Gram-positive cell wall
Bacteria are microscopic organisms that can be classified into two major groups based on the structure of their cell wall: Gram-positive and Gram-negative. The cell wall is a rigid layer that surrounds the cell membrane and protects the bacterial cell from environmental stress and mechanical damage. The cell wall also determines the shape of the bacterial cell and provides attachment sites for various molecules and structures.
The Gram-positive cell wall is named after the Danish bacteriologist Hans Christian Gram, who developed a staining technique in 1884 to differentiate between different types of bacteria. The Gram stain involves applying a series of dyes to a bacterial smear and observing the color under a microscope. Gram-positive bacteria retain the primary dye (crystal violet) and appear purple, while Gram-negative bacteria lose the primary dye and take up the counterstain (safranin) and appear pink.
The reason for this difference in staining is the composition and thickness of the cell wall. The Gram-positive cell wall is thick (15–80 nm) and more homogenous than that of the thin (2 nm) Gram-negative cell wall. The Gram-positive cell wall contains large amount of peptidoglycan, a polymer of sugar and amino acids that forms a mesh-like layer that gives strength and rigidity to the cell wall. Peptidoglycan constitutes about 40–80% of the dry weight of the Gram-positive cell wall.
In addition to peptidoglycan, the Gram-positive cell wall contains other components that are unique to this group of bacteria. These include teichoic acids, teichuronic acids, polysaccharides, proteins, and lipids. These components account for up to 50% of the dry weight of the wall and 10% of the dry weight of the total cell. They play various roles in the structure, function, and interaction of the Gram-positive bacteria with their environment.
In this article, we will explore the composition and role of these components in more detail, starting with teichoic acids and their functions.
The main component of the Gram-positive cell wall is peptidoglycan, a polymer of alternating sugar units (N-acetylglucosamine and N-acetylmuramic acid) and short peptide chains. Peptidoglycan forms a thick and rigid layer that surrounds the plasma membrane and gives shape and strength to the bacterial cell. The peptidoglycan layer can be up to 80 nm thick and account for up to 80% of the dry weight of the cell wall.
The peptidoglycan layer is cross-linked by peptide bridges between adjacent chains. The degree and type of cross-linking vary among different species of Gram-positive bacteria. For example, in Staphylococcus aureus, the peptide bridges consist of five amino acids, while in Bacillus subtilis, they consist of four amino acids. The cross-linking provides mechanical stability and resistance to osmotic pressure.
The peptidoglycan layer also contains other molecules that are covalently attached to it or embedded within it. These include:
- Teichoic acids, polymers of glycerol or ribitol phosphate that extend from the peptidoglycan layer to the cell surface. Teichoic acids can be classified into wall teichoic acids (WTA), which are linked to the peptidoglycan, and lipoteichoic acids (LTA), which are anchored to the plasma membrane. Teichoic acids have various functions, such as serving as antigens, binding metal ions, regulating autolysis, and facilitating adhesion.
- Teichuronic acids, polymers of sugar acids that replace teichoic acids when phosphate is limiting. Teichuronic acids have similar functions as teichoic acids, but they are less common and less studied.
- Polysaccharides, chains of sugars that are attached to the peptidoglycan or the teichoic acids. Polysaccharides can act as antigens, receptors, or adhesins. For example, in Streptococcus pneumoniae, the polysaccharide capsule is a major virulence factor that protects the bacterium from phagocytosis.
- Proteins, such as enzymes, transporters, or toxins that are associated with the peptidoglycan layer or the cell surface. Some proteins are covalently linked to the peptidoglycan by a mechanism called sortase-mediated anchoring. Other proteins are non-covalently bound to the cell wall by electrostatic or hydrophobic interactions. Proteins can have various roles in metabolism, transport, secretion, or pathogenesis.
Teichoic acids are polymers of polyribitol phosphate or polyglycerol phosphate that are attached to the peptidoglycan layer of the Gram-positive cell wall. They contain ribitol and glycerol units, which may have sugar or amino acid substitutes. Teichoic acids are of two types: wall teichoic acid (WTA) and lipoteichoic acid (LTA). WTA is covalently linked to the peptidoglycan, while LTA is anchored to the plasma membrane lipids.
Teichoic acids have many functions in the Gram-positive cell wall, such as:
- They act as major surface antigens of some Gram-positive bacteria, such as Streptococcus pneumoniae and Streptococcus pyogenes. They bear antigenic determinants that can elicit immune responses and can be used for serological classification of bacteria.
