4: Bacteria - cell walls (2023)

Overview of bacterial cell walls

Outside the cell membrane is a cell wall, not just in bacteria but in all organisms. It is an additional layer that normally provides some resistance that the cell membrane lacks due to its semi-rigid structure.

The walls of gram-positive and gram-negative cells contain a component known aspeptidoglicano(also known asMurein). This particular substance has not been found anywhere else on Earth except in the cell walls of bacteria. But both types of bacterial cell walls contain other components that make the bacterial cell wall a complex structure overall, especially when compared to the cell walls of eukaryotic microbes. The cell walls of eukaryotic microbes usually consist of a single component, such as cellulose in algal cell walls or chitin in fungal cell walls.

The bacterial cell wall performs several functions in addition to the general strength of the cell. It also helps maintain the cell's shape, which is important for how the cell grows, reproduces, receives nutrients, and moves. Protects the cell fromosmotic lysis, when the cell moves from one environment to another or transports nutrients from its environment. Because water is free to move across the cell membrane and cell wall, the cell is at risk for an osmotic imbalance that can put pressure on the relatively weak plasma membrane. In fact, studies have shown that the internal pressure of a cell is similar to the pressure in a fully inflated car tire. That's a lot of pressure that the plasma membrane has to withstand! The cell wall can keep out certain molecules like toxins, especially for gram negative bacteria. And finally, the bacterial cell wall may contribute to the pathogenicity or disease-causing ability of the cell to certain bacterial pathogens.

peptidoglycan structure

Let's start with peptidoglycan, as it is an ingredient that both bacterial cell walls have in common. Peptidoglycan is a polysaccharide formed by two derivatives of glucose,N-acetylglucosamine (NAG)YN-acetylmuramic acid (NAM), alternating in long chains. The chains are joined by atetrapeptideextending from the NAM sugar unit, allowing the formation of a lattice-like structure. The four amino acids that make up the tetrapeptide are:L-alanine, D-glutamine, L-lysine bzwmeso-diaminopimelic acid (DPA),YD-alanine. Cells normally use only the L-isomeric form of amino acids, but the use of the mirror image of D-amino acids provides protection against proteases that can compromise cell wall integrity by attacking peptidoglycan. Tetrapeptides can bedirectly on the networkto each other, with the D-alanine of one tetrapeptide binding to the L-lysine/DPA of another tetrapeptide. In many gram-positive bacteria, there is a five-amino acid cross-bridge, such as glycine (peptide bridge), which serves to link one tetrapeptide to another. In both cases, crosslinking serves to increase the strength of the overall structure, from which more strength follows.complete networks, where each tetrapeptide is linked in some way to a tetrapeptide on a different NAG-NAM chain.

While much is still unknown about peptidoglycan, research over the last decade suggests that peptidoglycan is synthesized as a cylinder with a coiled substructure, with each spiral crosslinking with the neighboring coil, creating an even stronger overall structure.

Peptidoglycan structure.

staple cell walls

The cell walls of Gram-positive bacteria consist mainly of peptidoglycan. In fact, peptidoglycan can make up as much as 90% of the cell wall, forming layer upon layer around the cell membrane. NAM tetrapeptides normally cross-link with a peptide bridge, and complete cross-linking is common. All of these combine to form an incredibly strong cell wall.

The additional component in a gram positive cell wall isteichoic acid, a glycopolymer embedded in peptidoglycan layers. It is believed that teichoic acid performs several important functions for the cell, such as: B. the generation of the cell's net negative charge, which is essential for the development of a proton motive force. Teichoic acid contributes to the general rigidity of the cell wall, which is important for maintaining cell shape, particularly in rod-shaped organisms. There is also evidence that teichoic acids are involved in cell division by interacting with the peptidoglycan biosynthetic machinery. Finally, teichoic acids seem to play a role in resistance to adverse conditions, such as high temperatures and high salt concentrations, and to β-lactam antibiotics. Techoic acids can be covalently linked to peptidoglycan (wandteic acids or WTA) or attached to the cell membrane through a lipid anchor, in which case we speak oflipoteichonsäure.

