Some General Structural and Functional Properties of Bacterial Cell Envelopes

Some General Structural and Functional Properties of Bacterial Cell Envelopes 

The next barrier present in gram-negative bacteria is also a continuous, but a much more compact, layer of closely packed macromolecules. This entity, referred to as the outer membrane, consists predominantly of lipopolysaccharides (LPS), proteins, and phospholipids. The proteins have been estimated to contribute to about 50% of the surface area of this layer. The lipid-protein-polysaccharide components of the outer membrane form a matrix which appears in cross section as a typical double-tracked structure, similar to a phospholipid bilayer. This matrix is involved in a multiplicity of interactions of the cell surface with its exterior and interior environment. The outer membrane is known to function as a barrier to a large number of compounds, such as the antibiotics, actinomycin D, penicillin G, polymyxin, and bacitracin. In addition to preventing the entry of certain compounds, the outer membrane can also be viewed as a means of keeping enzymes of the periplasmic space trapped within the domain of the cell. 

The last barrier that our hypothetical molecule will have to cross is the plasma membrane or the inner membrane. The area between the outer and the protoplasmic membrane has been designated as periplasmic space, which appears to harbor a number of transport proteins and degra-dative enzymes. The inner membrane s composition and functions have been described extensively and can be summarized here as follows Structurally, the membrane consists of an organized and dynamic phospholipid-protein array in which the proteins (enzymes) are complexed with lipids to form units whose function is largely dependent on the state of the neighboring lipid environment. A schematic representation of the various surface structures of a gram-negative bacterium is shown in Fig. 1. 

Numerous functions essential for the survival of the cell have been ascribed to enzyme systems which are components of or closely associated with the inner membrane. Some of the more important functions will be briefly reviewed here. 

Mu rein Sacculus Globular Protein Lipopolysaccharide Lipid-Protein Complex 

---