## 2 Sampling Techniques
These adverse (bio)chemical reactions range from simply stains to acidification, discoloration, and utilization of the materials as nutrients and weaken the structure causing further disintegration and delamination. Figure 1: A SEM micrograph showing a matured biofilm consisting of both grown cells and extracellular polymeric materials on surface of a rock under moist conditions (scale bar, 2 \(\upmu\)m). Earlier sampling techniques required relatively large amount of materials to be taken from the object before chemical and especially microbiological analyses could be carried out due to the lower sensitivity of the analytical techniques involved. Because of this, the focus of this article is on the current sampling techniques with a minimum and non-invasive approach coupled with highly-sensitive microbiological analysis. Currently, a slightly different version of the same principle has been developed as a sampling technique using sterilized plastic adhesive sheet or tape to remove microbial biofilms and loosely-attached materials including degradation products from surfaces, regardless of the surface physical morphologies and material types [57] (Figure 2). This sampling device was initially developed by the Japan Space Agency for sampling surfaces for microbiological assessment on the International Space Station and then it was then adopted in the cultural heritage conservation work at Angkor for more than a couple of decades [31; 33; 58; 59; 60; 61; 62]. This sampling technique, with some small variations, has also been used in Europe for microbiological analysis of cultural heritage ([PERSON] and [PERSON], 2001). The device, consisting of an adhesive on a plastic sheet and a supporting paperboard, is made under sterile conditions, and can be conveniently carried to sampling locations. At the sampling site, after peeling the paperboard from the adhesive and attaching the adhesive surface to the sampling location to obtain the materials from the surface onto adhesive (Figure 3), further microbiological analyses can be carried out on the materials picked up on the adhesive. This sampling technique shows a higher recovery rate for microbial populations than the conventional cotton swab method [63] or stamp agar [57]. By applying the adhesive technique in sampling, samples could be collected as many times as required at a selected location to obtain spatial distribution information and composition of the microbial community from the sandstone bas-relief wall of Bayon temple of Angkor Thom in Cambodia [31; 33]. Specifically, samples were analyzed to establish the relationship between the specific colors on the sandstone wall and the dominant microorganisms [33]. By working closely with Apsara Authority and the Ministry of the Environment, Kingdom of Cambodia, this sampling technique was systematically applied in the conservation and protection research of Angkor monuments over the past 20 years or more to accumulate data on multiple dimensions of the research objectives [31; 33; 51]. Figure 2: The adhesive sheet sampling device developed for non-invasive/non-destructive sampling from various solid surfaces. The adhesive sheet can collect biofilm (the microbial community and the decomposed materials on the solid surface) from the surface to the deepest layer by a series of continuous peeling-off sampling of the biofilm at the same sampling position to analyze the stratified microbiome structure. The adhesive sampling technique allows analysis of samples for microbial community and composition on these sheets afterward [31; 33]. For example, the characteristics of the stratified structure of bacteria in the biofilm were analyzed and characterized by using 16S rRNA-based PCR-denaturing gradient gel electrophoresis (DGGE) analyses among bacterial communities after non-destructive collection of biofilm samples onto adhesive sheets from the sandstone surface of Bayon temple at Angkor Thom, Cambodia [33].