Lysozyme

Lysozyme breaks the bond that links the alternating N-acetyl-glucosamine and N-acetylmuramic acid molecules and thus destroys the structural integrity of the glycan chain, the backbone of the peptidoglycan molecule.

3.6 Cell Wall 61

Porin protein

Lipopolysaccharide (LPS)

Peptidoglycan

Porin protein

Lipopolysaccharide (LPS)

Peptidoglycan

Lipoprotein

Outer membrane (lipid bilayer)

Periplasm

Cytoplasmic membrane (inner membrane; lipid bilayer)

Transport protein

Lipoprotein

Outer membrane (lipid bilayer)

Periplasm

Cytoplasmic membrane (inner membrane; lipid bilayer)

Transport protein

Capsule

Outer membrane

Periplasm

Cytoplasmic membrane

Figure 3.34 Gram-Negative Cell Wall (a)The Gram-negative cell wall is characterized by a very thin layer of peptidoglycan surrounded by an outer membrane. (b)The peptidoglycan layer is made up of only one or two sheets of interconnected glycan chains.The outer membrane is a typical phospholipid bilayer, except the outer leaflet contains lipopolysaccharide. Porins span the membrane to allow specific molecules to pass. Periplasm fills the region between the cytoplasmic and outer membranes. (c) A transmission electron micrograph of a typical Gram-negative cell wall (Pseudomonas aeruginosa, 120,000x). Surrounding this particular bacterium is a capsule.

Capsule

Outer membrane

Periplasm

Cytoplasmic membrane

Lysozyme is sometimes used in the laboratory to remove the peptidoglycan layer from bacteria for experimental purposes. Treating a Gram-positive bacterium with the enzyme creates a protoplast, which completely lacks a cell wall. The process is more complicated when working with Gram-negative bacteria, however, because their outer membrane naturally excludes lysozyme. To counter this problem, the cells must first be treated with a compound called EDTA (ethylenediaminetetracetic acid). This interacts with ions that otherwise stabilize the outer membrane, allowing lysozyme access to the peptidoglycan. The action of lysozyme on an EDTA-treated Gram-negative cell cre ates a spheroplast, which retains part of the outer membrane. Because they lack their rigid cell wall, protoplasts and sphero-plasts both become spherical regardless of the original cell shape. Due to osmosis, they will burst unless maintained in a solution that has the same relative concentration of ions and small molecules as the cytoplasm.

Differences in Cell Wall Composition and the Gram Stain

Differences in the cell wall composition of Gram-positive and Gram-negative bacteria account for their staining characteristics. It

Chapter 3 Microscopy and Cell Structure

O-specific polysaccharide (varies in length and composition)

Core polysaccharide

Lipid A (embedded in lipid bilayer)

Characteristics of Bacteria that Lack a Cell Wall

Some bacteria naturally lack a cell wall. Species of Mycoplasma, one of which causes a mild form of pneumonia, have an extremely variable shape because they lack a rigid cell wall (figure 3.36). As expected, neither penicillin nor lysozyme affects these organisms. Mycoplasma and related bacteria can survive without a cell wall because their cytoplasmic membrane is stronger than that of most other bacteria. They have sterols in their membrane; these rigid, planar molecules stabilize membranes, making them stronger.

Cell Walls of the Domain Archaea

As a group, members of the Archaea inhabit a wide range of extreme environments, and so it is not surprising they contain a greater variety of cell wall types than do members of the Bacteria. However, because most of these organisms have not been studied as extensively as the Bacteria, less is known about the structure of their walls. None contain peptidoglycan, but some do have a similar molecule, pseudopeptidoglycan.

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