The Potential of Aloe vera and Opuntia ficus-indica Extracts as Biobased Agents for the Conservation of Cultural Heritage Metals

Abstract

Background/objectives: Biocorrosion, driven by microbial colonization and biofilm formation, poses a significant threat to the integrity of metal artifacts, particularly those composed of copper and its alloys. Pseudomonas aeruginosa, a bacterial species that reduces nitrates, plays a key role in this process. This study explores the potential of two metabolite-rich plant extracts, Aloe vera and Opuntia ficus-indica, as sustainable biobased inhibitors of microbial-induced corrosion (MICOR).

Methods: The antibacterial and antibiofilm activities of the extracts were evaluated using minimal inhibitory concentration (MIC) assays, time-kill kinetics, and biofilm prevention and removal tests on copper, bronze, and brass samples. Spectrophotometric and microbiological methods were used to quantify bacterial growth and biofilm density.

Results: Both extracts exhibited significant antibacterial activity, with MIC values of 8.3% (v/v). A. vera demonstrated superior bactericidal effects, achieving reductions of ≥3 log₁₀ in bacterial counts at lower concentrations. In antibiofilm assays, both extracts effectively prevented biofilm formation and reduced established biofilms, with A. vera exhibiting greater efficacy against them. The active metabolites-anthraquinones, phenolics, flavonoids, and tannins-likely contribute to these effects.

Conclusions: These findings highlight the dual role of A. vera and O. ficus-indica extracts as both corrosion and biocorrosion inhibitors. The secondary metabolite profiles of these plants support their application as eco-friendly alternatives in the conservation of metal cultural heritage objects.

Keywords: Aloe vera (L.) Burm.f.; Opuntia ficus-indica (L.) mill.; Pseudomonas aeruginosa; antibacterial activity; biocorrosion; biofilm inhibition; copper alloys; cultural heritage conservation; natural corrosion inhibitors; plant secondary metabolites.

Figure 1

MIC analysis for (a) O. ficus-indica and (b) A. vera extracts/P.A: P. aeruginosa/1.5: 2.075 (v/v); 2.6: 4.15 (v/v); 3.7: 8.3 (v/v); 4.8: 16.6 (v/v). NC: negative control—PC: positive control.

Figure 2

Time kill profiles for samples treated with solutions of O. ficus-indica. (a) by spectrophotometric analysis and (b) by the spread plate method.

Figure 3

Time kill profiles for samples treated with solutions of A. vera. (a) by spectrophotometric analysis and (b) by the spread plate method.

Figure 4

Bacterial densities on (a) brass samples, (b) copper samples, and (c) bronze samples. Composition of treatment solutions: B1, C1, Br 1: Bacteria/B2, C2, Br2: Bacteria + Antibiotic/B3, C3, Br3: Bacteria + O. ficus-indica extract/B4, C4, Br4: Bacteria + A. vera extract).

Figure 5

Stained bacterial biofilm from (a) copper samples, (b) brass samples, and (c) bronze samples.

Figure 6

Metal surface condition after biofilm removal and control samples of entreated metals, Bronze (Br), Copper (Cu), and Brass (B). Control samples bacteria: P. aeruginosa./1: Bacteria, 2: Bacteria + Antibiotic, 3: Bacteria + O. ficus-indica extract, 4: Bacteria + A. vera extract.