A glycocalyx, literally meaning "sugar coat", is a network of polysaccharides that project from cellular surfaces of bacteria, which classifies it as a universal surface component of a bacterial cell, found just outside the bacterium cell wall. A distinct, gelatinous glycocalyx is called abacterial capsule, whereas an irregular, diffuse layer is called a slime layer . This coat is extremely hydrated and stains with ruthenium red.
Glycocalyx
This diagram depicts the different types of glycocalyx. 1) Bacterial capsule is a well-organized layer outside the cell. 2) Slime layer is diffuse and irregular in structure. 3) Bacteria can form biofilms by fusing their glycocalyxes.
Bacteria growing in natural ecosystems, such as in soil, bovine intestines, or the human urinary tract, is surrounded by some sort of glycocalyx-enclosed microcolony. It serves to protect the bacterium from harmful phagocytes by creating capsules or allowing the bacterium to attach itself to inert surfaces, like teeth or rocks, via biofilms (e.g. Streptococcus pneumoniae attaches itself to either lung cells, prokaryotes, or other bacteria which can fuse their glycocalyxes to envelop the colony).
Capsule
The cell capsule is a very large structure of some bacterial cells. It is a layer that lies outside the cell wall of bacteria. It is a well organized layer, not easily washed off, and it can be the cause of various diseases. It usually consists of polysaccharides, but can be composed of other materials (e.g., polypeptide in B. anthracis). Most capsules are difficult to stain using standard stains because most stains do not adhere to the capsule. For examination under the microscope, the bacteria and their background are stained darker than the capsule, which doesn't stain.
The capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease (i.e. prevents phagocytosis). The capsule can protect cells from engulfment by eukaryotic cells, such as macrophages. A capsule-specific antibody may be required for phagocytosis to occur. Capsules also contain water which protects bacteria against desiccation. They also exclude bacterial viruses and most hydrophobic toxic materials such as detergents.
Slime layer
When the amorphous viscid secretion (that makes up the capsule) diffuses into the surrounding medium and remains as a loose undemarcated secretion, it is known as slime layer. A slime layer in bacteria is an easily removed, diffuse, unorganized layer of extracellular material that surrounds bacteria cells. Specifically, this consists mostly of exopolysaccharides, glycoproteins, and glycolipids.
The function of the slime layer is to protect the bacteria cells from environmental dangers such as antibiotics and desiccation. The slime layer also allows bacteria to adhere to smooth surfaces such as prosthetic medical devices and catheters. It may permit bacterial colonies to survive chemical sterilization with chlorine, iodine, and other chemicals, leaving autoclaving or flushing with boiling water as the only certain methods of decontamination.
The slime layer is not to be confused with the S-layer, a separate and highly organised glycoprotein layer surrounding many bacterial cells.