Remedial Treatment of Corroded Iron Objects by Environmental Aeromonas Isolates

Abstract

Using bacteria to transform reactive corrosion products into stable compounds represents an alternative to traditional methods employed in iron conservation. Two environmental Aeromonas strains (CA23 and CU5) were used to transform ferric iron corrosion products (goethite and lepidocrocite) into stable ferrous iron-bearing minerals (vivianite and siderite). A genomic and transcriptomic approach was used to analyze the metabolic traits of these strains and to evaluate their pathogenic potential. Although genes involved in solid-phase iron reduction were identified, key genes present in other environmental iron-reducing species are missing from the genome of CU5. Several pathogenicity factors were identified in the genomes of both strains, but none of these was expressed under iron reduction conditions. Additional in vivo tests showed hemolytic and cytotoxic activities for strain CA23 but not for strain CU5. Both strains were easily inactivated using ethanol and heat. Nonetheless, given a lesser potential for a pathogenic lifestyle, CU5 is the most promising candidate for the development of a bio-based iron conservation method stabilizing iron corrosion. Based on all the results, a prototype treatment was established using archaeological items. On those, the conversion of reactive corrosion products and the formation of a homogenous layer of biogenic iron minerals were achieved. This study shows how naturally occurring microorganisms and their metabolic capabilities can be used to develop bio-inspired solutions to the problem of metal corrosion.IMPORTANCE Microbiology can greatly help in the quest for a sustainable solution to the problem of iron corrosion, which causes important economic losses in a wide range of fields, including the protection of cultural heritage and building materials. Using bacteria to transform reactive and unstable corrosion products into more-stable compounds represents a promising approach. The overall aim of this study was to develop a method for the conservation and restoration of corroded iron items, starting from the isolation of iron-reducing bacteria from natural environments. This resulted in the identification of a suitable candidate (Aeromonas sp. strain CU5) that mediates the formation of desirable minerals at the surfaces of the objects. This led to the proof of concept of an application method on real objects.

Keywords: Aeromonas; artifacts; corrosion; iron; reduction; vivianite.

FIG 1

Iron reduction and biogenic mineral formation on corroded iron plates treated with strains CA23 and CU5. (A) Visual appearance (top) and scanning electron microscopy (SEM) images (bottom) of the iron coupons treated with strains CA23 and CU5 after 1, 2, and 4 weeks of incubation compared to an untreated coupon. The red letters f, s, and c indicate foil-, sphere-, and cube-like aggregates, respectively. (B) X-ray diffraction (XRD) diffractograms of corroded iron coupons treated with CU5 and CA23 for 4 weeks, the abiotic controls (Ac), and the untreated coupons (NT). (C) Confocal microscopy images of iron coupons. Left, plate treated with strain CU5 for 4 weeks. Right, untreated plate.

FIG 2

Phylogenomic analyses and potential solid iron reduction pathway of the bacterial strains CA23 and CU5. (A) A cladogram indicating the phylogenetic positions of the two isolates CA23 and CU5 (green) within the genus Aeromonas. (B) Homologous genes identified in the genomes of CA23 and CU5, based on the analysis of the genome of A. hydrophila and the iron reduction pathway of S. oneidensis (25).

FIG 3

Investigation of the pathogenic potential of Aeromonas sp. strains CA23 and CU5. (A) Hemolysis and CAMP factor detection tests using Columbia sheep blood agar. (B) Measurements of the toxicology of Aeromonas sp. strains CA23 and CU5 on Caenorhabditis elegans strain N2 (ancestral). Nematode relative activity (compared to the control fed with Escherichia coli OP50 at an OD of 0.7) was measured after 48 h in contact with bacteria at different initial ODs. (C) Images showing the inactivation of the two bacterial strains after treatment with 23% and 35% ethanol (with different exposure times) and heat at 60°C overnight.

FIG 4

Optimized bacterial treatment of archaeological iron objects. (A) Visual aspect of the iron objects treated for 1 month. Left, iron artifacts partially treated with the bacterium CU5 under aerobic and anaerobic conditions and having their bottom parts used as abiotic controls (gel delivery system without bacteria). Right, a full iron object treated with CA23 under anaerobic conditions in comparison to untreated nails. (B) Microscopic images of an archeological nail treated for 2 months under anaerobic conditions with CU5. (C) Raman spectra obtained from the surfaces of the treated archaeological objects after 1 (gray line) and 2 (black line) months of treatment with CU5 under anaerobic conditions. Corrosion compounds were identified as goethite (Go), vivianite (Vi), and siderite (Si).