The Density of Competitors in a Stratified Environment Determines the Relative Fitness of Biofilm Structures
Multicellular organisms consist of many billions of individual cells, whose spatial location, gene expression and reproduction are closely coordinated to produce a functional whole. Understanding how cells assemble into three dimensional, multicellular structures, and how these structures' spatial organization influences their function, is a fundamental challenge in biology and biophysics, while controlling cellular aggregation and growth for regenerative or therapeutic purposes is a major goal in bioengineering and medicine. Biofilms are three dimensional sessile communities of interacting unicellular organisms, and even in a biofilm of genetically identical clones the constituent cells can differentiate into different patterns of gene expression and growth rates; this differentiation is often linked to the cells' positions in the biofilm structure, which controls transport and intercellular contacts in the biofilm. P. aeruginosa is an opportunistic human pathogen and a widely studied model system for biofilm formation. It has recently been found that P. aeruginosa tends to form multicellular aggregates even when grown in shaken liquid culture. Here, we use a combination of confocal laser scanning microscopy experiments and individual based computer simulations to track the fate of pre aggregated and initially un aggregated cells as they compete for resources during the development of a biofilm in a flow cell. We find that pre formed aggregates do not have an intrinsic growth advantage over single cells, but nevertheless biofilm regions that are initiated by pre formed aggregates can outgrow regions that are initiated by single cells if competition for growth substrate is high. Our results show that preformed aggregates can assist a strain in colonizing a surface, under conditions of high competition from other strains. However, within the aggregate itself, a form of cooperation exists, in which cells on the top of an aggregate benefit at the expense of lower cells.