7.3 Bacteria engage in altruistic suicide

http://arstechnica.com/journals/science.ars/2007/10/26/bacteria-engage-in-altruistic-suicide

Bacteria engage in altruistic suicide

By John Timmer | Published: October 26, 2007 - 01:47PM CT

In multicellular animals, it makes a certain degree of sense for cells to commit suicide. As long as there's more to take their place, the loss of a few cells won't hurt the organism as a whole. Plus, if the cells involved are damaged, infected, or superfluous, they can actually improve the overall health of the organism by dying in an organized fashion. The same logic, however, doesn't obviously hold for single celled animals, which occupy environments where it's generally thought to be every cell for itself. That view has been receding under the weight of results that suggest that bacteria can act as a multicellular collective, organizing biofilms and identifying the number of fellow species-members through quorum-sensing signaling molecules. A paper in Science takes this sort of behavior to the next level, suggesting that bacteria can coordinate cellular suicide.

The system they use for doing this has previously been described in E. coli. In short, the bacteria constantly express a stable enzyme that, when active, will chew up the RNA in the cell, killing it. At the same time, they also express a less-stable inhibitor. When some sort of stress causes the bacteria to stop producing proteins, the inhibitor protein degrades first, setting its lethal target loose on the cell. The new data showed that this lethal combination is also held in check by a signaling system that bacteria use to sense their population density.

The authors found that they could trigger enough stress to cause E. coli to engage in cell suicide by hitting them with a brief dose of antibiotic, but only if the cells were growing in a dense culture; dilute cultures grew unperturbed. They next showed that the the bacteria sensed culture density through a soluble molecule by getting rid of the bacteria in a dense culture, and then using the remaining liquid to enable stress-based cell suicide in a dilute culture. Careful fractionation allowed them to isolate the signaling molecule involved, which is a five amino acid long peptide.

So, not only do bacteria engage in cell suicide, but they do so on the basis of signals from their fellow bacteria—it all sounds suspiciously like a multicellular organism, as the authors themselves note. The logic behind it also seems very similar to that which explains altruistic behavior in multicellular animals. In a dense, rapidly growing culture, most of the bacteria would be expected to be genetically related. When a source of stress, such as a virus or antibiotic, starts harming cells, the most efficient way to preserve their shared inheritance may be for some of the cells to sacrifice themselves.

Science, 2007. DOI: 10.1126/science.1147248

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