Chitin is a chemical compound containing carbon, nitrogen, oxygen and hydrogen that naturally occur in the external skeleton of insects and crustaceans. However, fungi are the only organisms that have chitin as a component of their cell walls. Chitin is responsible for the rigidity of cell walls of most fungi, including Basidiomycetes, Ascomycetes, Phycomycetes and some species of Oomycetes.
Basidiomycetes include macroscopic fungi, such as stinkhorns, mushrooms, shelf fungi and puffballs, as well as microscopic species, some of which are plant or human pathogens. Rigidoporus microporus causes white root rot disease on tropical plants, such as the rubber tree, while Rhizoctonia solani attacks cucumbers and other food crops. The genus Cryptococcus is responsible for a form of meningitis that attacks humans. Edible fungi, such as chanterelles (Cantharellus), also contain chitin in their wall cells.
Also known as sac fungi, Ascomycetes are the most numerous fungi phylum, with more than 60,000 identified species. The species Paracoccidioides brasiliensis is a pathogen that contains long fibers of chitin in its cell walls. Beneficial species, which also have chitin fibers, include Saccharomyces cerevisiae, a yeast used in the production of bread, wine and beer, as well as Saccharomyces boulardii, which is part of the intestinal flora of humans.
Phycomycetes, or Zygomycota, is a small phylum of fungi, with just over 1,000 species. Most species are molds, which are found in terrestrial habitats, often growing on decaying plant material or animal feces. The black bread mold (Rhizopus stolonifer) is a common example of this phylum. Like other fungi species, Phycomycetes have cell walls with chitin fibers to protect their internal cellular structures.
Although most species of Oomycetes do not contain chitin, but cellulose and other chemical compounds, species of the genera Achlya and Saprolegnia, as well as the plant pathogen Pythium ultimum, have chitin in their cell walls. Also known as water moulds, even preferring land habitats, oomycetes are microscopic and responsible for numerous plant diseases, such as downy mildew and potato blight.