Bacteria may be classified according to differences in cell wall structure. The differences are shown by using the Gram stain. • A Gram-positive bacterium has a cell wall mainly composed of a thick layer of peptidoglycan (murein). • A Gram-negative bacterium has a more complex cell wall. This wall is composed of a much thinner layer of peptidoglycan and an outer layer known as the outer membrane. Escherichia coli is a Gram-negative bacterium. Fig. 3.1 is a diagram through the cell surface membrane and the cell wall of E. coli. [Figure 3.1]