Dr Clare Sanson, Senior Associate Lecturer in Biological Sciences, writes on Sophie Downes’ talk on fungi and conservation in heritage buildings.The Department of Biological Sciences’ contributions to Birkbeck Science Week 2017 focused on ‘Microbes in the Real World’. Apart from that over-arching theme, however, the two sessions could hardly have been more different. The Week kicked off with a lecture by PhD candidate Sophie Downes on the interactions between fungi and heritage buildings. As far as I am aware, Sophie is the first Birkbeck student to have given a Science Week lecture; she spoke with confidence and clarity, and held her audience well.
Nicholas Keep, Executive Dean of the School of Science at Birkbeck, introduced Sophie as a graduate of the University of Lincoln who had worked in textile conservation before moving to Birkbeck to study for a doctorate in Jane Nicklin’s mycology lab. She began her lecture by explaining the context of her research: her job had been based in a large Elizabethan house that had problems with pests and condensation, particularly in the show rooms. The need to find out how best to preserve and repair organic material in buildings like this one led directly to her PhD studies.
In the UK we have a huge number of historic buildings, many of which are popular tourist attractions and play an important role in the local economy, particularly in rural areas. A large number of these are maintained by the National Trust or English Heritage, and many are open to the public for the majority of the year. The thousands of visitors drifting through properties will affect the number and types of micro-organisms, particularly fungi, found there. Sophie’s project included a year-long survey, starting in the autumn of 2013, of fungi found in 20 historic buildings in England, Wales and Northern Ireland. These included cottages and wartime tunnels as well as the more usual castles and mansions, so the survey could be expected to provide a snapshot of fungi and fungal damage in a wide range of historic properties in the UK.
When we think of fungi, we tend to think of so-called ‘macro’ fungi: this category includes the mushrooms we eat and poisonous toadstools, but also dry rot. Micro-fungi are harder to spot, but they are at least as pervasive and colonise an enormous range of organic matter, producing spores. For example, they are responsible for the blue colouration often found on stale bread and preserves. Micro-fungi will colonise almost any organic object that they find in their way, which, in the context of a historic building, might include wood, tapestry, leather book bindings and silk wall hangings. Sophie used air sampling and sterile swabs to obtain representative fungal samples from one outdoor and four indoor locations at each building and recorded the position of and features in each room or area selected, with its temperature and relative humidity.
Sophie landed up with a total of 4,000 samples to analyse, which, given her limited time, was too many for wholescale sequencing. She started by separating these according to colour and morphology and then selected representative samples for DNA extraction and ‘barcode screening’, and fewer for DNA sequencing. A total of 158 different fungal species from 77 genera were identified, with the most abundant genera being Aspergillus, Cladosporium and Penicillium. Some of the organisms found in smaller quantities, including fungal plant pathogens probably from the outside air and bacteria, were shed from visitors’ skin scales. Both the number of colony forming units and the diversity of fungal species recorded increased during the summer months.
Resident fungi can carry a small risk to human visitors to the buildings and perhaps a slightly higher risk to curators, given their higher exposure times. Fortunately, only a small fraction of the fungi identified were ‘nasty’ human pathogens, and all but one of these were classified in the lowest-risk group, Category 2. A larger number were recognised as of potential risk to particularly vulnerable individuals with damaged immune systems, and more still are only hazardous to the external environment.
The temperature, the height of the building, the type of room and amount of furnishings were found to be the most important factors in determining the extent of fungal growth within buildings and if high colony forming units would be observed, and the three most common fungal species in both the air and the swab samples – Penicillium brevicompactum, Cladosporium cladosporioides and Aspergillus versicolor – have frequently been reported in organic material in historical collections worldwide.
Fungi damage textiles and other organic materials by secreting enzymes that break down polymers, forming secondary metabolic products that cause further degradation. This process has important effects on the physical, chemical and mechanical properties of the materials. Sophie described how she had evaluated each of these, starting with the effect of fungal growth on the physical properties of cotton. Cladosporium infestation is known to cotton fibres, causing an unattractive colour change that cannot be removed by cleaning. She incubated new cotton strips with several fungal species and monitored them for 12 weeks using a technique known as colorimetry. Each fungus caused a gradual colour change, with Cladosporium causing by far the darkest stains. She also reconstructed images of fungi colonising woven cotton fibres in 3D with confocal fluorescence scanning microscopy.
Most fungi have long, filamentous structures called hyphae that secrete enzymes at their tips as they grow. These enzymes break down large and small organic molecules into nutrients; it is the breakdown of large molecules – polymers such as collagen, cellulose, fibroin and keratin – that cause chemical damage to heritage materials. Chitin and keratin are among the most complex organic substrates that fungi can digest and require several enzymes to break them down. Nevertheless, the three commonest species of fungi all managed to reduce the protein content of protein-containing fibres significantly, with Penicillium causing particularly serious damage to collagen. Fungal digestion also changed the local structure of protein fibres. And one net result of this chemical degradation is a change in the mechanical properties of the materials; for example, fungal infestation tends to cause silk to become more brittle.
But what are the implications of these results for the conservation of objects in historic buildings? All the test were conducted on modern materials, and aged ones, which are already worn, are bound to be more vulnerable. Sophie ended a fascinating talk by suggesting that this research will help to inform conservation protocols for the handling, treatment and risk factors involved with fungal contamination of historic collections.. .