Good bacteria: can microbes be altruistic?

Surprisingly, even some of the smallest organisms on Earth are capable of altruism. George Salmond, from the University of Cambridge, studies altruism among bacteria and he spoke with Chris Smith. First, Chris asked George about the circumstances in which bacteria can be altruistic....

George - Bacteria, like every other living organism on the planet are susceptible to virus infection. For bacteria these are called bacteriophages, or bacterial viruses, and these viruses only attack bacteria. The virus adsorbs to the bacteria, injects its genetic material, and essentially turns the bacterial cell into a factory for virus production. Then generally, these infected cells burst open to release lots of new virus which then goes on to infect other bacteria in the population.

So if you’re a bacterium, there’s a whole series of different strategies that have evolved to resist the potentially lethal effects of viral infection. And the one that we have found - the bacteria appear to commit suicide after viral infection. This superficially doesn’t seem like a clever strategy but, actually, that leads to the termination of replication of any invading virus and that means there’s no release of any new virus, and that means the rest of the bacterial population are protected from subsequent viral infection.

Chris - The bacteria are essentially putting themselves into isolation, albeit terminally, in order to prevent the spread of that infection through their population. How do they know they’ve been infected?

George - That’s a very good question. We know that only certain bacterial viruses can induce this system - it’s called abortive infection and essentially they must be titrating a particular viral signal, which we are currently trying to investigate. In their cells they have a thing called a toxin-antitoxin system and somehow the viral product seems to stimulate or destabilise this toxin-antitoxin system which leads to the killing of the bacterial cell - the suicidal event.

Chris - I suppose you could think of it a bit like a seesaw where you’ve got a balance. There’s this toxin trying to tell the cell to die, an antitoxin telling it to stay alive and when the virus comes along it adds weight to the end of the seesaw that says “die.” And that triggers the cell to then say well, I must wipe myself out because if I allow this virus to replicate in me I’m going to then infect the rest of my population. What’s the benefit to the population as a whole of that bacterium surrendering itself?

George - You need to think of bacterial populations, of course, as clonal.

Chris - What does that mean?

George - They’re all identical in a population.

Chris - Genetically because they’re splitting, so when one becomes two, becomes four, becomes eight, and so on it’s just because the cells are dividing, so they’re sharing exactly the same genetic information?

George - That’s right. They’re identical in that sense. So you can view that if one cell gets infected by a virus and give up it’s life then, essentially, it’s protecting the rest of the clones in that population.

Chris - So the genes survive because the same genes are running in that entire group of bacteria, but the individual cell that got infected has wiped itself out but it doesn’t really care because the genes are being propagated?

George - Absolutely.

Chris - How do you think that this behaviour evolves in the first place because that’s pretty complicated that they’ve been able to do that?

George - The system we’ve been investigating, we don’t know that that system is titrating the availability of virus infection in every case. We think that there maybe other environmental signals that also can stimulate this suicidal event, for example, nutritional stress.

Chris - So the bacteria, if they’re going hungry they could, for instance, think well, if I wipe myself out some of my friends may still make it?

George - That may be one strategy, yeah.

Chris - Are you saying that the same sort of system could be co-opted in response to lots of different stresses and pressures that the bacteria could just ultimately use this to minimise the number of viable organisms, therefore minimise the demand on the food supply that maximise the chances that strain or species will survive?

George - That may be happening in some cases. This is quite a complicated area because the amount of information on the mechanisms involved there’s a paucity of information there. But in our case, studying viral replication, it’s very clear that the population survives in the presence of the virus.