As peptidoglycan is relatively porous, most substances can pass through the cell wall of Gram-positive bacteria without much difficulty. However, some nutrients are too large for the cell to depend on for use.exoenzyme. These extracellular enzymes are produced in the cell's cytoplasm and then secreted beyond the cell membrane through the cell wall, where they work outside the cell to break large macromolecules into smaller components.

gram negative cell walls

The cell walls of Gram-negative bacteria are more complex than those of Gram-positive bacteria and contain more components overall. They also contain peptidoglycan, albeit only in certain layers, which make up 5-10% of the total cell wall. Most notable about the gram negative cell wall is the presence of a plasma membrane located outside the peptidoglycan layers known as theouter membrane. This makes up most of the gram negative cell wall. The outer membrane consists of a lipid bilayer very similar in composition to the cell membrane, with polar heads, fatty acid tails, and integral proteins. It differs from the cell membrane by the presence of large molecules known asLipopolissacárido (LPS), which are anchored to the outer membrane and protrude from the cell into the environment. The LPS consists of three different components: 1) theO antigen or O polysaccharide, which represents the outermost part of the structure, 2) thekernpolysaccharide, e 3)lipid A, which anchors LPS to the outer membrane. LPS is known to perform many different functions for the cell, such as: B. contributing to the cell's net negative charge, stabilizing the outer membrane, and providing protection against certain chemicals by physically blocking access to other parts of the cell wall. Furthermore, LPS plays a role in the host response to pathogenic gram-negative bacteria. The O antigen triggers an immune response in an infected host, triggering the production of antibiotics specific to that part of the LPS (think ofE. coli o157). Lipid A acts as a toxin, particularly inendotoxin, giving rise to general symptoms of the disease, such as fever and diarrhea. A large amount of lipid A released into the bloodstream can induce endotoxic shock, a bodywide inflammatory response that can be fatal.

The outer membrane provides a barrier to the cell. While you'll want to keep certain molecules out, like antibiotics and toxic chemicals, there are nutrients you want to let in, and the extra lipid bilayer provides a formidable barrier. Large molecules are broken down by enzymes so they can pass through LPS. Instead of using exoenzymes (like Gram-positive bacteria), Gram-negative bacteria useperiplasmic enzymethat are stored inperiplasm. Where is the periplasm, you ask? It is the space between the outer surface of the cell membrane and the inner surface of the outer membrane and contains the gram negative peptidoglycan. Once nutrients have been broken down by periplasmic enzymes into smaller molecules that can pass through LPS, they still need to be transported across the outer membrane, specifically the lipid bilayer. Use gram negative cellsTurvar, which are transmembrane proteins composed of a trimer of three subunits that form a pore across the membrane. Some porins are nonspecific and carry any suitable molecule, while other porins are specific and only carry substances that they recognize through a binding site. After the molecules traverse the outer membrane and periplasm, they make their way through the porous peptidoglycan layers before being transported across the cell membrane by the integral proteins.

The peptidoglycan layers are connected to the outer membrane through the use of a lipoprotein known asBraun's Lipoprotein(good old Dr. Braun). Lipoprotein is covalently attached to peptidoglycan at one end, while the other end is embedded in the outer membrane through its polar head. This connection between the two layers provides additional structural strength and integrity.

Unusual, wallless bacteria.

Having emphasized the importance of a cell wall and the peptidoglycan ingredient for gram-positive and gram-negative bacteria, it seems important to note some exceptions as well. Bacteria belonging to the tribe.chlamydiathey appear to lack peptidoglycan, although their cell walls are Gram-negative in structure in all other respects (ie, outer membrane, LPS, porin, etc.). It has been suggested that they use a protein coat that works similarly to peptidoglycan. This has the advantage of providing the cell with resistance to β-lactam antibiotics (such as penicillin) that attack the peptidoglycan.

Bacteria belonging to the tribe.tenerifethey lack a cell wall, making them extremely susceptible to osmotic changes. They usually strengthen the cell membrane a little by addingsterols, a substance normally associated with eukaryotic cell membranes. Many members of this tribe are pathogens that hide in the host's protective environment.