Peptidoglycan - Online Tutor, Practice Problems & Exam Prep

Peptidoglycan is a structural polysaccharide in bacterial cell walls, composed of alternating sugar units: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked by β-1,4-glycosidic bonds. Bacteria are classified as gram-positive, with a thick peptidoglycan layer, or gram-negative, with a thin layer and an outer membrane, enhancing antibiotic resistance. Understanding these differences is crucial for microbiology, as they influence bacterial behavior and treatment responses.

Peptidoglycan

In this video, we're going to introduce the polysaccharide peptidoglycan. So the polysaccharide peptidoglycan is a heteropolysaccharide, which, of course, we know from our previous lesson videos just means that the peptidoglycan structure consists of at least 2 different types of repeating sugar units. And so these repeating sugar units are going to be the NAG sugar molecule, which is the same sugar molecule that we covered in our previous lesson video of chitin, and the NAM sugar molecule, which is new in this lesson video. And so the NAG recall is just an abbreviation for N-acetylglucosamine. And the NAM, on the other hand, is an abbreviation for N-acetylmuramic acid. Now in peptidoglycan structure, the NAG and the NAM sugar units are going to be covalently linked to each other via these beta-1,4-glycosidic linkages. And of course, we have another beta configuration in the glycosidic linkage, which reminds us that the function is once again going to be a structural function. And so in our next lesson video, we'll talk about how peptidoglycan is the main structural component of bacterial cell walls. And so, of course, the organism is going to be bacteria. Now in terms of being a branched polysaccharide, we can say that peptidoglycan is not going to be branched, so no branches in peptidoglycan.

Now you might be wondering about the prefix "peptido" that we see in peptidoglycan. It kinda sounds like a peptide, which is a protein, but I thought peptidoglycan was a carbohydrate, not a protein. So why this peptido prefix? Well, it turns out that in peptidoglycan structure, there actually is a short peptide crosslinking the polysaccharide structures. And so that is exactly where the Peptido prefix comes from. And so notice if we take a look at our image over here, we're showing you peptidoglycan structure. Notice that this and this are the polysaccharide portions, so the carbohydrate portions. And then notice down below, these linking structures are the peptide crosslinks. So you can see the pink highlights represent the polysaccharide, and these green highlights that you see beneath here are all going to be peptide crosslinks.

And so, if we take a look at the polysaccharide portion, which is again this highlighted pink portion here. If we zoom in on this little region right here and blow it up in this image, you'll note that the sugar units are NAG, NAG molecules, which again are the same exact sugar molecule that we talked about in the chitin topic. And then we also have over here the NAM sugar unit, which is, of course, N-acetylmuramic acid. And so you'll note that NAM still has an amino sugar here, but it also has this region over here which would be a carboxylic acid if it weren't connected to the peptide link down below. And so that's what makes this group right here highlighted in yellow is really what distinguishes NAM from NAG. But really the main takeaway is just to know NAG and NAM are going to be linked together in peptidoglycan structure. And of course, the linkage is going to be a beta1,4-glycosidiclinkage as we can see here. The 2 carbon atoms are 1 and 4. And so, really, this concludes our introduction to peptidoglycan structure. And in our next lesson video, we'll talk about the differences between gram-positive and gram-negative bacteria. So I'll see you guys in that video.

Peptidoglycan

So in our last lesson video, we said that peptidoglycan is a structural polysaccharide that makes up the structures of bacterial cell walls, and so that leads us to 2 different categories of bacteria, gram-positive bacteria and gram-negative bacteria. Bacteria can be categorized based on whether or not they absorb the gram stain. We're not going to get into the details of the procedure of the Gram stain in this video, but what you guys should know is that gram-positive bacteria are going to react positively to the gram stain, meaning that they absorb the gram stain. However, gram-negative bacteria, on the other hand, are going to react negatively to the gram stain, which means that they do not absorb the Gram stain.

In terms of their structures, we can say that gram-positive bacteria have their cell membrane surrounded by just a thick cell wall made up of peptidoglycan. Just for context and comparison, this thick cell wall is about 250Å thick. Again, this is not a number that you guys need to memorize; it's just for comparison purposes to the gram-negative bacteria. Notice that gram-negative bacteria have their cell membrane surrounded by more than just a thick cell wall. They are surrounded by both a thin cell wall and another complex outer membrane. The thin cell wall of gram-negative bacteria, just for context, is only about 30Å thick, which is about 8 times less thick than the cell wall of gram-positive bacteria.

What's important to note about the gram-negative bacteria is that the outer membrane, this complex outer membrane that they have, will actually make the gram-negative bacteria more resistant to some antibiotics. That's something important to keep in mind. Alright. If we take a look at our image down below on the left-hand side, notice that we're showing you gram-positive bacteria. Over here, we have our gram-positive bacterial cell. If we take a little cross-section of the perimeter of our gram-positive bacterial cell and pull that out over here and then zoom into this little cross-section, you'll find that the perimeter of the gram-positive bacterial cell consists of a cell membrane here in blue and a thick peptidoglycan cell wall. Again, we can write over here that this is a thick peptidoglycan wall that is about 250Å thick.

