Bacteria feasting on the last supper

It’s not breaking news that bacteria play a major role in food deterioration, and I would not be surprised if from my title you thought I was going to talk about how they found the fossil remnants of the last supper and the bacteria that ate it. That discovery would be such a low probability event that I would not even be able to pronounce the number and anyway that’s not what this post is about. So did I mislead you with my title? Not exactly, because I am referring to a different “last supper” the one painted by Leonardo Da Vinci in 1496. By observing the mural painting with a closer look you can notice evident signs of microbial degradation. 

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So why do bacteria eat art? How? Let’s find out..  

First of all we must understand that when artists wanted to create a mural painting they were mixing water and their pigments on a wet plaster. The exposure to air was fixing the pigment particles in the plaster, so the painting would literally become part of the wall. This technology allowed the ancients to create and preserve art for centuries. Moreover, if you think about the Greeks or the Egyptians or the Romans, they were building everything in stone or also using forms of plasters, concrete and mortar to confer immortality to their artistic crafts. 

The plasters were mainly composed of calcium carbonate or calcium sulfate dihydrate (or a mix of the two). The concretes were a mix of calcium silicates and aluminates and the mortars were a mix of calcium carbonate and concrete. The ability of these materials to withstand the onslaught of time was depending on the presence of impurities or air-filled voids in the structure that would dictate the ability of the material to resist frost damage (Sayre, 1976). 

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So what these materials have in common is a high concentration of calcium, therefore a high susceptibility to acid corrosion. Bacteria have been living on the surface of statues or mural paintings for centuries yet their activity was only having a minor impact on the art beneath. The obvious change from the past is the pollution, in our atmosphere today we have high concentrations of sulfur dioxide, it’s a pollutant gas. There are bacteria such as the Thiobacillus tioparis (from now on every time I mention bacteria, I refer to them) that are capable of interacting with these gasses and to wreak chemical havoc. The bacteria can in fact, convert the sulfur dioxide in the air into the highly corrosive sulfuric acid (Huber, Herzog, Drewes, Koch, & Müller, 2016). This acid, especially on surfaces rich in calcium carbonate such as concrete, marble or the plaster of mural paintings, can react with the calcium carbonate forming CO2 and calcium sulfate, which is a weak form of plaster, easily dissolved by rain and humidity. The bacteria use the CO2 produced in this reaction as a source of carbons, so they literally feed on dissolved art.

To give you a better idea about the scale of the accelerated degradation of monuments, I will give you the example of the Parthenon in Greece, more marble has been destroyed in the last 35 years than in the previous 300 years. At this rate not much of it will be left for future generations (Ref Book 1).  

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To convey an exact information, I also have to mention that the degradation of an ancient building is not only caused by the chemical attacks of bacteria. There are also other organisms that can impact the integrity of these structures via physical activities. For example, there are fungi that grow on these structures and push their hyphae (their branching filaments) deep into the rock, cracking and pulverizing it. 

Another agent that can determine the longevity of the artistic creation are also the contaminants used while preparing their materials. In the past, in fact, it was quite common to include in the plasters formulation some natural organic binding materials such as albumin, keratin, casein or other types of glues. While these were conferring temporary stability, but in the long term these polymers have the tendency to degrade, weakening the whole structure (Sayre, 1976).

Probably one of the most aggressive molecule to art is water. To be precise, pure water has little effects on plasters and concretes, however, there are often dissolved acidic gasses in the water the reaches these materials (carbon, sulfur, nitrogen oxide), and as I mentioned earlier, acids can easily dissolve calcium carbonate. For this reason, there is considerable effort in keeping mural paintings and other forms of arts in dry environments. It prevents bacteria growth and also the degradation caused by humidity. 

So what to do about it?  

Honestly, I don’t see people spraying antibiotics on buildings any time soon. That would not even be an effective measure (@justtryme90 recently wrote a post explaining why antibiotic may not always be effective). Chemicals such as the isothiazolinone chloride could be used but there are obviously some downsides. Just ask the Cambodian Army workers, in Angkor the army was called to brush this fungicidal solution over an ancient temple afflicted by these parasites. The isothiazolinone chloride was effective in removing the fungi, but it also removed most of the mural paintings in the temple.   

Even if one day we find a magic formula that would get rid of most parasites from statues and cathedrals, that would not fix the damage done. Statues cannot re-grow skin and the marble of ancient temples is not capable of self-healing. Prevention is key, and if pollutant gasses seem to exacerbate this problem, probably this would be one more reason to take more care of our planet. 

References:

• Huber, B., Herzog, B., Drewes, J. E., Koch, K., & Müller, E. (2016). Characterization of sulfur oxidizing bacteria related to biogenic sulfuric acid corrosion in sludge digesters. BMC Microbiology, 16(1), 153. https://doi.org/10.1186/s12866-016-0767-7 
• Sayre, E. V. (1976). Deterioration and restoration of plaster, concrete and mortar. In Preservation and conservation: principles and practices: proceedings of the North American International Regional Conference (pp. 191–201). 
• Ref Book 1: Microbiology, principles and explorations 7th edition

Below there is another video made by artists that were invited to promote my region, here there is my way to support them. This video was made by Timmy Henny and it's entitled "Mini Matera"