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Figure 1. Electron micrograph of an intact Escherichia coli cell. LPs, lipopolysaccharide or outer membrane which is present in Gram-negative bacteria and contains various porins which allow diffusion of small molecules (up to penta-lysine) into the so-called peri-plasmic space. Not seen in the micrograph is the rigid, electron dense peptidoglycan layer of the cell wall, present in all Eubactericiae which provides the cell with its shape and allows bacteria to accumulate metabolites again large concentration gradients without bursting due to influx of water. Closely opposed to the peptidoglycan layer is the plasma membrane (PM).