Cooperation among Microorganisms

September 2006 | Volume 4 | Issue 9 | e299 One of the organizing principles of life on Earth is that cells cooperate. This is evident in the case of multicellular organisms, from nematodes to humans, but it also appears to apply widely among single-celled organisms such as bacteria, fungi, and amoeba. In many cases, the label “single-celled” applies to only part of the life cycle of these organisms. For example, the model amoeba Dictyostelium discodium is singlecelled under conditions of nutritional abundance, but upon starvation, it communicates to form aggregates that subsequently pass through multicellular stages of slug and fruiting body. Indeed, in light of recent discoveries of communication among bacteria and the importance and prevalence of bacterial biofi lms, “single-celled” may turn out to be a misnomer even for these organisms. Here we highlight some of the better-studied examples of cooperation among microorganisms and attempt to identify some of the important questions in this emerging fi eld. Understanding cooperation among microorganisms presents conceptual and mathematical challenges at the interface of evolutionary biology and the theory of emergent properties of independent agents, two of the most exciting areas in modern mathematical biology. Most of the best-studied cases of cooperation among microorganisms concern intraspecies cooperation. An example of this is quorum sensing among bacteria, in which cells produce, secrete, and detect small molecules, called autoinducers. At high enough autoinducer concentrations (high cell densities), the bacteria enter a new mode of existence characterized by expression of genes associated with collective behaviors that are best carried out in concerted fashion by many cells [1]. These behaviors include the formation of protective biofi lms, the expression of virulence factors to attack a host, the production of light, the establishment of competence to exchange DNA (a bacterial form of sexual recombination), and many others. The signaling pathway for one of the betterstudied quorum-sensing circuits, that of Vibrio cholerae, the human pathogen, is shown in Figure 1. Another well-studied example of intraspecies cooperation concerns the cyanobacterium Anabaena, which grows in long chains, in which approximately one cell out of ten differentiates into a heterocyst that provides fi xed nitrogen for the neighboring cells (Figure 2) [2]. Dictyostelium is probably the moststudied model for cooperation among eukaryotic microorganisms, but even in the nonmotile eukaryote Saccharomyces cerevisiae, hyphal growth (that is, fi lamentous growth) can be viewed as a cooperative mechanism for foraging. Cooperation between different microorganism species is much less understood, or studied, partially for practical reasons, but also because the ubiquity of communication among microorganisms has only recently been appreciated. Nevertheless, it has been clear for many years that bacteria form Cooperation among Microorganisms Ned S. Wingreen, Simon A. Levin