Ch Oh

teichos, meaning wall). These are chains of a common subunit, either ribitol-phosphate or glycerol-phosphate, to which various sugars and D-alanine are usually attached. They are attached to the peptidoglycan molecule through covalent bonds to N-acetylmuramic acid. Some, which are called lipoteichoic acids, are linked to the cytoplasmic membrane. Teichoic acids and lipoteichoic acids both stick out above the peptidoglycan layer and, because they are negatively charged, give the cell its negative polarity.

The Gram-Negative Cell Wall

The cell wall of Gram-negative bacteria is far more complex than that of Gram-positive organisms (figure 3.34). It contains only a thin layer of peptidoglycan. Outside of that layer is the

The Lipopolysaccharide Molecule The lipopolysaccharide molecule is extremely important from a medical standpoint. When purified lipopolysaccharide is injected into an animal, it elicits symptoms characteristic of infections caused by live bacteria. The same symptoms occur regardless of the bacterial species. To reflect the fact that the toxic activity is an inherent part of the cell wall, it is called endotoxin. ■ endotoxin, p. 475

Two parts of the LPS molecule are notable for their medical significance (figure 3.35):

■ Lipid A is the portion that anchors the LPS molecule in the lipid bilayer. Its chemical make-up plays a significant role in our body's ability to recognize the presence of invading bacteria. When Lipid A is introduced into the body in small amounts, such as when microbes

60 Chapter 3 Microscopy and Cell Structure

Peptidoglycan (cell wall)

Peptidoglycan (cell wall)

Figure 3.33 Gram-Positive Cell Wall (a)The Gram-positive cell wall is characterized by a relatively thick layer of peptidoglycan. (b) It is made up of many sheets of interconnected glycan chains. In turn, these sheets are interconnected to make the thick peptidoglycan molecule. (c)Transmission electron photomicrograph of a typical Gram-positive cell wall (Bacillus fastidiosis, 90,000x)

Figure 3.33 Gram-Positive Cell Wall (a)The Gram-positive cell wall is characterized by a relatively thick layer of peptidoglycan. (b) It is made up of many sheets of interconnected glycan chains. In turn, these sheets are interconnected to make the thick peptidoglycan molecule. (c)Transmission electron photomicrograph of a typical Gram-positive cell wall (Bacillus fastidiosis, 90,000x)

0.2 ^m contaminate a small lesion, the defense system responds at an appropriate level to effectively eliminate the invader. If, however, large amounts of Lipid A are present, such as when Gram-negative bacteria are actively growing in the bloodstream, the magnitude of the response damages even our own cells. This response to Lipid A is responsible for the symptoms associated with endotoxin.

■ The O-specific polysaccharide side chain is the portion of LPS directed away from the membrane, at the end opposite that of Lipid A. It is made up of a chain of sugar molecules, the number and composition of which varies among different species of bacteria. This characteristic can be exploited to identify certain species or strains. For example, the "O157" in E. coli O157:H7 refers to that particular strain's characteristic O-side chain.

Antibacterial Compounds that Target Peptidoglycan

Compounds that interfere with the synthesis of peptidoglycan or alter its structural integrity weaken the rigid molecule to a point where it is not strong enough to prevent the cell from bursting. These compounds include the antibiotic penicillin and the enzyme lysozyme, which is found in many body fluids including tears and saliva.

You Are What You Eat

You Are What You Eat

Nutrition is a matter that people spend their careers learning about and requires volumes of books to explain. My objective is to instruct you how to consume a healthy nutritional diet that aids your body in burning off fat instead of storing it. You do not require overwhelming science to get this.

Get My Free Ebook


Post a comment