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MICROBIAL SIDEROPHORES - GATEWAY FOR IRON REMOVAL
R. Balagurunathan & M. Radhakrishnan
Post Graduate Department of Microbiology
Sri Sankara Arts & Science College,
Kanchipuram - 631 561. Tamilnadu.
E-Mail: rbalaguru@yahoo.com


Introduction:
Siderophores (from the Greek: iron carriers) are defined as relatively low molecular weight, ferric ion specific chelating agents synthesized by bacteria, actinomycetes, fungi and certain algae growing under low ironic stress. Chemically siderophores are iron binding proteins with molecular weight ranging from 400 - 1500 Da. The role of these compounds is to scavenge iron from the environment and to make the mineral, which is almost always essential, available to the microbial cell. Research in this field begun about six decades ago, and interest in it accrued with the realization that most aerobic and facultative anaerobic microorganism synthesize atleast one siderophore. Siderophores have been related to virulence mechanisms in microorganism pathogenic to both animals and plants. In addition, they have applications in clinical, agricultural and environmental fields. At present nearly 500 siderophores are reported from selected microorganisms.

Importance of iron in the environment and microbiology:
Iron is an important element for most microorganisms owing to its importance in biogeochemical reactions including respiration, photosynthetic transport, nitrate synthesis, nitrogen fixation and detoxification of oxygen radicals. The aerobic atmosphere of the planet has caused the surface iron to become converted to oxyhydroxide polymers of very sparing solubility. The concentration of free ferric ion at neutral pH is dictated by the solubility product constant of ferric hydroxide. In spite of the earth's crust, dissolved iron concentrations are particularly low.

A great variety of means of acquisition, way of uptake and methods of storage are used by microbes to ensure a supply of the essential metal. Two methods commonly used by microbes for acquisition of iron are reduction and chelation. Solubilization of insoluble iron polymers is the first step in iron assimilation. Microorganisms growing under aerobic conditions need iron for a variety of functions including reduction of oxygen for the synthesis of ATP, for formation of heme and for other essential purposes. A level of atleast one micromolar iron is needed for optimum growth.

Siderophores accumulate iron and it transports the metal into the cell by membrane receptor molecules, these molecules are encoded by five genes in operon. These genes used to mobilize, transport and uptake of this essential element for the metabolism. The extreme focus on the need for iron is reflected by its requirement for the proper function of enzymes that facilitates electron transport, oxygen transport and other life sustaining processes.

Important siderophore groups:
Important groups of siderophores include hydroxamate siderophores, catecholate (phenolates) siderophores and carboxylate siderophores. Hydroxamate siderophores contain three secondary hydroxamate groups, in which each hydroxamate groups provides two oxygen molecules, which form a bidentate ligand with iron. Hydroxamate siderophores are produced by both bacteria and fungi. Ferrichrome produced by the fungus Ustilago sphaerogena, was the first siderophore to be isolated and shown to be a growth factor for other microbes.

Catecholate siderophores are produced only by certain bacteria. Each catecholate group provides two oxygen atoms for chelation with iron so that a hexadentate ochtahedral complex is formed as in the case of hydraxamate siderophores. The best example for catecholate siderophore is enterobactin produced by E.coli. Carboxylates produced by bacteria (certain Rhizobium and Staphylococcus strains) and fungi belonging exclusively to mucorales, coordinating iron with carboxyl and hydroxyl groups.

Siderophore producing microorganisms:
Siderophore synthesis, their structure, properties and uses have been studied for many terrestrial microorganisms. Wide range of bacteria, fungi and countable number of actinomycetes and algae are reported as produce different kinds of siderophores

Bacteria:
Bacteria are common inhabitants of metal contaminated sites, where they accumulate and immobilize heavy metals. The cell walls of gram positive bacteria have strong metal binding properties. Some bacteria produce extra cellular polysaccharide sheaths that bind metals. In addition, metals such as manganese, nickel and iron are absorbed through specific uptake receptors. Under iron stress, bacteria produce siderophores that bind ferric iron and transport into an iron siderophore receptor. Bacteria produce four types of siderophores: hydroxymate, catecholate, salicylate and carboxylate. These siderophores play an important role in the extra cellular solubilization of iron from minerals or organic substances. Some important siderophore producing bacteria includes Escherichia coli, Salmonella, Klebsiella pneumoniae, Vibrio cholerae, Vibrio anguillarum, Aeromonas, Aerobacter aerogens, Enterobacter, Yersinia and Mycobacterium species.

