Microbiologia

4
The Cell Wall Matrix

Chemical and morphological structures that comprise the envelope can no longer be dealt with as unrelated entities. The investigator must now ask how these structures interact dynamically—both temporally during their assembly and functionally during their operation. Loretta Leive, Bacterial Membranes and Walls, 1973

4.1. Introduction
Cell walls of prokaryotes aremore than just structural frameworks for the cells. The cell wall in bacteria is a highly complex region that contains different molecular components in addition to the peptidoglycan and the S-layer. In Gram-negative bacteria, the cell wall encompasses an outer membrane that is unique in that it has a highly organized structure and functions as a permeability barrier for many compounds. Varioustypes of ionized molecules are attached to the peptidoglycan of Gram-positive bacteria and they distribute charges within the cell wall as well as at the surface. These extra components have diverse but specific roles and may even enable bacteria to grow in hostile chemical environments. From various research reports concerning the cell walls of archaea, it can be inferred that these organisms alsocontain molecules in addition to those that are responsible for cell shape and cell stability. This chapter examines the chemical structure and biological functions of polysaccharides, acidic polymers, and proteins in the cell walls of prokaryotes. On the basis of the Gram-staining reaction, two distinct types of prokaryotes are differentiated and analysis of the cell wall of these two groups oforganisms indicates a fundamental difference in chemical composition. Besides providing structural rigidity for both Gram-positive and Gram-negative bacteria, the cell wall has additional functions that include the following: (1) transports nutrients from the environment to the cell membrane; (2) enhances virulence of pathogens by attaching microbial cells to host animal cells through specific bindingactivities; (3) protects the cell membrane against enzymes, bile salts, and immune proteins that would lead to destruction of the cell; and (4) influences synthesis of peptidoglycan biosynthesis through the modulation of
136

4.2. Cell Walls of Gram-Positive Bacteria

137

biosynthetic enzymes and the binding of antibiotics that inhibit peptiglycan formation. To understand the differentmechanisms for achieving these functions, it is crucial to examine the composition of lipids, proteins, and carbohydrates found in the cell walls of prokaryotes. Not only is it important to focus on the polymeric nature of these compounds but it is also necessary to examine the mechanisms that are used by microbes to localize these compounds in the cell wall region.

4.2. Cell Walls ofGram-Positive Bacteria
To detail fully all the structures in the cell wall of a prokaryote would be tedious, and in most instances all molecular components present have not been resolved. There is a continuum of cellular material from the plasma membrane to the surface of the cell, resulting in the cell wall structure being highly infiltrated with carbohydrates and proteins. Some investigators refer to thissomewhat diffuse segment of the cell as the cell envelope, which would include the cell wall and the surface materials. As the designation envelope has specific meaning in virology, it may be best to avoid employing this term when discussing bacteria. While the cell wall region of Grampositive bacteria may appear to be relatively simple on examination by electron microscopy, analytical biologicaland chemical studies reveal that a variety of materials exist in the cell wall region. Like those of bacteria, the cell walls of archaea may also harbor numerous compounds; however, generalizations about the nonstructural segments of archaeal cell walls cannot be made until additional research has been conducted.

4.2.1. Polysaccharides
Neutral polysaccharides are found in the cell walls of...