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Heritage Micribiology and Science
Microbes, Monuments and Maritime Materials
By Eric May, Mark Jones, Julian Mitchell
The Royal Society of ChemistryCopyright © 2008 The Royal Society of Chemistry
All rights reserved.
HERITAGE RESEARCH AND PRACTICE: TOWARDS A BETTER UNDERSTANDING?
LBW - Bioconsult, Schwarzer Weg 27, 26215 Wiefelstede, Germany
The importance of microbial impacts in the alteration and deterioration of cultural artefacts made of mineral, metallic or organic materials has been widely acknowledged in the course of many recent investigations. In the past, biodeterioration problems on cultural artefacts were often approached without a detailed analysis and, as a result, simply controlled by biocidal treatments. A much deeper interdisciplinary understanding of the environmental factors and material properties regulating biogenic damage would allow actions that were more specific and practically adequate. Thus long-term efficacy in the conservation of historical artefacts, whether derived from dirty, anoxic excavations, archives kept in dark and humid conditions or objects openly exposed to corrosive and nutritive atmospheric pollutants, is mainly dependent on a profound interdisciplinary anamnesis of the prevailing damage functions and the consequent formulation of conservation strategies.
In the course of the excavation of archaeological objects, made of glass, metals or wood, considerable changes of the prevailing physicochemical conditions assessed by environmental site analysis (i.e. temperature, moisture, redox potential, oxygen, nutrients) have to be taken into account in order to describe the possible (re-)activation of biodeteriorating impacts before and after exposure. In archives, the impact of microorganisms to the enzymatic deterioration of paper, parchment, leather and textiles is mainly determined by the availability of water determined by building physics and climate control. However, precise definitions of humidity levels favourable for the microbial growth are still missing and difficult to assess, since the climatic properties of the building and building materials (e.g. diffusion, absorbency), the maintenance of objects (e.g. cleaning) and types of materials affected differ from case to case and are presently not fully understood. In this regard material science helps to define the biosusceptibility of mineralic materials even for open exposed monuments (i.e. porosity, open surface, diffusivity, pH) and to understand the consequent function of biofilms as biodeteriorating (i.e. precursor of crusts) or bioprotecting (i.e. as protective barriers) factors.
If a better understanding of the prevailing environmental, moisture-related and material-specific parameters of biodeterioration by interdisciplinary anamnesis could be reached, the formulation of effective countermeasure strategies would be far easier to access, and it would open possibilities for an environmental-friendly approach in conservation, based on physical, chemical and biological interventions. This paper will consider the value of such an approach and elucidate the need by reference to different case studies from China, Denmark, Cambodia and Turkey, discussing various conservation strategies for the protection and conservation treatment of objects at these historic sites under different environmental conditions.
2 EVALUATION OF BIODETERIORATION PROCESSES
2.1 Biodeterioration Mechanisms
Whether as a single or catalytic, enhancing factor, microorganisms such as algae, cyanobacteria, lichens, bacteria and fungi influence, due to their contamination, growth and metabolic activity, the complex interaction between various types of materials and the surrounding physical as well as chemical damage functions.
In the course of biofouling (e.g. presence of colloidal microbial biofilms on or inside the materials), besides an aesthetic-impairing discoloration by biogenic pigments (e.g. green chlorophyll, brownish melanin, red carotinoids), the microflora leads to the alteration of physicochemical characteristics of the materials with regard to their (i) mechanical properties, (ii) surficial absorbency/hydrophobicity, (iii) diffusivity and (iv) thermal-hygric behaviour.
Subsequently, the microbial consortia may cause biocorrosive attack (e.g. microbial induced respectively influenced corrosion on materials), leading to the alteration of the structure and stability of materials by (i) phototrophic enrichment of organic biomass, (ii) selective cellular enrichment and redox processing of cations and anions (e.g. iron, manganese), (iii) the excretion of immediate corrosive metabolic products (e.g. organic and inorganic acids) as far as the (iv) enzymatic mineralisation of respective organic materials. Over and above these effects, germs, spores, dead cells and microbial toxins (e.g. endotoxins, mycotoxins) can all potentially exhibit allergenic, pathological effects, affecting restorers and conservators as well as visitors and users of cultural artefacts, especially in libraries and archives.
2.2 Environmental Conditions for Biodeterioration Processes
During the anamnesis of biodeterioration processes on cultural artefacts it is important to signal at an early stage the environmental conditions that favour microbial infection, contamination and biodeterioration in particular, identifying the basic parameters in order to consider and establish effective countermeasure strategies.
2.2.1 Exogenic Parameters. The microbial contamination on and in materials is basically determined by the availability of water provided by rainwater, rising dampness and condensation moisture, depending on the sorption isotherms of the respective materials. Fungal growth will be enabled at a water activity (aw)a > 0.6 and a time of wetness TOW > 0.5 (e.g. more than 12 h during a day); optimal conditions for their growth will be given with aw ~ 0.75. Other microorganisms, such as algae or bacteria, probably need a higher moisture supply (aw > 0.9), but in the widespread presence of moisture-conserving biofilms these microorganisms may survive in infected materials even under more unfavourable external moisture conditions.
