Differential Siderophore Utilization and Iron Uptake by Soil and Rhizosphere Bacteria

Abstract

The differential availabilities of the hydroxamate siderophores ferrioxamine B (FOB) and ferrichrome (FC) and the pseudobactin siderophores St3, 7NSK2, and WCS 358 as sources of Fe for soil and rhizosphere bacteria were studied. About 20% of the total bacterial CFU from the rhizospheres of four plant species were able to use FOB as the sole Fe source in an Fe-deficient medium, while about 12, 10, 2, and >1 % were able to use FC and pseudobactins 7NSK2, St3, and WCS 358, respectively. Of the 165 colonies isolated from plates containing pseudobactins, 64 were able to use the pseudobactin on which they were isolated as the sole Fe source in pure culture. Cross-feeding tests showed that almost all of these 64 strains were also able to use at least one of the other siderophores studied (pseudobactin, FOB, or FC). Pseudomonas putida StS2, Pseudomonas maltophilia 7NM1, and Vibriofluvialis WS1, which were originally isolated on pseudobactins St3, 7NSK2, and WCS 358, respectively, were selected for their ability to grow with pseudobactin St3 as the sole Fe source. They incorporated 55Fe3+ mediated by pseudobactin St3 at various rates (71.5, 4, and 23 pmol/min/mg [dry weight] of cells, respectively). Similarly, P. putida St3 was shown to incorporate 55Fe3+ mediated by FOB and FC. We suggest that the ability of bacteria to utilize a large variety of siderophores confers an ecological advantage. Fe forms insoluble hydroxides at neutral and basic pHs (20). Microorganisms have therefore evolved efficient uptake mechanisms to obtain sufficient amounts of this essential element (26). Most aerobic and facultative aerobic bacteria possess a high-affinity Fe transport system in which siderophores are excreted and the consequent Fe complex is taken up via the cognate-specific receptor and a transport pathway * Corresponding author. their ability to utilize exogenous siderophores. Receptors for

Fitness in soil and rhizosphere of Pseudomonas fluorescens C7R12 compared with a C7R12 mutant affected in pyoverdine synthesis and uptake

“…The ability of PL1 to incorporate several heterologous pyoverdines would enable the pvd 3 mutant to compete as e¤ciently as the wild-type C7R12 with the indigenous micro£ora, despite the lower rate of Fe-pyoverdine uptake by PL1 compared with C7R12. The possible ecological advantage to bacteria conferred by the ability to utilize a large variety of siderophores was previously suggested by Jurkevitch et al [42]. This hypothesis was recently supported by the data of Loper and Henkels [43], showing, with the help of the ice nucleation reporter gene inaZ, that bacteria producing heterologous pyoverdines enhanced the iron availability for an introduced mutant £uorescent pseudomonad impaired in pyoverdine synthesis but still able to use heterologous pyoverdines.…”

A Biological Sensor for Iron Available to Bacteria in Their Habitats on Plant Surfaces

“…commonly utilize a ferric pyoverdine(s) produced by other strains of Pseudomonas spp. (5,22) and ferric siderophores produced by members of diverse genera of bacteria and fungi (16). Pseudobactin, a pyoverdine produced by P. fluorescens B1O, was detected recently in a rhizosphere at a concentration of 3.5 x 10-' M by an immunoassay in which specific monoclonal antibodies were used (4).…”

Cross-utilization and expression of outer membrane receptor proteins for siderophore uptake by Diamondback mothPlutella xylostella(Lepidoptera: Plutellidae) gut bacteria

“…The competition for iron in an ecological niche, such as a host insect gut, occurs in two stages. First, competition between the excreted siderophores for the available iron and, second, competition between the microorganisms for the resulting iron-siderophore complex formed (Jurkevitch et al, 1992). The expression of an OMRP by a bacterial strain is essential to take up its own iron-loaded siderophores or those of others (cross-utilization).…”