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Hofmann M, Retamal-Morales G, Tischler D. Metal binding ability of microbial natural metal chelators and potential applications. Nat Prod Rep 2020; 37:1262-1283. [DOI: 10.1039/c9np00058e] [Citation(s) in RCA: 29] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Metallophores can chelate many different metal and metalloid ions next to iron, make them valuable for many applications.
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Affiliation(s)
- Marika Hofmann
- Institute of Biosciences
- Chemistry and Physics Faculty
- TU Bergakademie Freiberg
- 09599 Freiberg
- Germany
| | - Gerardo Retamal-Morales
- Laboratorio de Microbiología Básica y Aplicada
- Facultad de Química y Biología
- Universidad de Santiago de Chile
- Santiago
- Chile
| | - Dirk Tischler
- Microbial Biotechnology
- Ruhr-Universität Bochum
- 44780 Bochum
- Germany
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2
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Khan A, Gupta A, Singh P, Mishra AK, Ranjan RK, Srivastava A. Siderophore-assisted cadmium hyperaccumulation in Bacillus subtilis. Int Microbiol 2019; 23:277-286. [DOI: 10.1007/s10123-019-00101-4] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2019] [Revised: 09/12/2019] [Accepted: 09/16/2019] [Indexed: 01/23/2023]
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3
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Insights into copper effect on Proteus hauseri through proteomic and metabolic analyses. J Biosci Bioeng 2015; 121:178-85. [PMID: 26194304 DOI: 10.1016/j.jbiosc.2015.06.011] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2014] [Revised: 06/04/2015] [Accepted: 06/18/2015] [Indexed: 11/23/2022]
Abstract
This is the first-attempt to use liquid chromatography coupled with tandem mass (LC-MS-MS) in deciphering the effects of copper ion on Proteus hauseri. Total 941 proteins in copper-addition (+Cu) group and 898 proteins in non-copper-addition (-Cu) group were found, which containing 221 and 178 differential proteins in +Cu and -Cu group, respectively. Differential proteins in both groups were defined into 14 groups by their functional classification which transport/membrane function proteins were the major different part between the two groups, which took 19.5% and 7.7%, respectively. The result of BioCyc and KEGG analyses on metabolic pathway indicated that copper could interrupted the pathway of chemotaxis CheY and inhibited the swarming of P. hauseri, which provided a potential in controlling the pathogenicity of this strain.
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Johnstone TC, Nolan EM. Beyond iron: non-classical biological functions of bacterial siderophores. Dalton Trans 2015; 44:6320-39. [PMID: 25764171 PMCID: PMC4375017 DOI: 10.1039/c4dt03559c] [Citation(s) in RCA: 245] [Impact Index Per Article: 27.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Bacteria secrete small molecules known as siderophores to acquire iron from their surroundings. For over 60 years, investigations into the bioinorganic chemistry of these molecules, including fundamental coordination chemistry studies, have provided insight into the crucial role that siderophores play in bacterial iron homeostasis. The importance of understanding the fundamental chemistry underlying bacterial life has been highlighted evermore in recent years because of the emergence of antibiotic-resistant bacteria and the need to prevent the global rise of these superbugs. Increasing reports of siderophores functioning in capacities other than iron transport have appeared recently, but reports of "non-classical" siderophore functions have long paralleled those of iron transport. One particular non-classical function of these iron chelators, namely antibiotic activity, was documented before the role of siderophores in iron transport was established. In this Perspective, we present an exposition of past and current work into non-classical functions of siderophores and highlight the directions in which we anticipate that this research is headed. Examples include the ability of siderophores to function as zincophores, chalkophores, and metallophores for a variety of other metals, sequester heavy metal toxins, transport boron, act as signalling molecules, regulate oxidative stress, and provide antibacterial activity.
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Affiliation(s)
- Timothy C Johnstone
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
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Chaturvedi KS, Henderson JP. Pathogenic adaptations to host-derived antibacterial copper. Front Cell Infect Microbiol 2014; 4:3. [PMID: 24551598 PMCID: PMC3909829 DOI: 10.3389/fcimb.2014.00003] [Citation(s) in RCA: 88] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2013] [Accepted: 01/06/2014] [Indexed: 12/11/2022] Open
Abstract
Recent findings suggest that both host and pathogen manipulate copper content in infected host niches during infections. In this review, we summarize recent developments that implicate copper resistance as an important determinant of bacterial fitness at the host-pathogen interface. An essential mammalian nutrient, copper cycles between copper (I) (Cu(+)) in its reduced form and copper (II) (Cu(2+)) in its oxidized form under physiologic conditions. Cu(+) is significantly more bactericidal than Cu(2+) due to its ability to freely penetrate bacterial membranes and inactivate intracellular iron-sulfur clusters. Copper ions can also catalyze reactive oxygen species (ROS) generation, which may further contribute to their toxicity. Transporters, chaperones, redox proteins, receptors and transcription factors and even siderophores affect copper accumulation and distribution in both pathogenic microbes and their human hosts. This review will briefly cover evidence for copper as a mammalian antibacterial effector, the possible reasons for this toxicity, and pathogenic resistance mechanisms directed against it.
