Finally, teichoic acids provide much of the wall's antigenic specificity and thus make it possible to identify gram-positive bacteria by certain laboratory tests. They may also assume a role in cell growth, preventing extensive wall breakdown and possible cell lysis. Because of their negative charge (from the phosphate groups), teichoic acids may bind and regulate the movement of cations (positive ions) in to and out of the cell. There are two classes of teichoic acids: lipoteichoic acid, which spans the peptidoglycan layer and is linked to the plasma membrane, and wallteichoic acid, which is linked to the peptidoglycan layer. In addition, the cell walls of gram-positive bacteria contain teichoic acids, which consist primarily of an alcohol (such as glycerol or ribitol) and phosphate. By contrast, gram-negative cell walls contain only a thin layer of peptidoglycan. In most gram-positive bacteria, the cell wall consists of many layers of peptidoglycan, forming a thick, rigid structure. Larger, hydrophobic molecules cannot pass, partly accounting for the resistance of gram-negative cells to many antimicrobial agents, dyes, disinfectants, and lysozyme (Pommerville et al., 2011). These proteins form pores in the outer membrane through which small, hydrophilic molecules (sugars, amino acids, some ions) pass into the periplasm. The outer membrane also contains unique proteins called porins. On cell death, lipid A is released and represents an endotoxin that can be toxic if ingested. The so-called O polysaccharide is used to identify variants of a species (e.g., strain O157:H7 of E. However, the outer half is composed primarily of lipopolysaccharide (LPS), which consists of polysaccharide attached to a unique lipid molecule known as lipid A. The inner half of the outer membrane contains phospholipids similar to the cell membrane. The peptidoglycan layer is located in the periplasm and attached to lipoproteins in the cell membrane. This gel-like compartment contains digestive enzymes and transport proteins to speed entry of nutrients into the cell. The unique feature of the gram-negative cell wall is the presence of an outer membrane, which is separated by a gap, called the periplasm, from the cell membrane. This is one reason why it loses the crystal violet dye during the Gram stain. The peptidoglycan layer is two dimensional the glycan strands compose just a single layer or two. The cell wall of gram-negative bacterial cells is structurally quite different from that of the gram-positive wall. It contains porin proteins and the outer half is unique in containing lipopolysaccharide (Pommerville et al., 2011). Note the structure of the outer membrane in this figure. Moreover, an outer membrane overlies the peptidoglycan layer such that both comprise the cell wall. (C) In the gram-negative cell wall, the peptidoglycan layer is much thinner, and there is no teichoic acid. (B) The cell wall of a gram-positive bacterial cell is composed of peptidoglycan layers combined with teichoic acid molecules. (A) The structure of peptidoglycan is shown as units of NAG and NAM joined laterally by amino acid cross-bridges and vertically by side chains of four amino acids. Because the cell walls of gram-negative bacteria contain only a small amount of peptidoglycan, they are more susceptible to mechanical breakage (Tortora et al., 2010).Ī Comparison of the cell wall of gram-positive and gram-negative bacterial cells. Gram-negative cell walls do not contain teichoic acids. The periplasm contains a high concentration of degradative enzymes and transport proteins. The peptidoglycan is bonded to lipoproteins (lipids covalently linked to proteins) in the outer membrane and is in the periplasm, a gel-like fluid between the outer membrane and the plasma membrane. The cell walls of gram-negative bacteria consist of one or a very few layers of peptidoglycan and an outer membrane. Print page Microbiology for agronomists Gram-Negative Outer Membrane and Gram-Positive Cell Envelope Gram-Negative Bacteria
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