By Jason Horowitz

As early as 1595, descriptions of stains and discolouration began to appear in accounts of a sarcophagus in the graceful chapel that Michelangelo created as the final resting place of the Medicis. In the ensuing centuries, plasters that were used to incessantly copy the masterpieces he sculpted atop the tombs left discolouring residues. His ornate white walls dimmed.

Nearly a decade of restorations removed most of the blemishes, but the grime on the tomb and other stubborn stains required special, and clandestine, attention. In the months leading up to Italy’s COVID-19 epidemic and then in some of the darkest days of its second wave as the virus raged outside, restorers and scientists quietly unleashed microbes with good taste and an enormous appetite on the marbles, intentionally turning the chapel into a bacterial smorgasbord.

“It was top secret,” said Daniela Manna, one of the art restorers.

On a recent morning, she reclined — like Michelangelo’s allegorical sculptures of Dusk and Dawn above her — and reached into the shadowy nook between the chapel wall and the sarcophagus to point at a dirty black square, a remnant showing just how filthy the marble had become.

She attributed the mess to one Medici in particular, Alessandro Medici, a ruler of Florence, whose assassinated corpse had apparently been buried in the tomb without being properly eviscerated. Over the centuries, he seeped into Michelangelo’s marble, the chapel’s experts said, creating deep stains, button-shaped deformations and, more recently, providing a feast for the chapel’s preferred cleaning product, a bacteria called Serratia ficaria SH7.

“SH7 ate Alessandro,” Monica Bietti, former director of the Medici Chapels Museum, said as she stood in front of the now gleaming tomb, surrounded by Michelangelos, dead Medicis, tourists and an all-woman team of scientists, restorers and historians. Her team used bacteria that fed on glue, oil and apparently Alessandro’s phosphates as a bioweapon against centuries of stains.

In November 2019, the museum brought in Italy’s National Research Council, which used infrared spectroscopy that revealed calcite, silicate and other, more organic, remnants on the sculptures and two tombs that face one another across the New Sacristy.

That provided a key blueprint for Anna Rosa Sprocati, a biologist at the Italian National Agency for New Technologies, to choose the most appropriate bacteria from a collection of nearly 1,000 strains, usually used to break down petroleum in oil spills or to reduce the toxicity of heavy metals. Some of the bugs in her lab ate phosphates and proteins, but also the Carrara marble preferred by Michelangelo.

“We didn’t pick those,” Bietti said.

Then the restoration team tested the most promising eight strains behind the altar, on a small rectangle palette spotted with rows of squares like a tiny marble bingo board. All of the ones selected, she said, were nonhazardous and without spores.

“It’s better for our health,” said Manna, after crawling out from under the sarcophagus. “For the environment, and the works of art.”

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Above: (FROM LEFT TO RIGHT) Michelangelo’s allegorical sculpture of Dawn at the Medici Chapel in Florence, Italy. | Stains on the back of a tomb at the Medici Chapel in Florence. A team of scientists, restorers and historians deployed bacteria to clean the Medici Chapel, adorned with Michelangelo’s sculptures of Dusk and Dawn. | Strains of bacteria are tested behind an altar on a small square palette at the Medici Chapel.

Sprocati said they first introduced the bacteria to Michelangelo’s tomb for Giuliano di Lorenzo, Duke of Nemours. That sarcophagus is graced with allegorical sculptures for Day, a hulking, twisted male figure, and Night, a female body Michelangelo made so smooth and polished as to seem as if she shined in moonlight. The team washed her hair with Pseudomonas stutzeri CONC11, a bacteria isolated from the waste of a tannery near Naples, and cleaned residue of casting moulds, glue and oil off her ears with Rhodococcus sp. ZCONT, a strain that came from soil contaminated with diesel in Caserta.

It was a success. But Paola D’Agostino, who runs the Bargello Museums, which oversees the chapels and which will officially reveal the results of the project in June, preferred to play it safe on Night’s face. So did Bietti and Pietro Zander, a Vatican expert who joined them. They allowed the restorers to give her a facial of micro-gel packs of xanthan gum, a stabilizer often found in toothpaste and cosmetics that is derived from the Xanthomonas campestris bacteria. The head of Duke Giuliano, hovering above his tomb, received similar treatment.

Then, in February 2020, COVID hit, closing the museum the next month and interrupting the project.

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Above: From left: Donata Magrini, Anna Rosa Sprocati, Daniela Manna, Paola D’Agostino, Monica Bietti and Marina Vincenti at the Medici Chapel in Florence, Italy, on 24 May, 2021. The team of scientists, restorers and historians deployed bacteria to clean the Medici Chapel, adorned with Michelangelo’s sculptures of Dusk and Dawn.

