Box 31 Mesosomes the structures that never were

When looked at under the electron microscope, Gram-positive bacteria often contained localised in-foldings of the plasma membrane. These were given the name mesosomes, and were thought by some to act as attachment points for DNA during cell division, or to play a role in the formation of cross-walls. Others thought they were nothing more than artefacts produced by the rather elaborate sample preparation procedures necessary for electron microscopy. Nowadays, most microbiologists support the latter view.

Figure 3.9 The Gram-negative cell wall. Note the thinner layer of peptidoglycan compared to the gram-positive cell wall (Figure 3.8). It accounts for <10% of the dry weight. Beyond this lies the outer membrane, with its high lipopolysaccharide content. Channels made of porins allow the passage of certain solutes into the cell. From Henderson, B, Wilson, M, McNab, R & Lax, AJ: Cellular Microbiology: Bacteria-Host Interactions in Health and Disease, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

Figure 3.9 The Gram-negative cell wall. Note the thinner layer of peptidoglycan compared to the gram-positive cell wall (Figure 3.8). It accounts for <10% of the dry weight. Beyond this lies the outer membrane, with its high lipopolysaccharide content. Channels made of porins allow the passage of certain solutes into the cell. From Henderson, B, Wilson, M, McNab, R & Lax, AJ: Cellular Microbiology: Bacteria-Host Interactions in Health and Disease, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

Members of the Archaea have a cell wall chemistry quite different to that described above (see Chapter 7). Instead of being based on peptidoglycan, they have other complex polysaccharides, although a distinction between gram-positive and gram-negative types still occurs.

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