The Borrelia recurrentis bacteria causes recurring fever disease and, now, a scientific team has analyzed ancient DNA of this microorganism to determine when it evolved to spread through lice instead of the ticks, and how it won and lost genes in the process.
This transition may have coincided with changes in the human lifestyle, such as the closest coexistence and the beginning of wool trade, researchers from the Francis Crick British Institute and the University College of London conclude.
The findings, published in Science, underline how ancient DNA can illuminate the origins and evolution of infectious diseases and how pathogens such as B. Recurrentis have been molded by human social transformations.
The majority of recurring fever bacteria – characterized by repetitive fever episodes – that infect humans spread through ticks, but Borrelia recurrentis is unique when transmitted through body lice.
In this research, scientists sequenced the full genome of four samples of this bacteria. These remains found in Great Britain date from 2,300 and 600 years ago and include the oldest B. Recurrentis genome to date.
The teeth of individuals contained remains of bacteria DNA. Two of the samples had relatively high amounts of the pathogen, suggesting that these people could have died of a serious and acute infection, or that the DNA was especially well preserved, reports a statement from Francis Crick.
The researchers analyzed the differences between the ancient and current genomes of B. Recurrentis to determine how it has changed over time, and established, thanks to advanced techniques, that the species probably diverged from its nearest ‘cousin’ transmitted by ticks, B. Duttonii, between 6,000 and 4,000 years ago.
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Ancient DNA reveals how a pathogen began to use lice to infect humans
They compared the genomes of both and found that much of it had been lost during the tick transition to Piojo, but that new genes had also been won over time.
These genetic changes affected the ability of the bacteria to hide from the immune system and also to share DNA with neighboring bacteria, which suggests that B. recurrentis had specialized to survive within the human louse.
This divergence between bacteria occurred during the Neolithic transition to the Bronze Age. It was a time of changes in the lifestyle of humans, which began to domesticate animals and to live in densest settlements.
This was able to facilitate the propagation of B. recurrentis from one person to another -now the disease can also occur more in places with bad sanitary conditions or overcrowding.
Researchers also raise the possibility that the development of sheep breeding to obtain wool at the time has been an advantage for the pathogens transmitted by lice, since wool offers better conditions for them to put eggs.
They conclude that the evolution of B. recurrentis highlights that a combination of genetic and environmental changes can contribute to pathogens spread and infect more easily to populations.
The researcher Pooja Swali points out that the recurring fever transmitted by lice is a negligible disease with few modern genomes, which makes it difficult to study its diversity.
“Adding four ancient genomes of B. Recurrentis to the mixture has allowed us to create an evolutionary temporal series and shed light on how the genetics of the bacteria has changed over time.”
With EFE information.
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