Antibacterial Effect of Zinc Oxide-Based Nanomaterials on Environmental Biodeteriogens Affecting Historical Buildings

Abstract

The colonization of microorganisms and their subsequent interaction with stone substrates under different environmental conditions encourage deterioration of materials by multiple mechanisms resulting in changes in the original color, appearance and durability. One of the emerging alternatives to remedy biodeterioration is nanotechnology, thanks to nanoparticle properties such as small size, no-toxicity, high photo-reactivity, and low impact on the environment. This study highlighted the effects of ZnO-based nanomaterials of two bacteria genera isolated from the Temple of Concordia (Agrigento's Valley of the Temples in Sicily, Italy) that are involved in biodeterioration processes. The antimicrobial activities of ZnO-nanorods (Zn-NRs) and graphene nanoplatelets decorated with Zn-NRs (ZNGs) were evaluated against the Gram positive Arthrobacter aurescens and two isolates of the Gram negative Achromobacter spanius. ZNGs demonstrated high antibacterial and antibiofilm activities on several substrates such as stones with different porosity. In the case of ZNGs, a marked time- and dose-dependent bactericidal effect was highlighted against all bacterial species. Therefore, these nanomaterials represent a promising tool for developing biocompatible materials that can be exploited for the conservation of cultural heritage. These nanostructures can be successfully applied without releasing toxic compounds, thus spreading their usability.

Keywords: ZNGs; antimicrobial; biodegradation; cultural heritage; stones.

Figure 1

Effect of zinc oxide nanorods (ZnO-NRs) on different strains viability. Bacteria were treated or not (UT: untreated) with different concentrations of nanorods for 2 h (left column) or 24 h (right column) and bacterial survival was evaluated by CFU counting analysis. (a–c) indicate respectively A. aurescens TC4, A. spanius TC1, and A. spanius TC7. A one-way ANOVA analysis with the Bonferroni post-test was used to assess statistical significance (ns not significant; * p < 0.05 and *** p < 0.001 with respect to UT).

Figure 2

LIVE/DEAD staining after treatment with ZnO-NRs. Fluorescence microscope images of untreated bacterial strains (left) and treated cells with a concentration of ZnO-NRs of 125 μg/mL (right). (a–c) indicate respectively A. aurescens TC4, A. spanius TC1, and A. spanius TC7.

Figure 3

Effect of ZnO-NRs on biofilm formation. Different strains were grown in the absence (UT: untreated) or presence of 125 μg/mL ZnO-NRs. (a–c) indicate respectively A. aurescens TC4, A. spanius TC1, and A. spanius TC7, after 24 h of treatment. For statistical analysis one-way ANOVA method coupled with the Bonferroni post-test was used (*** p < 0.001 with respect to UT).

Figure 4

(a) LIVE/DEAD staining after A. spanius TC1 treatment with Zn-NRs (ZNGs). Fluorescence microscope images of untreated bacterial strains (on the left) and treated cells with a concentration of ZNGs of 10 μg/mL; (b) Effect of ZNGs on biofilm formation on A. spanius TC1. Bacteria were grown in the presence of 125 μg/mL ZNGs while samples without nanomaterials were taken as controls. To assess statistical analysis a one-way ANOVA analysis with the Bonferroni post-test was used (*** p < 0.001 with respect to UT).

Figure 5

FE-SEM images of different untreated and sprayed with ZNGs 250 μg/mL specimens. (a–c) indicate Noto stone, Common yellow brick and Carrara marble, respectively.

Figure 6

Cell viability after treatment of stone specimens with ZNGs. CFU counting of the indicated species after 24 h of incubation on treated stones. (a–c) indicate respectively Noto stone, common yellow brick, and Carrara marble. To assess statistical analysis a one-way ANOVA analysis with the Bonferroni post-test was used (*** p < 0.001 with respect to UT).

Figure 7

FE-SEM micrograph of A. aurescens TC4 cells after exposure to Noto stone covered with ZNGs. Panel (a), bacterial cell after 24 h exposure; Panel (b), brick covered by ZNGs. Bar, 200 nm.