Microbes as Enemies and Allies in the World of Art Conservation
Art is an inherent part of human civilization. Artistic relics, from stone monuments to monumental paintings, offer important glimpses into the lives of those who lived before. However, art is not just a testament to human creativity; it is also home to diverse communities of microorganisms, including numerous species of bacteria and fungi. By degrading everything from stone to paint pigments, these microbes pose a threat to priceless art and artifacts. Unfortunately, strategies for preventing and mitigating such ‘biodeterioration’ often include chemical treatments and may be harmful to the artwork itself, making these less-than-ideal options for preservation. Ironically, this is where certain microbes come into play, not as art destroyers, but as art restorers, capable of shoring up softening stone or eating away at the grime that tarnishes paintings. As a result, although some microbes may be the bane of art conservators’ existence—others could be their closest allies.
Microbes As Art Destroyers: Biodeterioration
Biodeterioration is a process whereby microbial metabolism triggers undesirable changes in the properties of a material object. In the art world, such changes can be aesthetic (e.g., discoloration of pigments on a painting) or structural (e.g., degradation of stone monuments or paint bubbling on murals). These manifestations of biodeterioration often go hand in hand. For instance, structural aberrations are likely to alter the appearance of a piece.
The onset and progression of biodeterioration is influenced by microbial interactions with one another, their environment and the art itself. There are numerous organisms implicated in this process, with bacteria and fungi being the most well-studied. While multiple factors shape the microbial communities that coexist on a given art piece, the type and age of a substrate are critical. For instance, though there may be some overlap, the microbes inhabiting an easel painting rich in organic materials, like canvas, pigments and glues, will differ from those colonizing stone murals, which are mostly comprised of inorganic materials and thus require specific metabolic capabilities for nutrient extraction and survival.
Although the base substrate of a piece of art helps characterize the identity of its initial microbial colonizers, biochemical reactions catalyzed by certain microbial species can alter the substrate in ways that promote further colonization by other species. Biofilms are a particular nuisance in art restoration; the extrapolymeric substance that encases them serves as a trap for pollutants that can be harmful to the art, particularly in outdoor settings. Moreover, biofilms are sticky and hard to clean, produce deteriorative molecules and protect microbes from assault.
Communities of deteriorative microbes are also heavily influenced by the environment in which the art exists. Factors like temperature, humidity and ventilation promote and modulate the growth of some microbes while inhibiting others. Furthermore, humans directly and indirectly create conditions that facilitate survival of organisms that damage the appearance and structure of the artwork. For instance, air pollution associated with human activities provides nutrients that cling to stone works and support microbial growth.
Methods of Art Restoration
Ultimately, the capacity of microbes to weaken and destroy art means that monitoring and eliminating such destructive organisms is critical for preserving artistic relics. Indoor venues like museums carefully control conditions, including temperature and humidity, to minimize the risk of microbial growth. However, stringent control of the environment is less feasible in outdoor settings. Therefore, prevention of biodeterioration depends on routine monitoring and rapid response to problematic microbes.
Completely eliminating "biodeteriogens" from artwork is tricky business. A variety of factors must be considered when selecting a mitigation strategy, including hazard levels to humans and the environment, versatility of use, economic feasibility and environmental demands. The most widely available methods involve chemical, mechanical and physical disruption of microbial consortia, and all of these techniques come with their own set of risks.
For instance, chemically treating artwork with biocides and fungicides is cheap, effective against a wide range of organisms and can be applied in remote areas. However, these chemicals can be toxic, ineffective for long-term use and may promote the development of biocide-resistant microbial communities. Mechanical methods require tools, like brushes, scalpels, vacuums and pressure washers to remove contamination from art surfaces. These methods circumvent the need for hazardous chemicals, but they can be damaging to certain types of artworks and may even push microbes deeper into the substrate. For example, pressure washing stone can force microbes into its fissures. Blasting artworks with gamma irradiation or lasers has proven to be a successful tactic for killing microorganisms and, like mechanical disruption, does not require harmful compounds. However, these methods are costly and require specialized staff for administration, thus decreasing accessibility.
Because no elimination method is perfect, and there is an outstanding need for gentle, effective and economical restoration methods, art conservationists are increasingly turning to the very things they are trying to combat—microbes—for the solution.
