Impact of spatial heterogeneity for selection regimes in multispecies biofilms

Bacteria live in sprawling communities forming complex dynamic biofilm structures. The spatial heterogeneity found in biofilms may be a driver for the selection for optimized biofilm variants. Bacterial species that increase their matrix production can position favourably in the exterior biofilm regions in order to compete for valuable substrates. Here, we study both the spatial organisation and growth of bacterial cells in different bacterial communities over time to determine links between the structure of interspecies biofilms and selection for phenotypes adapted to growth and persistence in biofilms. This leads to the identification of driving mechanisms behind the dynamic spatial organisation combined with the performance of individual species over time in the spatial biofilm structure.

Four species, previously co-isolated from soil, were used in different combinations. We examined the formation and spatial organisation of biofilms in distinct experimental model systems, as we hypothesized that their interaction would change dependent on the specific environment. The biofilm models used included the static Calgary biofilm device assay and two flow systems: the microfluidic BioFlux model (liquid bulk flow), and the drip flow reactor (liquid-air interphase). Both chromosomal fluorescent markers and FISH were used to visualize the organisation within biofilms by confocal laser scanning microscopy.

We reveal how the changes in biofilm structure affect the overall performance of the biofilm community as well as the individual species in the biofilm. Our data indicates that a favourable localization of the individual species in a multispecies biofilm reduces selection for competitive phenotypes. Furthermore, we also observed that changes in matrix production could serve to stabilise the interspecific interaction between bacteria. This highlights the specific structural composition of a biofilm community as important for explaining biofilm dynamics.