Siderophore-producing Bacteria Research Articles

Dynamics, Diversity and Function of Endophytic Siderophore-Producing Bacteria in Rice

Siderophore production confers to bacteria competitive advantages to colonize plant tissues and to exclude other microorganisms from the same ecological niche. This work shows that the community of endophytic siderophore-producing bacteria (SPB) associated to Oryza sativa cultivated in Uruguayan soils is dynamic and diverse. These bacteria were present in grains, roots, and leaves, and their density fluctuated between log(10) 3.44 and log(10) 5.52 cfu g(-1) fresh weight (fw) during the plant growth. Less than 10% of the heterotrophic bacteria produced siderophores in roots and leaves of young plants, but most of the heterotrophic bacteria were siderophore-producers in mature plants. According to their amplified restriction DNA ribosomal analysis (ARDRA) pattern, 54 of the 109 endophytic SPB isolated from different plant tissues or growth stages from replicate plots, were unique. Bacteria belonging to the genera Sphingomonas, Pseudomonas, Burkholderia, and Enterobacter alternated during plant growth, but the genus Pantoea was predominant in roots at tillering and in leaves at subsequent stages. Pantoea ananatis was the SPB permanently associated to any of the plant tissues, but the genetic diversity within this species-revealed by BOX-PCR fingerprinting- showed that different strains were randomly distributed along time and plant tissue, suggesting that a common trait of the species P. ananatis determined the interaction with the rice plant. Several isolates were stronger IAA producers than Azospirillum brasilense or Herbaspirillum seropedicae. In vitro inhibition assays showed that SPB of the genus Burkholderia were good antagonists of pathogenic fungi and that only one SPB isolate of the genus Pseudomonas was able to inhibit A. brasilense and H. seropedicae. These results denoted that SPB were selected into the rice plant. P. ananatis was the permanent and dominant associated species which was unable to inhibit two of the relevant plant growth-promoting bacteria.

The siderophore-producing bacterium, Bacillus subtilis CAS15, has a biocontrol effect on Fusarium wilt and promotes the growth of pepper

In this study, the Chrome Azurol Sulphonate (CAS) agar plate assay for siderophores was used to screen bacteria from the rhizospheric soil of pepper in Hainan, China. As a result, Bacillus subtilis CAS15, which produced a large orange halo, was obtained. A dual-culture test showed that CAS15 strongly antagonized the growth of 15 plant fungal pathogens, with rates of inhibition ranging from 19.26 to 94.07%. Analysis of the siderophore production by the ESI-MS and DHB(G) assays showed that CAS15 produced the catecholic siderophore 2,3-dihydroxybenzoate-glycine-threonine trimeric ester bacillibactin, and that siderophore productivity was inhibited by iron. A pot culture experiment was used to study the effects of B. subtilis CAS15 on pathogen development and plant growth. CAS15 reduced the incidence of Fusarium wilt in pepper significantly, by 12.5–56.9%, which indicated that B. subtilis CAS15 induced systemic resistance to Fusarium wilt in pepper. Iron supplementation reduced this biocontrol effect. There were significant differences in plant height at 14, 21, 28 and 40 days, with treated plants growing 27.24–54.53% taller than controls. B. subtilis CAS15 also enhanced the yield of pepper by shortening the time to 50 percent flowering to 17.26 days, increasing the average fruit weight 36.92%, and increasing the average yield per plant 49.68%. This research showed that B. subtilis CAS15 has great potential for plant growth promotion and biological control.

Potential of siderophore-producing bacteria for improving heavy metal phytoextraction

Phytoremediation holds promise for in situ treatment of heavy metal contaminated soils. Recently, the benefits of combining siderophore-producing bacteria (SPB) with plants for metal removal from contaminated soils have been demonstrated. Metal-resistant SPB play an important role in the successful survival and growth of plants in contaminated soils by alleviating the metal toxicity and supplying the plant with nutrients, particularly iron. Furthermore, bacterial siderophores are able to bind metals other than iron and thus enhance their bioavailability in the rhizosphere of plants. Overall, an increase in plant growth and metal uptake will further enhance the effectiveness of phytoremediation processes. Here, we highlight the diversity and ecology of metal resistant SPB and discuss their potential role in phytoremediation of heavy metals.

