Scientists Sequence Black Death Bacteria

First time accepted submitter Quince alPillan writes "The bacteria behind the Black Death has a very unusual history. Its ancestor is an unassuming soil bacterium and the current strains of Yersinia pestis still infects thousands of people annually, but no longer causes the suite of horrifying symptoms associated with the medieval plagues. The radical differences, in fact, had led some to suggest that we had been blaming the wrong bacteria. Now, researchers have obtained DNA from some of London's plague victims, and confirmed that Y. pestis appears to be to blame. But the sequences also suggest that the strains of bacteria we see today may be different from the ones that rampaged through Europe."

Not necessarily

[Moraelin]

Well, the actual question is: exactly how different. Yes, it's clear that some mutations are inevitable, but unless there's some clear evolutionary pressure, you may still find a bacterium that works by and large just like its ancestors.

Now it may seem that for a parasitic bacterium, not killing its host would be an advantage. And indeed in some other bacteria we can see a sort of a survival-of-the-sickest kind of selection.

But this is a soil bacterium. If it ends up in some host and kills it, worst that can happen is that it ends up back in the soil. It has nothing to lose by killing its host, and in fact everything to gain, since once the host is dead there's no more immune system killing the bacteria.

This kind of bacteria that have nothing to lose by killing the host are the most deadly and dangerous. Not just this, but see for example cholera too. That's a bacterium that not only has nothing to gain by peacefully staying inside you and not killing you, but is actually trying to get out of your body ASAP. Whether you live or die in the process, meh, it makes no difference for that one.

Additionally, for Y Pestis, the capability of clotting blood and forming colonies that plug blood vessels actually helped it spread too. The same mechanism makes it plug the stomach of fleas. The flea then will literally starve to death no matter how much blood it sucks, and driven by hunger, will go infect another host too.

So we have a bacterium for which the plasmid that kills its host:

1. isn't detrimental to the bacterium, since it can live just as well in a dead host or in soil, and

2. is actually beneficial to the bacterium, since it makes fleas spread it around.

That's one tough combo to evolve out of. There is no real survival benefit in losing those genes.

So while, yes, you would expect that bacteria can and will mutate in time, but it's not clear at all why this one would change in exactly that aspect.

Yet something seems to have changed. What and why? Those are the questions.