Art-loving bugs: the resurrection of Spinello Aretino from Pisa's cemetery.

During 1391-1392, Spinello Aretino painted a cycle of frescoes in Pisa's cemetery on the theme "Storie dei Santi Martiri Efisio e Potito", highly prized by contemporaries and by Vasari. Twenty years ago, one of these frescoes, "Conversione di S. Efisio e Battaglia" (Conversion of S. Efisio and Battle), because of discolouring and bad damage caused by humidity and atmospheric pollution, was removed from the walls using the "tear-off" technique, consisting in covering the surface with a strong cloth bound to the surface with generous layers of formaldehyde-treated glue. As luck would have it, this large fresco (3.50x7.80 m) was abandoned in a storehouse for more than 20 years. When the curators attempted to remove the cloth, much to their dismay they found that the glue resisted any attempt at digestion, even when treated with concoctions of the most aggressive proteolytic enzymes available on the market. It is likely that during the long storage the glue became slowly cross-linked by the formaldehyde to the point of forming an intricate mass of untreatable proteinaceous material. Thus, although poor Spinello died presumably as a bona fide Christian, his painting was condemned to wear the burka, Muslim-fashion, for the rest of its life. When we recently treated the fresco with a suspension of viable Pseudomonas stutzeri cells, these bacteria, although agnostic, were able to fully digest the hardened glue and restore to life Spinello's glorious painting. We show here how proteomics helped us solve the riddle of how these bacteria acted on the burka-obscured fresco.