Genetic diversity of siderophore-producing bacteria of tobacco rhizosphere.

The genetic diversity of siderophore-producing bacteria of tobacco rhizosphere was studied by amplified ribosomal DNA restriction analysis (ARDRA), 16S rRNA sequence homology and phylogenetics analysis methods. Studies demonstrated that 85% of the total 354 isolates produced siderophores in iron limited liquid medium. A total of 28 ARDRA patterns were identified among the 299 siderophore-producing bacterial isolates. The 28 ARDRA patterns represented bacteria of 14 different genera belonging to six bacterial divisions, namely β-, γ-, α-Proteobacteria, Sphingobacteria, Bacilli, and Actinobacteria. Especially, γ-Proteobacteria consisting of Pseudomonas, Enterobacter, Serratia, Pantoea, Erwinia and Stenotrophomonas genus encountered 18 different ARDRA groups. Results also showed a greater siderophore-producing bacterial diversity than previous researches. For example, Sphingobacterium (isolates G-2-21-1 and G-2-27-2), Pseudomonas poae (isolate G-2-1-1), Enterobacter endosymbiont (isolates G-2-10-2 and N-5-10), Delftia acidovorans (isolate G-1-15), and Achromobacter xylosoxidans (isolates N-46-11HH and N-5-20) were reported to be able to produce siderophores under low-iron conditions for the first time. Gram-negative isolates were more frequently encountered, with more than 95% total frequency. For Gram-positive bacteria, the Bacillus and Rhodococcus were the only two genera, with 1.7% total frequency. Furthermore, the Pseudomonas and Enterobacter were dominant in this environment, with 44.5% and 24.7% total frequency, respectively. It was also found that 75 percent of the isolates that had the high percentages of siderophore units (% between 40 and 60) belonged to Pseudomonas. Pseudomonas sp. G-229-21 screened out in this study may have potential to apply to low-iron soil to prevent plant soil-borne fungal pathogen diseases.

Enhanced phytoextraction of an agricultural Cr- and Pb-contaminated soil by bioaugmentation with siderophore-producing bacteria

Bioaugmentation-assisted phytoextraction may enhance the phytoextraction efficiency thanks to larger metal mobilization by microbial metabolites. Green fluorescent protein-tagged cells of Pseudomonas aeruginosa, Pseudomonas fluorescens or Ralstonia metallidurans, able to produce siderophores, were inoculated in an agricultural soil containing Cr (488 mg kg −1) and Pb (382 mg kg −1) and maize was cultivated. Bacteria were inoculated as free or immobilized cells in Ca–alginate beads, with skim milk in the aim at improving both the bacterial survival and the in situ siderophore production. Skim milk addition increased inoculated Pseudomonads concentration in soil. Soil inoculation with free cells of R. metallidurans supplied with skim milk increased Cr accumulation in maize shoots by a factor of 5.2 and inoculation with immobilized P. aeruginosa cells supplied with skim milk increased Cr and Pb uptake by maize shoots by a factor of 5.4 and 3.8, respectively. However total metal taken up by the whole plant decreases almost always with bioaugmentation. Translocation factor also increased with P. aeruginosa or R. metallidurans by a factor of 6 up to 7. Inoculated bacteria concentration in soil was correlated with metals in the exchangeable fraction. Cr and Pb concentrations in the exchangeable fraction were correlated with metal contents in shoots or roots. Our results suggest that bioaugmentation-assisted phytoextraction is a relevant method in the aim at increasing the phytoextraction rate which usually limits the use of phytoremediation technologies.

