A mummified ancestor of Boris Johnson didn’t die of syphilis, shocked scientists have revealed – and the true culprit is something we’ve never seen before.

Branded ‘Switzerland’s most famous mummy’, the corpse made headlines in 2018 when it was identified as Anna Catharina Bischoff, Johnson’s sixth great-grandmother.

Her remains, which were found in 1975, contained high levels of mercury – historically, a treatment for syphilis – so it was assumed that the disease had killed her.

But now an analysis of the microbes in her mummified organs has revealed not syphilis, but high levels of a bacterium previously unknown to science.

Microbiologist Mohamed Sarhan from Eurac Research said: ‘The initial assumption was based on the mercury presence in her body, especially in the lungs.

‘This might indicate inhalation treatment for syphilis, as this was the followed protocol back then.

‘We therefore analysed many samples from every organ in her body to see whether we can find any DNA traces of the syphilis-causing pathogen, but we couldn’t.

‘Instead we found this new bacterium that was highly abundant in the brain tissues and correlated with the highest mercury concentration in the brain.’

Comparing the mysterious old bacteria with bacteria from today revealed something intriguing.

It contained similar sets of genes to those found in modern bacteria that cause bone lesions and pulmonary symptoms.

Bone lesions, which are visible in Bischoff’s remains, are a known symptom of untreated late-stage syphilis.

So she may have been misdiagnosed with the sexually-transmitted infection, when the true cause of her sickness was something unknown.

For Dr Sarhan, it’s enough to rule out syphilis as a cause of death.

He said: ‘The assumption that she might have died of syphilis can be excluded even if she had it.

‘Syphilis at an advanced stage has very clear signs that she did not have.

‘Additionally, she died at age of 69, so not very young.

‘Also, she had other health issues – for example: she was overweight and had gallstones, and had other issues that are currently under research.

‘The mercury treatment might have weakened her body and immune system over time, but was not really the main cause of her death.’

Dr Sarhan’s conclusions aren’t without controversy, however.

Gerhard Hotz of the Natural History Museum of Basel, where the mummy is now kept, said: ‘This is the point where we have different opinions.

‘In the late stages of syphilis you don’t find a lot of bacteria in the body anymore. So it was very difficult to find the old genome of the bacteria.

‘So it’s not proof that she didn’t have it – for me personally, I still think she had it. Her skull clearly shows signs of syphilis. But we can’t prove it by genomes.’

Whatever the case, Bischoff’s illness was certainly assumed to be syphilis in her time.

And for the wealthy widow of a priest, that was a damning diagnosis.

Dr Hotz said: ‘Nobody wanted to talk about it. Normally when people died of her social class from Basel, there was a written obituary about the person, who she was and so on.

‘We found it about everybody, but not about her. So we think she died, and she was very quickly and privately buried in the church.’

He said her diagnosis would have barred her from using public baths and even being treated in a normal hospital.

But it didn’t necessarily indicate any illicit sexual activity.

Dr Hotz said the scrutiny of the churchgoing community would have made an affair difficult to conceal, and letters from her husband detailing his own illnesses give no symptoms of syphilis.

He continued: ‘We don’t think it was an affair, either from her husband or from herself.

‘But there’s another explanation – because she was the wife of a priest, she had to visit sick people, to console them.

‘And in Strasbourg, near where she lived, there was a hospital for syphilis, so we think she was going there to visit sick people.

‘And if somebody was newly infected, you can easily be infected.’

Bischoff was born to a wealthy family in Strasbourg, France, in 1719.

Her father, himself a priest, ministered to the Swiss families of the city, but when he died aged 40 the rest of the family rejoined their relations in Basel.

Bischoff’s husband would ultimately take her father’s old job in Strasbourg, and she lived there for more than 40 years, having seven children – four of whom survived to adulthood.

Her oldest daughter, also called Anna Katharina, is Boris Johnson’s fifth great-grandmother.

After her husband’s death, Bischoff returned once more to Basel, where she died in 1787.

And when she was buried in Barfüsser Church, the mercury that had been used to treat her slowed her putrefaction and turned her into a mummy.