Celebrating centuries: Pink-pigmented bacteria from rosy patinas in the House of Bicentenary (Herculaneum, Italy)

The archaeological site of Herculaneum (Campania, Italy), which was buried as a result of the eruption of Mt. Vesuvius in the first Century C.E., was first excavated in the XVIII century. It has been included on the UNESCO World Heritage List since 1997, and in 2001 the Herculaneum Conservation Project (HCP) was started, which has carried out, among other activities, studies and conservation interventions across the site. Up until this time, little data has been available on the growth of biological agents that could cause biodeterioration of wall paintings and archaeological structures. Particularly, the presence of rosy discoloration is frequent on ancient monuments of the Vesuvian area, even if such phenomenon has so far been largely neglected. In this study, we describe, for the first time, the pink patina distribution and the microbial species isolated from the House of the Bicentenary in Herculaneum. By combining culture-based approaches with molecular and phylogenetic analyses we reliably isolated the pink-producing bacterial species and attributed them primarily to Arthrobacter agilis, and secondarily to Rhodococcus corynebacterioides, and Methylobacterium extorquens. Strains closely related to Dietzia maris and Gordonia rubripertincta were also isolated. With the exception of M. extorquens, a proteobacterium, all of the other isolates belong to the phylum Actinobacteria. All isolates produced carotenoid pigments, suggesting that they can participate in the development of such peculiar coloration. Our data indicate the presence of a large number of pink-pigmented bacterial species in the community, even if the presence of bacteria in viable but not-culturable state, such as Rubrobacter radiotolerans, is not excludible. The culture-based approach had the advantage of (i) obtaining bacterial isolates, (ii) showing their differential ability to produce pink discolorations, and (iii) testing conditions for in vitro growth. Moreover, field observations showed an association of pink patinas with dry conditions, saline efflorescence and moderate solar radiation. Some seasonal variations were also detected, with an increase in late spring and summer.