Self-Assembly of Bacteria in Alternating-Current Electric Fields

Self-assembly of bacteria in electric fields is a promising route to fabricate biomaterials with reversible and specific structures. However, due to relatively less studies, our understanding of the self-assembly of bacteria in electric fields is still incomplete. Particularly, how different bacterial species behave differently in their field-mediated self-assembly behavior remains to be disclosed. In this study, we choose four bacterial species, including Shewanella oneidensis, Pseudomonas aeruginosa, Bacillus subtilis and Lactococcus lactis as model systems, and investigate their self-assembly behavior in alternating-current (AC) electric fields for both diluted and concentrated suspensions. The phase diagrams in the plane of applied field strength vs frequency are obtained. The results show that in diluted suspensions, a transition sequence of isotropic-paranematic-string-columnar phases is observed in all strains as the field strength increases. Details of the assembled structures are quantitatively differentiated among different strains. In concentrated suspensions, besides the isotropic and paranematic phases, a higher ordered phase with interdigitating rectangular crystal domains (OIR) and an ordered phase with smectic A liquid crystal domains are observed for S. oneidensis and P. aeruginosa, respectively. Our findings shed new light on fabricating potential biomaterials by assembling cells of appropriately chosen bacterial species that have desired surface properties under AC electric fields.