Detection and identification of the latent microorganisms in the corrosion layers of ancient bronze mirrors

On an ancient bronze mirror, we carried out a scanning electron microscopic (SEM) examination of fractured corrosion layers, a biological microscopic observation of a gram-stained sample and DNA analyses of the samples removed from corroded sites. From these scientific and technical researches, we obtained the following results. Fine particles about 2 μm in length were confirmed in the corrosion layer by the SEM observation. Microorganisms of the same size were observed in the gram-stained sample removed from the corrosion layer through a biological microscope. From these results, fine particles observed under the SEM should be considered as microorganisms. A lot of fine particles were confirmed especially in the corroded sites by the SEM observation. So (certain kinds of) microorganisms may have played a role in the corrosion (deterioration) of the ancient bronze mirrors while the mirrors were buried in soil. From the base sequences obtained by DGGE analysis, two kinds of microorganisms were confirmed to exist in the corrosion layer of the ancient bronze mirror. One is 97.4% 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 such as Stenotrophomonas and Xanthomona. That is, the sequence is derived from a strain belonging to the Xanthomonadaceae family. The other base sequence is 97.4% homologous to the 16S rDNA of the Bacteroidales order such as uncultured Bacteroidales bacterium (Accession number: AY 859647). That is, the sequence is derived from a strain belonging to the Bacteroidales order. Genes of microorganisms, presumed to belong to the Acetobacter, Gluconacetobacter genuses and Fe (III)-reducing bacterium, Shewanella algae were detected from the base sequence analysis by cloning. From the results above, microbial activity is assumed to be high around the ancient bronze mirrors. The corrosion mechanism of bronze mirrors seems complicated, but a several types of microbes which possibly corrode bronze mirrors were verified.