Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Site of Action
Simplest Living Organisms
Escherichia coli

The biochemists' "best friend"—the bacterium Escherichia coli—is a typical harmless inhabitant of the intestinal tract in humans and animals. Because this bacterium is easily cultivated in laboratory settings, it has been studied at THE MOLECULAR LEVEL more thoroughly than any other Organism [4]. E. coli can be considered a quintessential representative of true Bacteria. E. coli Cells are rod-shaped, with a length of ~2 µm, a diameter of 0.8 µm, a volume of ~1 µm³, a density of ~1.1 g ∙ cm^-3, and a mass of ~ 1 ∙ 10^-12 g (1 pg, or 0.7∙10¹² daltons) [5]. E. coli is roughly 100 times larger than the smallest mycoplasma, yet Electron Cell/15.html">Microscopy reveals that its Structure is remarkably simple and strikingly similar to that of Mycoplasmas (Figs. 1-1 and 1-2, A).

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FIG. 1-1. Escherichia coli and several smaller bacteria.

¹ See the figure in the paper by Burnham J. C., Hashimoto T., Conti S. F., J. Bacteriol., 96, 1366 (1968).

Each E. coli cell contains from 1 to 4 identical DNA molecules (depending on the growth rate of The Cell), along with 15,000–30,000 Ribosomes. Occasionally, other structures are detected within the cell, such as nutrient reserves—fat droplets and granules of poly-β-hydroxybutyric acid (Fig. 1-2, B), which constitutes up to 25% of the dry weight of Bacillus megaterium cells, as well as occasional Glycogen granules. Granules of highly polymerized phosphoric acid (polymetaphosphate), commonly known as metachromatic granules, are also frequently present. Vacuoles—that is, droplets of an isolated aqueous phase—are likewise observed.

FIG. 1-2. A. Electron micrograph of an intact E. coli cell; platinum shadowed. Flagella (curved) and pili (straight) are visible. Note the absence of F-pili (sex pili). (Courtesy of Williams F. D., VanderMolen Gail E., Amstein C. F.) B. Electron micrograph of a Spirillum cell; negative staining with phosphotungstic acid. Note the polar tufts of flagella, the rough-appearing outer surface, dark inclusions of poly-β-hydroxybutyric acid, and light granules of unknown nature. (Courtesy of Williams F. D., VanderMolen Gail E., Amstein C. F.)

FIG. 1-2. C. Electron micrograph of an ultrathin section (~60 nm) of the freshwater Gram-negative bacterium Aquaspirillum fasciculus. Visible are DNA (light region), dark ribosomes, the double membrane characteristic of Gram-negative bacteria (Chap. 5, Sec. D.1), and The Cell wall. An internal "polar membrane" is visible near the cell pole, which may be related to flagellar motility. (Courtesy of Williams F. D., VanderMolen Gail E., Amstein C. F.) D. Electron micrograph of an ultrathin section of a dividing Gram-positive Streptococcus cell. Note the light-stained DNA filament. Part of the mesosome is visible in the center of the cell, along with the Cytology/cytology/16.html">Early stages of cross-wall (septum) formation. (Courtesy of Williams F. D., VanderMolen Gail E., Amstein C. F.)