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Two-Faced Microbes: Dirty Fungi And Cleaning Bacteria

King Tut's tomb with brown spot stains. © The J. Paul Getty Trust, 2001

Microbes can be an ugly pain-in-the-butt for artifacts.

Even if the bacteria and fungi growing on heritage buildings, frescoes, space suits and archival documents can be killed, they often leave behind some rather unpleasant stains that are really hard to clean off the sensitive surfaces of artifacts. That’s the situation in King Tut’s tomb, for example, where fungi have left behind dark brown spots on the beautifully painted walls.

Today the Harvard Gazette wrote about this issue: At the request of Egyptian heritage officials, researchers at the Getty Conservation Institute swabbed the walls of King Tut’s tomb, and sent samples of the brown muck to Ralph Mitchell, a Harvard microbiologist who specializes in cultural heritage science. Getty chemists figured out that the dark spots are actually melanin–the same pigment that builds up in your skin when you get a tan–while Mitchell’s team figured out that the fungi are dead and probably won’t be producing any more browny spots. Mitchell thinks that the fungi initially grew because the tomb was sealed before the paintings inside were dry, suggesting that the teenage king was buried in a hurry. The still-wet surface thus provided tempting real-estate for melanin-producing fungi.

Cyanobacteria growing on the Luca Signorelli frescoes in Italy's St. Brizio Chapel caused a rosy discoloration. Credit: Wikimedia Commons.

It turns out that melanin-producing fungi have also stained marble in Italian cathedrals after an ill-advised attempt to protect the marble using acrylic polymers. The acrylic on the marble attracted the staining microbes who found the plastic to be a tasty meal. But microbes will also grow on buildings, art and artifacts that haven’t received unwise “protection.” For example, orangey carotenoid pigments are often left behind by bacteria on stone buildings, Mitchell says, and frescoes have been stained rosy red due to the phycoerythrin pigments produced by cyanobacteria.

The question remains: How does one remove these unfortunate discolorations?

Mitchell is developing a technique that uses enzymes to eat away the unpleasant pigments without hurting the pretty parts below. Think of laundry detergents that advertize enzymes which can remove stains on your white clothing without doing damage to the textiles. Mitchell’s strategy obeys the same overall principle… and that’s about as much information as he was able to give because he’s in the middle of patenting the enzyme cleaning process.

So, all this could leave you with the impression that microbes are only bad for cultural heritage, but here’s a possible counterpoint: Researchers in Italy and Spain are experimenting with the idea of using bacteria to clean off the salty crusts that build up on frescoes.

Using bacteria to clean up artwork in the Church of Santos Juanes in Valencia. Credit: RUVID

Sort of weird but true: The researchers grow Pseudomonas

bacteria in a wet gel, and apply it to the frescoes. The bacteria then get to snacking on the salt scabs found on the frescoes until there’s no more unsightly crust. You can apparently kill the bacteria by wiping off the gel. This dries the surface and the bacteria die in the arid environment.

According to this press release: They’ve tried it out on frescoes in the 17th century Church of Santos Juanes in Valencia, Spain, and on Campo Santo di Pisa murals, in Italy. It’s too early to know whether harnessing microbes as cleaning work-horses will compete with the other ways to remove salt crusts, such as a mechanical scraping or nanotechnology.

And the take-home message? The world of cultural heritage research might be a bit like the human microbiome, where some of the trillions of microbes growing in and on you are problem-making pathogens while others are good guys that help you digest your food.


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  • Jun 16th 201121:06
    by Christine Herman


    very nice post! It’s so neat to learn how microbes can either be problematic or part of the solution, depending on the circumstances.

  • Jun 20th 201108:06
    by Sarah Everts


    Thanks Christine. I’m guessing that as researchers better understand microbes and their biochemistry, these tiny organisms may become increasingly more useful than problematic in cultural heritage work.

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