High cell density and close proximity of diverse species of microorganisms are typical of life in natural biofilms. These conditions give ample opportunity for both competitive and cooperative interactions between individuals of the same and different species. Cooperative behaviour benefits the group of neighbouring microbes but comes at a fitness cost for the cooperating individuals. This creates a conflict of interest between the fitness of the individual and the fitness of the group. Individuals that defect from cooperation and therefore do not pay the cost but nevertheless benefit from the cooperative behaviour of others are called cheaters. Cooperative behaviour in the presence of cheaters constitutes altruism towards the cheaters. The aim of this review is two-fold: first, to introduce key concepts from kin selection and group selection theory that allow us to understand how cooperative behaviour can evolve in the face of cheaters; secondly, to draw attention to the conflicts of interest prevalent in biofilms yet largely ignored in the biofilm literature. Examples discussed comprise growth restraint in stationary phase as an instance of the Prisoner's Dilemma, growth restraint to allow channel formation, restraint in resource consumption or economical use of resources as altruistic behaviour, population heterogeneity as insurance against environmental changes, cooperative investment in diffusible exoenzymes, cooperation of pathogens and virulence, diffusion sensing versus quorum sensing and the inflation of signals, antibiotic resistance as collective action, and programmed cell death.
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