Bacteria are classified into three different shapes: (A) spheres (cocci), (B) rods (bacilli), and (C) spiral cells (spirochetes). (D) They are also classified as Gram-positive or Gram-negative. Bacteria such as Streptococci and Staphylococci have a single membrane and a thick cell wall made of cross-linked peptidoglycan. They retain the violet dye used in the Gram staining procedure and are thus called Gram-positive. Gram-negative bacteria such as E. coli and Salmonella have two membranes, separated by a periplasmic space (see Figure 11-17). The peptidoglycan layer in the cell wall of these organisms is located in the periplasmic space and is thinner than in Gram-positives; they therefore fail to retain the dye in the Gram staining procedure. The inner membrane of Gram-negative bacteria is a phospholipid bilayer, and the inner leaflet of the outer membrane is also made primarily of phospholipids; the outer leaflet of the outer membrane, however, is composed of a unique glycosylated lipid called lipopolysaccharide (LPS) (see Figure 25-40). (E) Cell-surface projections are important for bacterial behavior. Many bacteria swim using the rotation of helical flagella (see Figure 15-68). The bacterium illustrated has only a single flagellum at one pole; others such as E. coli are decorated with multiple flagella all over the surface. Straight pili (also called fimbriae) are used to adhere to surfaces in the host and to facilitate genetic exchange between bacteria. Both flagella and pili are anchored to the cell surface by large multiprotein complexes.