Michigan State E-Coli experiment

[joeactor]

Hey all,

Great blog about a new experiment done at Michigan State:
http://scienceblogs.com/loom/2008/06/02 ... lution.php

The upshot: they observed e-coli evolve under laboratory conditions.  The e-coli evolved over 44,000 generations to adapt their diet from glucose to citrate.  AND, they have samples of each generation to show the progression.

Gotta love it!,
JoeActor

[Will]

The bacteria in the flasks today breed 75% faster on average than their original ancestor. Lenski and his colleagues have pinpointed some of the genes that have evolved along the way; in some cases, for example, the same gene has changed in almost every line, but it has mutated in a different spot in each case. Lenski and his colleagues have also shown how natural selection has demanded trade-offs from the bacteria; while they grow faster on a meager diet of glucose, they've gotten worse at feeding on some other kinds of sugars.

After 33,127 generations Lenski and his students noticed something strange in one of the colonies. The flask started to turn cloudy. This happens sometimes when contaminating bacteria slip into a flask and start feeding on a compound in the broth known as citrate. Citrate is made up of carbon, hydrogen, and oxygen; it's essentially the same as the citric acid that makes lemons tart. Our own cells produce citrate in the long chain of chemical reactions that lets us draw energy from food. Many species of bacteria can eat citrate, but in an oxygen-rich environment like Lenski's lab, E. coli can't. The problem is that the bacteria can't pull the molecule in through their membranes. In fact, their failure has long been one of the defining hallmarks of E. coli as a species.

If citrate-eating bacteria invade the flasks, however, they can feast on the abundant citrate, and their exploding population turns the flask cloudy. This has only happened rarely in Lenski's experiment, and when it does, he and his colleagues throw out the flask and start the line again from its most recently frozen ancestors.

But in one remarkable case, however, they discovered that a flask had turned cloudy without any contamination. It was E. coli chowing down on the citrate. The researchers found that when they put the bacteria in pure citrate, the microbes could thrive on it as their sole source of carbon.

In nature, there have been a few reports of E. coli that can feed on citrate. But these oddballs all acquired a ring of DNA called a plasmid from some other species of bacteria. Lenski selected a strain of E. coli for his experiments that doesn't have any plasmids, there were no other bacteria in the experiment, and the evolved bacteria remain plasmid-free. So the only explanation was that this one line of E. coli had evolved the ability to eat citrate on its own.

Blount took on the job of figuring out what happened. He first tried to figure out when it happened. He went back through the ancestral stocks to see if they included any citrate-eaters. For the first 31,000 generations, he could find none. Then, in generation 31,500, they made up 0.5% of the population. Their population rose to 19% in the next 1000 generations, but then they nearly vanished at generation 33,000. But in the next 120 generations or so, the citrate-eaters went berserk, coming to dominate the population.

This rise and fall and rise suggests that the evolution of citrate-eating was not a one-mutation affair. The first mutation (or mutations) allowed the bacteria to eat citrate, but they were outcompeted by some glucose-eating mutants that still had the upper hand. Only after they mutated further did their citrate-eating become a recipe for success.

This is so fucking cool.  

[crocofish]

Isn't poop bacteria wonderful !?  

I have been following E. Coli since the 1970's.  When I was a teenager, I was fascinated by genetic engineering, and E. Coli was a favorite for genetic manipulation by the researchers.

It's not surprising that E. Coli could develop a mutation to eat citric acid.  We see bacteria develop resistance to antibiotics regularly.  But it is very significant that the mutation and selection process happened in a lab environment with a clear history recorded, nice detailed and direct evidence.

[tornado]

Very, very cool. Thanks for sharing this.