Flocking asbestos waste, an iron and magnesium source for Pseudomonas

Our plain-English summary of Flocking asbestos waste, an iron and magnesium source for Pseudomonas, by S.R. David, S. Fritsch, A. Forster, et al (The Science of the Total Environment, 2020).

The study

• Authors: S.R. David, S. Fritsch, A. Forster, D. Ihiawakrim, V.A. Geoffroy
• Journal: The Science of the Total Environment
• Published: 2020;709:135936
• Cited by: 12 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Iron and magnesium are essential nutrients for most microorganisms. In the environment, the availability of iron is low relative to that of magnesium. Microorganisms have developed various iron acquisition systems, which have been well studied, whereas few studies have examined magnesium acquisition. The production of siderophores is one of the efficient strategies widely used to sustain iron nutritional requirements. Many studies have shown that minerals, such as clays, iron oxides, and silicates, can serve as nutrient sources for bacteria. Asbestos, a natural fibrous silicate present in soil contains iron and/or magnesium, depending on the species of asbestos. Our aim was to study the acquisition of iron and magnesium from flocking asbestos waste by Pseudomonas aeruginosa and the involvement of the siderophores, pyoverdine and pyochelin. Flocking asbestos waste promoted growth under iron- and magnesium-limited conditions, together with a decrease in pyoverdine production, correlating with the dissolution of iron from the waste. In long-term experiments, flocking asbestos waste provided these two essential elements for bacterial growth and resulted in a decrease of iron in asbestos fibers. Among the enzymes required for pyochelin and pyoverdine synthesis, PchA and PvdJ were tagged with the fluorescent protein mCherry to analyze the expression patterns of proteins involved in siderophore production. Both enzymes were produced in the presence of flocking asbestos waste, suggesting a role of the pyoverdine and pyochelin pathway in asbestos dissolution. We investigated the involvement of each siderophore in iron and magnesium removal using mutants in one or both siderophore pathways. We observed a significant increase in iron extraction in the presence of siderophores and the absence of one of the two siderophores could be compensated by the other. Flocking asbestos waste represents an iron and magnesium source for P. aeruginosa, with iron removal linked to a siderophore-driven mechanism.

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In plain English

This laboratory study tested whether the bacterium Pseudomonas aeruginosa can take iron and magnesium from flocking asbestos waste, a chrysotile-gypsum insulation material from a Paris asbestos removal site. In a magnesium-free medium, the bacteria grew only once the waste was added. With the waste, they made 27 µM of the iron-gathering molecule pyoverdine, compared with 183 µM in a magnesium-containing medium, where growth was similar. Total iron released from the waste, in cells and liquid combined, was 0.27 mg per liter with bacteria versus 0.06 mg per liter in sterile medium. Electron microscopy showed less iron in the chrysotile fibers after long-term contact with the bacteria. Strains lacking either pyoverdine or a second such molecule, pyochelin, released about as much iron as normal bacteria; one lacking both released 0.09 mg per liter, similar to the control. Dissolved magnesium did not differ with or without bacteria; the authors suggested the medium was likely responsible. The authors concluded that the waste can serve as a nutrient source for the bacteria, suggesting an eco-friendly treatment for loose asbestos waste.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiments with bacterial cultures, including genetically modified bacterial strains and electron microscopy of fibers
• Who was studied: Pseudomonas aeruginosa strain PAO1 (wild type), mutants lacking pyoverdine, pyochelin, or both, and strains with fluorescent-tagged enzymes, grown with flocking asbestos waste from the Jussieu University of Paris asbestos removal site
• Exposure: Flocking asbestos waste (chrysotile-gypsum), an insulation material described as chrysotile fibers (5 to 90%) and gypsum (10 to 95%)
• Main results:
  • Without magnesium and without the waste, the bacteria did not grow over 24 hours.
  • With the waste added to the magnesium-depleted medium, the bacteria grew, which the authors took as use of magnesium from the waste.
  • Pyoverdine production with the waste was 27 µM. The positive control medium (with magnesium) produced 183 µM, with similar growth. The magnesium-depleted medium without the waste produced 2 µM, where the bacteria did not grow.
  • Total iron released from the waste was 0.27 mg per liter with bacteria, counting bacterial cells and supernatant together, versus 0.06 mg per liter for the waste in sterile medium. Of this total, 0.09 mg per liter was in the supernatant and 0.18 mg per liter in the bacterial cells.
  • No difference was found in dissolved magnesium with or without bacteria. The authors suggested that the bacteria may not influence magnesium dissolution and that the medium was likely responsible.
  • Over 16 repeated 8-hour cycles, bacterial numbers were higher with the waste in cycles C1 to C4 and again in C7 and C8, but not in the other cycles.
  • Electron microscopy (STEM-EDX) of chrysotile fibers showed a decrease in iron content after long-term contact with the bacteria. Magnesium content was similar with or without bacteria.
  • With the waste, fluorescence of the tagged siderophore-making enzymes was 547 (PvdJ) and 785 (PchA). In the positive control it was 1,064 and 1,331, and in magnesium-depleted medium 143 and 309.
  • Iron dissolved from the waste was 0.28 mg per liter with wild-type bacteria, 0.26 mg per liter with the pyoverdine-deficient mutant and 0.23 mg per liter with the pyochelin-deficient mutant. The control medium gave 0.08 mg per liter.
  • The mutant lacking both siderophores released 0.09 mg per liter iron, similar to the control.
  • Dissolved magnesium was not significantly different between the wild-type and mutant strains.
• The authors' conclusion: The authors concluded that flocking asbestos waste can serve as a nutrient source for the bacteria, with iron release driven by siderophores. They suggested that siderophore-producing Pseudomonas could be an eco-friendly bioremediation process for loose asbestos waste.

Topics

Medical terms in this study, explained in our Lexicon: Iron Acquisition, Silicate.

Related studies

• Engineering siderophore production in Pseudomonas to improve asbestos weathering — M. Lemare, H. Puja, et al. (2022)
• Iron removal from raw asbestos by siderophores-producing Pseudomonas — S.R. David, D. Ihiawakrim, et al. (2020)
• Biodeterioration of asbestos cement by siderophore-producing Pseudomonas — S.R. David, A. Jaouen, et al. (2021)
• Magnesium and silicon recovery from chrysotile asbestos waste of the deposit Zhitikara, Kazakhstan — R.A. Shayakhmetova, A.A. Mukhametzhanova, et al. (2024)
• Serpentine endophytic bacterium Pseudomonas azotoformans ASS1 accelerates phytoremediation of soil metals under drought stress — Y. Ma, M. Rajkumar, et al. (2017)
• A Review of Asbestos Bioweathering by Siderophore-Producing Pseudomonas: A Potential Strategy of Bioremediation — S.R. David, V.A. Geoffroy (2020)

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