Biocides in the Service of Naturalia: A Green Revolution for Conservation
Exploring the use of essential oils as natural biocides for preserving Naturalia collections in museums and heritage sites.
A Silent War in Museums
Imagine a collection of precious herbariums, carefully preserved for centuries, beginning to be covered with greenish spots. Visualize unique zoological specimens attacked by invisible molds. This microbial threat constantly hangs over the treasures of our natural heritage, collectively called "Naturalia".
Of heritage institutions report microbial damage to their collections
Biofilms can resist biocide concentrations 100 times higher than isolated bacteria
Faced with this danger, scientists and curators have long resorted to biocides, substances designed to neutralize harmful organisms. However, the use of these products raises a crucial question: how to protect our collections without compromising their long-term integrity, nor our environment? The answer may well come from nature itself, with the emergence of solutions based on essential oils, a revolutionary approach combining effectiveness and sustainability 1 .
Naturalia Under Influence: Between Protection and Threat
A Delicate Balance
Collections of Naturalia – these natural biological samples including dried plants, insects, fossils, animal skins and skeletons – represent an irreplaceable memory of biodiversity. They are nevertheless constantly threatened by an army of destructive microorganisms: fungi, bacteria and algae. These invisible invaders find ideal development conditions in collections: organic matter, darkness, and sometimes fluctuating relative humidity.
Once installed, these microorganisms excrete organic acids and enzymes that degrade materials, causing stains, discoloration, or worse, structural deterioration of specimens 1 .
Microbial Threats to Naturalia Collections
The Hidden Face of Traditional Biocides
Synthetic biocides – compounds based on formaldehyde, phenols or quaternary ammoniums – have long been the preferred solution for the disinfection of collections 1 . Their effectiveness is undeniable, but their intensive use is gradually revealing their limits.
Recent studies highlight a worrying phenomenon: bacteria and other microorganisms can develop cross-resistance to biocides and antibiotics 6 9 . This phenomenon is explained by several biological mechanisms:
Reduced Permeability
Reduction of cell membrane permeability, limiting the entry of active substances.
Efflux Systems
Activation of efflux systems, real "molecular pumps" that expel toxic compounds out of the cell.
Enzyme Production
Production of enzymes capable of degrading biocide agents.
Biofilm Formation
Formation of biofilms, structured microbial communities in a protective matrix, particularly resistant 3 .
The Green Revolution: Essential Oils to the Rescue of Collections
A Millennial Inspiration
The idea of using the antimicrobial properties of plants is not new. What is new, however, is the rigorous scientific approach now applied to validate their effectiveness in heritage conservation. Essential oils – these concentrated extracts of aromatic plants – contain complex mixtures of active molecules, mainly terpenoids and phenylpropanoids, endowed with remarkable antimicrobial properties 5 .
Unlike synthetic biocides which generally target a specific metabolic pathway, essential oils act on multiple targets simultaneously in the microbial cell 5 .
Advantages of Essential Oils vs Synthetic Biocides
This multi-target action significantly reduces the risk of resistance development, while presenting reduced toxicity for humans and the environment. Moreover, their biodegradable character and renewable origin make them ideal candidates for sustainable conservation 1 .
Natural Origin
Derived from renewable plant sources with minimal environmental impact.
Biodegradable
Break down naturally without leaving harmful residues in the environment.
Multi-Target Action
Attack multiple pathways in microbes, reducing resistance development.
Scientific Breakthrough: The Moscadim House Experiment
To concretely illustrate this potential, let's look at a recent study conducted in an exceptional historical setting: the Moscadim House, an 18th century mansion in Portugal 1 .
This site houses invaluable mural paintings, threatened by microbial biofilms. Faced with the delicacy of the work to be treated, the researchers undertook to evaluate the effectiveness of four essential oils against the microorganisms isolated from the paintings.
Methodology: A Rigorous Approach
The experiment followed a rigorous scientific protocol:
Microorganism Sampling
By swabbing contaminated areas of the mural paintings
Molecular Identification
Revealing four species responsible for degradation: two bacteria (Bacillus wiedmannii and Bacillus mobilis) and two fungi (Penicillium brevicompactum and Cladosporium cladosporioides)
Essential Oil Extraction
From four local aromatic plants: thyme (Thymus mastichina L.), fennel (Foeniculum vulgare Mill.), pennyroyal mint (Mentha pulegium L.) and green lavender (Lavandula viridis L'Hér.)
Effectiveness Tests
Evaluation by two complementary methods - direct contact and diffusion in controlled atmosphere
The "micro-atmosphere" approach was of particular interest: by using the natural volatility of essential oils, it opens the possibility of non-invasive treatments, particularly valuable for fragile surfaces 1 .
