Pyridine-2,6-Bis(Thiocarboxylic Acid) Produced by Pseudomonas stutzeri KC Reduces and Precipitates Selenium and Tellurium Oxyanions

Abstract

The siderophore of Pseudomonas stutzeri KC, pyridine-2,6-bis(thiocarboxylic acid) (pdtc), is shown to detoxify selenium and tellurium oxyanions in bacterial cultures. A mechanism for pdtc's detoxification of tellurite and selenite is proposed. The mechanism is based upon determination using mass spectrometry and energydispersive X-ray spectrometry of the chemical structures of compounds formed during initial reactions of tellurite and selenite with pdtc. Selenite and tellurite are reduced by pdtc or its hydrolysis product H 2 S, forming zero-valent pdtc selenides and pdtc tellurides that precipitate from solution. These insoluble compounds then hydrolyze, releasing nanometer-sized particles of elemental selenium or tellurium. Electron microscopy studies showed both extracellular precipitation and internal deposition of these metalloids by bacterial cells. The precipitates formed with synthetic pdtc were similar to those formed in pdtc-producing cultures of P. stutzeri KC. Culture filtrates of P. stutzeri KC containing pdtc were also active in removing selenite and precipitating elemental selenium and tellurium. The pdtc-producing wild-type strain KC conferred higher tolerance against selenite and tellurite toxicity than a pdtc-negative mutant strain, CTN1. These observations support the hypothesis that pdtc not only functions as a siderophore but also is involved in an initial line of defense against toxicity from various metals and metalloids.Siderophores are iron-specific chelators that are produced and excreted by microorganisms under iron-limiting conditions as a part of an iron acquisition system. Some siderophores chelate metals other than iron, forming soluble or insoluble metal compounds and affecting the mobility and toxicity of those metals (8,17,47). Pyridine-2,6-bis(thiocarboxylic acid) (pdtc) is a siderophore produced by Pseudomonas stutzeri KC and P. putida DSM 3601 and DSM 3602 that has fortuitous carbon tetrachloride degradation activity (21, 30). Recent research has shown that pdtc promotes iron transport into the cell (25). P. stutzeri KC, its spontaneous pdtc-negative mutant CTN1, and other P. stutzeri strains also produce proferrioxamines (pFOs), which are probably their primary siderophores (A. Zawadzka et al., unpublished data). pdtc is a broad-range metal chelator; it chelates many transition metals, some heavy metals, lanthanides, and actinides. In general, micronutrient metals chelated by pdtc are soluble, while toxic metals form insoluble precipitates with pdtc (8, 39). Selenium and tellurium oxyanions are among the toxic metalloids precipitated by pdtc and pdtc-producing Pseudomonas stutzeri KC cultures.

Roles of Siderophores, Oxalate, and Ascorbate in Mobilization of Iron from Hematite by the Aerobic Bacterium Pseudomonas mendocina

“…The ymp strain was also shown to have endogenous Fe(III)-reducing activity, which Hersman et al suggested could be involved in solubilizing ferric minerals (24). Likewise, closely related strains of Pseudomonas stutzeri have been shown to produce pyridine-2,6-bis(thiocarboxylic acid) (PDTC), which they can use in the reduction, transport, and detoxification of metals and metalloids (11,16,51). However, control experiments showed that P. mendocina did not secrete molecules that exhibited a significant amount of reducing activity under the conditions used in this study (see the supplemental material).…”

Thiol-Mediated Reduction of <i>Staphylococcus aureus</i> Tellurite Resistance

“…Nevertheless, lower 2-ME concentrations do not generate such a reduction, and the interaction between these two compounds is unknown. Zawadzka and co-workers reported that the interaction between tellurite and the SH-containing compound pyridine-2,6-bis(thiocarboxylic acid) resulted in the generation of a Te-containing intermediate that is reduced to elemental tellurium, sulfur and dpa [16]. The formation of a similar intermediate may occur during tellurite and 2-ME interaction.…”

Iron Transport and Signaling in Pseudomonads

“…PDTC contributes to the chelation and uptake of zinc [44] as well as other transition metals [45]. It also has activity in dechlorination reactions [46], and a role for PDTC in the defence against toxicity from various metals and metalloids has been proposed [47,48]. P. stutzeri can synthesize multiple siderophores including desferriferrioxamine E (also known as nocardamin) [49].…”