Subaerial biofilms: new horizons in stone biodeterioration research

Science and technology interact with art and culture in many ways: the arts draw both inspiration and new materials from science, while the scientific examination of art and artifacts has provided us with important insights into the progress of human civilizations. Now, science - in particular biological science - has an even more important role: to protect and conserve mankind's often fragile, cultural heritage for future generations. The evidence that many of the world’s most precious artworks are made of stone and have a finite life drives the interest in this topic. Their irreversible deterioration due to biological attack is a worldwide concern. Microorganisms colonize outdoor lithic surfaces and develop into biofilms at the interface solid/air (subaerial biofilms, SABs), which, in turn might cause aesthetic, chemical and physical decay. Although it has been estimated that at least 99% of the world's microbial biomass exists in biofilms, the role and behavior of microorganisms within the biofilm matrix and their complex interactions with the environment is still unknown. In this talk I will approach the complex interaction between SABs and cultural heritage assets from two different angles: 1) the establishment of a laboratory model system of SABs relevant to cultural heritage studies, and 2) the urgent need to develop preventive antibiofilm strategies. 1) One of the main gaps challenging our understanding of the physiology and the activity of biofilms inhabiting outdoor stone heritage is the lack of a model system of SABs. To overcome this limitation, we developed a methodology to obtain a laboratory model of a dual-species SAB that is relevant to cultural heritage studies. The results underscore the ability of the dual-species SAB model to underpin functional traits characteristic of biofilms inhabiting lithic substrate such as: i) microcolonies of aggregated bacteria; ii) network-like structure following surface topography; iii) deconstruction of the complexity of environmental samples into their main component parts (e.g. phototroph-heterotroph interactions); iv) ability to change the chemical parameters that characterize the microhabitats; v) survival in harsh environments and vi) biocide tolerance. To the best of our knowledge, this is the first time that a phototroph-heterotroph association at the stone/air interface has been successfully obtained at laboratory scale starting from two introduced, controlled species and not from an environmental microbial consortium. The current study has the potential to significantly advance our mechanistic understanding of the biofilm-stone-air interplay that has proven difficult to study in field experiments due to the inaccessibility of samples and the complexity of the ecosystem under investigation. 2) SABs inhabiting artistic surfaces, as any other biofilms, show a remarkable resistance to biocides, making them recalcitrant to traditional cleaning procedures. Plants offer a virtually inexhaustible and sustainable resource of very interesting classes of biologically active, low-molecular-weight compounds (parvome). In the past, the plant parvomes were screened mainly for their lethal effects, disregarding concentrations and ecologically relevant functions of these molecules in the natural context. Testing sub-lethal concentrations of plant-derived compounds mimicking environmental levels may be critical to revealing mechanisms subtler than the killing activity, e.g. those influencing the multicellular behavior, offering an elegant way to develop novel biocide-free antibiofilm strategies. In a cross-disciplinary fashion, I will illustrate recent successes of sub-lethal concentrations of the plant-derived compound zosteric acid, its mechanism of action, and the structural characteristics responsible for the anti-biofilm activity, envisioning applications on cultural heritage assets.