The bacterium Vibrio cholerae, the causative agent of the epidemic, diarrheal disease cholera. (C) The protozoan parasite Trypanosoma brucei (purple) in a field of erythrocytes (red blood cells; pink). This parasite causes African sleeping sickness, a potentially fatal disease of the central nervous system. (D) This clump of Ascaris nematodes was removed from the obstructed intestine of a two-year-old boy. (A, courtesy of Robert Grant, Stephan Crainic, and James M. Hogle; B, photograph courtesy of John Mekalanos; C, CDC, Department of Health and Human Services; D, from J.K. Baird et al., Am. J. Trop. Med. Hyg. 35:314–318, 1986. Photograph by Daniel H. Connor.) lipopolysaccharide (LPS) outer leafet of outer membrane pore protein
Figure 23–3 Bacterial shapes and cell-surface structures. (A) Bacteria are traditionally classified by shape. (B and C) They are also classified as Gram positive or Gram negative. (B) Gram-positive bacteria such as Streptococcus and Staphylococcus have a single membrane and a thick cell wall made of cross-linked peptidoglycan. They are called Gram positive because they retain the violet dye used in the Gram-staining procedure. (C) Gram-negative bacteria such as Escherichia coli (E. coli) and Salmonella have two membranes, separated by the periplasm (see Figure 11–17). The peptidoglycan cell wall of these organisms is located in the periplasm and is thinner than in Gram-positive bacteria; they therefore fail to retain the dye in the Gram-staining procedure. The inner membrane of both Gram-positive and Gram-negative bacteria is a phospholipid bilayer. The inner leaflet of the outer membrane of Gram-negativebacteria is also made primarily of phospholipids, whereas the outer leaflet of the outer membrane is composed of a unique glycosylated lipid called lipopolysaccharide (LPS). (D) Cell-surface appendages are important for bacterial behavior. Many bacteria swim using the rotation of helical flagella. The bacterium illustrated has only a single flagellum at one pole; however, many have multiple flagella. Straight pili (also called fimbriae) are used to adhere to various surfaces in the host, as well as to facilitate genetic exchange between bacteria. Some kinds of pili can retract to generate force and thereby help bacteria move along surfaces.
Bacteria Are Diverse and Occupy a Remarkable Variety of Ecological Niches