## 1 Introduction  
Protection of cultural heritage, a social'resource', has been recognized by many countries as both an economic asset and also an important factor for promotion of social integration [1]. At the same time, world cultural heritage and archaeological sites are the bases for the global tourism industry and economy [2; 3; 4]. World cultural heritage is broadly defined by including physical or \"tangible\" assets,including archaeological sites, architecture, monuments, buildings, sculptures etc. and intangible attributes, such as performing arts, oral traditions, rituals etc. Cultural heritage can be physically movable, e.g., books, documents, movable artworks, machines, clothing, and other artifacts worthy of preservation for the future. Meanwhile, the physical non-movable heritage of buildings, architectural ornaments, other historic sites, and monuments is the legacy of history and humanity, allowing selective value to pass from generation to generation. The recognition of the necessity of the historical value of the civilization and of the objects that possess their value and significance promotes the necessity for preservation of culture and heritage for conservation and protection through scientific understanding and better management [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. Conservation science studies the processes that lead to physical changes in historical objects or artifacts that affect their longevity and integrity as a result of various processes or mechanisms of deterioration. These deteriorative mechanisms inform the scientists or conservators about the potential risk factors and strategies to mitigate adverse environmental impacts through management policy to effectively assist long-term conservation [17; 18; 19; 20; 21; 22]. Conservation science has gradually evolved into an interdisciplinary research subject by embracing physics, chemistry and biology, microbiology, engineering, materials science, and engineering as well as archaeology, anthropology, and art history [23; 24; 25; 26; 27; 28]. In the current conservation research, significant efforts are being made by non-biological analyses, mostly chemistry and material characterization, while an emerging direction has been in analyzing microorganisms on these objects for a more comprehensive knowledge and preservation practice [29; 30]. Fungi, bacteria, archaea, and lichens have been observed on a variety of historic artifacts such as mural paintings and bas-reliefs in caves, churches and catacombs, stone monuments and architectural surfaces in outdoor environments, and historical/archaeological relics and sites [31; 32; 33; 34; 35; 36; 37; 38; 39]. The biodeterioration caused by microorganisms not only influences the aesthetical appearance of the cultural heritage, but also sometimes results in destruction of the structural integrity of the valuable cultural heritage via (bio)chemical processes such as acidification, salt crystallization, calcium carbonate precipitation, water retention, accumulation of pollutants through biofilm formation, and progression [30; 38; 40; 41; 42]. As the world cultural heritage is both unique and irreplaceable, is hardly possible to estimate the economic value of the damage caused by microbial degradation. Microbiological research on this subject is becoming recognized as an indispensable part of the overall conservation science and the application of current scientific analytical techniques enables microbiologists to determine the microbial community and composition as well as biodeterioration potential of the colonizing microorganisms on the cultural heritage under a given environmental condition. Such knowledge informs how deterioration is likely to occur on cultural heritage and reveals important clues for conservators to monitor or mitigate the adverse effects and to prevent the potential risk from deterioration of historical artworks and objects caused by microorganisms [43; 44].