Cell Walls Of Microorganisms

Microbial cell walls have been shown to both accumulate in soil and become byproducts for microbial growth and synthesis of humic substances. Many different bacteria such as Streptomyces, Pseudomonas, Bacillus, and Clostridium degrade fungal cell walls. A major component of fungal cell walls is chitin, which contributes significant quantities of amino sugars to soil. The acetylglucosamine structure of chitin (Fig. 12.13) contains N, making its degradation unlikely to be limited by N. Recent studies have shown that available soil N is highly related to amino sugar content. Most amino sugars in soil are of microbial origin. The cell walls of Phytophthera contain fibers of cellulose and ,5(1-4) glucans that may interact with other cell components to affect its decomposition. Polymers of ,(1-3) and ,(1-6)

(amino linkage)

FIGURE 12.13 The structure of chitin showing the ,(1-4) linking acetylglucosamine residues.

(amino linkage)

FIGURE 12.13 The structure of chitin showing the ,(1-4) linking acetylglucosamine residues.

glucan are found in organisms such as Fusarium. Hyphae containing both chitin and glucans require both chitinase and glucanase to be degraded. Dark-colored pigments, often referred to as melanins, provide further protection and contribute to the low turnover rates of fungal cell wall constituents.

Bacterial cell walls are composed of a rigid layer of two sugar derivatives, N-acetylglucosamine and N-acetylmuramic acid chains. Gram-negative bacteria contain additional layers outside the ridged layer, creating more complex cell walls. These chains are linked by a limited number of amino acids through peptide bonds (Fig. 12.14). The cross-linkage peptidoglycan (murein) structures are composed of repeating units of L-alanine, D-alanine, D-glutamic acid, and either lysine or diaminopimelic acid. These components are connected to form a repeating structure called the glycan tetrapeptide. In G~ bacteria, about 10% of the cell wall is peptidoglycan, in contrast to G+ bacteria, in which peptidoglycan can make up to 90% of the cell wall. The thick wall of G+ bacteria contains peptidoglycans linked to other wall constituents that include a variety of polysaccharide and polyphosphate

N-Acetylglucosamine N-Acetylmuramic acid

CH2OH

CH2OH

Peptide Cross-links

CH2OH

CH2OH

Lysozyme-sensitive bond

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