Used in microbiology for the detection of indole production by bacteria, particularly in the identification of Enterobacteriaceae. When added to a bacterial culture grown in tryptophan-containing medium, it reacts with indole to form a red-violet ring, indicating a positive result. This test helps differentiate bacterial species, such as distinguishing *Escherichia coli* (indole positive) from *Salmonella enterica* (indole negative). Commonly used in clinical labs for bacterial identification and quality control in food and water testing.