Gram-Negative Cell Walls - Online Tutor, Practice Problems & Exam Prep

Gram-negative bacteria possess a unique cell wall structure characterized by a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides (LPS), also known as endotoxins. LPS consists of three components: lipid A, core polysaccharide, and O antigen, which can vary among species. The outer membrane also features porins, integral proteins that facilitate passive transport. Understanding these structures is crucial for grasping the pathogenicity and antibiotic resistance of gram-negative bacteria, which are often associated with severe infections.

Gram-Negative Cell Walls

In this video, we're going to talk more about the details of gram-negative cell walls. Recall from our previous lesson videos that gram-negative cells do not absorb or retain the gram stain, unlike gram-positive cells that do. Also recall that the gram-negative cell walls have a thin layer of peptidoglycan, instead of having a thick layer of peptidoglycan like gram-positive cells do. These gram-negative cells, in addition to their plasma membrane, have an additional outer membrane that gram-positive cells do not have. This outer membrane is very important for these gram-negative bacteria. It serves as the outermost layer for these gram-negative cells. The outer membrane is actually anchored to the peptidoglycan layer via these lipoproteins. Lipoproteins, as their name implies, with the lipid and the protein, is a protein with a hydrophobic lipid tail, basically a protein with a lipid attached to it. This lipoprotein, in this case, serves as an anchor to the peptidoglycan layer.

Take a look at this image over here, showing a gram-negative cell with a gram-negative cell wall. Notice that the peptidoglycan layer, which is the light blue layer in between, is really thin in comparison to the peptidoglycan layer in gram-positive cells. If we zoom into this specific region right here and blow it up in size, that's what we have right here. Notice there is a thin peptidoglycan layer and up at the top, there is an outer membrane. This outer membrane, a second additional membrane that gram-negative cells have. Gram-positive cells do not have this additional membrane. Notice that the plasma membrane is at the very bottom, which all cells have. But notice that the outer membrane at the top here is anchored to the peptidoglycan layer via these lipoproteins that you can see at these positions. These lipoproteins serve as an anchor.

This here concludes our brief introduction to gram-negative cell walls. We will be able to apply these concepts and also learn more about gram-negative cell walls as we move forward in our course. So I'll see you all in our next video.

The Outer Membrane

In this video, we're going to talk more details about the outer membrane of gram-negative bacteria. Unlike the cytoplasmic or LPS for short. And so, lipopolysaccharides or LPS are a major component of the outer membrane of gram-negative bacteria. These lipopolysaccharides or LPS, they are large complex molecules with both lipid and carbohydrate components. Lipopolysaccharides or LPS are also called endotoxins, and they can be very damaging and deadly inside the host. That's why gram-negative bacteria can be associated with deadly bacterial infections.

If we take a look at our image down below, we can get a better understanding of the outer membrane of gram-negative bacteria. Once again, the gram-negative bacteria are going to have two membranes, an inner cytoplasmic plasma membrane and an outer membrane, which is on the border here. When we zoom into this region here and take a look at the outer membrane of the gram-negative bacteria, what you'll notice is that it is going to contain these LPS molecules, these lipopolysaccharides, or once again LPS, which are also known as endotoxins. Once again, the lipopolysaccharides are only found in the outer membrane of gram-negative bacteria and are not found in gram-positive bacteria.

This here concludes our brief introduction to the outer membrane and how it contains these lipopolysaccharide or LPS or endotoxin molecules. We'll be able to get some practice applying these concepts and learn more as we move forward. So, I'll see you all in our next video.

The cell wall of Gram-negative organisms:

• A. Has a thick peptidoglycan layer.
• B. Is more permeable to various molecules than the Gram-positive cell wall.
• C. Is characterized by an outer membrane containing LPS.
• D. Has a thin peptidoglycan layer AND is characterized by an outer membrane containing LPS.

Which of the following components of the gram-negative cell wall anchors the outer membrane to the thin peptidoglycan layer?

• A. Lipoproteins.
• B. Lipopolysaccharide.
• C. The periplasm.
• D. Cytoplasmic membrane phospholipids.

Structure of Lipopolysaccharide (LPS)

In this video, we're going to talk details about the structure of lipopolysaccharide or LPS. Now recall from our previous lesson videos that lipopolysaccharide or LPS is also known as endotoxin. And so LPS or endotoxin, they actually have 3 structural parts that we have numbered down below, 1, 2, and 3. Now before we actually get to these structural parts, let's take a quick look at our image down below. And notice on the left hand side we're showing you an image of a gram-negative bacterial cell. And so recall that the gram-negative bacterial cells have an outer membrane and a thin layer of peptidoglycan. Here in this image, we're only showing you a small bit of the thin layer of peptidoglycan, but we're showing you the outer membrane right here. And recall that lipopolysaccharides are a major component of the outer membrane of gram-negative cells. And so, zooming into the actual lipopolysaccharide molecule, again you'll see that it has 3 structural components. And so the first structural component number 1 is lipid A. And lipid A is going to anchor the entire lipopolysaccharide to the outer membrane's lipid bilayer. And so you can see that the lipid A portion is right down below right here. And its job is to literally anchor the entire lipopolysaccharide to the outer membrane. The second structural component is the core polysaccharide. And the core polysaccharide is at the core or the middle of the lipopolysaccharide. And so it's a structural molecule that's going to be in the middle and therefore it's going to connect the lipid A portion to the O antigen polysaccharide. Now the O antigen polysaccharide is actually the third component here. And so the O antigen polysaccharide is going to be a sugar polymer that extends outward from the membrane. And so you could think that the O is for the O in outward. And so if we take a look at the image down below, notice that right here at the core in the middle of the lipopolysaccharide is where we have the core polysaccharide. And then, towards the top here in green, what we have is the O antigen. And the O antigen is going to be pointing outwards, from the cell. And so, that is really the structure of the lipopolysaccharide. Now the O antigen polysaccharide is actually very variable. It will change a lot depending on the specific species. And so by identifying the O antigen polysaccharide, that can actually be used to identify a specific species. Now the core polysaccharide tends to remain relatively constant for the most part even across different species. Now one thing to note is that this lipid A portion that you see down below of the lipopolysaccharide, the lipid A is actually what is responsible for the damaging and the extremely deadly effects of lipopolysaccharide or endotoxin. And so really it's the lipid A portion that's responsible for some bacterial infections being so deadly and so damaging. And so this year concludes our brief lesson on the structure of Lipopolysaccharide, how it consists of these three components, lipid A, the core polysaccharide, and the O antigen polysaccharide. And we'll be able to get some practice applying these concepts as we move forward, so I'll see you all in our next video.