Special conditions allow binding of the siderophore salmochelin to siderocalin (NGAL-lipocalin)

Siderocalin is part of the innate immune system and is secreted by epithelial cells in the early stages of inflammation. This protein binds catecholate siderophores such as enterobactin with high affinity. As a consequence, the iron supply and growth of the siderophore-producing bacteria is disturbed. Recombinant siderocalin isolated from strains of Escherichia coli K-12 contained bound enterobactin. Very low amounts of siderocalin were isolated from another K-12 strain unable to produce enterobactin, which indicated that enterobactin might stabilize the recombinant protein. Siderocalin isolated from E. coli strain Nissle 1917, which produces the glucosylated enterobactin salmochelin, contained a mixture of bound salmochelin (55%) and enterobactin (45%). The growth of an enterobactin-producing E. coli K-12 strain but not of the same strain carrying a plasmid encoding iroBCDEN and therefore able to produce salmochelin was suppressed when siderocalin was added to the medium, which indicated that salmochelin is bound by siderocalin before siderocalin folds into its final conformation in the cell, and that binding of salmochelin to matured siderocalin is not possible. Salmochelin is mainly produced by pathogenic enterobacteria. Glucosylation of enterobactin may be a mechanism by which these bacteria evade trapping of the enterobactin by siderocalin.

Etch pit formation on iron silicate surfaces during siderophore-promoted dissolution

Understanding the effects of microbiota on mineral alteration requires the ability to recognize evidence of bacteria-promoted dissolution on mineral surfaces. Although siderophores are known to promote mineral dissolution, their effects on mineral surfaces are not well known. We have utilized atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Mirau vertical scanning interferometry (VSI) to investigate surfaces after incubation with the siderophore desferrioxamine-B mesylate (DFAM) and under colonies of bacteria. Iron-silicate glass planchets chemically similar to hornblende were incubated in buffered growth medium with siderophore-producing bacteria ( Bacillus sp.) for 46 days with parallel abiotic experiments conducted with and without 240 μM DFAM, with and without 0.01 g l − 1 of microbially produced extracellular polysaccharides (EPS, alginate or xanthan gum). Some glass planchets were protected by dialysis tubing from direct contact with the EPS. Weekly sampling and analysis of all filtered sample solutions showed negligible Fe and Al release in the control experiments and significant release of Fe and Al in the presence of DFAM, with negligible changes in pH. Concentration of Fe in the filtered solutions after incubation with bacteria was below detection, consistent with uptake of Fe by cells. Release of Fe, Al, and Si in control, xanthan-only, and alginate-only experiments was negligible. Release of these elements was enhanced in all experiments containing DFAM, and greatest in alginate + DFAM experiments. AFM and VSI analyses reveal widespread, small etch pits and greater root mean squared roughness on siderophore-exposed surfaces and fewer, localized, larger etch pits on bacteria-exposed surfaces. This is the first documented case of etch pit development during siderophore-promoted dissolution. Roughness was not affected by the growth medium, alginate, or xanthan gum alone. The roughness trends among samples correlate with trends in Fe depletion documented by XPS. Enhanced dissolution and roughness cannot be attributed to direct contact with EPS because no significant chemical or physical differences were observed between surfaces directly exposed to EPS and those protected by dialysis tubing. Acetate released from the EPS may have enhanced the siderophore-promoted dissolution. Siderophores produced by Bacillus sp. may be responsible for some of the ‘biopits.’ The difference in size and distribution of the biopits may be related to colonization.

Cooperation and virulence in acute Pseudomonas aeruginosa infections.

BackgroundEfficient host exploitation by parasites is frequently likely to depend on cooperative behaviour. Under these conditions, mixed-strain infections are predicted to show lower virulence (host mortality) than are single-clone infections, due to competition favouring non-contributing social 'cheats' whose presence will reduce within-host growth. We tested this hypothesis using the cooperative production of iron-scavenging siderophores by the pathogenic bacterium Pseudomonas aeruginosa in an insect host.ResultsWe found that infection by siderophore-producing bacteria (cooperators) results in more rapid host death than does infection by non-producers (cheats), and that mixtures of both result in intermediate levels of virulence. Within-host bacterial growth rates exhibited the same pattern. Crucially, cheats were more successful in mixed infections compared with single-clone infections, while the opposite was true of cooperators.ConclusionThese data demonstrate that mixed clone infections can favour the evolution of social cheats, and thus decrease virulence when parasite growth is dependent on cooperative behaviours.

