Capsules are layers that are well organized and not easily washed off. They are most often composed of polysaccharides, but some are constructed of other materials. For example, Bacillus anthracis (the anthrax bacterium) has a proteinaceous capsule composed of poly-d-glutamic acid.
Capsules are clearly visible in the light microscope when negative stains or specific capsule stains are employed; they also can be studied with the electron microscope.
Capsules are not required for growth and reproduction in laboratory cultures. However, they confer several advantages when bacteria grow in their normal habitats. They help pathogenic bacteria resist phagocytosis by host phagocytes. Streptococcus pneumoniae, which causes ear infections, pneumonia, and other diseases, provides a dramatic example.
When it lacks a capsule, it is phagocytosed easily and does not cause disease. On the other hand, the capsulated variant quickly kills mice. Capsules can also protect against desiccation because they contain a great deal of water. They exclude viruses and most hydrophobic toxic materials such as detergents.
A slime layer is a zone of diffuse, unorganized material that is removed easily. It is usually composed of polysaccharides but is not as easily observed by light microscopy. Gliding bacteria often produce slime, which in some cases has been shown to facilitate motility.
A bacterial glycocalyx is a layer consisting of a network of polysaccharides extending from the surface of the cell. The term can encompass both capsules and slime layers because they usually are composed of polysaccharides. The glycocalyx aids in attachment to solid surfaces, including tissue surfaces in plant and animal hosts.
Bacterial Slime Layers
Many bacteria have a regularly structured layer called a Slime layer on their surface. The S-layer has a pattern of something like floor tiles and is composed of protein or glycoprotein. In typical Gram-negative bacteria, the S-layer adheres directly to the outer membrane; it is associated with the peptidoglycan surface of typical Gram-positive cell walls.
S-layers are of considerable interest not only for their biological roles but also in the growing field of nanotechnology. Their biological roles include protecting the cell against ion and pH fluctuations, osmotic stress, enzymes, or predatory bacteria.
The S-layer also helps maintain the shape and envelope rigidity of some cells, and it can promote cell adhesion to surfaces. Finally, the S-layer seems to protect some bacterial pathogens against host defenses, thus contributing to their virulence.
The potential use of S-layers in nanotechnology is due to the ability of S-layer proteins to self-assemble; that is, S-layer proteins contain the information required to spontaneously associate and form the S-layer without the aid of any additional enzymes or other factors.
Thus S-layer proteins could be used as building blocks for the creation of technologies such as drug-delivery systems and novel detection systems for toxic chemicals or bioterrorism agents.