5-Bromo-3-indolyl nonanoate (BIN) is a chromogenic enzyme substrate often used for the detection of esterase activity. It is hydrolyzed by esterase to produce a blue product, which can be detected visually or by spectrophotometry. BIN is particularly useful for the detection of esterase activity in bacteria and fungi, and can be used to differentiate between different types of organisms based on their esterase activity patterns.