Microorganisms Attack Synthetic Polymers in Items Representing Our Cultural Heritage

With advancements in materials science over the past few decades, there has been a dramatic increase in the use of synthetic polymers by both artists and conservators. Synthetic polymers in items representing our cultural heritage occur either as original constituents of works of art or as materials used for conservation treatment, and these polymers include adhesives, consolidants, and protective coatings. In the 1980s there was a change in the perception of plastics from consumer goods and disposable materials to fashionable, highly collectable pieces with historical and technological significance (27, 34). Now, in their 20th and 21st century collections, most museums and galleries possess objects made from the thousands of different plastics that have been produced. As museums keep acquiring objects that reflect both everyday life and technological and historical events, the proportion of plastics in museums is increasing dramatically. Plastics may be present in objects of everyday life, such as housewares, jewelry, equipment, furniture, information technology, photography, and toys, and more of these objects are entering museum collections and contemporary art (57). In addition, synthetic polymers have been widely employed for treatment of items representing our cultural heritage as adhesives, consolidants, and protective coatings to preserve many artifacts from further deterioration (20, 45). Synthetic polymer conservation has been formally recognized as a research area only since the 1990s, and it was in this period that the world's most important organization in the field of cultural heritage conservation, the Committee for Conservation of the International Council of Museums, established the Modern Materials and Contemporary Art Working Group. Indeed, owners and curators have begun to notice that objects made of plastics degrade with time, sometimes very rapidly. Importantly, many synthetic polymers appear to deteriorate faster than other materials in museum collections and have a useful lifetime of just decades (57). Synthetic polymeric materials can suffer different forms of deterioration, including chemical (e.g., oxidation), physical (e.g., UV light), and biological. Although many reports in the scientific literature claim that microorganisms are capable of degrading synthetic resins (35, 42, 59, 68), the microbial contamination of synthetic polymers that are used as materials for conservation treatment (29, 32) and in contemporary collections (50) is still underestimated. Indeed, it was only in the 2005-2008 program that the Committee for Conservation of the International Council of Museums Modern Materials and Contemporary Art Working Group embraced “(microbial) biodeterioration” as a research topic (http://icom-cc.icom.museum/Documents/WorkingGroup/ModernMaterials/Modern-materials2005-2008.pdf). Microorganisms can damage the structure and function of synthetic polymers. According to Flemming (22), the main types of damage include (i) biological coating masking surface properties, (ii) increased leaching of additives and monomers that are used as nutrients, (iii) production of metabolites (e.g., acids), (iv) enzymatic attack, (v) physical penetration and disruption, (vi) water accumulation, and (vii) excretion of pigments. Table ​Table11 describes microorganisms and their modes of action for degrading synthetic resins (polyvinyl chloride [PVC], polyurethane, nylon, and acrylics). Barbie dolls, together with many other toys, clothes, and electrical insulation found in museums, are made from PVC (58). The instability of plasticized PVC is frequently manifested as migration of the plasticizers. Colonization of PVCs by fungi, especially black fungi, due to the availability of platicizers on the surface has been assessed several times (30, 51, 67). Webb et al. (67) identified fungal isolates obtained from PVC by PCR amplification and partial sequencing of the internally transcribed spacer regions and the 5.8S rRNA gene or the V3 domain of the 28S rRNA gene. TABLE 1. Microorganisms degrading the synthetic polymers PVC, polyurethane, nylon, and acrylics and their mode of action It has been suggested that biodeterioration of polyurethane polymers, which are products of a polyol based on either a polyester or polyether and a di- or polyisocyanate, occurs through enzymatic action of hydrolases, such as ureases, proteases, and esterases (18, 21, 52). Degradation of polyurethanes by microorganisms in 20th century museum textiles has been reported by many researchers (36, 63). Polyurethanes can also be found in products such as furniture, adhesives, paints, elastomers, coatings, and contemporary art (33, 34, 52). Biodeterioration due to enzymes, presumably including a manganese peroxidase of the basidiomycete Bjerkandera adusta, was also observed for the aliphatic polyamide Nylon-6 fiber (24). Damage to the polymer was assessed by microscopic examination, differential scanning calorimetry, and evaluation of changes in viscosity. One of the reasons for introducing synthetic consolidants and protective compounds in conservation treatments was the expectation that these materials would be more resistant to microbial attack than natural organic products. In 1968 the superintendents at Ostia Antica (Rome, Italy) decided to replace natural organic compounds, which are easily degraded by microorganisms, with acrylic compounds in conservation treatments. Frescoes detached with Paraloid did not show any biodeterioration problem for the first 3 years after application (4). However, as early as the 1950s, some experiments on biodeterioration of polyvinyl acetate resins were reported by the Istituto Centrale del Restauro in Rome, Italy (26). Generally, filamentous fungi were the agents causing deterioration of these materials that were studied the most, especially in early experiments (17, 41, 54, 61), although some bacteria, yeasts, algae, and lichens that are capable of growing on synthetic polymers have been found or isolated (14). Historically, identification of filamentous fungal species has been based on morphological characteristics, both macroscopic and microscopic. These methods may often be time-consuming and inaccurate, which has required the development of identification protocols that are rapid, sensitive, and precise. In the last decade molecular approaches for rapid characterization of fungi on painted items representing our cultural heritage and paint coatings have been developed (46, 53). A protocol for efficient extraction of fungal DNA from micromycetes colonizing painted art objects was developed by Mohlenhoff et al. (46), who claimed to have successfully removed any inhibitors. In particular, melanin can also be present, which is highly resistant to UV light, enzymatic digestion, and chemical breakdown and might be a potent inhibitor of DNA amplification (46). PCR amplification of the 28S rRNA gene and denaturing gradient gel electrophoresis analysis were used to characterize fungal communities. According to Saad et al. (53), fungi are commonly found on paint films as spores other than mycelium; hence, it is necessary to ensure that DNA extraction is effective also for propagules. The method used involves spore lysis by incubation of a specimen with the enzyme Lyticase, followed by bead beating. DNA is then purified from the lysate with a QIAamp DNA mini kit (53). There have also been case studies related to biodeterioration agents other than filamentous fungi. Bacterial biofilms composed of Pseudomonas aeruginosa, Ochrobactrum anthropi, Alcaligenes denitrificans, Xanthomonas maltophila, and Vibrio harveyi formed readily on the surfaces of synthetic materials being considered for use in space applications (28). A yeast isolated from a bronze statue treated with the acrylic-based coating Incralac was found to accelerate the deterioration of the coating itself, as determined by scanning electron microscopy and electrochemical impedance spectroscopy (44). Stones impregnated with Ahydrosil Z, a silicone resin, were recolonized by algae and fungi more quickly than untreated specimens (40). Rapid recolonization by the alga Stichococcus bacillaris was also noticed in the Roman archaeological site at Luni in northern Italy after treatment with an epoxy resin and an acrylic-siliconic resin (19). Finally, lichens were reported to deteriorate a synthetic polyester resin that was used as a consolidant of stucco walls and column capitals in the Roman city Baelo Claudia in Spain (2). The ecological succession of fungi over 10 months on two Brazilian buildings painted with a white acrylic paint was described by Shirakawa et al. (60). Prior to painting, the walls were treated with hypochlorite. In addition to Cladosporium, the main fungal genus identified during the experiment, the other fungal genera detected were Alternaria, Curvularia, Epicoccum, Helminthosporium, Coelomycetes, Monascus, Nigrospora, and Aureobasidium. The yeast population fell to undetectable levels after the third week, and this microbial group was not detected again until 7 months, after which the number of cells increased.