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...

The type VI secretion system (T6SS) mediates interactions between a diverse range of Gram-negative bacterial species. Recent studies have led to a drastic increase in the number of characterized T6SS effector proteins and produced a more complete and nuanced view of the adaptive significance of the system. While the system is most often implicated in antagonism, in this review we consider the case for its involvement in both antagonistic and non-antagonistic behaviors. Clarifying the roles that T6S plays in microbial communities will contribute to broader efforts to understand the importance of microbial interactions in maintaining human and environmental health, and will inform efforts to manipulate these interactions for therapeutic or environmental benefit.

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