So we are now headed off to a new, shiny server which will hopefully make us all happy and responsive again! And what better way to start it off than with a paper that BLOWS MY MIND?!?!
I’ll admit that Ed actually blogged it before me (though I had intended to blog it for a while), and so at first I thought I’d leave it alone, but then Ed and Razib put on the peer pressure to have me blog it, too. I’m a doormat for those guys. 🙂
But really, it’s a great paper to blog. When I first looked at it, I thought “oh, maybe it’s your basic viral gene transfer and they got it looking all nice”. But when I saw the specific results…my mind was blown that much more.
So here we go, let’s talk about some antibiotic resistant bacteria.
I’m sure you’ve all heard about antibiotic resistance in bacteria. Really basically, this results from evolution in action (insert action music). When you give antibiotics to a pile of bacteria, you run the risk of a few of them having mutations which make them resistant to that antibiotic. So rather than getting killed, they can make it, and keep reproducing. Because they keep reproducing (and with their less resistance brethren around, it means they have more space and resources to reproduce into), you end up with larger populations of bacteria that are resistant to antibiotics. This is obviously a problem, because it means that bacterial infections that we could formerly treat easily are now VERY difficult to fight.
But when bacteria grow resistant to an antibiotic, do the non-resistant mutants ALL die out? Apparently not. And the paper wanted to know WHY.
So they exposed a colony of your basic E. Coli to the antibacterial drug norfloxacin (an antibiotic used to treat things like urinary tract infections). They carefully measured the dose of antibiotic so that roughly 60% of the bacteria would kick it in each bacterial culture. You might think that this wouldn’t be very useful to kill the infection, and indeed it’s not. What they wanted to see was how the bacteria populations evolved and changed over time as a population, rather than as single bacteria.
Sure enough, as has been seen loads of time by now, the colonies developed resistance to the antibiotic. But the scientists noticed something interesting. When they took individual cells out of the population and looked at them, they saw that they had a few bacteria that were REALLY resistant to the antibiotic (highly resistant isolates or HRIs). What was surprising was that MOST of the bacteria were NOT really resistance to the antibiotic! They were Less resistance isolates (LRIs), and somehow, they were still making it by being associated with their HRI buddies.
So what was causing this sharing of antibiotic resistance? It turns out the resistant cells were producing lots of indole. Indole is a compound that is naturally produced by growing, healthy cells. But it’s got a role to play, it is a chemical signal to other bacteria in the area to harden the f**k up.
(My name is indole, and I’m telling you that Bacteria need to harden the f**k up)
In the presence of indole, bacteria will increase production and activity of drug-efflux pumps (which can then pump the antibiotic out of the bacterial cells), and will also turn on mechanisms to protect the bacteria from oxidative stress. Normal, healthy cells produce indole. Dead and dying cells do not.
But what is it about those few hardened up indole producing cells that allows them to survive? Well, it’s not the indole itself. Instead, it’s the resistant bacteria. They had mutations which allowed them to survive, producing more drug-efflux pumps and resisting chemical damage. Then those cells, which had the hardcore mechanisms allowing them to be healthy, produced indole like normal healthy cells, telling the non-resistant cells to harden the f**k up.
The model they developed looks like this:
The far left is a section of normal, healthy cells. You can see they are all producing indole (in green). The middle panel is cells under stress from an antibiotic (in red). There is no green indole to be seen because the red cells are dead or dying. In the far right, though, a resistant bacteria comes on the scene. He feels just fine and starts producing indole again. Cells in the vicinity pick up on the indole signals and push the antibiotic out. The resistant cell is protecting his neighbors.
But this isn’t all free and dandy for the resistant cells. Pumping out indole actually does take energy, and so the resistant bacteria have a COST to helping the cells around them. The authors hypothesize that this is actually bacteria level kin selection. Because bacteria reproduce asexually, all the bacteria share highly similar genes with each other. So the resistant bacteria has a vested interest in protecting her kin, who may then develop other beneficial mutations as a result of being able to stay alive. You could even call it a form of altruism.
Sci thinks this is all incredibly cool, but she does have one question. If a normal, healthy cell produces indole anyway, and the resistant bacteria are producing indole because they are resistant and therefore normal and healthy in the presence of antibiotics, why does producing indole have a cost for these bacteria? I mean, maybe producing indole ALWAYS has a cost, and if it does, why would it have MORE of a cost in these drug-resistant bacteria? It’s not addressed in the paper (not that they necessarily could, it’s only four pages, as required by letters to Nature, which Sci thinks is rather foolish and not necessarily to their benefit, as they then put every flipping thing into the supplementary material, but that’s a rant for another time), but does anyone know? Any microbiologists out there know about indole production? I think that’d be some good follow up. How much does kin protection really cost?
Lee HH, Molla MN, Cantor CR, & Collins JJ (2010). Bacterial charity work leads to population-wide resistance. Nature, 467 (7311), 82-5 PMID: 20811456