Evaluation of efficiency of six biocides against microorganisms commonly found on Feilaifeng Limestone, China

Abstract Feilaifeng is listed as a cultural heritage site because of its abundant Buddhist statues possibly dating back to the Five Dynasties period (907 AD–960 AD). However, microorganism growth on the surface of stone results in esthetic and structural damage. Biocide treatment is commonly used to eradicate the microorganisms, and we assessed the effectiveness of four chemicals (benzalkonium chloride, octhilinone, tebuconazole, and thiabendazole) and two commercial biocides (ACTICIDE® 50 X (A) and AW-600) in inhibiting microbial colonization. The minimal inhibitory concentrations of various biocides were determined, and their efficacies were compared using the inhibition zone method. The most effective biocides were applied in situ to Feilaifeng Limestone, and the overall effects were monitored by photographic recordings and microbial cultures for a month. The factors affecting the efficacy of each biocide, such as thermal stability and loss-resistance, were also evaluated. The biocides varied markedly in their ability to inhibit the proliferation of different microorganisms. The three species of fungi tested herein demonstrated the highest susceptibility to octhilinone, but benzalkonium chloride showed the best antibacterial activity to the bacteria, Pseudomonas sp. Based on the results of in vitro assays, octhilinone, as well as its respective commercial product AW-600, were the best biocides for treating microorganisms, and 0.5% was the optimal concentration. Although the biocides on disc papers could be easily diluted by water, they were resistant to degradation by high temperatures. These findings suggest that frequent rainfall might have been the reason for the recolonization of microorganisms in a short time after octhilinone and AW-600 treatment in situ. Our results provide new insights on how to protect Feilaifeng Limestone, which is prone to damage caused by various microorganisms.