In the long-term, the material structure (e.g. surface roughness, absorbency / hydrophobicity, porosity and inner surface), determines the adhesion, colonisation and spreading of the microorganisms on and within the material. Its chemical composition may additionally support the microbial succession, by providing internal inorganic and organic nutrients. Further decomposable nutrient sources may be offered by exposure to light, leading to the enrichment of photosynthetic biomass, as well as the deposition of natural and anthropogenic aerosols (e.g. ammonia, nitrate or combustible, biogenic hydrocarbons). When evaluating the nutritive conditions for a particular microbial consortia, it is important to consider that microorganisms settling on material surfaces are able to survive or even grow under oligotrophic conditions (i.e. low concentrations of nutrients). The contamination process will even be extended when the material in question possesses a capacity to buffer biogenic metabolic compounds with acidic properties, since the optimum pH for most of the microorganisms studied on cultural artefacts varies around neutrality.
The optimal temperature for most of the microorganisms involved in biodeterioration of cultural artefacts ranges between 16 and 35°C. The oxygen supply will not exclude microbial activity, but will determine the type of the respective metabolic pathways, whether oxidative or fermentative. Finally, the possible routes of contamination (e.g. airborne, soil, vegetation, infected materials) have to be analysed and considered as potential causes of microbial infection and biodeterioration processes on historic objects.
2.2.2 Biofilm - a stabilising microniche. It is important to stress the fact that a material-specific microflora preferably is embedded in a colloidal slime layer, called biofilm (Figure 4 + 5). The biofilm protects the microorganisms by balancing changes in humidity, temperature, osmotic and pH, the latter due to the presence of colloidal polymeric substances. Based on its considerable ion-exchange capacity, the biofilm even resists the penetration of biocides, detergents or antibiotics, impeding the control of the microbial contamination and biodeterioration processes in the long-term.
Over and above the above effects, the arrangement of microbial consortia in a biofllm matrix leads to the stimulation of their metabolic activity by (i) the extension of the colonisation area, (ii) the deposition and enrichment of nutrients on the adhesive surface, (iii) the promotion of a microbial metabolic network (cross-feeding) and (iv) the support of the intracellular communication by the exchange of genetic information. Therefore, in contrast to medical microbiology, the 'pathogenic' impacts of microorganisms on materials refer only rarely to the activity of one species, but are more often caused by complex microbial consortia characterized by a high adaptability and flexibility during the biodeterioration process.
2.3 Microbiological Assessment of Biodeterioration Impacts
The attention of restorers and conservators of cultural heritage to biodeterioration problems has revealed a growing demand for complete evaluation of the importance of microbial impacts interacting with material properties as well as natural and anthropogenic influences during the deterioration process. According to the proposed analytical strategies of May and Lewis as well as Becker et al. the appropriate analytical approach, in order, comprises:
object anamnesis (e.g. damage description, object history, climatic/environmental conditions, material properties, previous protective treatments);
non-destructive observations (e.g. videomicroscopy, remission spectroscopy, respiration/photosynthesis measurement, assessment of ATP-content);
microscopical studies (e.g. biofllm staining procedures-PAS/FDA, light and fluorescence microscopy, SEM, CLSM);
biochemical measurements (e.g. quantification of proteins/phospholipids as biomass, analysis of pigments); and finally
microbiological investigations (e.g. enumeration of air-borne and material-associated microorganisms, characterisation and taxonomical classification of the microflora, simulation studies, toxicological studies).
Above all, the effects of biodeterioration need to be demonstrated by quantification of complementary changes in material properties (e.g. discoloration, loss of weight, weakened stability, increased roughness, altered structure/porosity, increased absorbency/hydrophobicity). In this approach, changes in the physicochemical behaviour of the material to the environment should be addressed, such as its thermal-hygric stresses due to the darkening of the material surface by biogenic pigments, its tendency for an increased deposition of pollutants due to the presence of a sticky biofilm and alteration in moisture transport due to the impact of pore-filling biofilms. In specific cases, the potential hazardous impacts of microbial metabolites to human health (e.g. allergenic spores, toxins, pathogenic microorganisms) need to be considered, analysed and evaluated in parallel.
3 MICROBIOLOGY AND ARCHAEOLOGY - CASE STUDIES
The microbial impacts at archaeological sites include three major phases: (1) initial decay of vulnerable organic materials soon after burial due to limited maintenance and care of the site in the initial months and years, (2) transforming biodeterioration processes during the burial and uncontrolled exposure to prevailing environmental conditions over centuries and (3) post-excavation biodeterioration after the uncovering, safeguarding and conservation of historical artefacts within days and months.
The preservation of archaeological sites and inherent historical artefacts is thus basically favoured by low temperatures, natural dry conditions, artificial and natural preservation (i.e. salts) as well as low oxygen content of the surrounding environment.
Excerpted from Heritage Micribiology and Science by Eric May, Mark Jones, Julian Mitchell. Copyright © 2008 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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