Escherichia coli

The biochemist’s best friend is Escherichia coli, an ordinarily harmless inhabitant of our intestinal tract. This bacterium is easy to grow in the laboratory and has become the best understood organism at the molecular level. 4,9 It may be regarded as a typical true bacterium or eubacterium. The cell of E. coli (Figs. 1-1, 1-2) is a rod ~2 µm long and 0.8 µm in diameter with a volume of ~1 µm3 and a density of ~1.1 g/cm3. The mass is ~1 x 10–12 g, i.e., 1 picogram (pg) or ~0.7 x 1012 daltons (Da) (see Box 1-B). 4 It is about 100 times bigger than the smallest mycoplasma but the internal structure, as revealed by the electron microscope, resembles that of a mycoplasma. Each cell of E. coli contains from one to four identical DNA molecules, depending upon how fast the cell is growing, and ~15,000–30,000 ribosomes. Other particles that are sometimes seen within bacteria include food stores such as fat droplets and granules (Fig. 1-3). The granules often consist of poly- - hydroxybutyric acid10 accounting for up to 25% of the weight of Bacillus megaterium. Polymetaphosphate, a highly polymerized phosphoric acid, is sometimes stored in “metachromatic granules.” In addition, there may be droplets of a separate aqueous phase, known as vacuoles.