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Affiliation(s)
- Kaveri S Chaturvedi
- Division of Infectious Diseases, Department of Internal Medicine, Center for Women's Infectious Diseases Research, Washington University School of Medicine St. Louis, MO, USA
| | - Jeffrey P Henderson
- Division of Infectious Diseases, Department of Internal Medicine, Center for Women's Infectious Diseases Research, Washington University School of Medicine St. Louis, MO, USA
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Gaonkar T, Bhosle S. Effect of metals on a siderophore producing bacterial isolate and its implications on microbial assisted bioremediation of metal contaminated soils. CHEMOSPHERE 2013; 93:1835-1843. [PMID: 23838040 DOI: 10.1016/j.chemosphere.2013.06.036] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/27/2012] [Revised: 05/26/2013] [Accepted: 06/07/2013] [Indexed: 05/28/2023]
Abstract
A bacterial isolate producing siderophore under iron limiting conditions, was isolated from mangroves of Goa. Based on morphological, biochemical, chemotaxonomical and 16S rDNA studies, the isolate was identified as Bacillus amyloliquefaciens NAR38.1. Preliminary characterization of the siderophore indicated it to be catecholate type with dihydroxy benzoate as the core component. Optimum siderophore production was observed at pH 7 in mineral salts medium (MSM) without any added iron with glucose as the carbon source. Addition of NaCl in the growth medium showed considerable decrease in siderophore production above 2% NaCl. Fe(+2) and Fe(+3) below 2 μM and 40 μM concentrations respectively, induced siderophore production, above which the production was repressed. Binding studies of the siderophore with Fe(+2) and Fe(+3) indicated its high affinity towards Fe(+3). The siderophore concentration in the extracellular medium was enhanced when MSM was amended with essential metals Zn, Co, Mo and Mn, however, decreased with Cu, while the concentration was reduced with abiotic metals As, Pb, Al and Cd. Significant increase in extracellular siderophore production was observed with Pb and Al at concentrations of 50 μM and above. The effect of metals on siderophore production was completely mitigated in presence of Fe. The results implicate effect of metals on the efficiency of siderophore production by bacteria for potential application in bioremediation of metal contaminated iron deficient soils especially in the microbial assisted phytoremediation processes.
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Affiliation(s)
- Teja Gaonkar
- Department of Microbiology, Goa University, Taleigao Plateau, Goa 403 206, India
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Herter S, Schmidt M, Thompson ML, Mikolasch A, Schauer F. Investigating the effects of metals on phenol oxidase-producing nitrogen-fixing Azotobacter chroococcum. J Basic Microbiol 2012; 53:509-17. [PMID: 22961388 DOI: 10.1002/jobm.201100443] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2011] [Accepted: 03/03/2012] [Indexed: 11/05/2022]
Abstract
Expression of phenol oxidases (PO) in bacteria is often observed during physiological and morphological changes; in the nitrogen-fixing strain Azotobacter chroococcum SBUG 1484, it is accompanied by the formation of encysted cells and melanin. Herein, we studied the effects of copper and the depletion of the nitrogenase-relevant metals molybdenum and iron on physiological characteristics such as culture pigmentation, release of ortho-dihydroxylated melanin precursors, and expression of PO activity in A. chroococcum. Biomass production and melanogenic appearance were directly affected by the depletion of either iron or molybdenum, or in the absence of both metals. Only nitrogen-fixing cells growing in the presence of both metals and cultures supplemented with iron (molybdenum starved) showed the ability to produce an intensively brown-black melanin pigment typically associated with A. chroococcum. Accordingly, PO production was only detected in the presence of both metals and in iron-supplemented cultures starved of molybdenum. The total amount of catecholate siderophores produced by nitrogen-fixing melanogenic cells was considerably higher than in cultures starved of metal ions. Induction of enhanced PO activity was stimulated by additional copper sulfate, possibly related to cellular processes involved in the detoxification of this particular metal, and revealed distinct release of the ortho-dihydroxylated melanin precursors catechol and 3,4-dihydroxybenzoic acid.
