Microbial deterioration of cultural heritage and works of art — tilting at windmills?

Katja Sterflinger Guadalupe Piar Microorganisms (bacteria, archaea and fungi), in addition to lichens and insect pests, cause problems in the conservation of cultural heritage because of their biodeteriorative potential. This holds true for all types of historic artefacts, and even for art made of modern materials, in public buildings, museums and private art collections. The variety of biodeterioration phenomena observed on materials of cultural heritage is determined by several factors, such as the chemical composition and nature of the material itself, the climate and exposure of the object, in addition to the manner and frequency of surface cleaning and housekeeping in museums. This study offers a review of a variety of well-known biodeterioration phenomena observed on different materials, such as stone and building materials, objects exhibited in museums and libraries, as well as human remains and burial-related materials. The decontamination of infected artefacts, exhibition rooms and depots incurs high expenditure for museums. Nevertheless, the question has to be raised: whether the process of biodeterioration of cultural heritage can or should be stopped under all circumstances, or whether we have to accept it as a natural and an implicit consecution of its creation. This study also highlights critically the pros and cons of biocide treatments and gives some prominent examples of successful and unsuccessful conservation treatments. Furthermore, an outlook on the future research needs and developments in this highly interesting field is given. - Biodeterioration can be defined as any undesirable change in a material brought about by the vital activities of organisms (Allsopp 2011). Bacteria, archaea, fungi and lichens as well as insect pests are constantly causing problems in the conservation of cultural heritage because of their biodeteriorative potential. This holds true for all types of historic artefacts and even for art made of modern materials (e.g., polymers; Sabev et al. 2006) in public museums and in private art collections. Fungi, bacteria and lichens are also found on mural paintings in churches, caves and catacombs, and even as biodeteriogens of architectural surfaces and stone monuments in outdoor environments (Ettenauer et al. 2010; Piar and Sterflinger 2009; Saarela et al. 2004; Steiger et al. 2011; Sterflinger 2000; Urz 2004). The oldest and most precious objects suffering from serious fungal invasions are rock art caves, such as the caves of Lascaux in France (Bastian and Alabouvette 2009). Although the history of biodeterioration of houses and art is long and cases of red and green leprosies in houses have been described in the Bible (e.g., Leviticus Chap. 14, v. 36), its importance has been neglected for a long time, during which chemical and physical processes were believed to be the dominant factors of material decay. In recent decades, however, the dogma has changed and it is now generally agreed that fungi and bacteria not only cause serious aesthetical destruction of paintings, costumes, ceramics, mummies, books and manuscripts, they inhabit and penetrate into the materials, resulting in material loss, due to acid corrosion, enzymatic degradation and mechanical attack. Decontamination of infected artefacts, exhibition rooms and depots results in high expenditure for museums (Allsopp et al. 2004; Cappitelli et al. 2009; Koestler et al. 2003; Mesquita et al. 2009; Nittrus 2000a; Pangallo et al. 2009; Sterflinger 2010). Allsopp (2011) stated that the annual world loss of non-food materials due to fungal attack is US$40 billion. However, the cultural and historical value of many paintings, books and manuscripts is inestimable and thus, cannot be expressed merely in terms of money. Nevertheless, the question has to be raised: whether the process of biodeterioration of cultural heritage can or should be stopped under any circumstances, or whether we have to accept it as a natural and implicit consecution of its creation. Microorganisms associated with biodeterioration phenomena observed on materials of cultural heritage The biodeterioration phenomena observed on materials of cultural heritage are determined by several factors: (1) the chemical composition and nature of the material itself, (2) the climate and exposure of the object, (3) the manner and frequency of surface cleaning and housekeeping in museums. Some well-known examples are detailed below. Biodeterioration of stone and building materials Microorganisms contribute significantly to the overall deterioration phenomena observed on stone and other building materials, such as concrete, mortar, slurries and paint coatings, glass and metals used in architecture (Piar and Sterflinger 2009). On building stone exposed to the environment, fungi may be the most important biodeteriorative organisms because they are extremely erosive (Scheerer et al. 2009; Sterflinger 2000). Depending on the physical properties of the material, fungi may penetrate inside the stone. The phenomenon of bio-pitting the formation of pits in sizes ranging up to 2 cm in diameter and depth in stone is caused mainly by black fungi. Bio-pitting occurs predominantly on marble and limestone, but it has also been observed on antique glass (Piar et al. 2013a).There are two major morphological and ecological groups of stone-inhabiting and stone-dwelling fungi. These have adapted to different environmental conditions. In moderate or humid climates, the fungal communities on rock are dominated by hyphomycetes (mold) that form mycelia (hyphal networks) in the porous space of the stones (Sterflinger 2000; Rosling et al. 2009). Since the settlement of spores from the air is the first step for fungal colonization, the species diversity of stone fungi is rather similar to the diversity of common airborne spores. Alternaria , Cladosporium , Epicoccum , Aureobasidium and Phoma are the most important species (Sterflinger and Prillinger 2001). In arid and semi-arid environments, such as those found in the Mediterranean area, the climatic conditions are too extreme for most hyphomycetes, therefore the communities shift towards the so-called black yeasts and microcolonial fungi. Black fungi belonging to the genera Hortaea , Sarcinomyces , Coniosporium , Capnobotryella , Exophiala, Knufia and Trimmatostroma form small black colonies on and inside the stone and often occur in close association with lichens (Sterflinger 2005). Due to the thick walls they develop, fungi also resist chemical attack and, therefore, resist biocides and other anti-microbial treatments. Black fungi dwell deep inside granite, calcareous limestone and marble, which they erode by both chemical and mechanical attack. They are the main culprits for the phenomenon of bio-pitting. Due to the strong melanization of the cell walls, stones colonized by these fungi exhibit black spots or may be completely covered by a black layer. In addition to outdoor environments (...truncated)