Bacterial membranes and cell wall

Bacteria are bounded by a cell wall. The cell wall defines the shape of the microorganism, exerts some control as to what enters and exits the bacterium, and, in the case of infectious microorganisms , can participate in the disease process.

Many bacteria can be classified as either Gram-positive or Gram-negative. The Gram stain is a method that differentiates bacteria based on the structure of their cell wall. Gram-positive bacteria retain the crystal violet stain that is applied to the bacteria, and appear purple. In contrast, gram-negative bacteria do not retain this stain, but are "counterstained" red by the safranin stain that is applied later. The basis of these different staining behaviors lies in the composition of the cell walls of each Gram type.

Gram-positive bacteria have a cell wall that consists of a single membrane and a thick layer of peptidoglycan . Gramnegative bacteria have a cell wall that is made up of two membranes that sandwich a region known as the periplasmic space or periplasm . The outermost membrane is designated the outer membrane and the innermost one is known as the inner membrane. In the periplasm lies a thin peptidoglycan layer, which is linked with the overlaying outer membrane.

The cell wall of Gram-positive bacteria tends to be 2 to 8 times as thick as the Gram-negative wall.

When thin sections of bacteria are viewed in the transmission electron microscope , the membranes appear visually similar to a railroad track. There are two parallel thickly stained lines separated by an almost transparent region. The dark regions are the charged head groups of molecules called phospholipids . Bacterial phospholipids consist of the charged, hydrophilic ("water-loving") head region and an uncharged, hydrophobic ("water-hating") tail. The tail is buried within the membrane and forms most of the electron-transparent region evident in the electron microscope.

Phospholipids make up the bulk of bacterial membranes. In Gram-positive bacteria and in the inner membrane of Gram-negative bacteria the phospholipids are arranged fairly evenly on either "leaflet" of the membrane. In contrast, the outer membrane of Gram-negative bacteria is asymmetric with respect to the arrangement of phospholipids. The majority of the phospholipids are located at the inner leaflet of the membrane. The outer leaflet contains some phospholipid, and also proteins and a lipid molecule termed lipopolysaccharide.

The asymmetrical arrangement of the Gram-negative outer membrane confers various functions to the bacterium. Proteins allow the diffusion of compounds across the outer membrane, as long as they can fit into the pore that runs through the center of the protein. In addition, other proteins function to specifically transport compounds to the inside of the bacterium in an energy-dependent manner. The lipopolysaccharide component of the outer membrane is capable of various chemical arrangements that can influence the bacterium's ability to elude host immune defenses. For example, when free of the bacterium, lipopolysaccharide is referred to as endotoxin, and can be toxic to mammals, including humans.

The presence of the outer membrane makes the existence of the periplasm possible. The periplasm was once thought to be just functionless empty space. Now, however, the periplasm is now known to have very important functions in the survival and operation of the bacterium. The region acts as a buffer between the very different chemistries of the external environment and the interior of the bacterium. As well, specialized transport proteins and enzymes are located exclusively in this region. For example, the periplasm contains proteins that function to sense the environment and help determine the response of a bacterium to environmental cues, such as occurs in the directed movement known as chemotaxis.

Not all bacteria have such a cell wall structure. For example the bacteria known as mycobacteria lack a peptidoglycan and have different components in the cell membrane. Specifically, a compound called mycolic acid is present. Other bacteria called Mycoplasma lack a cell wall. They need to exist inside a host cell in order to survive.

The synthesis of the cell wall and the insertion of new cell wall material into the pre-existing wall is a highly coordinated process. Incorporation of the new material must be done so as not to weaken the existing wall. Otherwise, the bacterium would lose the structural support necessary for shape and survival against the osmotic pressure difference between the interior and exterior of the bacterium. Wall synthesis and insertion involves a variety of enzymes that function in both the mechanics of the process and as sensors. The latter stimulate production of the cell wall as a bacterium readies for division into two daughter cells.

See also Bacterial ultrastructure; Bacterial surface layers