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Affiliation(s)
- Susanne Herter
- Institute of Microbiology, Department of Applied Microbiology, University of Greifswald, Greifswald, Germany.
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Chaturvedi KS, Hung CS, Crowley JR, Stapleton AE, Henderson JP. The siderophore yersiniabactin binds copper to protect pathogens during infection. Nat Chem Biol 2012; 8:731-6. [PMID: 22772152 PMCID: PMC3600419 DOI: 10.1038/nchembio.1020] [Citation(s) in RCA: 205] [Impact Index Per Article: 17.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2012] [Accepted: 06/05/2012] [Indexed: 12/13/2022]
Abstract
Bacterial pathogens secrete chemically diverse iron chelators called siderophores, which may exert additional distinctive functions in vivo. Among these, uropathogenic Escherichia coli often coexpress the virulence-associated siderophore yersiniabactin (Ybt) with catecholate siderophores. Here we used a new MS screening approach to reveal that Ybt is also a physiologically favorable Cu(II) ligand. Direct MS detection of the resulting Cu(II)-Ybt complex in mice and humans with E. coli urinary tract infections demonstrates copper binding to be a physiologically relevant in vivo interaction during infection. Ybt expression corresponded to higher copper resistance among human urinary tract isolates, suggesting a protective role for this interaction. Chemical and genetic characterization showed that Ybt helps bacteria resist copper toxicity by sequestering host-derived Cu(II) and preventing its catechol-mediated reduction to Cu(I). Together, these studies reveal a new virulence-associated function for Ybt that is distinct from iron binding.
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Affiliation(s)
- Kaveri S. Chaturvedi
- Center for Women’s Infectious Diseases Research, Washington University School of Medicine, St. Louis, Missouri, United States of America
- Division of Infectious Diseases, Washington University School of Medicine, St. Louis, Missouri, United States of America
- Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Chia S. Hung
- Center for Women’s Infectious Diseases Research, Washington University School of Medicine, St. Louis, Missouri, United States of America
- Division of Infectious Diseases, Washington University School of Medicine, St. Louis, Missouri, United States of America
- Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Jan R. Crowley
- Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Ann E. Stapleton
- Department of Medicine, Division of Allergy and Infectious Diseases, University of Washington, Seattle, Washington, United States of America
| | - Jeffrey P. Henderson
- Center for Women’s Infectious Diseases Research, Washington University School of Medicine, St. Louis, Missouri, United States of America
- Division of Infectious Diseases, Washington University School of Medicine, St. Louis, Missouri, United States of America
- Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri, United States of America
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Nakouti I, Sihanonth P, Hobbs G. A new approach to isolating siderophore-producing actinobacteria. Lett Appl Microbiol 2012; 55:68-72. [PMID: 22537552 DOI: 10.1111/j.1472-765x.2012.03259.x] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
AIMS This study was conducted to investigate the application of 2,2'-dipyridyl as a new approach to isolating siderophore-producing actinobacteria. METHODS AND RESULTS Isolation of actinobacteria from soil was conducted by a soil dilution plate technique using starch-casein agar. Iron starvation was fostered by the incorporation of the iron chelator 2,2'-dipyridyl in the isolation medium. Pretreatment of the samples at an elevated temperature (40°C) ensured that the majority of nonsporulating bacteria were excluded. The survivors of this treatment were largely actinobacteria. Of the viable cultures grown in the presence of 2,2'-dipyridyl, more than 78-88% (average of three separate studies) were reported to produce siderophore-like compounds compared to 13-18% (average of three separate studies) when grown on the basic media in the absence of the chelating agent. The most prolific producers as assessed by the chrome azurol sulphate (CAS) assay were further characterized and found to belong to the genus Streptomyces. CONCLUSIONS Selective pressure using 2,2'-dipyridyl as an iron-chelating agent in starch-casein media increased the isolation of siderophore-producing actinobacteria compared to the unamended medium. SIGNIFICANCE AND IMPACT OF THE STUDY The study described represents a new approach to the isolation of siderophore-producing actinobacteria using a novel procedure that places a selection on cell population based upon the incorporation of a chelating agent in the medium.