What part of Lipopolysaccharide is the endotoxin during bacterial infections?

• A. O-antigen.
• B. Lipid-A.
• C. Core Polysaccharide.
• D. LPS is only an endotoxin when all 3 parts are together.

Porins

In this video, we're going to introduce porins, which are found in the outer membrane of gram-negative bacteria. These porins are integral membrane proteins that are integrated into the outer membrane of gram-negative bacteria. They help to facilitate passive transport of molecules across the outer membrane. These porins span the entire membrane as cylinder-shaped pores, creating an entire passageway or a tunnel through the outer membrane to allow molecules to diffuse across passively. Porins always have identical subunits and are found in the outer membrane, forming in groups of three. You'll see these three identical subunits that associate with each other to form the entire porin structure.

If we take a look at this image below, we're showing you a gram-negative bacterial cell, focusing specifically on its outer membrane. You can see the lipopolysaccharides that are embedded here, and you can also see the porins themselves, which are these cylindrical-shaped pores. These porins form in groups of three, with units of three that associate with each other to create a pathway for molecules to diffuse across passively. When you zoom into the structure of the porin, you can see how it consists of these three subunits. They associate with each other to allow for passive diffusion. Here's a top view that shows how these cylindrical-shaped subunits come together to create the pore for specific molecules to diffuse across.

This concludes our brief introduction to these porins, and we'll be able to get some practice applying this knowledge as we move forward. I'll see you all in our next video.

Which of the following statements regarding porins is TRUE?

• A. They are found in the outer membrane of gram-positive cells.
• B. They are proteins that anchor on the outer of the membrane to peptidoglycan.
• C. They form in groups of 4 in the membrane.
• D. They facilitate passive transport of molecules across the outer membrane of gram-negative cells.
• E. They are the main structural component of bacterial cell walls.

Here’s what students ask on this topic:

What are the main differences between Gram-negative and Gram-positive cell walls?

Gram-negative and Gram-positive bacteria differ primarily in their cell wall structures. Gram-negative bacteria have a thin peptidoglycan layer and an additional outer membrane containing lipopolysaccharides (LPS), also known as endotoxins. This outer membrane is anchored to the peptidoglycan layer via lipoproteins. In contrast, Gram-positive bacteria have a thick peptidoglycan layer and lack an outer membrane. The thick peptidoglycan layer in Gram-positive bacteria retains the crystal violet stain used in Gram staining, making them appear purple under a microscope, whereas Gram-negative bacteria do not retain the stain and appear pink after counterstaining.

What is the role of lipopolysaccharides (LPS) in Gram-negative bacteria?

Lipopolysaccharides (LPS), also known as endotoxins, are a major component of the outer membrane of Gram-negative bacteria. LPS molecules consist of three parts: lipid A, core polysaccharide, and O antigen. Lipid A anchors the LPS to the outer membrane and is responsible for the toxic effects of LPS. The core polysaccharide connects lipid A to the O antigen, which extends outward from the cell. LPS plays a crucial role in the structural integrity of the bacteria and can trigger strong immune responses in the host, often leading to severe infections.

How do porins function in the outer membrane of Gram-negative bacteria?

Porins are integral membrane proteins found in the outer membrane of Gram-negative bacteria. They form cylindrical pores that span the entire membrane, facilitating the passive transport of molecules across the outer membrane. Porins are composed of three identical subunits that associate to create a passageway or tunnel, allowing molecules to diffuse through. This passive diffusion is essential for the uptake of nutrients and the expulsion of waste products, contributing to the bacteria's survival and adaptability.

Why are Gram-negative bacteria often associated with severe infections?

Gram-negative bacteria are often associated with severe infections due to the presence of lipopolysaccharides (LPS) in their outer membrane. LPS, also known as endotoxins, can trigger strong immune responses in the host, leading to inflammation, fever, and septic shock. The lipid A component of LPS is particularly toxic and can cause significant damage to host tissues. Additionally, the outer membrane of Gram-negative bacteria provides an extra barrier to antibiotics, contributing to their resistance and making infections harder to treat.

What are the structural components of lipopolysaccharides (LPS) in Gram-negative bacteria?

Lipopolysaccharides (LPS) in Gram-negative bacteria consist of three structural components: lipid A, core polysaccharide, and O antigen. Lipid A anchors the LPS to the outer membrane's lipid bilayer and is responsible for the toxic effects of LPS. The core polysaccharide is located in the middle of the LPS and connects lipid A to the O antigen. The O antigen is a sugar polymer that extends outward from the membrane and is highly variable among different bacterial species, aiding in species identification.