General Microbiology - Schlegel, H. 1987

Degradation of Natural Substances
Chitin

Chitin can formally be considered as Cellulose in which the hydroxyl groups at the C-2 carbon atoms of the glucose residues are replaced by acetylated amino groups (Sec. 2.2.3). The high stability of chitin can be explained by the presence of Hydrogen Bonds involving the lateral N-acetyl groups. Serving as a structural skeletal substance, chitin is widespread in both the animal and plant kingdoms. It forms the exoskeleton of many invertebrates. Planktonic copepods produce many millions of tons of chitin annually. Chitin is also continuously produced in the soil, serving as a major Cell wall component in many Fungi, particularly Basidiomycetes and ascomycetes.

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Fig. 14.3. Colonies of Cytophaga fermentans var. agarovorans on Agar plates. Agar degradation is indicated by the depression of the colonies (A). Flooding the agar plate with an iodine-potassium iodide solution reveals the diffusion zones of agar-hydrolyzing exoenzymes (B). (Veldkamp H., J. gen. Microbiol., 26 [1961], 33 .)

It is therefore not surprising that many soil and aquatic Bacteria are capable of utilizing chitin. Approximately 50 species of chitin-degrading bacteria have been isolated from field soils, including species of Flavobacterium, Bacillus, Cytophaga, Pseudomonas, Streptomyces, Nocardia, and Micromonospora. Among fungi, species of Aspergillus and Mortierella possess The ability to degrade chitin. One gram of soil can contain up to 10⁶ chitin-utilizing microbial Cells. Such Abundance indicates that chitin is a constantly available substrate in the soil.

When finely ground chitin is added to the soil, actinomycetes respond most rapidly with vigorous growth. Consequently, agar media containing chitin as the sole carbon and nitrogen source serve as excellent selective media for streptomycetes.

Microorganisms act on chitin by means of exoenzymes. Streptomyces griseus secretes two Enzymes: chitinase and chitobiase. The breakdown of chitin evidently results from the simultaneous action of chitinase on multiple sites of the polymer chain, yielding primarily chitobiose and chitotriose alongside only small amounts of N-acetylglucosamine. The chitotrioses and chitobioses are subsequently cleaved into monomers by chitobiase.