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Affiliation(s)
- I Nakouti
- School of Pharmacy and Biomolecular Sciences, Liverpool John Moores University, Liverpool, UK
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Xiao R, Kisaalita WS. Purification of Pyoverdines of Pseudomonas fluorescens 2-79 by Copper-Chelate Chromatography. Appl Environ Microbiol 2010; 61:3769-74. [PMID: 16535157 PMCID: PMC1388593 DOI: 10.1128/aem.61.11.3769-3774.1995] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Three pyoverdines, Pf-A, Pf-B, and Pf-C, were purified with copper-chelate Sepharose and Sephadex G-15 columns from Pseudomonas fluorescens 2-79, and the yields (per 100 ml of culture supernatant) were 2.8, 21.6, and 3.2 mg, respectively. The absorption and fluorescence spectra of these pyoverdines were strongly pH dependent. Characteristic changes in the maximal absorbance wavelengths were observed when Fe(sup3+) or Cu(sup2+) was added. The addition of Cu(sup2+) shifted the pyoverdine Pf-B absorbance spectrum so that it exhibited a single peak at 410 nm but did not give rise to a new absorbance maximum at approximately 460 nm, which appeared when Fe(sup3+) was added. Fluorescence quenching experiments revealed that the forward reaction rate constant with pyoverdines was much higher with Cu(sup2+) (10(sup4) to 10(sup5) M(sup-1) s(sup-1)) than with Fe(sup3+) (10(sup2) M(sup-1) s(sup-1)). However, Cu(sup2+)-pyoverdine complexes were completely dissociated by EDTA at a low concentration (0.1 mM), while the level of Fe(sup3+)-pyoverdine complex dissociation at the same EDTA concentration was relatively low. The dissociation of Fe(sup3+)-pyoverdine complexes was EDTA concentration dependent. Formation of free pyoverdine was observed when the three types of Fe(sup3+)-pyoverdine complexes were incubated separately with P. fluorescens 2-79 cells, thus demonstrating that pyoverdines Pf-A, Pf-B, and Pf-C mediate iron transport.
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Hu X, Boyer GL. Siderophore-Mediated Aluminum Uptake by Bacillus megaterium ATCC 19213. Appl Environ Microbiol 2010; 62:4044-8. [PMID: 16535439 PMCID: PMC1388977 DOI: 10.1128/aem.62.11.4044-4048.1996] [Citation(s) in RCA: 82] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The bacterium Bacillus megaterium ATCC 19213 is known to produce two hydroxamate siderophores, schizokinen and N-deoxyschizokinen, under iron-limited conditions. In addition to their high affinity for ferric ions, these siderophores chelate aluminum. Aluminum was absorbed by B. megaterium ATCC 19213 through the siderophore transport receptor, providing an extra pathway for aluminum accumulation into iron-deficient bacteria. At low concentrations of the metal, siderophore-mediated uptake was the dominant process for aluminum accumulation. At high concentrations of aluminum, passive transport dominated and siderophore production slowed the passive transport of aluminum into the cell. Siderophore production was affected by the aluminum content in the media. High concentrations of aluminum increased production of siderophores in iron-limited cultures, and this production continued into stationary phase. Aluminum did not stimulate siderophore production in iron-replete cultures. The production of siderophores markedly affected aluminum uptake. This has direct implications on the toxicity of heavy metals under iron-deficient conditions.
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Illmer P, Buttinger R. Interactions between iron availability, aluminium toxicity and fungal siderophores. Biometals 2006; 19:367-77. [PMID: 16841246 DOI: 10.1007/s10534-005-3496-1] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2005] [Accepted: 09/24/2005] [Indexed: 11/24/2022]
Abstract
The influence of iron, aluminium and of the combined application of both metals on microbial biomass and production of siderophores by three fungi (Aspergillus nidulans, Neurospora crassa and Hymenoscyphus ericae) were investigated. All three species showed a strong iron regulation and Al-sensitivity of siderophore biosynthesis although several differences remained species dependent. Inhibitory effects of Fe and Al on siderophore-production were additive and the higher binding capacity of siderophores towards iron could be compensated by a higher Al-availability. Although pH itself is also important for regulation of siderophore biosynthesis, an indirect effect of Al on siderophore production via an Al-induced pH decrease could be outlined. The toxic effects of Al resulting in a reduced biomass production were compensated by high Fe-availability, whereas the addition of DFAM, a bacterial siderophore, enhanced Al-toxicity.
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Affiliation(s)
- Paul Illmer
- Institute of Microbiology, University of Innsbruck, Technikerstr. 25, A-6020, Innsbruck, Austria.