Fungi:
Fungi are the important siderophore producing microorganisms next to bacteria. Some important siderophore producing fungi includes Aspergillus nidulans, A. versicolor, Penicillium chrysogenum, P. citrinum, Mucor, Rhizopus, Trametes versicolor. Ustilago sphaerogina, Saccharomyces cerivisiae, Rhodotorula minuta and Debaromyces species.

Actinomycetes:
Actinomycetes are aerobic gram positive filamentous bacteria with high guanine + cytosine (G+C) content and form asexual spores. Mostly they are saprophytic in nature which prefer complex substrate for their growth and able to tolerate certain metals at high concentrations. Siderophore producing actinomycetes includes Actinomadura madurae, Nocardia asteroids, and Streptomyces griseus. Actinomycetes produce both hydroxymate and salicylate types of siderophores.

Algae:
Few algae also reported as siderophore producers. Schizokinen, a dihydroxymate type of siderophore, produced by Anabaena sp., reported to facilitate iron uptake. Anabaena flos-aquae and Anabaena cylindrica produce siderophores which accumulate copper.

Microbial siderophores and marine ecosystem:
Surprisingly very little is known about the nature of the siderophores produced by marine microorganism. Structure of anguibactin produced by a fish pathogen Vibrio anguillarum and Bisucabarin produced by a deep sea bacterium Alteromonas haloplanktis, are known. Natural Dessferrioxamine G was produced from marine Vibrio species. Strains of open ocean bacteria from the genera Vibrio, Alteromonas, Alkaligenes, Pseudomonas and Photobacterium have been surveyed for their ability to produce siderophores. Marine fungi includes Aspergillus versicolor, Cunninghamella elegans, Rhizopus species and Syncephalastrum racemosum were found to be more potent siderophore producers. In addition, marine phytoplankton Rhodomonas ovalis were found to be a producer of siderophores.

In open ocean, the iron content is low because of it's solubility nature in sea water. When iron is scarce, marine bacteria (and possibly some kinds of microalgae) make siderophores. Most iron chelators are probably siderophores or their break-down products. Siderophore production and binding is part of specialized biological machinery that helps these organisms to harvest iron. This machinery includes not only the siderophore-producing proteins, but also cell-membranebound proteins that enable ingestion of siderophore-complexed iron.

Siderophores effectively increase the solubility of iron in sea water, allowing ecosystems to support more microbes by making more iron available for essential biological processes. But chelated iron is available only to organisms that can take it in.

Alison Butler, Jennifer Martinez and their colleagues at UCSanta Barbara, in collaboration with Margo Haygood's research group at the Scripps Institution of Oceanography, have isolated and characterized several siderophores from marine microorganisms. These siderophores are structurally quite different from known terrestrial siderophores. Aquachelins from Halomonas aquamarina DS40M3 and marinobactins from Marinobacter sp. DS40M6 and DS40M8 both have an unusual water-insoluble fatty acid part and a water-soluble peptide part.

Applications of microbial siderophores:
The importance of microbial siderophores extends beyond their immediate role in microbial physiology and their role in biotechnology. Applications of microbial siderophores for sustainability of humans, animals, plants and environment is enormous.

Clinical applications:
As naturally occurring chelating agents for iron, siderophores might be expected to be some what less noxious for deferrization of patients suffering from transfusion induced siderosis. A siderophore from Streptomyces pilosus, desfarrioxamine B, is marketed as mesylate salt under the trade name Desferol and is advocated for removal of excess iron resulting from the suppurative therapy for Thalassemia. The potency of common antibiotics has been elevated by binding in to the iron binding functional groups of siderophores.

Siderophore from Klebsiella pneumoniae has been used as an antimalarial agent. Sideromycin is an iron chelating antibiotic produced by Streptomyces species showed good antimicrobial activity.

