Indole substrate reagent is a specialized reagent for microbial biochemical identification, primarily distinguishing Enterobacteriaceae species by detecting whether bacteria can decompose tryptophan to produce indole (indole substrate). Its core principle is: indole reacts with p-dimethylaminobenzaldehyde in the reagent under acidic conditions to form rose indole, exhibiting a vivid red color. Positive reactions are commonly observed in Escherichia coli, while negative results are frequently seen in Enterobacter aerogenes and similar species. This reagent finds extensive application in clinical diagnostics, food hygiene testing, and environmental microbial monitoring.

Identification of Escherichia coli (core use): Used for indole testing of Enterobacteriaceae bacteria, distinguishing between Escherichia coli (positive) and gas producing Escherichia coli (negative), etc. It is an important component of the IMViC series biochemical reactions.

Identification of other pathogenic bacteria: It can also assist in the identification of bacteria that produce tryptophan enzymes, such as Proteus mirabilis and Vibrio cholerae, as well as some non fermenting bacteria, harsh tolerant bacteria, and anaerobic bacteria.

Auxiliary clinical infection diagnosis: help to judge the internal environment of the intestinal tract by identifying the types of pathogens, and provide diagnostic basis for diarrhea, abdominal pain and other digestive tract infectious diseases.

Indigo reagent (Koch reagent) is often used in combination with peptone water as a detection indicator; Bacteria produce tryptophan enzymes, which break down tryptophan into indole. Indole reacts with reagents to form a red complex, indicating a positive indole test.

Indigo positive bacteria: Escherichia coli, Proteus mirabilis, Proteus mirabilis, Morganella, etc;

Indigo negative bacteria: Salmonella, Shigella, Klebsiella pneumoniae, Enterobacter aerogenes;

It can also be used for auxiliary biochemical identification of miscellaneous bacteria such as Staphylococcus aureus and Pseudomonas aeruginosa.