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Zou G, Boyer GL. Synthesis and properties of different metal complexes of the siderophore desferriferricrocin. Biometals 2005; 18:63-74. [PMID: 15865411 DOI: 10.1007/s10534-004-5786-4] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Desferriferricrocin is a cyclic hexa-peptide siderophore with three hydroxamates as primary coordination groups. It forms metal complexes with Fe(III), Cr(III), Al(III), Ga(III), Cu(II), and Zn(II). These complexes were prepared and characterized using UV-vis, circular dichroism spectroscopy (CD), nuclear magnetic resonance spectroscopy (NMR), and electrospray ionization mass spectroscopy (ESI-MS). The mononuclear trivalent metal complexes of desferriferricrocin were stable in aqueous solutions, and their coordination centers primarily adopted the lambda configuration. The formation of multinuclear complexes of desferriferricrocin was determined by ESI-MS. Desferriferricrocin was able to bind up to three Cu(II) and two Zn(II) respectively. Heteronuclear complexes containing one trivalent and one divalent were also determined. In these complexes, amide nitrogens were utilized as alternative binding groups of desferriferricrocin in addition to the primary binding groups, the hydroxamates.
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Affiliation(s)
- Guozhang Zou
- Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, New York 13210, USA
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Sebat JL, Paszczynski AJ, Cortese MS, Crawford RL. Antimicrobial properties of pyridine-2,6-dithiocarboxylic acid, a metal chelator produced by Pseudomonas spp. Appl Environ Microbiol 2001; 67:3934-42. [PMID: 11525988 PMCID: PMC93112 DOI: 10.1128/aem.67.9.3934-3942.2001] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Pyridine-2,6-dithiocarboxylic acid (pdtc) is a metal chelator produced by Pseudomonas spp. It has been shown to be involved in the biodegradation of carbon tetrachloride; however, little is known about its biological function. In this study, we examined the antimicrobial properties of pdtc and the mechanism of its antibiotic activity. The growth of Pseudomonas stutzeri strain KC, a pdtc-producing strain, was significantly enhanced by 32 microM pdtc. All nonpseudomonads and two strains of P. stutzeri were sensitive to 16 to 32 microM pdtc. In general, fluorescent pseudomonads were resistant to all concentrations tested. In competition experiments, strain KC demonstrated antagonism toward Escherichia coli. This effect was partially alleviated by 100 microM FeCl3. Less antagonism was observed in mutant derivatives of strain KC (CTN1 and KC657) which lack the ability to produce pdtc. A competitive advantage was restored to strain CTN1 by cosmid pT31, which restores pdtc production. pT31 also enhanced the pdtc resistance of all pdtc-sensitive strains, indicating that this plasmid contains elements responsible for resistance to pdtc. The antimicrobial effect of pdtc was reduced by the addition of Fe(III), Co(III), and Cu(II) and enhanced by Zn(II). Analyses by mass spectrometry determined that Cu(I):pdtc and Co(III):pdtc2 form immediately under our experimental conditions. Our results suggest that pdtc is an antagonist and that metal sequestration is the primary mechanism of its antimicrobial activity. It is also possible that Zn(II), if present, may play a role in pdtc toxicity.
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Affiliation(s)
- J L Sebat
- Environmental Biotechnology Institute and Department of Microbiology, Molecular Biology & Biochemistry, University of Idaho, Moscow, Idaho 83844-1052, USA
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Clarke SE, Stuart J, Sanders-Loehr J. Induction of siderophore activity in Anabaena spp. and its moderation of copper toxicity. Appl Environ Microbiol 1987; 53:917-22. [PMID: 2955743 PMCID: PMC203786 DOI: 10.1128/aem.53.5.917-922.1987] [Citation(s) in RCA: 72] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
Growth of Anabaena sp. strain 7120 (in the absence of chelators or added iron) was inhibited by the addition of 2.1 to 6.5 microM copper and was abolished by copper concentration of 10 microM or higher. When the copper was chelated to schizokinen (the siderophore produced by this organism in response to iron starvation), the toxic effects were eliminated. Analysis of culture filtrates showed that the cupric schizokinen remains in the medium, thereby lowering the amount of copper taken up by the cells. Although this organism actively transports ferric schizokinen, it apparently does not recognize the cupric complex. Thus, Anabaena sp. is protected from copper toxicity under conditions in which siderophore is being produced. For cells grown in low iron, the accumulation of extracellular schizokinen was observed to parallel cell growth and continue well into stationary phase. The actual iron status of the organism was monitored by using iron uptake velocity as an assay. Cultures grown on 0.1 microM added iron were found to be severely iron limited upon reaching stationary phase, thus explaining the continued production of schizokinen. These data show that the siderophore system in Anabaena spp. has developed primarily as a response to iron starvation and that additional functions such as alleviation of copper toxicity or allelopathic inhibition of other algal species are merely secondary benefits.
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