Siderophore production and cross-feeding assay
Siderophore production and cross-feeding assays were performed as reported previously [17]. The chrome azurol S (CAS) assay [27] was used to detect siderophores. On CAS agar plates, the formation of colony and siderophore halos were evaluated following 7 days of colony incubation at 30°C. All isolated strains were inoculated on four different CAS plates: (i) without any addition, (ii) with the addition of 0.1 nM DEF, (iii) with the addition of 0.1 nM of 3OC6-HSL or C8-HSL, (iv) and with the addition of 0.1 nM DEF plus 0.1 nM of 3OC6-HSL or C8-HSL.
The induced siderophores of unknown strains were partially isolated and compared with desferroixamine by examination with the Csaky test [15] and the Arnow reaction [1] for their hydroxamate and catechol functionality respectively. In these assays, hydroxylamine and 2,3-dihydroxybenzoic acid, respectively, were used as the standards.