The plague is responsible for some of the worst catastrophes in human history, killing millions of people worldwide in at least three major epidemics. It was caused when the bacteria Yersinia pestis spread from rodents to humans via insects.
But that’s all history right?
Unfortunately not. There are currently 155 strains of this bacteria in circulation, some infecting thousands of people around the world every year. While most cases can be treated simply with antibiotics, there are a few of these strains which are showing the signs of antibiotic resistance. Which we all know is a VERY BAD THING.
So we need to find out where this disease is going. And actually, the best way to do that is to find out where it’s been.
Which brings me to teeth. Because teeth are incredible.
Teeth are made up in part of the most highly mineralised material in the body – enamel. This surrounds the dentine that makes up the bulk of your tooth, and contained within this is the pulp cavity, which is the living structure of your tooth – fed by the body’s blood and nervous system.
Now, this is the same system that the plague bacteria hijack in order to spread throughout your body. Which means that once infected, the bacteria can actually spread to inside your teeth. And if you should happen to die, the bacteria can become trapped inside.
Now enamel, because it’s so highly mineralised degrades muuuch slower than bones after you die, so it continues to protect the dentine and pulp cavity and any bacteria contained within, for hundreds or even thousands of years. Long after the evidence of the disease has degraded or been contaminated in the rest of your remains (in fact, long after the rest of your remains have degraded completely).
Making teeth like tiny little time capsules.
And time capsules, well they can be opened.
By sawing open a tooth’s enamel, you can drill directly into the dentine and pulp cavity, creating a sort of powder of biological matter, from which you can then extract any DNA that is present.
Once the DNA has been sequenced and identified, if any bacteria DNA is present – like Yersinia pestis – it can then be compared against other known strains, either from historical or modern samples.*
This is how we know that all of the human-infecting strains currently in circulation are all direct descendents of the strain which caused the Black Death during the medieval period. This is helping researchers to understand the bacteria’s mutations over time giving us information on its origins, transmission, adaptations, and therefore importantly its possible futures. Which is a very good thing.
However, recent DNA evidence from teeth almost 1500 years old has shown that the strains responsible for the medieval plague and the earlier Roman plague are actually distinct – emerging independently as epidemics.
This suggests the plague bacteria may be capable of rapidly mutating, but remaining highly infectious – and deadly – to humans.
So, while another epidemic is unlikely, please remember to brush your teeth – just in case we need them in the future.
* Yes, I am aware that this is a very simplified explanation – for more information on this process see my next blogpost on polymerase chain reaction (PCR) and ancient DNA (aDNA).
For more information on the archaeology and aDNA identification of the 1500 year old teeth that were recently in the news, see this great (open access) article by Harbeck, et al. 2013.