Identification of bacteria in a biodegraded wall painting by denaturing gradient gel electrophoresis of PCR-amplified gene fragments coding for 16S rRNA.

Medieval wall paintings are often affected by biodecay. An inventory of the existing microorganisms associated with the damage to the paintings is not yet an integral part of the restoration process. This stems from the lack of effective means for such a stocktaking. Nevertheless, fungi and bacteria cause severe damage through mechanical processes from growth into the painting and its grounding and through their metabolism. Detailed information on the bacterial colonization of ancient wall paintings is essential for the protection of the paintings. We used a molecular approach based on the detection and identification of DNA sequences encoding rRNA (rDNA) to identify bacteria present on an ancient wall painting without prior cultivation of the organisms, since it has been shown that most of these bacteria cannot be cultivated under laboratory conditions. To trace the noncultivated fraction of bacteria, total DNA from a biodegraded wall painting sample from a 13th century fresco was extracted and 194-bp fragments of the 16S rDNA were amplified with eubacterial primers. The 16S rDNA fragments of uniform length obtained from the different bacterial species were separated according to their sequence differences by denaturing gradient gel electrophoresis (DGGE). By sequencing excised and reamplified individual DNA bands, we characterized the phylogenetic affiliation of the corresponding bacteria. Using this approach, we identified members or close relatives of the genera Halomonas, Clostridium, and Frankia. To our knowledge, these groups of bacteria have not yet been isolated and implicated by conventional microbiological techniques as contributing to the biodegradation of wall paintings.