The same microbes that live in and on us are also slowly eating our most treasured monuments.
A silent and invisible war is being waged on the surfaces of the world's most cherished cultural monuments. From the ancient temples of Angkor to Renaissance masterpieces, our collective heritage is under constant assault not by the elements alone, but by thriving communities of microorganisms—bacteria, fungi, and algae—that use stone and paint as their home and food source. The field of cultural heritage microbiology has emerged as a vital scientific frontier, dedicated to identifying these microscopic invaders and developing strategies to stop them.
For conservators and scientists, the challenge is twofold: they must detect and analyze these destructive microbial communities without causing further damage to the irreplaceable artifacts they're trying to save. This has led to the development of incredibly sensitive, non-invasive sampling techniques and cutting-edge molecular technologies that can reveal not just which microbes are present, but which are actively causing damage. The insights gained are revolutionizing how we preserve our past for future generations.
Biodeterioration—the breakdown of materials by living organisms—is a complex process affecting cultural heritage sites worldwide. The microorganisms responsible are not inherently malicious; they are simply surviving and thriving in their environment, which unfortunately happens to be priceless historical objects.
Microbial growth and migration, such as fungal hyphae penetrating tiny cracks in stone and expanding them over time 4 .
The pioneer species in this destructive process are often cyanobacteria and green algae (such as Gloeocapsa, Phormidium, and Chlorella), which can thrive on rock surfaces with minimal nutrients 4 . When these phototrophic organisms die, they release organic matter that creates an environment favorable for heterotrophic bacteria and fungi (including Bacillus, Penicillium, and Aspergillus species), which accelerate the physico-chemical deterioration 4 .
| Microorganism Type | Example Genera | Primary Damage Caused |
|---|---|---|
| Cyanobacteria | Gloeocapsa, Phormidium | Initial biofilm formation, discoloration, stone dissolution via acids |
| Fungi | Penicillium, Aspergillus, Fusarium | Physical penetration of materials, pigment production, acid secretion |
| Heterotrophic Bacteria | Bacillus, Pseudomonas | Chemical deterioration through metabolic acids, biofilm formation |
| Algae | Chlorella, Chlorococcum | Surface discoloration, moisture retention that enables other microbes |
The fundamental principle in cultural heritage conservation is that the object itself is irreplaceable, and any analysis must cause minimal to no damage. This has driven the development of sophisticated non-invasive and micro-invasive sampling techniques that can recover sufficient material for analysis without visually or structurally compromising the artifact.
One of the most effective methods involves using sterilized plastic adhesive sheets or tapes to remove microbial biofilms and loosely-attached materials from surfaces 1 . This technique, initially developed by the Japan Space Agency for the International Space Station, was adapted for cultural heritage work at Angkor and has since been used globally 1 .
The process is elegantly simple: a sterile adhesive is carefully applied to the sampled surface, gently pressed to ensure contact, and then peeled away with the microbial community attached 1 . The collected sample can then be subjected to various analytical methods. This approach works regardless of surface physical morphologies and material types, making it versatile for different heritage objects 1 .
Non-invasive collection of microbial biofilms from delicate surfaces
Depending on the situation and conservation constraints, several other techniques may be employed:
Once samples are collected, the real detective work begins. Scientists now employ a multi-layered approach to identify both the composition of microbial communities and their activity levels.
Culture-independent molecular techniques have revolutionized cultural heritage microbiology by revealing microbial communities that cannot be grown in laboratory settings 1 .
This approach sequences specific genetic markers, such as the V3 and V4 hypervariable regions of the 16S rDNA gene for bacteria, or the ITS regions for fungi 9 . This provides a census of which microorganisms are present.
Going beyond identification, this method sequences all the genetic material in a sample, allowing researchers to understand the functional potential of the microbial community 9 .
While molecular methods dominate current research, traditional culturing techniques still play a valuable role. By growing microorganisms from samples on various nutrient media, scientists can obtain live isolates for further study 9 .
| Technique | What It Reveals | Key Advantage |
|---|---|---|
| 16S/ITS Amplicon Sequencing | Identity of bacteria and fungi present | Cost-effective for biodiversity surveys |
| Whole Metagenome Sequencing | All genes present in the community | Reveals functional potential of microbes |
| Metatranscriptomics | Which genes are actively being expressed | Identifies metabolically active microbes and processes |
| Quantitative PCR (qPCR) | Quantity of specific microbial groups | Fast, quantitative assessment of contamination |
To understand how these techniques work together in practice, consider a research approach applied at the Angkor temple complex in Cambodia, where scientists have been studying microbial deterioration for decades.
Researchers used sterile adhesive tapes to sample biofilms from various locations on sandstone surfaces at the temples, focusing on areas showing visible discoloration or deterioration 1 .
DNA and RNA were extracted from these samples. The RNA was particularly important as it helped identify the metabolically active members of the microbial community directly involved in biodeterioration processes 1 .
The genetic material was subjected to next-generation sequencing, revealing the complete microbial community composition 1 .
By repeating samples across different locations and seasons, researchers built a comprehensive picture of how microbial communities change over space and time 1 .
The analysis revealed a stratified bacterial structural organization in the biofilms on the limestone and sandstone monuments, with different microbial communities occupying specific niches 1 . Furthermore, the research demonstrated distinct spatial and temporal dynamics in these communities, influenced by environmental conditions such as temperature, humidity, and exposure to sunlight 1 .
Most importantly, by comparing DNA-based (total community) and RNA-based (active community) results, scientists could identify which microorganisms were merely present versus those actively contributing to deterioration at specific times. This crucial insight helps conservators develop targeted, effective treatment strategies rather than broad-spectrum approaches that might be unnecessary or even damaging.
| Item | Function in Research |
|---|---|
| Sterile Adhesive Tapes/Sheets | Non-invasive sampling of biofilm microorganisms from surfaces |
| DNA/RNA Extraction Kits | Isolation of genetic material from minute samples without contamination |
| PCR Reagents | Amplification of specific genetic markers for identification |
| 16S rDNA & ITS Primers | Target-specific sequences for identifying bacteria and fungi |
| Next-Generation Sequencers | High-throughput analysis of microbial community composition |
| Various Culture Media | Enrichment and isolation of specific microorganisms for further study |
| RNA Stabilization Solutions | Preserving the integrity of RNA to identify active microbes |
Perhaps the most surprising development in cultural heritage microbiology is the concept of using microorganisms to protect and even treat damaged artworks.
When classical chemical and mechanical methods fail or produce poor results, certain benign microorganisms can be applied for targeted restoration 9 .
Some bacteria naturally produce calcium carbonate, which can be used to consolidate damaged stone, effectively filling micro-cracks and strengthening the material 9 .
As research continues, the field is moving toward even more sophisticated approaches. Third-generation sequencing technologies, such as Oxford Nanopore's MinION devices, offer the potential for on-site, real-time analysis with longer sequencing reads that provide better species-level identification 9 . The integration of metabolomics and metatranscriptomics with metagenomic studies will significantly increase our understanding of the microbial processes occurring on different materials under various environmental conditions 9 .
On-site analysis with devices like Oxford Nanopore's MinION
Combining genomics, transcriptomics, and metabolomics
Machine learning for predictive modeling of deterioration
What begins as an invisible threat may ultimately be controlled through invisible solutions—using our growing knowledge of microbial ecology to protect humanity's greatest visible achievements. The silent war continues, but we are increasingly equipped with the tools to ensure our cultural heritage survives for generations to come.