4.1 Gram-positive and Gram-negative bacteria

Bacteria are divided into two groups based on how the cell wall appears when they are stained using Gram straining. This procedure allows the composition of the wall to be visualised.

In Gram-positive bacteria, the cell wall has a thick peptidoglycan layer which is relatively porous, allowing substances to pass through it quite easily.

In Gram-negative bacteria, this peptidoglycan layer is greatly reduced and is further protected by a second, outer membrane (Figure 10).

Figure 10 Arrangement of the cell wall in (a) Gram-positive and (b) Gram-negative bacteria.

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This diagram shows the differences in cell wall structure between Gram-positive and Gram-negative bacteria. In the Gram-positive bacteria in (a) the peptidoglycan is a thick external layer shown in brown, while in the Gram-negative bacteria in (b) the peptidoglycan layer is much thinner and is surrounded by an outer membrane of lipopolysaccharide and protein (as a green wavy line). The inner membrane in (a) and (b) (shown as a double green line) is separated from the peptidoglycan layer by the periplasmic space.

Figure 10 Arrangement of the cell wall in (a) Gram-positive and (b) Gram-negative bacteria.

This second, outer membrane of Gram-negative bacteria is an effective barrier, regulating the passage of large molecules such as antibiotics into the cell. In contrast, the thick, porous peptidoglycan layer in the cell wall of Gram-positive bacteria gives greater access to antibiotics, allowing them to more easily penetrate the cell and/or interact with the peptidoglycan itself.

You will learn more about the strategies antibiotics use to cross the cell wall in Week 3.