Melzer's reagent

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Lactarius rubidus spores stained with Melzer's reagent

Melzer's reagent (also known as Melzer's iodine reagent,

plant pathogens.[1]

Composition

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 remainder of the solution being 50% water and 50% chloral hydrate.[2][3] Melzer's is toxic to humans if ingested due to the presence of iodine and chloral hydrate.[4] Due to the legal status of chloral hydrate, Melzer's reagent is difficult to obtain in the United States.[4]

In response to difficulties obtaining chloral hydrate, scientists at Rutgers formulated Visikol[5] (compatible with Lugol's iodine) as a replacement. In 2019, research showed that Visikol behaves differently to Melzer’s reagent in several key situations, noting it should not be recommended as a viable substitute.[6]

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.

Reactions

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.[2]

Among the amyloid reaction, 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
    hemiamyloidity).[7]

Melzer's reactions are typically almost immediate, though in some cases the reaction may take up to 20 minutes to develop.[2]

The function of the chemicals that make up Melzer's reagent are several. The chloral hydrate is a

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 than 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.[7][8]

An experiment in which spores from 35 species of basidiomycetes were tested for reactions to both Melzer's and Lugol's showed that spores in a large percentage of the species tested display very different reactions between the two reagents. These varied from being weakly or non-reactive in Lugols, to giving iodine-positive reactions in Lugol's but not in Melzer's, to even giving dextrinoid reactions in Lugol's while giving amyloid reactions in Melzer's.[4]

Melzer's degrades into a cloudy precipitate when combined with alkaline solutions,

ammonium hydroxide
solutions. When potassium hydroxide is used as a pretreatment, the alkalinity must be first neutralized before adding Melzer's.

History

The use of iodine-containing solutions as an aid to describing and identifying fungi dates back to the mid-19th century.[4]

Melzer's reagent was first described in 1924[9] 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.[7] Melzer was a specialist in Russula, a genus in which the amyloidy on the spore ornamentation or entire spore is of great taxonomic significance.[10]

References

  1. ^
    ISSN 1882-0484
    .
  2. ^
    ISBN 0-916422-09-7
    . p 25–26.
  3. ISBN 0-7627-3109-5
    . p 549.
  4. ^ a b c d Leonard, Lawrence M. (2006). "Melzer's, Lugol's or Iodine for Identification of white-spored Agaricales?" (PDF). McIlvainea. 16 (1): 43–51. Archived from the original (PDF) on 2011-07-20.
  5. ^ http://otc.rutgers.edu/pdf/Simon-2012-146.pdf[permanent dead link]
  6. ^ Leonard, Lawrence M. (Summer 2019). "Melzer's Reagent Update, 2019" (PDF). Fungi. 12 (2): 10.
  7. ^ 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.
  8. ^ Kohn LM, Korf RP. 1975. Variation in ascomycete iodine reactions: KOH pretreatment explored. Mycotaxon 3:165–172.
  9. ^ Melzer, V. (1924). "L'ornementation des spores de Russules". Bulletin Trimestriel de la Société Mycologique de France (in French). 40: 78–81.
  10. ^ The Russulales News Team. 2007. Introduction to the Russulales: Characteristics of the russuloid fungi Archived 2013-07-07 at the Wayback Machine, Russulales News (website).

Further reading

  • Blackwell M, et al. 2001. The presence of glycine betaine and the dextrinoid reaction in basidiomata. Harvard Papers in Botany 6:35–41.
  • Rossman AY. 1980. The iodine reaction: Melzer's vs. IKI. MSA newsletter 31:22.
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