Melzer's reagent (also known as Melzer's iodine reagent, Melzer's solution or informally as Melzer's) is a

chemical reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

used by

mycologist Mycology is the branch of biology concerned with the study of fungi, including their taxonomy, genetics, biochemical properties, and use by humans. Fungi can be a source of tinder, food, traditional medicine, as well as entheogens, poison, and ...

s to assist with the identification of

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

, and by

phytopathologist Plant pathology or phytopathology is the scientific study of plant diseases caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Plant pathology involves the study of pathogen identification, disease ...

s for fungi that are

plant pathogen Plant diseases are diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like orga ...

Composition

Melzer's reagent is an aqueous solution of

chloral hydrate Chloral hydrate is a geminal diol with the formula . It was first used as a sedative and hypnotic in Germany in the 1870s. Over time it was replaced by safer and more effective alternatives but it remained in use in the United States until at ...

potassium iodide Potassium iodide is a chemical compound, medication, and dietary supplement. It is a medication used for treating hyperthyroidism, in radiation emergencies, and for protecting the thyroid gland when certain types of radiopharmaceuticals are u ...

, and

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

. 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. Melzer's is toxic to humans if ingested due to the presence of iodine and chloral hydrate. Due to the legal status of chloral hydrate, Melzer's reagent is difficult to obtain in the United States. In response to difficulties obtaining chloral hydrate, scientists at Rutgers formulated Visikol (compatible with

Lugol's iodine Lugol's iodine, also known as aqueous iodine and strong iodine solution, is a solution of potassium iodide with iodine in water. It is a medication and disinfectant used for a number of purposes. Taken by mouth it is used to treat thyrotoxic ...

) 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. 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 A microscope slide is a thin flat piece of glass, typically 75 by 26 mm (3 by 1 inches) and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is mounted (secured) on the slide, and then ...

preparation, and looking for any of three color reactions: *

Amyloid Amyloids are aggregates of proteins characterised by a fibrillar morphology of typically 7–13 nm in diameter, a β-sheet secondary structure (known as cross-β) and ability to be stained by particular dyes, such as Congo red. In the human ...

or Melzer's-positive reaction, in which the material reacts blue to black. * Pseudoamyloid or

dextrinoid In mycology a tissue or feature is said to be amyloid if it has a positive amyloid reaction when subjected to a crude chemical test using iodine as an ingredient of either Melzer's reagent or Lugol's solution, producing a blue to blue-black staini ...

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.Largent D, et al. 1977. ''How to Identify Mushrooms to Genus III: Microscopic Features''. Arcata, CA: Mad River Press. . p 25–26. Among the amyloid reaction, two types can be distinguished: * Euamyloid reaction, in which the material turns blue without

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which utili ...

(KOH)-pretreatment. * Hemiamyloid reaction, in which the material turns red in

Lugol's solution Lugol's iodine, also known as aqueous iodine and strong iodine solution, is a solution of potassium iodide with iodine in water. It is a medication and disinfectant used for a number of purposes. Taken by mouth it is used to treat thyrotoxico ...

, but shows no reaction in Melzer's reagent; when KOH-pretreated it turns blue in both reagents ( hemiamyloidity).Baral H-O. 1987
Lugol's solution/IKI versus Melzer's reagent: Hemiamyloidity, a universal feature of the ascus wall.
''Mycotaxon'' 29:399–450. Melzer's reactions are typically almost immediate, though in some cases the reaction may take up to 20 minutes to develop. 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 In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

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 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. An experiment in which spores from 35 species of

basidiomycetes Basidiomycota () is one of two large division (mycology), divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. Mor ...

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. Melzer's degrades into a cloudy precipitate when combined with alkaline solutions, hence it cannot be used in combination or in direct series with such common mycological reagents such as potassium hydroxide or

ammonium hydroxide Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although ...

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. Melzer's 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. Melzer was a specialist in ''

Russula ''Russula'' is a very large genus composed of around 750 worldwide species of fungi. The genus was described by Christian Hendrik Persoon in 1796. The mushrooms are fairly large, and brightly colored – making them one of the most recognizable ...

'', a genus in which the amyloidy on the

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

ornamentation or entire spore is of great taxonomic significance.The Russulales News Team. 2007. Introduction to the Russulales
Characteristics of the russuloid fungi
, ''Russulales News'' (website).

Composition

Reactions

History

References

Further reading