Chrome+azurol+S+(CAS)+assay

Chrome azurol S (CAS) is a method that can be used to detect the mobilization of iron. In many soils iron is not often in an abundant form that plants can readily uptake and use. Siderophores are produced by rhizosphere bacteria to help enhance plant growth by increasing the availability of iron near the root (1). A siderophore is a low molecular-weight Fe(III) specific ligand that is a chelator. Chelators form multiple bonds with metal ions and in the case of siderophores they assist in acquiring useable iron. Siderophore in greek literally means iron carrier. The role of siderophores is to scavenge iron from the environment and make the mineral available to the cell (6). Siderophores are not only used in the process of plants acquiring iron and other metals, they also are found on bacteria in the body on cells and in some cases the siderophores rob our own blood cells of iron. Escherichia coli strains have been used in CAS assays to demonstrate the production of siderophores (7). In an ecological study CAS was used to detect mobilization of iron by testing for siderophore presence in soil samples gathered from the study site (2). Thus, CAS can be useful in determining if siderophores are being produced in ecological samples as well as medical samples. To test mobilization of iron, chrome azurol S (CAS) media can be used. On CAS, iron mobilization is done via the production of complexing acids or siderophores. Since siderophore production increases availability of iron, measuring the production of these gives us a measure of iron mobilization ability of organisms. Alexander and Zuberer (1991) demonstrated that CAS agar effectively differentiated bacteria that were able to excrete large amounts of siderophore. Frey-Klett //et al.// (2005) used this same method however the CAS media was modified. Consequently, it seems that many bacteria fail to grow on CAS agar because of a large presence of HDTMA which is a cationic detergent that is added to CAS agar to stabilize the Fe-CAS indicator and gives the medium a blue color (1). CAS can be prepared in a solution or as CAS agar plates.
 * Chrome azurol S (CAS) assay**

Specific chemical components of the CAS media can all be found in the study performed by Schwyn and Neilands (1987). To perform CAS analysis a strong ligand (e.g. siderophore) is added to a highly colored iron dye complex. When the iron ligand complex is formed the release of the free dye is accompanied with a color change (7). Since CAS assay is high in sensitivity it is able to be used on agar plates. The Fe(III) gives the agar a rich blue color and concentration of siderophores excreted by iron starved organisms results in a color change to orange. After inoculating it takes only a short time before a color change will occur usually no more that 6 hours (7, 1). HDTMA is used as a detergent as mentioned above and the concentration of HDTMA is crucial to successful growth and testing of bacteria. If the concentration is too low the blue dye will precipitate and if it is too high the bacteria will die. This does present selectivity issues when culturing ecological samples especially. However, Alexander and Zuberer (1991) developed a modification to circumvent the problem of HDTMA toxicity. The amount of HDTMA used was reduced significantly and MES buffer was used to substitute as a detergent. This has been a much more successful method for growth and testing for ecological microbial samples.



For the liquid assay solution one only needs to observe the color change that occurs or more rigorous turbidity measurements can be taken. The diameter of discoloration on the CAS media is directly positively correlated to ability to mobilize iron. Since the diameter of discoloration area and efficacy of iron mobilization are correlated, CAS tests are inexpensive and simple methods for screening bacterial isolates for the ability to mobilize iron. Using this method only visual observation of bacterial growth and measurement of the diameter of discolored area are needed for analysis. Literature cited 1. Alexander, D.B. and Zuberer, D.A. 1991. Use of chrome azurol S reagents to evaluate siderophore production by rhizosphere bacteria. Biology and Fertility of Soils 12: 39-45. 2. Calvaruso, C, Turpault, M.P., Leclerc, E and Frey-Klett, P. 2007. Impact of Ectomycorrhizosphere on the functional diversity of soil bacterial and fungal communities from a forest stand in relation to nutrient mobilization processes. Microbial Ecology 54: 567-577. 3. Duval, B.D. and Hungate, B.A. 2008. The long road to nitrogen fixation. Nature Geoscience 1: 213-214. 4. Frey-Klett, P., Chavatte, M, Clausse, M.L., Courrier, S, Le Roux, C, Raaijmakers, J, Martinotti, M.G., Pierrat, J.C. and Garbaye, J. 2005. Ectomycorrhizal symbiosis affects functional diversity of rhizosphere fluorescent pseudomonads. New phytologist 165: 317-328. 5. Gao, W, Liu, Y, Giometti, C.S., Tollaksen, S.L., Khare, T, Wu, L, Klingeman, D.M., Fields, M.W. and Zhou, J. 2006. Knock-out of S01377 gene, which encodes the member of a conserved hypothetical bacterial protein family COG2268, results in alteration of iron metabolism, increased spontaneous mutation and hydrogen peroxide sensitivity in Shewanella oneidensis MR-1. BMC Genomics 7:76. 6. Neilands, J.B. 1995. Siderophores: Structure and function of microbial iron transport compounds. The Journal of Biological Chemistry 45: 26723-26726. 7. Schwyn, B. and Neilands, J.B. 1987. Universal Chemical Assay for the Detection and Determination of Siderophores. Analytical Biochemistry 160: 47-56.