Assessing the Optimum Geometry of Pre-formed Aggregates in Biofilms

Until recently, the initial stage of biofilm development was thought to predominantly involve the attachment of planktonic cells to the surface of interest. However, recent studies involving Pseudomonas aeruginosa and Staphylococcus aureus, have found that planktonic cells can form suspended aggregates and exhibit much of the phenotypical behaviour of attached biofilms, e.g. packing density, extracellular DNA, comparable growth rates, matrix structure, ability to survive lethal treatments of antibiotics, and mutation frequency. It seems probable that initial colonisation of the surface by single planktonic cells may also be accompanied by attachment of pre-existing aggregates that are already in a biofilm-like mode of growth. Since it is likely that those cells that begin biofilm-life in a preformed aggregate may develop structure differently, it is of fundamental interest to investigate the initial configuration of cell aggregates on the surface, and the influence that these aggregates have on the overall structure on the biofilm. For example: cells within a pre-formed aggregate may have either better or worse access to nutrients; the presence of aggregates might affect clonal structure of the biofilm; and considering that an aggregate is likely to have a higher initial cell density, one might expect aggregate cells to initiate cooperation through quorum sensing much earlier than cells that are randomly distributed on the surface. Using agent based computer simulations, we investigate whether there is an optimum configuration for a clump of cells of size N to arrange on the surface, considering competitive effects from a surrounding mat of individual cells. The fitness of these clumps is assessed using a surface contact angle as the order parameter. Results reveal that there is an interplay between initial surface coverage and aggregate height. For a given density of surrounding cells, there is a transition angle at which clump height becomes more important than surface coverage. Our results show a direct correlation between this angle and the density (competition) of the surrounding cells.