Question

How does the molecular composition of the parts of a bacterial cell envelope determine their functions?

Answer 1

The bacteria cell envelope is a complex multilayered structure which serves to protect these organisms from their sometimes unpredictable and often hostile environment. The envelopes of the cell of most bacteria fall into one of two major groups. The groups are Gram-negative bacteria, which are surrounded by a thin peptidoglycan cell wall. By itself it is surrounded by an outer membrane which contains lipopolysaccharide. Another one is Gram-positive bacteria. It lacks an outer membrane but they are surrounded by some layers of peptidoglycan which are many times thicker than is found in the Gram-negatives. Threading through these layers of peptidoglycan are long anionic polymers are called teichoic acids. The composition and organization of these envelope layers and recent insights into the mechanisms of cell envelope assembly are discussed.
Since the late 1830's it has been well known that all living organisms are composed of fundamental units called cells. The cell is a finite entity with a definite boundary, also known as the plasma membrane. That means that the essence of the living state must be contained within the biological membrane. It is a defining feature of all living things. Everything that exists outside of the biological membrane is nonliving. Some chemists tend to think of membranes as self assembling but biological membranes do not self assemble; they require energy to be established and maintained. In almost all cells this structure is a phospholipid bilayer. It surrounds and contains the cytoplasm. In addition to lipid components biological membranes are composed of proteins; the proteins are what make each membrane unique. Despite its obvious importance, membranes and their associated functions remained poorly understood until the 1950s. Before this, the membrane was viewed as a semipermeable bag. This view persisted for a number of years, particularly in the case of bacteria, because it could not be envisioned how such a “simple” organism could have anything but a simple membrane.

The bacterial cell envelope or the membrane and the other structures which surround and protect the cytoplasm, is anything but a simple membrane. Unlike cells of higher organisms, the bacterium is faced with an unpredictable, dilute and often hostile environment. To survive, bacteria have evolved a sophisticated and complex cell envelope that protects them. But it allows selective passage of nutrients from the outside and waste products from the inside. The following discussion concerns the organisation, composition, and the functions of the various layers and compartments that make up this remarkable cellular structure. It is easily appreciated that a living system cannot do what it does without the ability to establish separate compartments in which components are segregated. Specialized functions occur within different compartments because the types of molecules within the compartment can be restricted. Membranes do not simply serve to segregate different types of molecules. They also function as surfaces on which reactions can occur. Recent advances in microscopy, have revealed strikingly nonrandom localization of envelope components.
More than 100 years ago Christian Gram developed a staining procedure. It allowed him to classify nearly all bacteria into two large groups, and this eponymous stain is still in widespread use. One group of bacteria retain Christian's stain, named as Gram-positive, and the other do not, named as Gram-negative. The basis for the Gram stain lies in fundamental structural differences in the cell envelope of these two groups of bacteria. For discussion of the Gram-negative bacterial cell envelope Escherichia coli will be used, an extensively-studied organism that has served as a model for understanding a number of fundamental biological processes. In comparing Gram-negative and Gram-positive cell envelopes Staphylococcus aureus will be used as a reference point but will highlight specific differences between it and Bacillus subtilis. B. subtilis is the major Gram-positive model organism and a substantial knowledge base exists for it, but the cell envelope of S. aureus has been studied more extensively because of interest in how surface features mediate interactions with the environment in the course of infection. Care should be taken in generalizing from examples drawn from particular microorganisms. For example, E. coli inhabits the mammalian gut. Accordingly, E. coliand other enteric bacteria must have a cell envelope which is particularly effective at excluding detergents such as bile salts. This need not be a pressing issue for other Gram-negative bacteria, and their envelopes may differ in species- and environmentally specific ways. The ability to use the Gram stain to categorize bacteria suggests that the basic organizational principles we present are conserved. In addition to this many bacteria express an outermost coat, the S-layer, that is composed of a single protein which totally encases the organism. S-layers and capsules, which are coats composed of polysaccharides, are beyond the scope of this review.

Bacteria's cell envelopes are complex, dynamic structures. It plays a variety of protective and adaptive roles. The major conserved component of all bacterial cell envelopes is peptidoglycan. It is essential for stabilizing cell membranes against high internal osmotic pressures. But peptidoglycan alone is not enough to enable bacteria to survive in their different environments. In addition to peptidoglycan, the outer membrane of Gram-negative bacteria the, dense array of negatively charged polymers embedded in Gram-positive peptidoglycan and the complex outer layers of the Corynebacterineae play important roles in cell envelope integrity. One of the major challenges in the next decade will be to define the mechanisms by which these complex structures are assembled and regulated in response to changing environmental conditions.
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