Microbes are typically surrounded by different strains and species with whom they compete for scarce nutrients and limited space. Given such challenging living conditions, microbes have evolved many phenotypes with which they can outcompete and displace their neighbours: secretions to harvest resources, loss of costly genes whose products can be obtained from others, stabbing and poisoning neighbouring cells, or colonising spaces while preventing others from doing so. These competitive phenotypes appear to be common, although evidence suggests that, over time, competition dies down locally, often leading to stable coexistence of genetically distinct lineages. Nevertheless, the selective forces acting on competition and the resulting evolutionary fates of the different players depend on ecological conditions in a way that is not yet well understood. Here, we highlight open questions and theoretical predictions of the long-term dynamics of competition that remain to be tested. Establishing a clearer understanding of microbial competition will allow us to better predict the behaviour of microbes, and to control and manipulate microbial communities for industrial, environmental, and medical purposes.

The term "cheating" is used in the evolutionary and ecological literature to describe a wide range of exploitative or deceitful traits.Although many find this a useful short hand, others have suggested that it implies cognitive intent in a misleading way, and is used inconsistently. We provide a formal justification of the use of the term "cheat" from the perspective of an individual as a maximizing agent. We provide a definition for cheating that can be applied widely, and show that cheats can be broadly classified on the basis of four distinctions: (i) whether cooperation is an option; (ii) whether deception is involved; (iii) whether members of the same or different species are cheated; and (iv) whether the cheat is facultative or obligate. Our formal definition and classification provide a framework that allow us to resolve and clarify a number of issues, regarding the detection and evolutionary consequences of cheating, as well as illuminating common principles and similarities in the underlying selection pressures.

Since Hamilton published his seminal papers in 1964, our understanding of the importance of cooperation for life on earth has evolved beyond recognition. Early research was focused on altruism in the social insects, where the problem of cooperation was easy to see. In more recent years, research into cooperation has expanded across the entire tree of life, and has been revolutionised by advances in genetic, microbiological, and analytical techniques. We highlight ten insights that have arisen from these advances, which have illuminated generalisations across different taxa, making the world simpler to explain. Furthermore, progress in these areas has opened up numerous new problems to solve, suggesting exciting directions for future research. Relatedness: from anecdotes to broad generalisationsInclusive fitness theory shows how cooperation can be favoured by kin selection if it is directed towards relatives, who carry the gene for cooperation (relatives) 2 (Box 1). But how widespread is the importance of kin selection? And if it is important, how do we explain variation between individuals in the level of cooperation, or explain why cooperation is favoured in some species and not others?Recent research has shown that the relatedness (R) between interacting individuals has a clear and consistent influence on the evolution of cooperation, with both theory and data suggesting that the same factors play analogous roles at all levels of biology, from simple replicators and viruses, to complex animal groups (see below; Supplementary Tables 1 & 2). This role of relatedness has been demonstrated with a combination of methodologies, including observational, experimental, experimental evolution, across-species comparisons, and genomic.1a. Group formation. Consistent with kin selection favouring cooperation, there is considerable evidence that population structure and how groups form is a major determinant of whether cooperation is favoured 2 (Fig. 2).(i) Individuals are more cooperative in species where social groups form by staying together, compared with species where social groups form by aggregation, across a range of taxa, including bacteria, fungi, slime moulds, insects, and birds [19][20][21][22][23] .(ii) Individuals are more cooperative in species where females mate monogamously or with few males, in birds, mammals, insects and shrimps 13,19,21,[24][25][26] .(iii) When population structure is manipulated experimentally, in a range of microorganisms, including bacteria, fungi and viruses, the conditions which lead to high relatedness favour greater cooperation 15,[27][28][29][30][31][32] .(iv) The role of group formation has also been supported by sequence data. If a trait is favoured by kin selection, then lower relatedness will lead to weaker selection for that trait, and so the removal of deleterious mutations will be slower and less likely [33][34][35][36] . Consistent with kin selection playing an important role: (a) ant and bee species with either multiple mating or multiple queens, and hence reduced relatedn...

Pseudomonas aeruginosa, is an opportunistic, bacterial pathogen causing persistent and frequently fatal infections of the lung in patients with cystic fibrosis. Isolates from chronic infections differ from laboratory and environmental strains in a range of traits and this is widely interpreted as the result of adaptation to the lung environment. Typically, chronic strains carry mutations in global regulation factors that could effect reduced expression of social traits, raising the possibility that competitive dynamics between cooperative and selfish, cheating strains could also drive changes in P. aeruginosa infections. We compared the expression of cooperative traits - biofilm formation, secretion of exo-products and quorum sensing (QS) - in P. aeruginosa isolates that were estimated to have spent different lengths of time in the lung based on clinical information. All three exo-products involved in nutrient acquisition were produced in significantly smaller quantities with increased duration of infection, and patterns across four QS signal molecules were consistent with accumulation over time of mutations in lasR, which are known to disrupt the ability of cells to respond to QS signal. Pyocyanin production, and the proportion of cells in biofilm relative to motile, free-living cells in liquid culture, did not change. Overall, our results confirm that the loss of social behaviour is a consistent trend with time spent in the lung and suggest that social dynamics are potentially relevant to understanding the behaviour of P. aeruginosa in lung infections.

No abstract