Jacques-Louis David's portrait of Marat made his death famous.Public domain

The radical French revolutionary Jean-Paul Marat died, famously, in a bathtub. He was soaking in one when his assassin, Charlotte Corday, plunged a kitchen knife into his chest in 1793. And he was soaking in a bath because of a mysterious condition that left his skin intensely itchy and blistered. The bath was his only relief, and the bath was where he died.

In the centuries since, people have speculated endlessly about the origins of his skin condition. Marat himself blamed the time he spent hiding from his political enemies, sleeping in cellars in damp, dirty clothes. Doctors have speculated about conditions with ever more complicated names: syphilis, scrofula, scabies, leprosy, diabetic candidiasis, atopic eczema, seborrheic dermatitis, dermatitis herpetiformis, bullous pemphigoid, and histiocytic proliferative disorder. Recently, geneticists decided to look at the only physical evidence that remains: the bloodstained newspapers he was annotating at the time he was killed.

A new study uses DNA from the newspapers to identify potential pathogens in Marat’s blood. (A preprint of the study was posted on bioRxiv last month, and it has not yet been peer-reviewed.) This technique, the authors are careful to say, can be used to narrow down the list of possible afflictions, but it cannot offer a conclusive diagnosis. Based on the DNA, they suggest that Marat may have suffered from a fungal infection, subsequently superinfected with bacteria, which led to an itchy condition called seborrheic dermatitis.

This idea to use new DNA-sequencing technology to investigate old medical mysteries came from Philippe Charlier, whom The New York Times has dubbed “France’s most famous forensic sleuth.” Charlier made a name for himself analyzing the purported remains of famous people, including Richard the Lionheart and Joan of Arc. In 2010, Charlier and Carles Lalueza-Fox, a paleogeneticist at CSIC-Universitat Pompeu Fabra, began analyzing DNA from a handkerchief supposedly dipped in Louis XVI’s blood during his execution (not actually his blood, according to their subsequent DNA analysis). When Charlier persuaded the Bibliothèque Nationale de France, in Paris, to allow a forensic swab across Marat’s bloodstained newspapers, he got in touch again with Lalueza-Fox.

A forensic swab was used to take DNA from Marat’s bloodstained newspapers. (de-Dios et al.)

The team extracted DNA from the swab and sequenced it using metagenomics. This technique involves chopping up all the genetic material in a sample into small segments, which are then sequenced and reassembled using the genomes of known organisms as a map. The organism that most interested the team at first was human: In other words, was this actually Marat’s blood? They found a Y chromosome and southern-French ancestry, which fit what is known about Marat, and they had no particular reason to doubt the provenance of the bloody newspapers. So they proceeded to look at DNA from microbes in the sample.

They did not find genetic evidence of pathogens causing several previously suggested diseases: syphilis, leprosy, scrofula, diabetic candidiasis, or scabies. They ruled these out.

But they did find some particularly noteworthy pathogens. First, a fungus called Malassezia restricta that is known to cause seborrheic dermatitis, that itchy-skin condition. And second, a bacterium called Cutibacterium acnes—the specific strain looked similar to ones that cause soft- and deep-tissue infections today. The DNA from these particular pathogens also looked like it had been damaged over time, which, for the purposes of this study, was a good thing. Any DNA that came from pathogens infecting Marat would be more than 200 years old. “If we could identify bits of DNA with this damage, we could have more confidence it was a species at the time,” says Lucy van Dorp, a postdoctoral researcher at University College London who co-authored the study. The team have submitted the paper for publication, and a TV documentary about the findings is also in the works.

“I was charmed by it,” says Matthew Collins, who studies ancient DNA and proteins at the University of Cambridge, of the study. But it also has the limitations of any study that uses metagenomics. The short segments of DNA are assembled using, as a guide, the genomes of previously sequenced microbes, and scientists have previously tended to focus on microbes that cause human disease. “There may be some intriguing organisms that inhabit paper archives but we don’t know about them,” Collins says. That means the short DNA segments belonging to an unknown paper microbe might be erroneously assigned to a known pathogen.

The metagenomics findings also cannot rule out an entirely autoimmune disease, such as dermatitis herpetiformis, which results from gluten sensitivity. Marat’s own DNA might offer some hints, but unfortunately, van Dorp says, not enough of his genome was sequenced to analyze that. Ultimately, the study sheds some light on Marat’s suffering but doesn’t offer conclusive answers. And we may never find them. Marat was buried as a martyr, but when his reputation later suffered in the French Revolution, his remains were moved and their exact location lost.

Yet the fascination with his skin condition endured. After Marat’s death, his friend Jacques-Louis David painted his famous portrait The Death of Marat as tribute and as propaganda. The painting was widely disseminated and copied. Its composition is striking; Marat’s face is almost beatific. If not for the painting, the bathtub assassination might never have become so iconic and Marat’s skin affliction so discussed. “Now we are used to seeing photos of things instantly,” says Clifford Conner, the author of two biographies of Marat. “That was one of the first international news flashes that had an illustration.” That image of Marat in the bathtub is still vivid two centuries later.

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