PrefaceMost natural environments harbor a stunningly diverse collection of microbial species. Within these communities, bacteria compete with their neighbors for space and resources. Laboratory experiments with pure and mixed cultures have revealed many active mechanisms by which bacteria can impair or kill other microbes. Additionally, a growing body of theoretical and experimental population studies indicate that the interactions within and between bacterial species can profoundly impact the outcome of competition in nature. The next challenge is to integrate the findings of these laboratory and theoretical studies, and to evaluate the predictions they generate in more natural settings. IntroductionExamples of true charity and altruism in human societies are highly lauded, and rightfully so, but are far from the norm. Competition is a fact of modern life, with individuals and institutions vying to gain advantage in terms of finances, material resources, and status. In capitalist societies, competition is thought to continually hone the attributes of competing entities, improving their efficiency and defining their activities and structure. The high level of competition in human society in many ways mirrors the comparatively ancient and complex interactions observed at virtually every level in the natural world. The battle for resources through which organisms survive and pass on genes to the next generation can often be fierce and unforgiving. This leads to natural selection, which provides the driving force for innovation and diversification between competing organisms 1 .In animals and plants, there are a large number of well studied examples of populations which are held in balance, or driven to transition, by competitive forces. Connell's barnacles provide a classic example 2 . He found that in intertidal zones in Scotland, Balanus barnacles were always found closest to the shore, while Chthamalus barnacles grew further up the rocks. If he experimentally removed the Balanus barnacles from the lower areas, Chthamalus could grow there, but upon reintroduction of Balanus, Chthamalus would eventually be crowded out by the more competitive Balanus. However, Balanus could not grow further up the rocks, due to desiccation sensitivity. Thus, the habitat of Chthamalus was limited to areas where it could escape from competition with Balanus, an example of competitive exclusion.Similarly, most microorganisms face a constant battle for resources. Vast numbers of microbes are present in all but the most rarified environments. Tremendous microbial diversity has been revealed by new molecular methodologies such as metagenomic sequencing and deep microbial tag sequencing 3,4 . These approaches and others have begun to reveal that underlying the numerically dominant microbial populations is a highly diverse, low-abundance population (described as the rare biosphere, see 3 ). Members of the rare biosphere that are amplified under favorable conditions to which they are pre-adapted can give rise to discrete, abundant NIH Publ...

Quorum sensing is an example of community behavior prevalent among diverse bacterial species. The term "quorum sensing" describes the ability of a microorganism to perceive and respond to microbial population density, usually relying on the production and subsequent response to diffusible signal molecules. A significant number of gram-negative bacteria produce acylated homoserine lactones (acyl-HSLs) as signal molecules that function in quorum sensing. Bacteria that produce acyl-HSLs can respond to the local concentration of the signaling molecules, and high population densities foster the accumulation of inducing levels of acyl-HSLs. Depending upon the bacterial species, the physiological processes regulated by quorum sensing are extremely diverse, ranging from bioluminescence to swarming motility. Acyl-HSL quorum sensing has become a paradigm for intercellular signaling mechanisms. A flurry of research over the past decade has led to significant understanding of many aspects of quorum sensing including the synthesis of acyl-HSLs, the receptors that recognize the acyl-HSL signal and transduce this information to the level of gene expression, and the interaction of these receptors with the transcriptional machinery. Recent studies have begun to integrate acyl-HSL quorum sensing into global regulatory networks and establish its role in developing and maintaining the structure of bacterial communities.

The importance of accurate demographic information is reflected in the United States Constitution, Article 1, which provides for a decennial census of this country's human population. Bacteria also conduct a census of their population and do so more frequently, more efficiently, and as far we know, with little if any of the political contentiousness caused by human demographers. Many examples have been found of particular bacterial genes, operons, or regulons that are expressed preferentially at high cell densities. Many of these are regulated by proteins related to the LuxR and LuxI proteins of Vibrio fischeri, and by a diffusible pheromone called an autoinducer. LuxR and LuxI and their cognate autoinducer (3-oxohexanoyl homoserine lactone, designated VAI-1) provide an important model to describe the functions of this family of proteins. LuxR is a VAI-1 receptor and a VAI-1-dependent transcriptional activator, and LuxI directs the synthesis of VAI-1. VAI-1 diffuses across the bacterial envelope, and intracellular concentrations of it are therefore strongly increased by nearby VAI-1-producing bacteria. Similar systems regulate pathogenesis factors in Pseudomonas aeruginosa and Erwinia spp., as well as T1 plasmid conjugal transfer in Agrobacterium tumefaciens, and many other genes in numerous genera of gram-negative bacteria. Genetic analyses of these systems have revealed a high degree of functional conservation, while also uncovering features that are unique to each.