Siderophore Detection Using Chrome Azurol S and Cross-Feeding Assays

More than 500 siderophores that bind ferric iron have been characterized and grouped by type based on their chemical structure. The chrome azurol S (CAS) assay is a universal colorimetric method that detects siderophores independent of their structure. In this assay, siderophores scavenge iron from an Fe-CAS-hexadecyltrimethylammonium bromide complex, and subsequent release of the CAS dye results in a color change from blue to orange. Solution-based experiments with CAS result in a quantitative measure of siderophore production, while an observable color change on CAS agar plates can be performed for qualitative detection of siderophores. Cross-feeding assays are another useful method to detect and characterize siderophores produced by bacteria. Under iron-limiting conditions, cross-feeding assays test the ability of an indicator strain to grow when supplied with a specific siderophore (from a test strain) to which it has a cognate receptor required for import into the cell. The cross-feeding assay can be tested with a variety of wild-type strains, siderophore biosynthesis mutants, and siderophore receptor mutants.