Modified chrome azurol S method for detection and estimation of siderophores having affinity for metal ions other than iron

Siderophores are small molecular weight (generally 1 kDa) ferric specific ligands produced by variety of organisms to chelate iron under iron limiting conditions. Various assays have been in use to detect and estimate different phenotypes of siderophores. Though there are various methods available for detection of iron specific siderophore or modified method for Cu specific siderophore [chalkophore], reports on modified methods for detection of siderophore having affinity for various other metal ions are scarce. In present study, a modified method was designed for screening siderophores that can bind to heavy metal ions such as Cu2+, Ni2+, Mn2+, Co2+, Zn2+, Hg2+ and Ag2+. Each of the modified chrome azurol S (CAS) solution when used to screen siderophore resulted in instant colour change similar to the traditionally used CAS solution for screening Fe3+ specific siderophores. Two bacterial cultures isolated from local rhizospheric soil produced hydroxamate and catecholate siderophores that could remove Cu2+, Ni2+, Mn2+, Co2+, Zn2+, Hg2+ and Ag2+ metal ions from CAS solution resulting in instant colour change. Similar observations were recorded on CAS agar amended with modified CAS solution. The order of complexation of metals with siderophore produced by Alcaligenes sp. RZS2 in CAS assay was as follows; Cu2+ > Ni2+ > Mn2+ > Fe3+ > Co2+ > Zn2+ > Hg2+ > Ag2+. The order of complexation of metals with siderophore produced by Pseudomonas aeruginosa RZS3 in CAS assay was as follows; Ni2+ > Co2+ > Mn2+ > Zn2+ > Fe3+ > Cu2+ > Hg2+ > Ag2+.