Melzer's reagent

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Melzer's reagent (also known as Melzer's solution or informally as Melzer's) is a chemical reagent used by mycologists to assist with the identification of fungi.

Melzer's reagent is an aqueous solution of chloral hydrate, potassium iodide, and iodine. Depending on the formulation, it consists of approximately 2.50-3.75% potassium iodide and 0.75–1.25% iodine, with the remainer of the solution being 50% water and 50% chloral hydrate.[1][2]

Melzer's is used by exposing fungal tissue or cells to the reagent, typically in a microscope slide preparation, and looking for any of three color reactions:

  • Amyloid or Melzer's-positive reaction, in which the material reacts blue to black.
  • Pseudoamyloid or dextrinoid reaction, in which the material reacts brown to reddish-brown.
  • Inamyloid or Melzer's-negative, in which the tissues do not change color, or react faintly yellow-brown.[1]

Among the amyloid rection, two types can be distinguished:

  • Euamyloid reaction, in which the material turns blue without potassium hydroxide (KOH)-pretreatment.
  • Hemiamyloid reaction, in which the material turns red in Lugol's solution, but shows no reaction in Melzer's reagent; when KOH-pretreated it turns blue in both reagents.[3]

Melzer's reactions are typically almost immediate, though in some cases the reaction may take up to 20 minutes to develop.[1]

The function of the chemicals that make up Melzer's reagent are several. The chloral hydrate is a clearing agent, bleaching and improving the transparency of various dark-colored microscopic materials. The potassium iodide is used to improve the solubility of the iodine, which is otherwise only semi-soluble in water. Iodine is thought to be the main active staining agent in Melzer's; it is thought to react with starch-like polysaccharides in the cell walls of amyloid material, however, its mechanism of action is not entirely understood. It has been observed that hemiamyloid material reacts differently when exposed to Melzer's then it does when exposed to other IKI solutions such as Lugol's, and that in some cases an amyloid reaction is shown in material that had prior exposure to KOH, but an inamyloid reaction without such pretreatment.[3][4]

Melzer's degrades into a cloudy precipitate when combined with alkaline solutions,[1] hence it cannot be used in combination or in direct series with such common mycological reagents such as potassium hydroxide or ammonium hydroxide solutions. When potassium hydroxide is used as a pretreatment, the alkalinity must be first neutralized before adding Melzer's.

The reagent is toxic to humans due to the presence of iodine and chloral hydrate.

[edit] History

Melzer's reagent is part of a class of iodine/potassium iodide (IKI)-containing reagents used in biology; Lugol's iodine is another such formula. The reagent was first described in 1924 and takes its name from its inventor, the mycologist Václav Melzer, who modified an older chloral hydrate-containing IKI solution developed by botanist Arthur Meyer.[3] Melzer was a specialist in Russula, a genus in which the amyloidy on the spore ornamentation or entire spore is of great taxonomic significance.[5]

[edit] References

  1. ^ a b c d Largent D, et al. 1977. How to Identify Mushrooms to Genus III: Microscopic Features. Arcata, CA: Mad River Press. ISBN 0-916-422-09-7. p 25–26.
  2. ^ Miller OK, Miller H. 2006. North American Mushrooms: A Field Guide to Edible and Inedible Fungi. Guilford, CT: Falcon Guide. ISBN 0762731095. p 549.
  3. ^ a b c Baral H-O. 1987. Lugol's solution/IKI versus Melzer's reagent: Hemiamyloidity, a universal feature of the ascus wall. Mycotaxon 29:399–450.
  4. ^ Kohn LM, Korf RP. 1975. Variation in ascomycete iodine reactions: KOH pretreatment explored. Mycotaxon 3:165–172.
  5. ^ The Russulales News Team. 2007. Introduction to the Russulales: Characteristics of the russuloid fungi, Russulales News (website).

[edit] Further reading

  • Blackwell M, et al. 2001. The presence of glycine betaine and the dextinoid reaction in basidiomata. Harvard Papers in Botany 6:35–41.
  • Leonard LM. 2006. Melzer's, Lugol's, or iodine for identification of white-spored Agaricales? McIlvainea 16(1).
  • Rossman AY. 1980. The iodine reaction: Melzer's vs. IKI. MSA newsletter 31:22.
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