layer. But unlike gram-positive bacteria, the gram-negative peptidoglycan layer is only one or two sheets thick.
Finally, further into the cell wall is the plasma membrane.
Gram staining is a common test used to determine whether a bacterium is of the gram-positive or gram-negative category. It is due to the difference in the cell wall composition in the gram-negative and gram-positive bacteria that they display different colors when gram stained. As a result, gram-positive bacteria stain purple while gram-negative bacteria stain reddish-pink.
Some agents are able to disturb the protective peptidoglycan layer of the cell wall, like crashing through the roof, causing bacterial cell death. One chemical is penicillin, an antibiotic that disrupts the ability of the bacterial cell to make the peptidoglycan layer of the cell wall. Consequently, cells cannot make new peptidoglycan layer during cell division, which weakens the cell causing it to burst.
The outer membrane of the gram-negative bacteria offers some protection against penicillin by physically separating the antibiotic from the peptidoglycan layer. This is why gram-negative bacteria are less easily killed by penicillin.Another substance that attacks the peptidoglycan layer is called lysozyme. Lysozyme can be found in egg whites, as well as our body’s tears, saliva, and mucus membranes. This enzyme acts as a natural form of protection against invading bacteria.
Lysozyme cleaves the bonds between the NAG and NAM in the glycan chain. Once again, gram-positive bacteria are more easily destroyed by lysozyme because of their peptidoglycan layer residing on the outermost surface of the cell.
Just as we are lucky to have a roof over our heads for protection against the elements, bacteria contain a cell wall that maintains their shape and osmotic pressure. The backbone of the cell wall is the peptidoglycan layer, which offers rigidity and strength. The peptidoglycan layer is composed of a chain of alternating molecules called N-acetylglucosamine and N-acetylmuramic acid. Gram-positive bacteria display a thicker peptidoglycan layer than the gram-negative bacteria.
However, gram-negative bacteria have a protective outer membrane making it less susceptible to penicillin and lysozyme.
Important Terms & Definitions
|Cell wall||surrounds the entire bacteria, holds the cell together and offers protection|
|Osmotic pressure||lets in the amount of water and ions that the cell needs|
|Peptidoglycan||a molecule found only in bacteria|
|N-acetylglucosamine (NAG) / N-acetylmuramic acid (NAM)||chains of alternating molecules|
|Tetrapeptide chain||branches of four amino acids|
|Plasma membrane||a lipid bilayer, or two fatty sheets that holds the cell cytoplasm together|
|Periplasm||a gel-like fluid|
|Outer membrane||a lipid bilayer, or two fatty sheets|
|Gram staining||common test used to determine whether a bacterium is gram-positive or gram-negative|
When you reach the end of the video lesson on the peptidoglycan layer, test your ability to:
- Define peptidoglycan and describe its appearance
- Outline the structure of the bacterial cell wall
- Note the purpose of gram staining
- Identify agents that disturb the peptidoglycan layer