An ancestor of former U.K. Prime Minister Boris Johnson dubbed "Switzerland's most famous mummy" died while carrying a mysterious disease - and not syphilis as originally claimed.

The mummy was dug up in 1975 in the Barfüsser church in Basel and mystified scientists for decades. In 2018, it was identified as Anna Catharina Bischoff - who died in 1787 and is the great-great-great-great-great-great-great grandmother of Johnson.

Several clues, including changes in the bones of her skull, initially suggested that the woman died while suffering from syphilis.

However, analyses conducted by a team from the Eurac Research centre's Mummy Research Institute found no traces of that particular pathogen were present when she died.

Using a new method, so far rarely applied to ancient DNA, it was possible to assemble the genome of a still unknown non-tuberculous mycobacterium and clarify that the woman did not die of syphilis.

The bacterium that had infected Ms. Bischoff belongs to a group of non-tuberculous mycobacteria, which are part of the family of bacteria to which the agents that cause leprosy and tuberculosis also belong.

The fact that Mrs. Bischoff died at the age of 68, however, probably had as much to do with the treatments used in Europe at the time: vapours or ointments of mercury. The concentration of mercury in the brain was extremely high. This, together with changes in the bones of her skull, had strengthened the initial hypothesis that Bischoff had syphilis. Most likely the mercury also favoured the mummification process.

In addition to fleshing out Johnson's family history, the possibility of discovering new and rare microorganisms even in very ancient genetic material allows science to investigate important aspects of the development of human infectious diseases.