Microcheck

Peptidoglycan is a molecule unique to bacteria that provides rigidity to the cell wall. The Gram-positive cell wall is composed of a relatively thick layer of peptidoglycan. The Gram-negative cell wall has a thin layer of peptidoglycan and an outer membrane, which contains lipopolysaccharide. The outer membrane excludes molecules with the exception of those that pass through porins; proteins are secreted via special mechanisms. Penicillin and lysozyme interfere with the structural integrity of peptidoglycan. Mycoplasma species lack a cell wall. Members of the Archaea have a variety of cell wall types.

■ What is the significance of Lipid A?

Figure 3.35 Chemical Structure of Lipopolysaccharide The Lipid A portion, which anchors the LPS molecule in the lipid bilayer, is responsible for the symptoms associated with endotoxin.The composition and length of the O-specific polysaccharide side chain varies among different species of bacteria.

is not the cell wall, however, but the inside of the cell that is stained by the crystal violet-iodine complex. The Gram-positive cell wall somehow retains the crystal violet-iodine complex even when subjected to the trauma of acetone-alcohol treatment, whereas the Gram-negative cell wall cannot. The precise mechanism that accounts for the differential aspect of the Gram stain is not entirely understood. Presumably, the decolorizing agent dehydrates the thick layer of peptidoglycan and in this dehydrated state the wall acts as a permeability barrier, retaining the dye. In contrast, the solvent action of acetone-alcohol easily damages the outer membrane of Gram-negative bacteria; their relatively thin layer of pep-tidoglycan cannot retain the dye complex. These bacteria lose the dye complex more readily than their Gram-positive counterparts. Also, as Gram-positive cells age, they often lose their ability to retain the dye. This probably results from damage to their peptidoglycan layer that occurs as a consequence of aging.

Figure 3.36 The Plastic Shape of Mycoplasma pneumoniae, Which Lacks a Rigid Cell Wall

Figure 3.36 The Plastic Shape of Mycoplasma pneumoniae, Which Lacks a Rigid Cell Wall

3.8 Filamentous Protein Appendages 63

■ How does the action of penicillin differ from that of lysozyme?

■ Explain why penicillin will kill only actively multiplying cells, whereas lysozyme will kill cells in any stage of growth.

■ How does the action of penicillin differ from that of lysozyme?

■ Explain why penicillin will kill only actively multiplying cells, whereas lysozyme will kill cells in any stage of growth.

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