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Bacteria trained to save art

· A conversation with biologist Giancarlo Ranalli ·

Not all bacteria are harmful: some are sadly notorious for their capacity to cause diseases and others are useful to man. If they are properly “trained” they can even help to clean ancient frescos and save them from decay. This is the case of the “bacteria-cleaners” put to work by Giancarlo Ranalli, a microbiologist at the University of Molise, and used to restore paintings in the churches of Santos Juanes in Valencia and in the Camposanto (Monumental Cemetery) of Pisa. In both cases, the frescos were removed from the walls with the aid of animal glues in the Spanish church in 1936 after a fire, and in the Camposanto in 1944 to save them from the bombing in the Second World War.

“To remove them from the walls” explained Claudio Giantomassi, who with his staff supervised both the Spanish and the Italian work sites, “animal glues were used, which, in the course of time, had become insoluble and impossible to remove with traditional methods. The bacteria are able to eat away this glue, leaving the painted part intact. Likewise other substances may be chosen; in the Spanish case, for example, the problem stemmed rather from nitrate s and other salts, which were also ‘comestible’”. We asked Giancarlo Ranalli, the man who invented this technique, to tell us about his “micro-collaborators”.

How do the bacteria-cleaners work?

The micro-organisms belong to the genus Pseudomonas stutzeri and before they can start work they must be “trained”. The cultures are fed only water and the substance they are to eat, in this case animal glue. The quickest to grow are selected. Certain enzymes are used to treat polychrome paintings and those on paper, but the use of living microbe cells as agents for bio-restoration in the context of the restoration of cultural goods is unprecedented; so far there has only been sporadic laboratory research.

The frescoes in the Monumental Cemetery in Pisa are among the illustrious “patients” being treated.

The Camposanto of Pisa is an impressive artistic complex that extends over a surface area of about 1,500 sq.m. An uninterrupted sequence of frescoes decorates the walls of the Cemetery’s great galleries, illustrating the events associated with the passion of Christ, with the destination of the soul and with the lives of the Pisan saints and the Old Testament. These frescoes were painted by celebrated artists. First, Francesco Traini from Pisa and later Florentine painters such as Buonamico Buffalmacco, the protagonist in many of Boccaccio’s stories and in those of Franco Sacchetti, recognizable in the cycle known as The Triumph of Death , and the mysterious Stefano, Giotto’s favourite student, Taddeo Gaddi and Andrea da Firenze. In addition, Antonio Veneziano, Spinello Aretino, Piero da Puccio from Orvieto and Benozzo Gozzoli decorated the northern gallery with biblical scenes, commissioned by Bishop Filippo de’ Medici between 1467 and 1484.

These works very soon gave rise to problems.

At the end of the 14th century and between the end of the 16th and the beginning of the 17 century, numerous interventions and repairs were already documented — which at that time involved treating various kinds of damage and redoing the frescoed areas — and continued in the following centuries. It was not by chance that the various topics discussed at the first congress of Italian scientists, held in Pisa in 1839, included the recovery of the frescoes that were then in a state of serious decay. When in 1944, a grenade of the American artillery destroyed the trusses of the the extensive roofing of the Cemetery, the painted surfaces suffered further serious damage. The operation to save and protect them encountered all sorts of delays and obstacles that were due to inertia, to the protracted war and to the unreliable means available.

To save them in this situation the sole remedy was to detach them from the walls, dividing them into many sections, with recourse to the delicate “a strappo” technique. To remove the pictorial surface from its original site, glue was applied and left to dry and a cotton fabric placed over it, then with a mixture of casein of calcium, it was stretched on a support consisting of one sheet of eternit (asbestos) fixed to wooden stretchers.

However, in the following years the frescoes were exposed to rapid deterioration. In the immediate postwar years, the restorers concentrated above all on remounting them and then, eventually, on touching them up. No care was taken to wipe away the salts or the many substances applied during the 19th century, with which they were permeated. At the end of the 1960s the actual survival of the paintings was at risk; the colour no longer adhered but curled, flaked, became detached and fell off. In 1996, a restoration project was funded by the Opera della Primaziale Pisana , as well as by the Regione Toscana and the Provincia di Pisa . From that moment the work made regular progress.

How are the “good” bacteria selected?

The selection of strains is a very important stage because the fast or slow recovery of the works depends on the action of the microorganisms. To determine which bacteria “would devour” the incrustations, the quickest and best, avant-guarde molecular biology techniques were used to detect in their dna those genes that codify the enzymes useful for the operation. Once the most efficient and quickest bacteria to remove the incrustations had been identified, we immediately turned our attention to the organisms that belong to the Pseudomonas genus.

These are rod-shaped bacteria, ubiquitous by nature and endowed with a large metabolic capacity. In fact, they can use more than 100 different compounds as a source of carbon and energy. Among the many species tested in vitro Pseudomonas stutzeri A29 attracted our attention because it is not pathogenic to human beings or to the environment.

Once removed from the depleted growth medium, the bacteria cells were found to be in a physiological condition characterized by an enormous “appetite”. Eight to twelve hours after the application of the bacterial suspension, at a concentration of about 100 million living cells per millilitre, cellular activity was so intense that the concerned areas were cleaned, easy to remove and caused no structural damage. The cells consumed the glue as food, a source of carbon and energy, releasing carbon dioxide in a gaseous form as a product of their aerobic metabolism.

Is this technique safe for frescoes?

The Pseudomonas stutzeri does not expose the frescoes to any risk because the cells are not sporogenous, that is, they are not equipped with faculties that enable them to change from an active vegetative form to a “dormant” form of spore. In comparison with the cost of enzymes such as protease and collagenase the use of bacterial cultures has proven to be more expedient.

A European project, funded for three years, has been developed. It will involve partners from five countries: the United Kingdom, Germany, Latvia and Greece, as well as the University of Milan, the University of Molise and the firm Syremont, the industrial partner. We are initiating the phase of application. In the first place, given the incidence of climatic conditions, on the activity of the micro-organisms, temperature and humidity, it was decided that the applications should be undertaken in two countries with decidedly opposite climates. One experiment will be carried out in Riga, Latvia, on a 19th-century building and the other in Greece, in archaeological sites and specifically on the ancient theatre of Epidaurus.




St. Peter’s Square

Nov. 19, 2019