Engineering siderophore production in Pseudomonas to improve asbestos weathering

Our plain-English summary of Engineering siderophore production in Pseudomonas to improve asbestos weathering, by M. Lemare, H. Puja, S.R. David, et al (Microbial Biotechnology, 2022).

The study

• Authors: M. Lemare, H. Puja, S.R. David, S. Mathieu, D. Ihiawakrim, V.A. Geoffroy, C. Rigouin
• Journal: Microbial Biotechnology
• Published: 2022;15(9):2351-2363
• Cited by: 12 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Iron plays a key role in microbial metabolism and bacteria have developed multiple siderophore-driven mechanisms due to its poor bioavailability for organisms in the environment. Iron-bearing minerals generally serve as a nutrient source to sustain bacterial growth after bioweathering. Siderophores are high-affinity ferric iron chelators, of which the biosynthesis is tightly regulated by the presence of iron. Pyoverdine-producing Pseudomonas have shown their ability to extract iron and magnesium from asbestos waste as nutrients. However, such bioweathering is rapidly limited due to repression of the pyoverdine pathway and the low bacterial requirement for iron. We developed a metabolically engineered strain of Pseudomonas aeruginosa for which pyoverdine production was no longer repressed by iron as a proof of concept. We compared siderophore-promoted dissolution of flocking asbestos waste by this optimized strain to that by the wild-type strain. Interestingly, pyoverdine production by the optimized strain was seven times higher in the presence of asbestos waste and the dissolution of magnesium and iron from the chrysotile fibres contained in flocking asbestos waste was significantly enhanced. This innovative mineral weathering process contributes to remove toxic iron from the asbestos fibres and may contribute to the development of an eco-friendly method to manage asbestos waste.

© 2022 The Authors. Microbial Biotechnology published by Society for Applied Microbiology and John Wiley & Sons Ltd.

In plain English

This laboratory study engineered Pseudomonas aeruginosa so that pyoverdine production was no longer switched off by iron, then tested whether this improved weathering of flocking asbestos waste (chrysotile fibres in a gypsum matrix) from an asbestos removal site in Paris. In the mutant strain PaM1 the iron-responsive promoter region of pvdS was replaced by an arabinose-inducible promoter. In rich medium pyoverdine reached 143 plus or minus 12 mg per litre for wild-type PAO1 and 574 plus or minus 10 mg per litre for PaM1 when induced by 0.5% arabinose; its production was independent of iron but relied on the arabinose inducer. Bacterial suspensions of 20 ml were added to 0.2 g samples of the waste for five 24-hour cycles with arabinose. At the end of the five cycles pyoverdine production by PaM1 was roughly seven times higher than the wild-type strain, and more metal was extracted: iron 64 plus or minus 8 micrograms by PAO1 (7.0 plus or minus 0.9%) versus 100 plus or minus 10 micrograms by PaM1 (10.9 plus or minus 1.1%), and magnesium 534 plus or minus 128 micrograms (3.6 plus or minus 0.9%) versus 1004 plus or minus 125 micrograms (6.8 plus or minus 0.8%). The authors concluded this is an original approach to improve bacterial alteration of asbestos.

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 experiment: construction of a metabolically engineered bacterial strain and in vitro asbestos-waste weathering assays
• Who was studied: Two Pseudomonas aeruginosa strains, the wild-type PAO1 and the engineered mutant PaM1, grown with flocking asbestos waste obtained from an asbestos removal site at Jussieu University, Paris (France); experiments in triplicate over five 24-hour cycles
• Exposure: 20 ml bacterial suspensions added to 0.2 g samples of flocking asbestos waste consisting of chrysotile fibres in a gypsum matrix, in magnesium-free casamino acid medium with arabinose induction
• Main results:
  • In rich medium, total pyoverdine was 143 plus or minus 12 mg per litre for PAO1 and 574 plus or minus 10 mg per litre for arabinose-induced PaM1; in minimal iron-depleted medium with arabinose, PAO1 produced 529 plus or minus 9 and PaM1 574 plus or minus 4 mg per litre
  • Pyoverdine production by PaM1 was independent of iron and depended on arabinose in a dose-dependent manner
  • In PAO1, pvdS transcripts were 123-fold higher without iron than with iron
  • Adding 0.5% arabinose raised pvdS transcript levels in PaM1 approximately 1100-fold, significantly higher than in PAO1 without iron
  • Higher pvdS transcript levels in PaM1 were not matched by increased pvdD or pvdA transcription compared with induced PAO1
  • With flocking asbestos waste, PaM1 pyoverdine reached 523 plus or minus 23 mg per litre at cycle 2, and at the end of the five cycles was about 7-fold higher than the wild-type strain
  • After five cycles, iron extracted from the 0.2 g asbestos waste samples was 64 plus or minus 8 micrograms by PAO1 (7.0 plus or minus 0.9%) and 100 plus or minus 10 micrograms by PaM1 (10.9 plus or minus 1.1%)
  • Magnesium extracted was 534 plus or minus 128 micrograms by PAO1 (3.6 plus or minus 0.9%) and 1004 plus or minus 125 micrograms by PaM1 (6.8 plus or minus 0.8%)
  • STEM-EDX of chrysotile fibres showed a lower iron mass percentage after alteration by PaM1 (0.9%) than by PAO1 (1.3%), with Fe/Si atomic ratios of 0.022 (PaM1) versus 0.029 (PAO1)
  • The authors estimated that only a minority of pyoverdine was iron-complexed: 23% for PAO1 and 12% for PaM1, with the seven-fold rise in pyoverdine giving only a two-fold rise in metal extraction
• The authors' conclusion: The authors concluded that they describe an original approach to improve the alteration of asbestos by bacteria, which they describe as low-energy and low-cost and potentially extendable to other asbestos wastes and bioleaching strategies
• Limitations the authors state:
  • The strain used, PAO1, is an opportunistic pathogen and the authors state it is not suitable for an innocuous waste-management process; work with non-pathogenic Pseudomonas putida is ongoing
  • The contribution of the second siderophore pyochelin to iron extraction was not assessed
  • Whether biofilm formation on the fibres explains the decline in pyoverdine and metal extraction over cycles was not determined
  • The arabinose inducer may perturb metabolism and be too costly at larger scale; other promoters remain to be tried

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile.

Related studies

• Biodeterioration of asbestos cement by siderophore-producing Pseudomonas — S.R. David, A. Jaouen, et al. (2021)
• Iron removal from raw asbestos by siderophores-producing Pseudomonas — S.R. David, D. Ihiawakrim, et al. (2020)
• Flocking asbestos waste, an iron and magnesium source for Pseudomonas — S.R. David, S. Fritsch, et al. (2020)
• Siderophore-mediated iron removal from chrysotile: Implications for asbestos toxicity reduction and bioremediation — S.K. Mohanty, C. Gonneau, et al. (2018)
• A Review of Asbestos Bioweathering by Siderophore-Producing Pseudomonas: A Potential Strategy of Bioremediation — S.R. David, V.A. Geoffroy (2020)
• Serpentine endophytic bacterium Pseudomonas azotoformans ASS1 accelerates phytoremediation of soil metals under drought stress — Y. Ma, M. Rajkumar, et al. (2017)

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