Microbial Mobilization of Uranium from Shale Mine Waste

The alum shale in the Billingen area in southern Sweden was mined in Ranstad for 5 years during the 1960s. The crushed tailings (processed and unprocessed) were left behind when the Ranstad mine was closed that has caused leaching of metals to the surroundings. The siderophore producing bacterium Pseudomonas fluorescens (Äspö, SE Sweden) was grown in batch cultures for 5 to 8 days with naturally weathered (unprocessed) uranium ore (0.0029% U by weight), kolm (0.52% U by weight) and acid-leached ore (0.0099% U by weight) in chemically defined media (unbuffered and buffered). Pseudomonas fluorescens grown with ore and unbuffered medium changed the pH from 4.7 to 9.3 and leached out 0.016 to 0.9% (normalized to surface area) of the total amount of U from the different ores. Incubation of the acid-leached ore with bacteria in buffered medium leached out 0.04% of the total U. Uranium was leached out selectively at all conditions, but this could be a pH effect, as pH increased at the same time as the U concentrations did. The observed release of Fe was most likely attributed to the production of microbial siderophores (Fe3+ specific chelators) since Fe3+ has a low solubility at pH > 4. As siderophores contain a number of chelating groups they may still function as complexators even in partly degraded form also for other metals than Fe. Thus, the production of microbial chelators could contribute to the elevated metal concentrations in the drainage water from the closed Ranstad mine, as abiotic processes cannot fully explain these high metal concentrations. In the extension: ligand promoted leaching of toxic elements could also be the key to bioremediation as there is a need for nontoxic cleanup methods for metal contaminated sites.

Isolation of Marine Bacteria by In Situ Culture on Media-Supplemented Polyurethane Foam

Polyurethane foam (PUF) supplemented with various agar media was used in situ to trap marine bacteria and it consequently provided a substrate on which they could be cultivated while exposed to natural seawater in the coral reef area. The bacterial population on the PUF blocks was analyzed by denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR)-amplified 16S rDNA fragments. Changing the composition of the cultivation medium in the PUF blocks and selecting different sampling sites resulted in different bacteria being detected on the PUF blocks. For example, iron-utilizing (IU) bacteria, siderophore-producing (SP) bacteria, and petroleum-degrading (PD) bacteria were isolated from PUF blocks and it was discovered that IU and SP contained iron and PD contained hydrocarbon. This method opens up the possibility for isolating novel and useful marine bacteria.

Inhibition of matrix metalloproteinase-2 activity by siderophores of Pseudomonas species.

To obtain a novel matrix metalloproteinase (MMP) inhibitor produced by bacteria, we have focused on the chelating activity of siderophores. Several siderophore-producing bacteria were isolated from soil using chrome azurol S agar plates and then the effect of siderophores on MMP-2 activity was assayed by gelatin zymography. The results showed that partially purified siderophores from ten isolated strains inhibited MMP-2 activity. Among these strains, two were non-fluorescent and eight were fluorescent Pseudomonas species. From these eight strains, pyoverdine-type siderophores were detected. The Zn(2+)-chelating activity of these siderophores correlated with the inhibition of MMP-2 activity. Therefore, it is considered that siderophores such as pyoverdines inhibit MMP-2 activity by chelating Zn(2+) on the active site of MMP-2.

Effect of exogenous siderophores on iron uptake activity of marine bacteria under iron-limited conditions.

