Bacteria may have one or more layers outside of the cell wall. The functions of some of these are well established, but that of others are unknown.
Many bacteria envelop themselves with a gel-like layer called a glycocalyx that generally functions as a mechanism of either protection or attachment (figure 3.37). If the layer is distinct and gelatinous, it is called a capsule. If, instead, the layer is diffuse and irregular, it is called a slime layer. Colonies of bacteria that form either of these extracellular layers often appear moist and glistening.
Capsules and slime layers vary in their chemical composition depending on the species of bacteria. Most are composed of polysaccharides such as dextrans and glucans. These take the form of tiny, short, hairlike structures or fibrils, which form a network on the outside of the cell wall. A few capsules consist of polypeptides made up of repeating subunits of only one or two amino acids. Interestingly, the amino acids are generally of the D-stereoisomeric form, one of the few places D-amino acids are found in nature. ■ polysaccharide, p. 29 ■ D-amino acid, p. 27
Some types of capsules and slime layers enable bacteria to adhere to specific surfaces, including teeth, rocks, and other bacteria. These often enable microorganisms to grow as a biofilm, a mass of bacteria coating a surface. One example is dental plaque, a biofilm on teeth. Streptococcus mutans uses sucrose to synthesize a capsule, which enables it to adhere to and grow in the crevices of the tooth. Other bacteria can then adhere to the layer created by the growth of S. mutans. Acid production by bacteria in the biofilm damages the tooth surface.
■ Streptococcus mutans, p. 602 ■ sucrose, p. 30 ■ dental caries, p. 602
Some capsules enable bacteria to thwart innate defense systems that otherwise protect against infection. For example, Streptococcus pneumoniae, the organism that causes bacterial pneumonia, can only cause disease if it has a capsule. Unencapsulated cells are quickly engulfed and killed by phagocytes, an important cell of the body's defense system.
■ Streptococcus pneumoniae, p. 576 ■ phagocytes, p. 384
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