The evolution of a gene has allowed the bacterium of the plague to modulate its virulence and the duration of life of infected guests over time.
The black plague was the most lethal pandemic in human history, capable of killing from a fifth to a quarter of the European population to its debut in the 14th century and to recur, with subsequent waves, until 1840. How did the plague bacterium evolved over time? How did he change his virulence, so as to regulate the life time of the bodies that infected – and therefore the time available to spread?
A search published on Science attributes the “spirit of adaptation” of the pathogen to the evolution of a particular gene, whose number of copies has determined on the one hand the lethality of the plague, on the other the duration of the survival of its guests.
The conclusions of the study respond to some fundamental questions about how the pathogens to pandemic potential relate to human populations. As viruses and bacteria do to evolve different levels of virulence So as to persist for a long time in infected organisms? What regulates the ability to modulate their degree of lethality?
Three pandemics. McMaster University scientists in Ontario, Canada and the Pasteur Institute in France examined hundreds of DNA champions of Yersinia Pestis taken from ancient and modern victims of the plague.
Historians identify three main pandemics of plague: the Justinian plague, which began in the mid -500 DC and continued for about three centuries, between Europe and the Middle East; the black plague, spread from the mid -14th century and later responsible for different epidemics in Europe, until the beginning of the 19th century; And a third, which started from China in 1855 and still in progress, although controlled almost everywhere in its lethal effects, thanks to antibiotics.
A crucial gene. Scientists have sought in the champions a gene known as planepresent in many copies in the genetic code ofY. Pestis. This gene allows the bacterium to penetrate the lymph nodes by escaping immune surveillance and from there to spread to the rest of the body.
Less and less copies. The total number of copies of the Pla was decreased in samples dating back to the latest pandemic waves of plague. And this decrease was hand in hand with a drop of 20% of the lethality of the bacterium and with a longer duration of the infection. It means that thanks to this reduction, the guests of the plague – the rats: men are in fact “accidental victims” – they could live longer and spread the virus for longer, before succumbing to pathogen.
On the other hand, when the Pla was present in a high number of copies, the disease was much more virulent and infected animals died faster. The importance of the Pla and its ability to modulate the lethality of the bacterium have been confirmed by studying the spread of the bubonic plague in the mice.
Let’s let them wander … In the end, even the strains with reduced copies of Pla went extinct, yet another change in the relationship between the pathogen and its guests. The ancient and modern strains of Y. Pestis they followed Very similar trajectories in the reduction of copies of the Pla gene. Both for the plague of Justinian and for the black plague, this change took place a hundred years after the first epidemics.
For scientists this allowed the infected rats to live longer and spread the infection, ensuring reproductive success of the bacterium. The rats lived in cities in close contact with humans. The plague bacterium needed rodents to survive long enough to transmit the plague to new guests and allow them to continue its transmission chain.
