2-32 Bacterial cells are often covered in glycocalyx

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• The outer surface of cells is often covered in a polysaccharide or protein material called glycocalyx.
• Glycocalyx can be formed from protein, polysaccharides, polyalcohols or amino sugars.
• Glycocalyx can function in attachment, protection from desiccation, and protection from attack by a hosts immune system.

The general term for any network of polysaccharide or protein containing material extending outside of the cell is the glycocalyx. Many bacteria produce such a coating on the outside of their cell, and they come in two types: capsules and slime layers. The difference between the two is somewhat arbitrary. A capsule is closely associated with cells and does not wash off easily, while a slime layer is more diffuse and is easily washed away. Figure 2-50 shows the capsule surrounding Klebsiella planticola.

There are many different types of proteins, polysaccharides, polyalcohols and amino sugars in glycocalyx and the exact makeup is species-specific. The structure can be thick or thin, rigid or flexible. Observing cells stained with India ink in the microscope shows dark cells with an outline around them, as the stain does not penetrate the glycocalyx.

There are several functions attributed to glycocalyx, one of which is to help cells attach to their target's environment. Streptococcus mutans produces a slime layer in the presence of sucrose. This forms a surface that allows other bacteria to aggregate on tooth surfaces and results in dental plaque. Vibrio cholerae, the cause of cholera, also produces a glycocalyx that helps it attach to the intestinal villi of the host. Glycocalyx can play other roles in pathogenesis as well. Bacteria that enter the body are always in danger of being attacked by phagocytes (host cells that protect you from invaders). Often the capsule makes it more difficult for phagocytes to attach to and engulf pathogens. As one example, Streptococcus pneumoniae, when encapsulated by a glycocalyx, is able to kill 90% of infected animals, while non-encapsulated forms cannot kill. In addition capsules and slime layers are largely hydrophilic, so they can bind extra water in the environment and contribute to the protection of the cell from desiccation. Capsules and slime layers can also provide protection from the loss of nutrients by holding them within the layer. These extra layers coating the surface of the cell may also potentially mask viral receptors making it more difficult for viruses to attach. Many of these functions manifest themselves in the form of biofilms, which allow the formation of communities of microorganisms all held together by glycocalyx. Biofilms are covered in more detail in the chapter on Environmental Microbiology.

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