Agricultural interest:
Fluorescent Pseudomonads form a line of siderophores comprised of a quinoline moiety, responsible for the fluorescence and a peptide chain of variable length bearing hydroxamic acid and - hydroxyl acid functions. Capacity to form these Pseudobactin/ pyoverdine type siderophores has been associated with improved plant growth either through a direct effect on the plant through control of noxious, organisms in the soil. Nitrogenase can said to be an iron-intensive enzyme complex and the symbiotic variety, as found in Rhizobium spp, may require an intact siderophore system for expression of this exclusively prokaryotic catalyst upon which all life depends.

Environmental applications:
Siderophores have potential ability to resolve various environmental problems like heavy metal accumulation, rust removal, biofouling, dye degradation, sewage treatment and bioleaching, etc

Siderophores in rust removal:
Rust consists predominantly of Fe2O3 molecules with trivalent iron ions. Traditional chemical rust removal is based on two principles: rust needs to be converted and then pickled. Traditional, inorganic rust converters consist predominantly of mineral acids, fat dissolvers and corrosion inhibitors. These substances react with the rust and form iron phosphate. The use of natural acids such as malic or citric acid has also proved successful. However, the use of natural products requires a lot of time (several hours to a whole day) before satisfactory results can be obtained.

Bacteria, fungi and plants are using biological chelating compounds in order to efficiently bind iron ions. So rust layers can be abated in a natural and gentle way. Biologists, mineralogists and biotechnology engineers have been investigating the process of biological rust removal for many years.

Conclusion:
Iron is an important element for all living cells. It is difficult to take up iron into cells due to its poor solubility. For the iron supply of living cells, some microorganisms include bacteria, fungi, actinomycetes and algae produce an iron chelator, siderophore, outside the cells and the siderophore is chelated with ferric iron. There is an enormous scope for the application of microbial siderophores for the sustainability of humans, animals and plants. Currently the applications of siderophores in clinical, agricultural and environmental sector are reported in some extent. But the siderophore research is not at all initiated in most of the microbiology research laboratories. So, there is a need to discover siderophores from normal and also extremophiles in the ecosystems like deep sea, desert and forest to exploit their applications for welfare of all living beings as well as for environment. Management Development of bioprocess technology is also needed for the industrial production of microbial siderophores, since it has multisector applications.

Organisms
Siderophores
Bacteria

Escherichia coli

Aeromonas hydrophila

Pseudomonas aeruginosa

Aerobacter aerogenes

Salmonella

Klebsiella sp.

Vibrio cholerae

Acinetobacter calcoacaticus

Mycobacterium tuberculosis

Yersinia pestis

Staphylococcus aureus


Fungi

Ustilago sphaerogina

Ustilago maydis

Aspergillus nidulans

Penicillium

Debaromyces

Rhodotoulaminuta

Actinomycetes

Actinomadura madurae

Nocardia sp.

Nocardia asteroides

Streptomyces griseus




Enterobactin

Amonabsactin

Pyoverdin and pyochelin

Aerobacin

Aerobactin

Aerobactin

Vibriobactin

Acinetobactin

Mycobactin

Yersniabactin

Aureochelin

Ferrichrome

Ferriaxamine B

Hydroxymate type

Hydroxymate type

Hydroxymate type

Hydroxymate type

Madurastatin

Nocobactin

Asterobactin

Desferrioxamine B

About the authors:
Dr. R. Balagurunathan, Ph.D. is a Senior Lecturer and Head, Department of Microbiology, Sri Sankara Arts & Science College, Kanchipuram. He has obtained his Ph.D from Centre of Advanced Study in Marine Biology, Annamalai University, Parangipettai, India and subsequently worked as Post Doctoral Research Fellow at Malaysia and China. He has successfully completed two research projects and recently received a research grant from DST, New Delhi for the discovery of Antituberculosis Drugs from Actinomycetes. He has 20 publications which includes, research and review papers and chapters in book. He has been working with high value metabolites from extremophilic Actinomycetes.

M. Radhakrishnan, M.Sc., M.Phil., is a Lecturer in Microbiology Department of Sri Sankara Arts and Science College, Kanchipuram. He has several publications which include research and review papers and a chapters in book. His field of interest is Actinomycetology - Actinomycetes Biology and Technology.

ENVIS CENTRE Newsletter Vol.5, July 2007 Back 

 

 
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