Used in microbiology as a selective agent in culture media to identify bacteria that can hydrolyze esculin, particularly in distinguishing members of the Enterococcus and Group D Streptococcus from other streptococci. When esculin is broken down, it reacts with iron salts in the medium to form a dark brown or black complex, providing a visual indicator of esculinase activity. Commonly found in bile esculin agar, it helps in the presumptive identification of enterococci and some Listeria species. Also used in some biochemical tests and research settings to study enzymatic hydrolysis and bacterial metabolism.