More than 60% of species examined from a total of 421 strains of heterotrophic marine bacteria which were isolated from marine sponges and seawater were observed to have no detectable siderophore production even when Fe(III) was present in the culture medium at a concentration of 1.0 pM. The growth of one such non-siderophore-producing strain, alpha proteobacterium V0210, was stimulated under iron-limited conditions with the addition of an isolated exogenous siderophore, N,N'-bis (2,3-dihydroxybenzoyl)-O-serylserine from a Vibrio sp. Growth was also stimulated by the addition of three exogenous siderophore extracts from siderophore-producing bacteria. Radioisotope studies using (59)Fe showed that the iron uptake ability of V0210 increased only with the addition of exogenous siderophores. Biosynthesis of a hydroxamate siderophore by V0210 was shown by paper electrophoresis and chemical assays for the detection of hydroxamates and catechols. An 85-kDa iron-regulated outer membrane protein was induced only under iron-limited conditions in the presence of exogenous siderophores. This is the first report of bacterial iron uptake through an induced siderophore in response to exogenous siderophores. Our results suggest that siderophores are necessary signaling compounds for growth and for iron uptake by some non-siderophore-producing marine bacteria under iron-limited conditions.

Fractionation of Fe isotopes by soil microbes and organic acids

Research Article| June 01, 2001 Fractionation of Fe isotopes by soil microbes and organic acids Susan L. Brantley; Susan L. Brantley 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Laura Liermann; Laura Liermann 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Thomas D. Bullen Thomas D. Bullen 2U.S. Geological Survey, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Geology (2001) 29 (6): 535–538. https://doi.org/10.1130/0091-7613(2001)029<0535:FOFIBS>2.0.CO;2 Article history received: 05 Sep 2000 rev-recd: 26 Feb 2001 accepted: 06 Mar 2001 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Susan L. Brantley, Laura Liermann, Thomas D. Bullen; Fractionation of Fe isotopes by soil microbes and organic acids. Geology 2001;; 29 (6): 535–538. doi: https://doi.org/10.1130/0091-7613(2001)029<0535:FOFIBS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Small natural variations in Fe isotopes have been attributed to biological cycling. However, without understanding the mechanisms of fractionation, it is impossible to interpret such variations. Here we show that the δ56Fe of Fe dissolved from a silicate soil mineral by siderophore-producing bacteria is as much as 0.8‰ lighter than bulk Fe in the mineral. A smaller isotopic shift is observed for Fe released abiotically by two chelates, and the magnitude of the shift increases with affinity of the ligand for Fe, consistent with a kinetic isotope effect during hydrolysis of Fe at the mineral surface. Fe dissolved abiotically without chelates shows no isotopic shift. The δ56Fe of the exchange fraction on soil grains is also lighter by ∼0.6‰–1‰ than Fe from both hornblende and iron oxyhydroxides. The kinetic isotope effect is therefore preserved in open systems such as soils. When recorded in the rock record, Fe isotopic fractionation could document Fe transport by organic molecules or by microbes where such entities were present in the geologic past. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Rates of bacteria-promoted solubilization of Fe from minerals: a review of problems and approaches

Understanding the effects of bacteria on mineral weathering will require careful, controlled experiments in chemical reactors. Unfortunately, many of the details of protocol for such experiments have not yet been addressed. In particular, experimenters need to find ways to normalize experiments for the mass of bacteria involved in reaction and need to measure the rate of change of this mass. Furthermore, experimenters will need to define whether mineral dissolution occurs during the log or stationary phase of growth. Bacteria cell mass should be quantified both before and after dissolution. In some cases, flow bioreactors will be useful to understand dissolution under steady state conditions. Finally, even the choice of medium will involve trade-offs between encouraging growth of bacteria while mimicking natural solutions. In the case study for mineral dissolution presented here, two bacterial species of the genera Streptomyces and Arthrobacter, each involved in the natural weathering of hornblende, were investigated in growth experiments in medium with hornblende. Experiments with and without desferrioxamine B (its mesylate salt deferoxamine mesylate, DFAM) were also completed. In the presence of bacteria or DFAM, Fe release from hornblende is accelerated by up to a factor of ∼20. Both bacteria produce catecholamide siderophores that are presumed responsible for the enhanced Fe release when bacteria are present. These results represent the first quantification of the rate of release of Fe from a mineral in the presence of the siderophore-producing bacteria, with subsequent identification of the siderophore. The rate of Fe release from hornblende increases non-linearly in bacteria-free experiments with increasing concentrations of DFAM. Such a rate–concentration function is consistent with formation of surface complexes on the hornblende surface. Surface complexation may also explain enhancement of Fe release in the presence of the catechol siderophores produced by the arthrobacter and streptomycete. The effects of siderophores on Fe transport could be significant in many soils.

