From the cell wall inward, most bacteria are much alike. The bacterial cell wall and materials adhering to its surface are the main ways in which we differentiate one species or strain from another. Some bacteria have an additional layer outside of the cell wall called the glycocalyx. This coating of macromolecules protects the cell, helps it adhere to surfaces and elude our immune defenses.
* Bacterial Cell Wall *
Prokaryotic eubacteria (bacteria) have a cell wall made of a unique substance called peptidoglycan. The rigid structure of peptidoglycan gives the bacterial cell shape, surrounds the cytoplasmic membrane and provides prokaryotes with protection from their environment.
* Types of Glycocalyces *
Beyond the cell wall, some bacteria have an additional layer called the glycocalyx. The structural features and chemical composition of glycocalyces differ depending on the species of bacteria, but in general this additional layer can come in one of two forms:
1. Slime Layer Type of Glycocalyx: A glycocalyx is considered a slime layer is when the glycoprotein molecules are loosely associated with the cell wall. Bacteria that are covered with this loose shield are protected from dehydration and loss of nutrients.
2. Capsule Type of Glycocalyx: The glycocalyx is considered a capsule when the polysaccharides are more firmly attached to the cell wall. Capsules have a gummy, sticky consistency and provide protection as well as adhesion to solid surfaces and to nutrients in the environment.
Bacteria that possess capsules are considered to be encapsulated, and generally have greater pathogenicity because capsules protect bacteria, even from phagocytic white blood cells of the immune system. The adhesive power of capsules is also a major factor in the initiation of some bacterial diseases.
* What Is a Biofilm? *
Glycocalyces are instrumental in the formation of biofilms. A biofilm is a living ecosystem made of millions of bacterial cells, their wastes and other extracellular products. These microbial biofilms can be found everywhere, from the layer of scum hugging the bowl of an unclean toilet to the inside of the human mouth.
* Oral Biofilm & Plaque *
The slime layer of Streptococcus mutans allows this bacteria and others to accumulate on tooth enamel (yuck mouth and one of the causes of cavities). Other bacteria in the mouth become trapped in the slime and form a biofilm, eventually building up as plaque.
Unchecked, this biofilm can easily reach a thickness of hundreds of cells on the surfaces of the teeth, and, over time, plaque build-up can become mineralized, eventually forming calculus (tartar).
* Medical Impact of Biofilms *
Biofilms also have other serious medical inplications. Persistent biofilms containing pathogenic bacteria can be problematic when they accumulate on damaged tissues and internal medical devices, such as catheters and pacemekers.
* Sources *
Bauman, R. (2005) Microbiology. Pearson Banjamin Cummings.
Park Talaro, K. (2008) Foundations in Microbiology. McGraw-Hill.