Wednesday, September 10, 2008

Altruism in Bacteria? Allowing Yourself to Die for the Good of the Species

ResearchBlogging.orgAltruism in general is an interesting concept from an evolutionary perspective. As defined by, altruism is "the principle or practice of unselfish concern for or devotion to the welfare of others," or more specifically, it is "behavior by an animal that may be to its disadvantage but that benefits others of its kind, such as a warning cry that reveals the location of the caller to a predator. " On the surface, this appears to be in direct opposition to the idea of survival of the fittest. In reality, this is not the case. However, the point of this article is not to delve into such evolutionary relationships.

Rather, I would like to point out that the practice of altruism is not limited to humans, or even animals. Animal behavioralists have described altruistic behavior in such species as chimpanzees, rats, dogs, and many others. A recent study by Ackermann, et al in last month's Nature, shows a form of altruistic behavior being practiced by Salmonella typhimurium.

S. typhimurium utilizes a Type III Secretion System (T3SS) which triggers inflammation when expressed in the gut of the bacteria's host. This inflammation kills off any competative organisms and allows the bacterium to colonize further into the gut tissue. Since the inflammation response is primarily directed at cells within the gut tissueAn example of a Burkolderia Type 3 SS. Image Credit: Chen Kang 2002, rather than the lumen, those bacteria residing within the tissue are at a high risk of death.

These researchers show that within the gut lumen, only 15% of the bacteria are expressing the T3SS (even if genetically clonal). Those that are found within the gut tissue are all expressing the T3SS. In fact, the T3SS is necessary for colonization of the gut tissue. So, in essence, those cells expressing this secretion system are able to enter the gut tissue and stimulate a response which can kill competitors (but also themselves). The 85% not expressing this T3SS in the lumen, can not enter the tissue and so are not at high risk for death, BUT are able to benefit from the death of the competiting organisms. One review of this study called the cells utilizing the T3SS "kamikaze bacteria" destroying themselves to benefit the greater good.

I believe that this observation, from a reductive standpoint, can help to show that complex social behaviors can be seen in "simpler organisms." This experiment also demonstrates how altruistic genes can be kept within a population, despite destruction of the organisms expressing the suicide genes.

From my understanding, how this phenotypic switching (or as the authors refer to as "phenotypic noise") occurs is as of yet unknown, and will be interesting to discover. The behavior of phenotypic switiching (whereby clonal organisms express a variant of their genes set in identical environments) is seen in other circumstances, including bacterial persistance in the face of antibiotics.

Ackermann, M., Stecher, B., Freed, N., Songhet, P., Hardt, W., & Doebeli, M. (2008). Self-destructive cooperation mediated by phenotypic noise Nature, 454 (7207), 987-990 DOI: 10.1038/nature07067