ELECTRIC-FIELDS INDUCE CURVED GROWTH OF ENTEROBACTER-CLOACAE, ESCHERICHIA-COLI, AND BACILLUS-SUBTILIS CELLS - IMPLICATIONS FOR MECHANISMS OF GALVANOTROPISM AND BACTERIAL-GROWTH

Directional growth in response to electric fields (galvanotropism) is known for eukaryotic cells as diverse as fibroblasts, neurons, algae, and fungal hyphae. The mechanism is not understood, but all proposals invoke actin either directly or indirectly. We applied electric fields to bacteria (which are inherently free of actin) to determine whether actin was essential for galvanotropism. Field-treated (but not contro l) Enterobacter cloacae and Escherichia coli cells curved rapidly towa rd the anode. The response was both field strength and pH dependent. T he direction of curvature was reversed upon reversal of field polarity . The directional growth was not due to passive bending of the cells o r to field-induced gradients of tropic substances in the medium. Field -treated Bacillus subtilis cells also curved, but the threshold was mu ch higher than for E. cloacae or E. coli. Since the curved morphology must reflect spatial differences in the rates of cell wall synthesis a nd degradation, we looked for regions of active wall growth. Experimen ts in which the cells were decorated with latex beads revealed that th e anode-facing ends of cells grew faster than the cathode-facing ends of the same cells. Inhibitors of cell wall synthesis caused spheroplas ts to form on the convex regions of field-treated cells, suggesting th at the initial curvature resulted from enhanced growth of cathode-faci ng regions. Our results indicate that an electric field modulates wall growth spatially and that the mechanism may involve differential stim ulation of wall growth in both anode- and cathode-facing regions. Elec tric fields may therefore serve as valuable tools for studies of bacte rial wall growth. Use of specific E. coli mutants may allow dissection of the galvanotropic mechanism at the molecular level.