Most Gram-positive cell walls contain considerable amounts of teichoic and teichuronic acids, which may account for up to 50% of the dry weight of the wall and 10% of the dry weight of the total cell. In addition, some Gram-positive walls may contain polysaccharide molecules.

1. Teichoic and teichuronic acids—The term teichoic acids encompass all wall, membrane, or capsular polymers containing glycerophosphate or ribitol phosphate residues. These polyalcohols are connected by phosphodiester link ages and usually have other sugars and d-alanine attached (Figure 1A). Because they are negatively charged, teichoic acids are partially responsible for the net negative charge of the cell surface. There are two types of teichoic acids: wall teichoic acid (WTA), covalently linked to peptidoglycan; and membrane teichoic acid, covalently linked to membrane glycolipid. Because the latter are intimately associated with lipids, they have been called lipoteichoic acids (LTA). Together with peptidoglycan, WTA and LTA make up a polyanionic network or matrix that provides functions relating to the elasticity, porosity, tensile strength, and electrostatic properties of the envelope. Although not all Gram-positive bacteria have conventional LTA and WTA, those that lack these polymers generally have functionally similar ones.

Fig1. A: Teichoic acid structure. The segment of a teichoic acid made of phosphate, glycerol, and a side chain, R. R may represent d-alanine, glucose, or other molecules. B: Teichoic and lipoteichoic acids of the Gram-positive envelope. (Reproduced with permission from Willey JM, Sherwood LM, Woolverton CJ: Prescott, Harley, and Klein’s Microbiology, 7th ed. McGraw-Hill; 2008. © McGraw-Hill Education.)

Most teichoic acids contain substantial amounts of d-alanine, usually attached to position 2 or 3 of glycerol or position 3 or 4 of ribitol. In some of the more complex teichoic acids, however, d-alanine is attached to one of the sugar residues. In addition to d-alanine, other substituents may be attached to the free hydroxyl groups of glycerol and ribitol (eg, glucose, galactose, N-acetylglucosamine, N-acetylgalactosamine, or succinate). A given species may have more than one type of sugar substituent in addition to d-alanine; in such cases, it is not certain whether the different sugars occur on the same or on separate teichoic acid molecules. The composition of the teichoic acid formed by a given bacterial species can vary with the composition of the growth medium.

The teichoic acids constitute major surface antigens of those Gram-positive species that possess them, and their accessibility to antibodies has been taken as evidence that they lie on the outside surface of the peptidoglycan. Their activity is often increased, however, by partial digestion of the peptidoglycan; thus, much of the teichoic acid may lie between the cytoplasmic membrane and the peptidoglycan layer, possibly extending upward through pores in the latter (Figure 1B). In the pneumococcus (Streptococcus pneumoniae), the teichoic acids bear the antigenic determinants called Forssman antigen. In Streptococcus pyogenes, LTA is associated with the M protein that protrudes from the cell membrane through the peptidoglycan layer. The long M protein molecules together with the LTA form microfibrils that facilitate the attachment of S. pyogenes to animal cells.

The teichuronic acids are similar polymers, but the repeat units include sugar acids (eg, N-acetylmannosuronic or d-glucosuronic acid) instead of phosphoric acids. They are synthesized in place of teichoic acids when phosphate is limiting.

2. Polysaccharides—The hydrolysis of Gram-positive walls has yielded, from certain species, neutral sugars, such as mannose, arabinose, rhamnose, and glucosamine, and acidic sugars, such as glucuronic acid and mannuronic acid. It has been proposed that these sugars exist as subunits of polysaccharides in the cell wall; the discovery, however, that teichoic and teichuronic acids may contain a variety of sugars (see Figure 1A) leaves the true origin of these sugars uncertain.

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مواضيع ذات صلة

The Acid-Fast Cell Wall

Special Components of Gram-Negative Cell Walls

The Peptidoglycan Layer

Bacterial Cultivation

Principles of Bacterial Cultivation : Environmental Requirements

Structure and Function of the Bacterial Cell

Bacterial Metabolism

Bacterial Genetics: Nucleic Acid Structure and Organization

Detection of Bacteremia: Culture Techniques

Detection of Bacteremia: Types of Blood Culture Bottle

Detection of Bacteremia: Specimen Collection

The Study of the Intestinal Microbiota