Probing Intercellular Interactions in Dynamic Clusters of Swarming Bacteria

Expansion of dense bacterial populations over surfaces with collective dynamics driven by flagella, known as bacterial swarming, is often considered as a precursor to the growth of biofilms. However, it is an open question whether these two major types of microbial communities are governed by similar mechanisms at the molecular and microscopic levels. Underlying questions can include: what types of cell-cell interactions exist between neighboring cells, what types of polymeric materials are present at the cell surface or in the extracellular space, what signaling or surface active molecules are involved, and what osmolytes are required, if any. To probe for intercellular interactions, we performed experiments on Pseudomonas aeruginosa and Enterobacter sp. SM3, which are two distinct species that are both strong swarmers. By observing the collective motion exhibited in the swarms of both species of bacteria, in particular the dynamic packs of motile SM3 cells upon dilution as compared with their motility in the planktonic state, our study demonstrates to what extent cell-cell interactions exist and affect collective behaviors. The experimental results of these studies will help elucidate the physical mechanisms that dictate the growth and behavior of bacterial communities.