Outgrowing the Plague

Every year about 10-15 people in the U.S. contract the plague. Just the sound of the world plague sounds ominous. But the illness is much less of a death sentence than it was during the Dark Ages. Now, a quick dose of antibiotics and the plagued person is right as rain.

After completing the first reconstruction of the plague’s genome, scientists have discovered why the Black Death is just an annoying bug to us now. They discovered that the disease itself hasn’t changed much in hundreds of years but we have.

After extracting DNA from dried blood in the teeth of Londoners who died from the Black Death which killed nearly half of 14th Century Europe, scientists reconstructed the genome using only skeletal remains.

Kirsten Bos, a PhD candidate at McMaster University in Ontario played dentist to the plague victims. The anthropologist who specializes in skeletal pathology and infectious disease in past human populations. She then removed 40 teeth from some of the 600 skeletons housed in the Museum of London, drilled into the pulp inside the teeth to extract a black powdery material, which was likely dried blood that contained DNA from the plague bacteria.

And when she was done, Bos returned the teeth, minus a little DNA, to the skeletons at the museum.

While studying the genome of the original Black Death bacteria and the strain commonly found today, the science team discovered astonishing similarity between them. The strain that ravaged Europe, killing 50 million Europeans between 1347 and 1351 and the plague strain today which sickens about 2,000 people each year are almost the same.

Scientists found only a few dozen changes among more than 4 million building blocks of DNA. And they found no discernible reason why the virulent pathogen of yore is so tame today.

Johannes Krause of the University of Tubingen in Germany says, “They’re almost identical. Even a mother and a child show more genetic differences than the ancient Black Death strain and modern plague strain.”

In the intervening years, the bacteria has changed very little while humans have changed a lot. Changes in medical treatment, sanitation and economics put people in a stronger position to fight the plague, which is generally passed from fleas on rodents to people or livestock.

The study authors say that the plague was so deadly because circumstances then were different not because the bacteria was particularly lethal.

Scientists also discovered that by being so devastating the plague essentially changed the human immune system. Study co-author Hendrik Poinar of McMaster University says, “It changed the human immune system, basically wiping out people who couldn’t deal with the disease and leaving the stronger to survive.”

Julian Parkhill, a disease genome expert at the UK Wellcome Trust Sanger Institute wasn’t involved in the Black Death genome project but has studied the bacteria. He says, “Getting an effectively complete genome sequence of a bacterium that lived nearly 700 years ago is incredibly exciting.”

Chris Chase, a veterinarian from South Dakota State University says this research has promise for understanding livestock diseases too. He says, “I could see that it could have big effects on cattle diseases that have been with us for a long time, pathogens like brucellosis and tuberculosis, and how they have changed under vaccine pressure from their ancestors. That can that help us to design new vaccines.”

This is the first time a science team has been able to reconstruct an pathogen of this size. Several years ago a team rebuilt the 1918 influenza virus contained in the lungs of frozen Eskimos. The genome of the plague bacterium is much larger than the flu virus.

Genome Sizes:
Black Death — 4,367,867 base pairs
1918 Flu — 582,970 base pairs
Human — 3,000,000,000 base pairs

Base Pair Basics

In molecular biology and genetics, the linking between two bases on opposite complementary DNA strands that are connected via hydrogen bonds is called a base pair (often abbreviated bp).

In basic Watson-Crick DNA base pairing, adenine (A) forms a base pair with thymine (T) and guanine (G) forms a base pair with cytosine (C). In RNA, thymine is replaced by uracil (U).

Some DNA- or RNA-binding enzymes can recognize specific base pairing patterns that identify particular regulatory regions of genes.

The size of an individual gene or an organism’s entire genome is often measured in base pairs because the structure of DNA is usually a double helix. Hence, the number of total base pairs is equal to the number of nucleotides in one of the strands (with the exception of non-coding single-stranded regions of telomeres).