The centuries-old Medici Chapel in Florence, Italy, was long overdue for a deep cleaning.
Although created to be a spectacular final resting place for members of Renaissance Italy’s most infamous family, the mausoleum had since accumulated a few layers of grime. Its formerly gleaming marble was discolored by glue residue, bits of plaster, oils… and organic matter from the decayed body of Alessandro de’ Medici, whose corpse was not properly eviscerated before being dumped into his father’s crypt in 1537.
After more than a decade of cleaning and restoration work, an all-women team of art historians, conservation scientists and restorers secretly turned to an unconventional method for busting its stubbornest stains last year: applying a paste of bacteria to literally eat them.
People don’t often associate bacteria with cleanliness. But, although there are some notorious germs in the bacterial world (looking at you, Staphylococcus and E. coli!), the vast majority coexist with us peacefully. Many of the microbes are industrious decomposers with a knack for breaking down stuff that other organisms can’t. Biologists and engineers have taken note of this talent and turned it towards cleaning up all sorts of tricky messes, from oil spills to corpse-stained marble.
“If you can just develop a microbial community that learns how to break down the stuff that you want to get rid of, then you can kind of let them do the work,” says Susannah Tringe, head of the Microbial Systems Group at Lawrence Berkeley National Laboratory.
Eat Dirt and Live
At the most basic level, all organisms are looking to break down something they can get energy and carbon from, says Tringe.
Because they’re small and simple, microbes can afford to metabolize more energy-intensive stuff than larger creatures like humans. They take the material that nobody else has the biochemical machinery to deal with and turn it into energy for themselves and products that other living things can use.
Bacteria have been digesting human waste for millennia, but it wasn’t until the early 1900s that microbiology advanced to the point where humans could deliberately put them to work in modern sewage treatment. Later, we realized that for nearly any kind of waste there were microbes that could be coaxed into breaking it down.
Microbial bioremediation is used nowadays to clean up pesticides, metals and organic industrial byproducts in water and soil. Bacteria are abundant and quick to adapt, so if given enough time to get used to a diet of industrial sludge, something will most likely find a way to live on it, says Tringe. Hydrocarbon-eating bacteria bloomed to feast on the Deepwater Horizon oil spill in 2010. And in 2016, scientists in Japan discovered a bacterium living near a recycling facility that had developed an appetite for polyethylene terephthalate, one of the plastics that the facility processed.
Tringe is part of a research effort to better understand the microbial communities involved in treating wastewater from the oil and gas industry. Bioreactors for industrial waste treatment are basically the ‘Hunger Games’ for bacteria; engineers put a bunch of different strains in and let natural selection do its thing.
“If they’re going to live, they’re going to have to live on this [wastewater],” says Bruce Rittmann, an environmental engineer at Arizona State University. “If you do several cycles of this, you’ll eventually enrich the community with organisms that are capable.” Then, scientists can characterize the winners to understand what makes them capable.
One of Rittmann’s research goals is to design a bio-based approach to breaking down per- and polyfluoroalkyl substances (PFAS), a particularly challenging class of pollutants with exceptionally strong chemical bonds. Bacteria can’t yet do it unassisted because PFAS are unlike any molecule found in nature — the microbes simply can’t recognize the potential food source. But Rittmann has found that if humans use their own chemistry to get the degradation process started, the bacteria can finish the job.
“We're the perfect team,” he says. “We’ve got our brains and our organizational ability, and the microorganisms have their incredible metabolic diversity to transform chemicals that we consider pollutants and they consider food.”
Although it uses some of the same organisms — Pseudomonas stutzeri, for example, has been used both for water treatment and to clean frescoes — restoring art with microbes requires a very different approach.
Chemically speaking, glue and corpse juice aren’t terribly challenging for bacteria to break down. The tricky part for art restorers is choosing microbes that will eat the grime but not the artwork itself. You probably don’t want to unleash, for example, a bacterial strain that has a taste for cellulose on an artifact made of paper. For this reason, bacterial treatments in the art world are most often applied to sculptures and architecture, using microbes that prefer organic material over minerals.
Marble, as in the Medici Chapel, is a match made in heaven for bacteria because they can be easily removed from the nonporous material after getting rid of stains. “I think it's kind of a textbook application for me,” says Francesca Casadio, executive director of conservation and science at the Art Institute of Chicago.
After narrowing down more than 1000 grime-eating bacterial options in the lab and testing eight promising contenders in a grid on the back of the Medici Chapel’s altar, the restorers chose to apply three to Michelangelo’s masterpieces: Pseudomonas stutzeri CONC11, Rhodococcus sp. ZCONT and Serratia ficaria SH7.
Choosing the right technique for cleaning a work of art, microbial or otherwise, is the result of close collaboration between art historians, restorers and scientists like Casadio. She likens this collaboration to a medical team, with the art as the patient. Conservation scientists must first use advanced imaging methods to diagnose the damage (for example, the chemical composition of a stain) so the restorers can choose the right treatment to lift the stain without damaging the artwork underneath.
“You need that combination of experts in order to do an almost genetically targeted treatment because there's not one single person that has all that expertise,” says Casadio.
Currently, the best way to see if a microorganism will break down a substance is to expose it to that substance and see what happens. Future advances in sequencing technology and metagenomics will help researchers understand how these organisms do what they do — and the better we understand them, the better we can help them tackle ever trickier chemistry in the name of cleaning.