Moscadim House
18th century Portuguese mansion where the essential oil experiment was conducted.
Results: Promising Performances
The results, summarized in the table below, revealed marked differences in effectiveness between the tested oils:
| Essential Oil | Action on Bacteria | Action on Fungi | Overall Efficacy |
|---|---|---|---|
| Fennel | Moderate | Very Strong | Excellent |
| Pennyroyal Mint | Strong | Strong | High |
| Green Lavender | Moderate | Strong | Good |
| Thyme | Weak to Moderate | Moderate | Moderate |
This exceptional performance against fungi, combined with a non-invasive application, makes it a candidate of choice for preventive conservation.
The Scientist's Toolkit: Research Methods and Materials
Evaluation Tools
Research on natural biocides relies on a range of sophisticated techniques allowing to evaluate both their effectiveness and their safety for treated materials. Among the most used methods:
Colorimetry
Measures color changes of treated surfaces, with a variation ΔE < 2 considered non-perceptible to the naked eye and therefore acceptable 5 .
Water Absorption Measurement
Evaluates modifications of hydrophilic properties of materials.
Scanning Electron Microscopy (SEM)
Observes morphological changes at the microscopic scale.
Gas Chromatography-Mass Spectrometry (GC-MS)
Identifies the precise chemical composition of essential oils 5 .
| Analysis Method | Parameter Measured | Importance in Conservation |
|---|---|---|
| Inhibition Tests | Inhibition zone around the biocide | Evaluates antimicrobial effectiveness |
| Colorimetry | Color change (ΔE) | Guarantees aesthetic safety of treatment |
| Chlorophyllometry | Chlorophyll a concentration | Measures algicidal effect on photosynthetic biofilms |
| GC-MS | Chemical composition of essential oil | Ensures reproducibility and standardization |
Reagents and Solutions in Research
Studies on biocides require specialized reagents to precisely evaluate their activity. The table below presents some of the essential tools used in this research field:
| Reagent/Solution | Main Function | Typical Application |
|---|---|---|
| Culture Media (BG-11, etc.) | Culture of test microorganisms | Propagation of strains for efficacy tests |
| Biocide Reagent Kits | Precise dosage of concentrations | Standardization of antimicrobial activity tests 4 |
| Reference Microbial Strains | Quality control of tests | Validation of experimental protocols |
| Color Indicators | Revealing metabolic activity | Rapid evaluation of microbial viability |
Future Directions: Towards Sustainable Conservation
Ongoing Research and Challenges to Address
The transition towards natural and sustainable biocides still raises many scientific questions. Several ambitious research projects are currently trying to answer them. The BIOCIDE project from the University of Gothenburg, for example, seeks to determine how antibacterial biocides contribute to the development of antibiotic-resistant bacteria in different aquatic ecosystems .
Standardization
Development of standardized application methods for consistent results across institutions.
Long-Term Effects
Studies of long-term effects on materials to ensure preservation without degradation.
Formulation Optimization
Optimization of formulations for prolonged release of antimicrobial activity.
Cost-Effectiveness
Evaluation of cost-effectiveness ratio for widespread adoption.
Integrated Strategies and "One Health" Approach
The sustainable conservation of Naturalia is increasingly part of a multidisciplinary and integrated perspective, recognizing the interconnection between ecosystem health, that of heritage collections and human health. This approach, qualified as "One Health", considers that the ability of bacteria and genes to move between humans, animals and the external environment requires a global vision to understand and effectively manage the development of resistance .
The One Health Approach in Conservation
In this perspective, the protection of Naturalia collections is no longer limited to the occasional application of products, but is integrated into a global environmental management: rigorous control of relative humidity and temperature, limitation of contamination by strict procedures, and regular monitoring of the microbiological state of collections.
Conclusion: A Green Future for Conservation
The awareness of the limits of traditional biocides has opened an exciting new chapter in the history of Naturalia conservation. Essential oils and other natural alternatives do not represent a simple product substitution, but rather a profound transformation of practices, aligning heritage preservation with the imperatives of sustainable development.
The Moscadim House experiment and similar research conducted worldwide 5 demonstrate that effective, safe and environmentally friendly solutions are within reach. The persistent technical and scientific challenges should not obscure the essential: we are witnessing the emergence of an ecological conservation that preserves both our natural heritage and the environment in which it is inscribed.
As we rediscover the protective virtues of the plants around us, it is a whole traditional knowledge that is rehabilitated and enhanced by modern science. Naturalia collections, witnesses of past and present biodiversity, will thus find in nature itself the means of their sustainability for future generations.