Now over here on the right-hand side, we're showing you gram-negative bacteria. Over here, we have our gram-negative bacterial cell. If we take a little cross-section of the perimeter of the gram-negative bacterial cell and pull it out and then zoom into that cross-section, you'll find that there's actually a very thin layer of peptidoglycan. There's a thin peptidoglycan cell wall that's only about 30Å thick. You can see that this thin peptidoglycan wall is actually surrounded by a membrane below which is going to be the inner membrane and it's also surrounded by another membrane above that is going to be the outer membrane. Notice that the outer membrane is more complex with these lipopolysaccharides that are attached to it, which are just polysaccharides with lipids attached. It's this complex outer membrane that allows gram-negative bacteria to be more resistant to some antibiotics.

What helps me remember the differences between gram-positive and gram-negative bacteria is that when I think about the positive, I think about the positive sign. And, of course, when I think about the negative, I think about the negative sign. This helps me remember that it's the gram-positive that have a thick peptidoglycan cell wall and the gram-negative that have a thin peptidoglycan cell wall. Then, of course, the gram-negative bacteria need to compensate for having a thin peptidoglycan cell wall by having a more complex outer membrane. Hopefully, this will help you guys remember the differences between gram-positive and gram-negative bacteria. And that concludes this video. See you guys in our next one.

What type of molecule provides the 'cross-links' between the peptidoglycan polysaccharides?

• A. Fatty acid.
• B. Lipid.
• C. Carbohydrate.
• D. Protein.
• E. Salt bridge.

Which of the following best describes the structure of the Gram-negative cell wall?

• A. A thin layer of chitin with an outer membrane on top.
• B. A thin peptidoglycan layer between two cell membranes.
• C. A thick peptidoglycan layer with an outer membrane on top.
• D. A thick peptidoglycan later under an outer membrane.

Here’s what students ask on this topic:

What is peptidoglycan and what is its structure?

Peptidoglycan is a structural polysaccharide found in bacterial cell walls. It is composed of alternating sugar units: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked by β-1,4-glycosidic bonds. These sugar units form long chains that are cross-linked by short peptide chains, giving the structure its rigidity and strength. The NAG and NAM units are covalently bonded, and the peptide cross-links provide additional stability. This structure is crucial for maintaining the shape and integrity of bacterial cells.

What is the difference between gram-positive and gram-negative bacteria in terms of peptidoglycan?

Gram-positive bacteria have a thick peptidoglycan layer, approximately 250 angstroms thick, which is the primary component of their cell wall. In contrast, gram-negative bacteria have a much thinner peptidoglycan layer, about 30 angstroms thick, and an additional complex outer membrane. This outer membrane contains lipopolysaccharides and provides extra protection, making gram-negative bacteria more resistant to certain antibiotics. The differences in peptidoglycan thickness and the presence of the outer membrane are key factors in how these bacteria respond to treatments and stains.

How do NAG and NAM contribute to the structure of peptidoglycan?

N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) are the two alternating sugar units that form the backbone of peptidoglycan. They are linked by β-1,4-glycosidic bonds, creating long chains. NAM has an additional carboxylic acid group that allows it to form peptide cross-links with other NAM units, providing structural integrity and rigidity to the bacterial cell wall. These cross-links are essential for the strength and stability of the peptidoglycan layer, which protects the bacterial cell from osmotic pressure and mechanical damage.

Why are gram-negative bacteria more resistant to antibiotics compared to gram-positive bacteria?

Gram-negative bacteria are more resistant to antibiotics primarily due to their complex outer membrane, which is absent in gram-positive bacteria. This outer membrane contains lipopolysaccharides and acts as an additional barrier, preventing many antibiotics from penetrating the cell. Additionally, the thin peptidoglycan layer in gram-negative bacteria is less accessible to antibiotics that target cell wall synthesis. The combination of the outer membrane and the thin peptidoglycan layer makes gram-negative bacteria more resilient against certain antibiotic treatments.

What role does peptidoglycan play in bacterial cell walls?

Peptidoglycan plays a crucial role in maintaining the structural integrity and shape of bacterial cell walls. It provides rigidity and strength, protecting the cell from osmotic pressure and mechanical damage. The alternating NAG and NAM sugar units form a strong, mesh-like structure that is cross-linked by short peptide chains. This network is essential for the survival of bacteria, as it prevents the cell from lysing in hypotonic environments. Additionally, peptidoglycan is involved in the process of cell division and growth, making it a vital component of bacterial physiology.