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Microorganisms frequently live in dense and diverse communities, termed biofilms, which can be surface- bound or free-floating and are usually encased in a secreted polymer matrix1,2. Biofilms are indispensable to global biogeochemical cycling3,4 and the healthy functioning of the microbiota of multicellular organisms5. By contrast, biofilms also cause antibiotic-tolerant infections6 and the destruction of surfaces and flow systems; they are therefore of great concern in medical and industrial settings79.
Biofilm-dwelling cells interact intimately and influence each others evolutionary fitness through social
phenotypes10,11 (BOX1). Many of these behaviours are simple forms of cooperation that benefit neighbouring cells, such as the secretion of nutrient chelators12,13, digestive enzymes14, surface adhesins15, wetting agents16, structural polymers17 and signalling molecules18. For example, outside human hosts, Vibrio cholerae forms biofilms on environmental particles of the structural polymer chitin, which it digests through communally beneficial chitinases19,20. Diverse biofilm-dwelling bacteria also produce siderophores, which are low-molecular-mass compounds that bind to and solubilize otherwise inaccessible iron, a frequently limiting nutrient in the abiotic environment and within host organisms12,21. Owing to their cooperative and collective behaviour, biofilm-dwelling cells have substantial advantages compared with solitary cells, including an increased resilience against external threats and a higher efficiency in digesting complex nutrients20,22. Microorganisms are thus fundamentally social organisms, and their cooperative phenotypes are pivotal to how they affect the world aroundthem.
Social interactions can also be competitive however, and cells within a microbial community should not be assumed to work together harmoniously11. Competition
for limited space and resources is pervasive23,24, and
many social phenotypes harm other strains and species. Antibiotic secretion, the direct injection of toxins into adjacent cells, and mechanisms for displacing or suffocating neighbours all target competitors for elimination and can substantially alter the composition of biofilms2528. For instance, Pseudomonasaeruginosa engages in bouts of type VI secretion system (T6SS)-mediated attack specifically in response to antagonism from other bacteria27,29. V.cholerae and Pseudomonas fluorescens produce extracellular matrix materials that give secreting cells a positional advantage over competitors, which are physically displaced30,31 or cut off from nutrient access32.
The spatial arrangement of different strains and species within biofilms strongly influences the relative fitness benefits of cooperative and competitive phenotypes. By altering the reproductive rates of neighbouring cells, social phenotypes can cause compositional and structural changes in microbial communities, thus...