PARENT SESSION
Oral Session #90: Evolutionary Ecology.
Presiding: R. del Castillo
Thursday, August 8. 1:00 PM to 3:45 PM. Apache Meeting Room, TCC.

Bacterial "quorum sensing" and cooperation: Why don't cheaters win?

Murray, Jill^*,1, ¹ University of California, Santa Barbara, Santa Barbara, CA

ABSTRACT- One of the most exciting developments in microbiology has been the recent discovery that bacteria engage in extensive chemical communication, often leading to density-dependent behavior ('quorum sensing'). First studied in bioluminescent bacteria, quorum sensing has since been found to regulate basic ecological processes in garden-variety bacteria. For example, quorum sensing has been tied to the release of extracellular enzymes, used in foraging, and the production of antibiotics, used in defensive and competitive behaviors. These observations have led to frequent speculation that bacteria are cooperative, but scant theory has been developed to test these speculations. In particular, little attention has been paid to the potential for cheaters to pervade populations and prevent the persistence of cooperative genotypes. The research described here used analytical and simulation models to predict when and how cooperative strategies should stabilize in bacterial populations. Results showed that for most behaviors, the success of cheating individuals should drive populations away from stable cooperation. An exception was found in the analysis of a strategy using communication to predict the onset of famine conditions and to govern the transition to an inactive state, i.e. 'stationary phase' or dormancy. This cooperative strategy was stable due to a unique feature of quorum sensing, that is, the autocatalytic property of the signal compound. In order to receive the benefit (gain information about imminent famine), an organism must also participate (produce information); cheaters enter starvation without the protective proteins transcribed by cooperators (communicators). Interestingly, bacteria have evolved with the linkage of many density-dependent, potentially cooperative behaviors to a limited number of regulatory systems (often just one). Thus, cooperative behaviors may stabilize in a novel way in bacteria. For example, 'antibiotic cheaters' might persist in the short run, but they may be expunged from populations during famine conditions due to higher mortality rates in 'starvation cheaters.' Cooperative strategies that would not persist on their own may be maintained by their linkage to a limited number of strategies that are resistant to cheating.

KEY WORDS: bacteria, evolution, cooperation, quorum sensing