Bioactivity of Serratiochelin A, a Siderophore Isolated from a Co-Culture of Serratia sp. and Shewanella sp.

Siderophores are compounds with high affinity for ferric iron. Bacteria produce these compounds to acquire iron in iron-limiting conditions. Iron is one of the most abundant metals on earth, and its presence is necessary for many vital life processes. Bacteria from the genus Serratia contribute to the iron respiration in their environments, and previously several siderophores have been isolated from this genus. As part of our ongoing search for medicinally relevant compounds produced by marine microbes, a co-culture of a Shewanella sp. isolate and a Serratia sp. isolate, grown in iron-limited conditions, was investigated, and the rare siderophore serratiochelin A (1) was isolated with high yields. Compound 1 has previously been isolated exclusively from Serratia sp., and to our knowledge, there is no bioactivity data available for this siderophore to date. During the isolation process, we observed the degradation product serratiochelin C (2) after exposure to formic acid. Both 1 and 2 were verified by 1-D and 2-D NMR and high-resolution MS/MS. Here, we present the isolation of 1 from an iron-depleted co-culture of Shewanella sp. and Serratia sp., its proposed mechanism of degradation into 2, and the chemical and biological characterization of both compounds. The effects of 1 and 2 on eukaryotic and prokaryotic cells were evaluated, as well as their effect on biofilm formation by Staphylococcus epidermidis. While 2 did not show bioactivity in the given assays, 1 inhibited the growth of the eukaryotic cells and Staphylococcus aureus.

Dissociation between Iron and Heme Biosyntheses Is Largely Accountable for Respiration Defects ofShewanella oneidensis furMutants

ABSTRACTIron, a major protein cofactor, is essential for most organisms but can simultaneously be toxic. Iron homeostasis thus has to be effectively maintained under a range of iron regimes. This may be particularly true withShewanella oneidensis, a representative of dissimilatory metal-reducing bacteria (DMRB), which are capable of respiring a variety of chemicals as electron acceptors (EAs), including iron ores. Although iron respiration and its regulation have been extensively studied in this bacterium, how iron homeostasis is maintained remains largely unknown. Here, we report that the loss of the iron homeostasis master regulator Fur negatively affects the respiration of all EAs tested. This defect appears mainly to be a result of reduced cytochromec(cytc) production, despite a decrease in the expression of reductases that are under the direct control of Fur. We also show thatS. oneidensisFur interacts with canonical Fur box motifs in F-F-x-R configuration rather than the palindromic motif proposed before. Thefurmutant has lowered total iron and increased free iron contents. Under iron-rich conditions, overproduction of the major iron storage protein Bfr elevates the total iron levels of thefurmutant over those of the wild-type but does not affect free iron levels. Intriguingly, such an operation only marginally improves cytcproduction by affecting hemebbiosynthesis. It is established that iron dictates hemeb/cytcbiosynthesis inS. oneidensisfur+strains, but thefurmutation annuls the dependence of hemeb/cytcbiosynthesis on iron. Overall, our results suggest that Fur has a profound impact on the iron homeostasis ofS. oneidensis, through which many physiological processes, especially respiration, are transformed.IMPORTANCEIron reduction is a signature ofS. oneidensis, and this process relies on a large number of typeccytochromes, whichper seare iron-containing proteins. Thus, iron plays an essential and special role in iron respiration, but to date, the nature of iron metabolism and regulation of the bacterium remains largely unknown. In this study, we investigated impacts of Fur, the master regulator of iron homeostasis, on respiration. The loss of Fur causes a general defect in respiration, a result of impaired cytcproduction rather than specific regulation. Additionally, thefurmutant is unresponsive to iron, resulting in imbalanced iron homeostasis and dissociation between iron and cytcproduction. These findings provide important insights into the iron biology of DMRB.

A Ferrous Iron Exporter Mediates Iron Resistance in Shewanella oneidensis MR-1

ABSTRACTShewanella oneidensisstrain MR-1 is a dissimilatory metal-reducing bacterium frequently found in aquatic sediments. In the absence of oxygen,S. oneidensiscan respire extracellular, insoluble oxidized metals, such as iron (hydr)oxides, making it intimately involved in environmental metal and nutrient cycling. The reduction of ferric iron (Fe3+) results in the production of ferrous iron (Fe2+) ions, which remain soluble under certain conditions and are toxic to cells at higher concentrations. We have identified an inner membrane protein inS. oneidensis, encoded by the gene SO_4475 and here called FeoE, which is important for survival during anaerobic iron respiration. FeoE, a member of the cation diffusion facilitator (CDF) protein family, functions to export excess Fe2+from the MR-1 cytoplasm. Mutants lackingfeoEexhibit an increased sensitivity to Fe2+. The export function of FeoE is specific for Fe2+, as anfeoEmutant is equally sensitive to other metal ions known to be substrates of other CDF proteins (Cd2+, Co2+, Cu2+, Mn2+, Ni2+, or Zn2+). The substrate specificity of FeoE differs from that of FieF, theEscherichia colihomolog of FeoE, which has been reported to be a Cd2+/Zn2+or Fe2+/Zn2+exporter. A complementedfeoEmutant has an increased growth rate in the presence of excess Fe2+compared to that of the ΔfeoEmutant complemented withfieF. It is possible that FeoE has evolved to become an efficient and specific Fe2+exporter in response to the high levels of iron often present in the types of environmental niches in whichShewanellaspecies can be found.

Microbial effects on phosphorus release in aquatic sediments

Microbial effects on phosphorus release were studied for the sediments of Tianjin source water by controlling DO and pH. The results show that: (1) In sterilised water, phosphorus began to release when pH = 9.1 and the stable release rate was 9.51 mg/(d·m2). It indicates that microorganisms may utilise anaerobic iron respiration to release Fe-P. (2) With unsterilised water, phosphorus release rate is 2.14 mg/(d·m2) when pH = 6.5, 8.60 mg/(d·m2) when pH is uncontrolled, and gets to 8.51 mg/(d·m2) when pH = 9.1. This indicates that microorganisms can dissolve insoluble phosphates to accelerate the ion exchange of OH− and PO43−, which are derived from iron-bound ortho-P and aluminium-bound ortho-P.

Anaerobic Respiration Using Fe3+, S0, and H2 in the Chemolithoautotrophic Bacterium Acidithiobacillus ferrooxidans

ABSTRACT The chemolithoautotrophic bacterium Acidithiobacillus ferrooxidans has been known as an aerobe that respires on iron and sulfur. Here we show that the bacterium could chemolithoautotrophically grow not only on H2/O2 under aerobic conditions but also on H2/Fe3+, H2/S0, or S0/Fe3+ under anaerobic conditions. Anaerobic respiration using Fe3+ or S0 as an electron acceptor and H2 or S0 as an electron donor serves as a primary energy source of the bacterium. Anaerobic respiration based on reduction of Fe3+ induced the bacterium to synthesize significant amounts of a c-type cytochrome that was purified as an acid-stable and soluble 28-kDa monomer. The purified cytochrome in the oxidized form was reduced in the presence of the crude extract, and the reduced cytochrome was reoxidized by Fe3+. Respiration based on reduction of Fe3+ coupled to oxidation of a c-type cytochrome may be involved in the primary mechanism of energy production in the bacterium on anaerobic iron respiration.