Microorganisms and soil physico-chemical conditions in the drilosphere of Lumbricus terrestris

The influence of organic matter input into the soil by Lumbricus terrestris on the spatial distribution of microorganisms and on physico-chemical soil characteristics was examined. An experiment was carried out with L. terrestris in soil cores of about 25 cm depth. Lettuce was supplied to the worms as a food source. In the control soil without earthworms, lettuce was manually mixed with the soil. The numbers of total bacteria, siderophore producing bacteria and fluorescent pseudomonads were greatest in the casts and burrowing walls of the 0–5 cm soil layer (1.5 × 10 9, 1.6 × 10 8 and 4.4 × 10 7 CFU g −1 of dry soil, respectively). Numbers in the uningested soil from the same soil layer were 60–320 times smaller. In the deeper soil layers (5–22 cm), bacterial numbers in the uningested soil were 6–32 times smaller than in the burrow walls. The bacterial counts for the control soil were intermediate between the counts for the casts + burrow walls and the counts for the uningested soil. Over the depth of soil studied, the NO 3 −N content of the casts and burrow walls was 1.2- to 2.1-fold greater than in the uningested soil, but it was only greater than the control soil (notably 1.66-fold) in the top layer (0–5 cm). The distribution of microorganisms and nutrients along the earthworm burrows was associated with good water availability and improved aeration. Based on these observations, the drilosphere of actively feeding L. terrestris is considered to be one of the most important microbial niches in soil.

Lumbricus terrestris in a soil core experiment: Nutrient-enrichment processes (NEP) and gut-associated processes (GAP) and their effect on microbial biomass and microbial activity

For 5 weeks, lettuce was supplied to the surface of three types of soil cores. In the first treatment, Lumbricus terrestris (L.) was added before the lettuce supply. The average specific growth rate of L. terrestris was 0.01 d −1 with an average feeding rate of 47 mg lettuce dw g −1 earthworm dw d −1 (2.1 mg lettuce Kjeldahl(Kj)-N g −1 earthworm dw d −1. In the second treatment, no worms were added, but the lettuce was mixed with the soil. Relative to this “Mixed control”, microbial biomass, nitrifying activity and respiration rate were significantly reduced in the presence of L. terrestris by 20, 25 and 49%, respectively. Proteolytic bacteria were significantly stimulated, but the inverse was noticed for fluorescent pseudomonads. Total plate counts, siderophore-producing bacteria and fungi were similar in both treatments. In the third treatment, no worms were added but the lettuce was left unmixed on the soil surface. The “Earthworm treatment” scored significantly higher for most values relative to this “Unmixed control”. Nutrient-enrichment processes (NEP) associated with the organic matter incorporation were distinguished from gut-associated processes (GAP) associated with the transit of soil and organic matter through the earthworm gut. It was concluded that the NEP rather than the GAP are responsible for the increased numbers of microorganisms reported in the presence of earthworms. On the other hand, the specific metabolic activity ( qCO 2) of the microbial biomass was decreased by 35% due to the GAP. The lower qCO 2 indicated a better energy to biomass conversion and an alteration in the microbial community in favour of a higher proportion of K-strategists.

