Detection and Identification of the Latent Microorganisms in the Altered Layers of Ancient Bronze Mirrors

摘要

We obtained ancient bronze mirrors and carried out: scanning electron microscopy (SEM) of fractured altered layers; biologic microscopic observation of Gram-stained samples; and DNA analyses of samples removed from altered sites. Fine particles about 2 mu m in length were confirmed in the altered layer by SEM. Microorganisms of identical size were observed in the Gram-stained sample removed from the altered layer through a biologic microscope. Fine particles observed under SEM were considered to be microorganisms. Many fine particles were confirmed, particularly in the altered sites, by SEM. Certain types of microorganisms may have played a part in the alteration (deterioration) of the ancient bronze mirrors while the latter were buried in soil. From the base sequences obtained by DGGE analysis, two types of microorganisms were present in the altered layer of the mirror. One was 94.7 percent homologous to the 16S rDNA of the uncultured bacterium (accession number: AY 053488). It was also highly homologous to the sequence derived from the 16S rDNA of the Xanthomonadaceae family (e.g., Stenotrophomonas, Xanthomona). That is, the sequence was derived from a strain belonging to the Xanthomonadaceae family. The other base sequence was 97.4 percent homologous to the 16S rDNA of the Bacteroidales order such as uncultured Bacteroidales bacterium (accession number: AY 859647). That is, the sequence was derived from a strain belonging to the Bacteroidales order. Genes of microorganisms, presumed to belong to the Acetobacter and Gluconacetobacter genera and the Fe(III)-reducing bacterium, Shewanella algae, were detected from base sequence analysis by cloning. Microbial activity around the mirrors was assumed to be high. The alteration containing corrosion mechanism of bronze mirrors appears complex, but several types of microbes that possibly altered the bronze mirrors were verified.