Lead-resistant microorganisms from red stains of marble of the Certosa of Pavia, Italy and use of nucleic acid-based techniques for their detection

Twenty-one microbial strains have been isolated from the red spots present on the marble of the facade of the Certosa of Pavia. These strains have been characterized and identified to the genus level using API test kits. As the pigmentation of these stains was previously attributed to minium (Pb₃O₄), the resistance to lead of the strains isolated was tested in a liquid medium. Thirteen strains showed resistance to lead nitrate at concentrations up to 500 ppm, while four, Pseudomonas vesicularis, Bacillus cereus, Staphylococcus sp. and Streptomyces sp., grew normally at 1300 ppm and with difficulty at 1500 ppm. In the presence of lead nitrate P. vesicularis and Streptomyces sp. showed a red and red-brown pigmentation in the cultures, respectively. The EDX analysis demonstrated that lead was associated with Streptomyces cells. Further investigations were carried out to assay the suitability of rapid molecular tests for the detection of isolates responsible for red stains directly on the biodeteriorated samples. The polymerase chain reaction technique was used on fifteen altered samples of artworks of different origins to detect B. cereus, chosen as test organism to set up the analytical method. The technique allowed for the detection of the organism in less than 24 h, including a 12-h enrichment step. Among the samples analyzed, B. cereus was detected via PCR only in samples from which it had been previously isolated with conventional techniques, i.e. samples from the Certosa of Pavia and Ca' D'Oro in Venice. The results indicate the potential usefulness of the molecular approach for the detection of alterative microorganisms on artworks. The advantages of the method in terms of speed, specificity and sensitivity in comparison with conventional ones are discussed.