Microbes As Art Restorers: Biorestoration
Microbial metabolism can be exploited to mitigate biodeterioration and manage the effects of general weathering and residue accumulation on art pieces. Notably, compared to traditional restoration methods, microbes are cheaper, less invasive, highly specific and easier to control. Furthermore, in terms of sheer effectiveness, "biorestoration" can outperform other tactics. When comparing chemical, laser and microbial cleaning methods for removing black crust (a sulfurous build-up that is common on stone in polluted environments) from the Cathedral of Florence, scientists concluded that microbes were the superior choice.
There are several ways in which microbes are used to restore pieces of art. Biocleaning involves application of microbes directly to art, where they metabolize problematic deposits, including organic matter and salts, to remove them from the artwork. A number of studies have evaluated the utility of this technique, including a recent report in which viable Pseudomonas stutzeri strain A29, a bacterium previously lauded for its biocleaning potential, was used to successfully clean nearly-400-year-old wall paintings at the Vatican Museums and Pisa Cathedral Cupola in Italy. Biocleaning has also been enlisted, with promising results, for graffiti-removal.
In addition to metabolically scrubbing art, microbes also strengthen it. Bacterial species like Desulfovbrio and Bacillus produce build-ups of calcium that bolster degrading stone. This is called "bioconsolidation," and it is one of the most well-studied microbe-based treatments for restoration of stone artworks. For example, researchers isolated bacteria from stone blocks at the San Jeronimo Monastery in Granada, Spain. After determining the biomineralization capabilities of the bacteria, they reapplied promising isolates to the stone. Thanks to the formation of abundant calcium carbonate, excellent surface consolidation was observed over a 2-year period. These microbes were well-adapted to the conditions of the stone found in that particular monastery environment, suggesting the potential of indigenous microbes for bioconsolidation.
Besides the tactics above, researchers are exploiting antagonistic relationships between microbes and/or their secretory products to eradicate deteriorative organisms. Bacillus species have garnered particular attention in this regard, since they produce a wide array of metabolic products with antimicrobial potential, including anti-fungal peptides and lipoproteins, among others. Several studies have tested the applicability of Bacillus to inhibit the growth of paint-dwelling microbes. One study found that spores of Bacillus (namely B. subtilis, B. pumilius and B. megaterium) could nearly completely inhibit the growth of fungi and bacteria isolated from a 17th century easel painting. Another report isolated secondary metabolites, including biocides and lipoproteins, from pure cultures of Bacillus species. These compounds mitigated the growth of fungi sourced from biodegraded mural paintings. Importantly, after 5 months, the treatment had not affected the appearance of representative mural pieces, suggesting this is also a reasonably safe technique. Nevertheless, more work is needed to ascertain that such biocontrol methods do not negatively affect art in the process of decontaminating it.
The relationship between art and microbes is a complicated one, and we still have a great deal to learn about both biodeterioriation and biorestoration. Driven by advancements in DNA sequencing, most work thus far has centered on cataloguing the microbes that inhabit artwork. Less research has been conducted to understand the metabolic functions of those microbes as they pertain to biodeterioration. Furthermore, not all microbes on a piece of art pose a threat, and some are actually useful.
To address the current gaps in knowledge, some studies have utilized RNA-based profiling of communities or metabolomic analyses to identify genes and compounds linked to deterioration. Ultimately, a multi-faceted approach using culture-independent and culture-dependent methodologies, coupled with functional analyses, will lend the most insight into the deteriorative potential of microbes on diverse artworks. Moreover, monitoring and relating changes in microbial communities to substrate composition and environmental conditions over time would be instrumental in determining the best methods for decontamination.
The success of biorestoration provides incentive for identifying new microbes with promising restorative potential and determining the contexts in which these microbes can, and should, be applied. Development of efficient formulas and techniques, which use restorative microbes to prevent destructive microbial growth, without also damaging the artwork will be a priority. Efficiency, utility and economic feasibility of such techniques will also be important. Notably, scientists recently created an agar-gauze gel enriched with viable bacteria to clean wall paintings, which caused significant reduction in organic substances on the paintings within only 3-12 hours of administration. Another group developed a dry biocleaning process, in which dehydrated yeast cells were applied to stone works. The water naturally present in the stone rehydrated the cells to metabolize build-up; a test on the Quattro Fontane in Rome showed exciting potential.
Overall, as we learn more about the microbes that colonize art, we will be better equipped to battle and possibly befriend them.
Not only can microbes destroy and restore art, they can also be used to create it! ASM's Agar Art contest, launched in 2015, aims to share the beautiful and diverse world of microbes with the public.