Sprocati took her bugs elsewhere. In August, her group of biologists used bacteria isolated from a Naples industrial site to clean the wax left by centuries of votive candles from Alessandro Algardi’s baroque masterpiece, a colossal marble relief in St. Peter’s Basilica in Rome of the Meeting of Attila and Pope Leo.

The bacteria strains got back to the Medici Chapel — which had reopened with reduced hours — in mid-October. Wearing white lab coats, blue gloves and anti-COVID surgical masks, Sprocati and the restorers spread gels with the SH7 bacteria — from soil contaminated by heavy metals at a mineral site in Sardinia — on the sullied sarcophagus of Lorenzo di Piero, Duke of Urbino, buried with his assassinated son, Alessandro.

“It ate the whole night,” said Marina Vincenti, another of the restorers.

The Medicis were more accustomed to sitting atop Florence’s food chain.

In 1513, Giovanni di Lorenzo de Medici became Leo X — the first Medici pope. He had big plans for a new sacristy for the interment of his family, including his father, Lorenzo the Magnificent, the powerful ruler of Florence who largely bankrolled the Renaissance. Il Magnifico is now also buried here, under a modest altar adorned with Michelangelo’s Madonna and Child, flanked by saints who also had their toes nibbled by cleansing bacteria. But back then his coffin waited, probably on the Old Sacristy floor. He was soon joined by Leo X’s brother, Giuliano, and his nephew, Lorenzo, the Prince to whom Machiavelli dedicated his treatise on wielding power.

“You had coffins waiting to be buried,” said D’Agostino. “It’s kind of gloomy.”

Pope Leo X hired Michelangelo to design and build the mausoleum. The pope then promptly died of pneumonia. In the ensuing years, Michelangelo carved the masterpieces and then ran afoul of his patrons.

In 1527, with the Sack of Rome, Florentines, including Michelangelo, supported a Republic and overthrew the Medicis. Among the ousted princes was Lorenzo di Piero’s sometimes volatile son, Alessandro, whom many historians consider a real piece of work. Michelangelo could not stand him, and when the Medicis stormed back, it was Michelangelo’s turn to flee.

In 1531, the Medici Pope Clement VII pardoned Michelangelo, who went back to work on the family chapel. But by that time, Alessandro had become Duke of Florence. Michelangelo soon left town, and the unfinished chapel, for good.

“Alessandro was terrible,” D’Agostino said.

Alessandro’s relative, known as the “bad Lorenzo,” agreed and stabbed him to death in 1537. The duke’s body was rolled up in a carpet and plopped in the sarcophagus. It is unclear if his father, Lorenzo, was already in there or moved in later.

“A roommate,” D’Agostino said.

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Above: Monica Bietti, a former director of the Medici Chapels Museum, who worked to clean and restore the chapel, points to Michelangelo’s allegorical sculpture of Night at the Medici Chapel in Florence, Italy, on 24 May, 2021.

In 2013, Bietti, then the museum’s director, realized how badly things had deteriorated since a 1988 restoration. The museum cleaned the walls, marred by centuries of humidity and handprints, revealed damages from the casts and iron brushes used to remove oil and wax, and reanimated the statues.

“Come and see,” Bietti said, pointing, Creation-of-Adam-style, at the toe of Night.

But the cleaner the chapel became, the more the stubbornly marred sarcophagus of Lorenzo di Piero stood out as an eyesore.

In 2016, Vincenti attended a conference held by Sprocati and her biologists. (“An introduction to the world of microorganisms,” Sprocati called it.) They showed how bacteria had cleaned up some resin residues on Baroque masterpiece frescoes in the Carracci Gallery at Palazzo Farnese in Rome. Strains isolated from mine drainage waters in Sardinia eliminated corrosive iron stains in the gallery’s Carrara marble.

When it came time to clean the Michelangelos, Vincenti pushed for a bacterial assist.

“I said, ‘OK,'” said D’Agostino. “‘But let’s do a test first.’”

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Above: A tourist admires the tomb of Giuliano di Lorenzo, Duke of Nemours, and Michelangelo’s allegorical sculptures of Dusk and Dawn, at the Medici Chapel in Florence, Italy, on 24 May, 2021.

The bacteria passed the exam and did the job. On Monday, tourists admired the downward pensive glance of Michelangelo’s bearded Dusk, the rising of his groggy Dawn and Lorenzo’s tomb, now rid of the remnants of Alessandro.

“It’s very strange, especially in this time of COVID,” Marika Tapuska, a Slovakian visiting Florence with her family, said when she learned that bacteria had cleaned up the sarcophagus. “But if it works, why not?”

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Banner image: The New Sacristy gleams after a lengthy period of restorations at the Medici Chapel in Florence, Italy.

All photographs courtesy of Gianni Cipriano for The New York Times.

Jason Horowitz c.2021 The New York Times Company