- They serve as substrates for autolytic enzymes, which are involved in cell wall remodeling and turnover. Autolytic enzymes can cleave the bonds between teichoic acids and peptidoglycan, leading to cell wall lysis and death.
- They bind magnesium ions and may play a role in supplying this essential element to the cell. Magnesium ions are important for many enzymatic reactions and for stabilizing the cell membrane.
- They play a role in normal functioning of the cell wall and provide an external permeability barrier to Gram-positive bacteria. They can regulate the passage of ions and molecules across the cell wall and maintain its integrity and rigidity.
- They help in attachment of bacteria to host cells or surfaces. LTA can form microfibrils with other surface proteins, such as M protein in S. pyogenes, that facilitate the adherence of bacteria to animal cells or tissues. This can enhance bacterial colonization and infection.
Teichoic acids are unique to Gram-positive bacteria and are essential for their survival and virulence. They are also potential targets for antimicrobial agents that can disrupt their synthesis or function.
Teichuronic acid is a type of polysaccharide that is found in some Gram-positive cell walls. It is composed of repeating units of sugar acids, such as N-acetylmannuronic acid or D-glucuronic acid. Teichuronic acid is synthesized when the phosphate supply to the cell is limited, and it replaces teichoic acids in the cell wall.
Teichuronic acid has several roles in the Gram-positive cell wall:
- It provides structural support and rigidity to the cell wall, as it forms cross-links with peptidoglycan and other polysaccharides.
- It acts as a surface antigen that can be recognized by the host immune system. For example, Bacillus anthracis, the causative agent of anthrax, has a capsule made of poly-D-glutamic acid that is attached to the cell wall by teichuronic acid.
- It modulates the permeability and charge of the cell wall, as it binds cations such as calcium and magnesium. This may affect the uptake of nutrients and antibiotics by the cell.
- It regulates the activity of autolysins, which are enzymes that degrade the cell wall. Teichuronic acid can inhibit or activate autolysins depending on the environmental conditions.
Teichuronic acid is a unique component of the Gram-positive cell wall that plays an important role in the survival and virulence of some bacteria.
Besides peptidoglycan and teichoic acids, the Gram-positive cell wall may contain other components that vary depending on the bacterial species. Some of these components are:
- Capsule: A capsule is a layer of polysaccharides or polypeptides that surrounds the cell wall and protects the bacterium from phagocytosis, desiccation, and antibiotics. Some examples of Gram-positive bacteria with capsules are Streptococcus pneumoniae, Bacillus anthracis, and Staphylococcus aureus.
- S-layer: An S-layer is a crystalline layer of protein or glycoprotein that covers the cell wall and provides structural support, adhesion, and protection. Some examples of Gram-positive bacteria with S-layers are Bacillus sphaericus, Lactobacillus acidophilus, and Clostridium difficile.
- Exopolysaccharides: Exopolysaccharides are polymers of sugars that are secreted by some bacteria and form a slimy layer on the cell wall. They help the bacteria to form biofilms, adhere to surfaces, and resist environmental stress. Some examples of Gram-positive bacteria that produce exopolysaccharides are Streptococcus mutans, Lactococcus lactis, and Bacillus subtilis.
- Lipoglycans: Lipoglycans are glycolipids that are anchored to the cell membrane and extend through the peptidoglycan layer. They have various functions such as cell wall synthesis, signal transduction, and antigenicity. Some examples of Gram-positive bacteria that have lipoglycans are Mycobacterium tuberculosis, Corynebacterium diphtheriae, and Nocardia asteroides.
- Surface proteins: Surface proteins are proteins that are either covalently attached to the peptidoglycan layer or non-covalently associated with the cell wall. They have diverse roles such as enzyme activity, nutrient uptake, adhesion, invasion, and immune evasion. Some examples of Gram-positive bacteria that express surface proteins are Streptococcus pyogenes (M protein), Staphylococcus aureus (protein A), and Listeria monocytogenes (internalin).
These components contribute to the structural integrity, physiological functions, and pathogenicity of the Gram-positive cell wall. They also serve as targets for antimicrobial agents and vaccines. Therefore, understanding their biosynthesis, regulation, and interactions is important for developing new strategies to combat Gram-positive infections.
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