Responses by iron-efficient and inefficient oat cultivars to inoculation with siderophore-producing bacteria in a calcareous soil

Rhizosphere bacteria may enhance plant uptake of Fe by producing siderophores that chelate sparingly soluble Fe3+ in calcareous soils. To evaluate the extent to which plants benefit from colonization of the roots by prolific siderophore-producing bacteria, we inoculated two oat cultivars with six strains of bacteria that produced high concentrations of siderophores under Felimiting conditions in vitro. Oat cv Coker 227, an Fe-efficient cultivar, which produces the phytosiderophore avenic acid, and cv TAM 0-312, and Fe-inefficient cultivar, which does not produce the phytosiderophore, were grown in a calcareous soil (Weswood silt loam) on a light bench in the laboratory. Half of the plants were fertilized with a nutrient solution containing 5 mM Fe and half with a nutrient solution containing no Fe. After 6 weeks of growth, we compared colonization of the roots by the inoculant bacteria and the dry weight and Fe content of roots and shoots. Three species of Pseudomonas colonized the roots of both oat cultivars in high numbers (≥106 cells g-1 root dry weight), whereas the remaining bacteria colonized the roots in substantially lower numbers (≤104 cells g-1 root dry weight). Plants fertilized with 5 mM Fe were larger and supported greater numbers or rhizosphere bacteria per gram of root than plants not supplied with Fe. Comparisons of the Fe content and dry weight of roots and shoots revealed few significant differences between inoculated and uninoculated plants, or among the plants inoculated with the different strains of siderophore-producing bacteria. The differences that were observed revealed no consistent response to inoculation. We conclude that inoculation of the roots of the two oat cultivars with bacteria that produce high concentrations of siderophores in response to an Fe deficiency had little or no effect on Fe acquisition by the plants.

Use of chrome azurol S reagents to evaluate siderophore production by rhizosphere bacteria

Siderophores produced by rhizosphere bacteria may enhance plant growth by increasing the availability of Fe near the root or by inhibiting the colonization of roots by plant pathogens or other harmful bacteria. To examine the populations of siderophore-producing bacteria colonizing the roots of two grass species that differed in their susceptibility to Fe deficiency, we inoculated serial dilutions of root samples onto chrome azurol S (CAS) agar and several other selective and non-selective culture meida. CAS agar effectively differentiated bacteria that were capable of excreting large amounts of siderophore, but the composition of the medium limited its usefulness for ecological studies. A large proportion (71–79%) of the bacterial population that grew on a non-selective medium (tryptic soy agar) failed to grow on CAS agar, and several isolates that showed no sign of siderophore production on CAS agar produced siderophore in liquid culture. Similar populations of siderophore-producing bacteria were observed on roots of St. Augustine grass, which frequently exhibits Fe chlorosis, and bermuda grass, which does not. Roots of both grasses were colonized by bacteria that produced siderophore in vitro at concentrations ranging from 100 to 230 μM. The CAS assay solution was also used to compare siderophore production by Pseudomonas fluorescens Q6, an isolate from bermuda grass, and by P. putida B 10, a plant growth-promoting pseudomonad. P. fluorescens Q6 produced 2.4 times more siderophore in vitro than P. putida B 10.

Distribution of siderophore-producing bacteria in the sediments of a small lagoon by Lake Biwa.

Siderophore-producing activity of heterotrophic bacteria isolated from sediments of a small lagoon by Lake Biwa was examined by bioassay with siderophore auxotrophs. About a half of the heterotrophic bacteria exhibited siderophore producing activity and there were not so much differences in the presence of siderophore-producing bacteria between in the surface layer of the sediment and in the subsurface layer, even though the concentration of ferrous iron in these sedi-ments was different. Though the concentration of ferrous iron in the interstitial water seemed sufficient to repress the siderophore production by these bacteria, siderophore was detected in the interstitial water of the sediment when assayed by hyroxamate suxotroph Arthrobacter flavescens JG-9 after extracted the siderophore from interstitial water.