Understanding the Past with Ancient Pathogens
Diana - Now it's time to join Meera Senthilingam. Meera's been out in London this week, finding out how the DNA of ancient pathogens, that's things like bacteria found in ancient human bones and tissues, can be extracted and sequenced to help us understand more about the evolution of diseases such as tuberculosis...
Meera - This week, I've come along to University College London to meet Dr. Helen Donoghue who's a senior lecturer in medical microbiology. Helen, you focus on the occurrence of particular pathogens in our past. So, what's the importance of understanding more about their occurrence in our past?
Helen - Well I happen to be in a department that has a long standing interest in a group of bacteria called mycobacteria that includes the bacteria that cause tuberculosis and leprosy. But it also so happens that these are particularly good subjects to look at in the past. This is because they are surrounded by very resistant cell walls which protects the DNA and the DNA itself, its chemical composition is more stable than that of some other bacteria.
Meera - What kind of samples do you work with and how old are they?
Helen - The age of the samples varies greatly. I think the oldest one I've ever looked at was from a Pleistocene Bison 17,500 years ago and the oldest human samples I've looked at were 9,000 years old. Generally, they're not quite so old as that. I've got large number of samples, for instance, from a collection from the 18th century in Hungary. So that would be the 17- or 1800s. And really, all the ages between then and ancient Egypt, everything in between.
Meera - Now Helen, we are here in your lab in the department of infection. You've got some samples here in front of us today. One particular one I can see looks like a rib. So tell me more about this sample.
Helen - Okay. Well, I'll take it out so we can see it a bit more clearly. It's from a child. It's a lower rib from the Hungarians from the 1800s. It's curved but it's about 15 centimetres long. We had quite a few children in this group of individuals. Children are not quite so susceptible as adults for tuberculosis but the results I've got so far do show that many of them were infected with TB. The reason why we're interested in looking at a rib is because tuberculosis is an infection we normally catch by breathing an infected aerosol, it goes into our lungs and the lungs are the main site of infection. And when somebody dies, the evidence of the disease can be left on the outer surface of the ribs that were next to the lung.
Meera - Now as well as the rib, what other parts of the body can you target when obtaining your samples?
Helen - Well once tuberculosis breaks out of the lung and gets into the bloodstream, it can go all around the body. Also, you can catch tuberculosis by swallowing infected sputum or by eating or drinking infected milk or meat, perhaps from an infected animal. But actually, that's very rate in the archaeological record. But I've certainly found evidence of tuberculosis in the abdomen and in bones that don't show any sign of any lesion, and that's because they were infected by the bloodstream.
Meera - How careful do you have to be with your samples, I imagine - first of all they must be rare, and second of all you must not want to destroy very much of it?
Helen - Well luckily, we don't need very much. So normally, when I do an extraction, I use between 20 and 60 milligrams of sample, that's if it's bony sample. You need rather more than that if it's mummified. You can have that much without destroying the bone.
Meera - So I guess having identified if particular individuals from the past had tuberculosis, how does this all come together? What does it tell us about our past?
Helen - Well let me just say something about one of the collections I've got which is from the 18th century Hungarians. They've got a very good archive so we know the age and family relationships of many of the individuals, and there are over 200 of them. That means we can do some epidemiology. We can look at family groups. We can look and see, because the samples were very well preserved, we can actually look at different lineages of the MTB (Mycobacterium tuberculosis) DNA.
We know there were at least two distinct lineages of the MTB DNA at that time and this is from an era before there were any antibiotics. We know there were many people in this population who lived to a great age even though they were infected with TB. We know there were other people who died early in life or in their early 20's of TB. This gives us the chance to look at the evolution of the bacterial infecting organism and its human host over time from an age before there was any distortion in how these bacteria evolve, because there was no antibiotic therapy. That enables us to compare that with the modern day trend and we can see that there is now a logarithmic, an exponential, increase in the rate of change of the organism.
Meera - And how important would you say it is then to understand more about tuberculosis?
Helen - Tuberculosis is probably the greatest cause of death from any single infectious disease in the world today. According to the World Health Organization, we think one third of the world's population is infected and in 2007, there were just under 2 million deaths. This is from a disease where we can treat it with antibiotics, even though drug resistance is becoming an increasing problem. So we really do need to understand our relationship between ourselves and this particular species.
Meera - How can this research help us understand more about our past and our ancestors?
Helen - Well, both tuberculosis and leprosy are examples of clonal diseases where the infecting bacterium has sometimes a lifelong relationship with its human host. And so, as humans migrate, these bacteria migrate with them. So by studying these organisms or the DNA from these organisms, from samples from different geographical sites around the world, we can understand how humans have migrated around the world.
Diana - That was Dr. Helen Donahue from University College London, talking to Meera Senthilingam about how reading an ancient pathogen's DNA can help us understand how the bacteria that cause diseases like tuberculosis have evolved alongside us.
Kat - And if you want to check out Helen's work, there's an exhibition showcasing her work on ancient tuberculosis called Body Matters which will be in the Leventis Gallery in the UCL Institute of Archaeology from the 17th of May.