As David Winter sets out to blog about the exciting Tuatara sequencing project, just in time for Monday Micro comes news that scientists have used sequencing to untangle a little more of the history of leprosy.
Just in case you need reminding, leprosy*, also known as Hansen’s disease, is caused by the extremely slow growing bacterium Mycobacterium leprae. Scientists still haven’t identified the correct nutrient mix to grow M. leprae in the lab, instead the organism has to be grown in the footpad of a mouse or by infecting a nine-banded armadillo. Leprosy was endemic in Europe in the middle ages, but these days the majority of new cases are in south east Asia, with 73% of the 219,075 cases reported globally in 2011 coming from this region.
A paper just out in the journal Science describes the whole genome sequencing of a number of M. leprae isolates – some taken from modern day patients and others from medieval skeletons excavated in Denmark, Sweden and the UK. So what can these sequences tell us? Well, firstly that there were at least two distinct genotypes of the M. leaprae bacterium circulating in Europe in medieval times – one described as “distinctly European” and which was spread to North America by European settlers, while the other is now more commonly seen in the middle east.
The other interesting information from the genomes is the number of pseudogenes – genes which have lost their ability to encode proteins or are no longer expressed. Normally these genes are lost by the bacterium, but in M. leprae half of its genome is made up of pseudogenes. Compare this to the related microbe M. tuberculosis which has only a handful of pseudogenes within its 4000 gene genome. As many of M. leprae‘s pseudogenes are genes that would have previously been involved in metabolism and nutrient uptake, this explains why the bacterium is so fastidious.
*Oh, and despite popular belief, leprosy doesn’t make your arms and legs fall off. But if can cause permanent damage to the skin, nerves, limbs and eyes if left untreated.