Biodeteriogens of mummies: diversity, threats, and control strategies

Deterioration of cultural heritage materials gives causes for great concern to conservators and restorers around the World. Biodeterioration is a category of deterioration, defined as any undesirable change in the properties of a material brought by the action of macro-and/or micro- organisms, generallly named biodeteriogens. Among different types of cultural heritage artifacts, mummified human remains are mostly exposed to biodeterioration since they contain a wide range of organic compounds (resin, wood, linen, soft tissues, etc.). Much research is currently focused on opportunistic biocenoses of fungi and bacteria associated with mummies and their action on the degradable materials. Resulting data from these investigations are essential to evaluate the cause- effect of the biodeterioration action, and aid to improve conservative treatments along with developing more specific biological, chemical, and biochemical methods of control and prevention. This work aimed to consider crucial steps of research dealing with microorganisms involved in the phenomena of biodeterioration observed on human mummies. Firstly, literature dealing with microbial diversity on mummified materials is considered, focusing on different characterization approaches, including isolation and culturing supporting morphological identification, and molecular techniques. With regard to these latter, examples deal with DNA-barcoding of fungal and bacterial cultures obtained from mummified surfaces, the creation and sequencing of clone libraries, metabarcoding by Illumina platform high-throughput sequencing, and metaproteomic approaches. As a second step, causative relation between the biodeteriogens and the damage to mummified materials is described. In particular, the mechanical action of microbial structures on the physical stability of different components of mummies, including human tissues, linen and decorative pigments, and the effects of enzymatic attacks on their chemical stability are examined.With this regard, examples deal with microscopic observations of microbial growths on deteriorated materials and with the spectroscopic characterization of chemical changes in different organic substrates. Finally, treatments adopted to devitalize and remove microbial communities from mummified materials are described. Despite of a strong necessity of experimental work on the suitability of different techniques to control fungal and bacterial growths on mummies, only a relatively scarse number of papers is available, focused on physical approaches, like UV and gamma-ray application, and the application of alternative biocidal products, such as essential oils. Moreover, the available investigation rarely report a complete information including a documented (quantitative) analysis of the treatment efficacy in terms of devitalization, and the evaluation of potential effects on the organic mummified substrate. With this regard, it is worth noting that my stage activity dealt with isolation of fungi from mummified substrates, finalized to the future evaluation in controlled conditions of the devitalization effectiveness of biocidal approaches and of their potential impact on the organic heritage surface. At the end of the work, advantages and limits of different approaches of biological diagnostics and of different conservation strategies are discussed, including a perspective section on promising tools for the future biodeterioration control on mummies.