StructureThe generalized structure of a bacterial cell is illustrated in Figure 8.2. The main features include:
prokaryotic cell structure: Although we have stressed that prokaryotes do not contain a nucleus, they also lack other internal structural compartments found in eukaryotic cells (see Chapter 10)
- Cell wall: This surrounds the plasma membrane of the cell and helps to retain its shape. The cell wall consists of peptidoglycan, a substance composed of polysaccharides linked together via peptides. The exact structure of the cell wall can be determined via the Gram staining test. If the bacteria bind the dye used in the staining process, they are said to be gram positive, and contain one layer of peptidoglycans (Staphylococcus is one example). If cells exclude the dye, they are gram negative and the cell wall contains two layers: inner layer is made up of peptidoglycan, and the outer contains lipoproteins and lipopolysaccharides. E. coli is a gram negative bacterium.
- Glycocalyx: The glycocalyx, which surrounds the cell wall, consists of a sticky mesh of polysaccharides, polypeptides, or both. If this structure is tightly organized, it forms a protective capsule. If it is loosely organized, it forms a slime layer that helps the bacterium attach to surfaces.
- Flagellum (plural: flagella): Flagella are long, winding protein chains extruding from the bacterial cell. Flagella rotate to move the cell.
- Pilus (plural: pili): Pili are short, filamentous proteins extending from the cell. They typically are present in large numbers. Pili play an important role in helping bacteria attach to surfaces or to other cells.