The cell membrane of bacteria, epithelia is surrounded by a pericellular matrix, which is known as Glycocalyx. It is a carbohydrate-enriched coating that covers both eukaryotic cells and prokaryotic cells, particularly bacteria
- It is made of glycoprotein and glycolipid.
- Martinez and Palomo first discovered a cell coat around the animal cells in 1970, it is termed as the glycocalyx.
- The external surface of plasma membranes in most animal epithelial cells contains a fuzz-like coating which is made of several carbohydrate moieties of membrane glycolipids and glycoproteins. It assists as backbone molecules for maintenance. The carbohydrate portion of the glycolipids helps in cell–cell recognition, communication, and intercellular adhesion.
- The body uses glycocalyx as an identifier to differentiate between its own normal cells and transplanted tissues, diseased cells, or invading organisms.
- The glycocalyx performs a significant role in the regulation of endothelial vascular tissue, including the modulation of red blood cell volume in capillaries.
- During the embryonic development, Glycocalyx permits the cells to adhere to each other and control the movement of cells.
- The slime layer at the surface of a fish is a great example of glycocalyx.
- The term glycocalyx came from the Greek word glykys = sweet, kalyx = husk which means “sugar coat”.
- It is a highly charged layer of membrane-bound biological macromolecules that are connected to a cell membrane.
- It functions as a barrier within a cell and its surrounding.
- It helps in cell-cell interactions and protects the cell membrane from physical forces and stresses. It also maintains the integrity of the membrane.
- They also help in the development and progression of many diseases.
Glycocalyx composition and structure
- It is made up of glycosaminoglycans, proteoglycans, and other glycoproteins bearing acidic oligosaccharides and terminal sialic acids.
- It is thick approximately 500-2000nm in diameter hydrated gel-like coating on the luminal exterior of the vascular endothelium.
- The term means “sweet husk”, which referrs to the high polysaccharide content.
- Most of the associated proteins of Glycocalyx are transmembrane and can be connected to the cytoskeleton. This connection helps in the transduction of signals from the external to the internal parts of a cell.
- It contains different types of polyanionic macromolecules such as Glycoproteins, Polysaccharides, Proteoglycans, Glycosaminoglycans (For example heparan sulfate, chondroitin sulfate, and hyaluronic acid). These are helps to repel circulating platelets
- Based on the local microenvironment the exact composition is varied.
- At any granted time, it further comprises constituents of the routine molecular traffic which moves within it or lodges within it, such as Plasma proteins, Enzymes and enzyme inhibitors, Growth factors, Cytokines, Amino acids, Cations, Water.
- It is fragile but self-repairing. It can restore itself within less than 1 second by adsorbing plasma constituents
Types of Glycocalyx
Based on the bacterial species the structural features and chemical composition of glycocalyces can change, but as usual, this extra coat can develop in one of two forms.
In the slime layer, the glycoprotein molecules are loosely connected with the cell wall. This type of layer prevents the dehydration and loss of nutrients within the bacterial cell
In Capsule, the polysaccharides are more tightly connected to the cell wall. This layer is gummy, sticky consistency, and provides protection to the cell. It also helps in adhesion to solid surfaces and to nutrients in the environment.
Capsule containing bacteria are considered as encapsulated and they have a greater pathogenicity (ability to cause disease). These capsules protect the bacterial cells from phagocytic white blood cells of the immune system as well as in adhesion, which is a major factor in the initiation of some bacterial diseases.
- Protection: It Cushions the plasma membrane and defends it from chemical damage
- Immunity to infection: It facilitates the immune system to identify and selectively attack the outside organisms.
- Inflammation regulation: In blood vessels, the rolling/binding of leukocytes in healthy states are prevented by the Glycocalyx coating on endothelial walls.
- Defense against cancer: In cancer cells, the changes in the glycocalyx permit the immune system to identify and neutralize them.
- Fertilization: It allows the sperm to identify and adhere to eggs.
- Transplant compatibility: Forms the basis for compatibility of blood transfusions, tissue grafts, and organ transplants.
- Embryonic development: Glycocalyx guides embryonic cells to their targets in the body
- Cell adhesion: It held cells together so that tissues do not fall apart.
What is Biofilm?
- Biofilms are the complex bacterial communities of pathogenic bacteria, as well as normal flora and many environmental bacteria.
- In biofilms, a group of bacteria are appended to a surface and embedded in a common secreted adhesive matrix, typically polysaccharide in nature.
- In biofilms, bacteria are able to communicate with each other by a process known as quorum sensing. They also interact with and adapt to their environment.
- These bacteria can resist attack by antibiotics.
- They also help in bacterial growth by trapping nutrients and remain in a favorable niche.
- These bacteria adhere to environmental surfaces and resist flushing.
- They can resist phagocytosis and attack by the body’s complement pathways.
- Biofilms contain water channels for delivering water and nutrients.
- The planktonic bacteria interact with an environmental surface by their motility or by random collision to initiate the biofilm formation. After that they use their pili or cell wall adhesins to attach to that surface. As a result of this attachment, it signals the expression of genes involved in quorum sensing and, ultimately, biofilm formation.
- The motile bacteria lose thier flagella and become non-motile due to the secretion of biofilm matrix.
Planktonic bacteria such as Pseudomonas aeruginosa move through water or mucus by using its polar flagellum and interact with a solid surface such as the body’s mucous membranes.
After that, it is attached to the epithelial cells of the mucous membrane by using pili and cell wall adhesins. As a result, it activates signaling and quorum sensing genes to finally allow the population of P. aeruginosa to initiate synthesizing a polysaccharide biofilm made of alginate.
The bacteria started to lose their flagella and become non-motile as biofilms started to grow. These bacteria secrets different types of enzymes which allows the population to obtain nutrients from the host cells.
The biofilms develop a water channel to supply water and nutrients to the bacterial cells. When too many bacterial cells gather within a biofilm the quorum sensing allows some of the Pseudomonas to construct flagella, leave the biofilm, and colonize in a distinct location.