INFLUENCE OF ELECTRIC-FIELDS AND PH ON BIOFILM STRUCTURE AS RELATED TO THE BIOELECTRIC EFFECT

Mixed species biofilms of Klebsiella pneumoniae, Pseudomonas fluoresce ns, and Pseudomonas aeruginosa were grown in a how cell fitted with tw o platinum wire electrodes. The biofilm growing on the wires reached a thickness of approximately 50 mu m after 3 days. When a voltage was a pplied with oscillating polarity, the biofilm attached to the wire exp anded and contracted. The biofilm expanded by approximately 4% when th e wire was cathodic but was reduced to 74% of the original thickness w hen the wire was anodic. The phenomenon was reproduced by alternately flushing the how cell with media adjusted to pH 3 and pH 10 with no el ectric current. At pH 10 the biofilm was unaltered, but it became comp acted to 69% of the original thickness at pH 3. We explained these phe nomena in terms of the molecular interactions between charged acidic g roups in the biofilm slime and the bacterial cell walls. Contraction o f the biofilm under acidic conditions may be caused by (i) the elimina tion of electrostatic repulsion from neutralization of negatively char ged carboxylate groups through protonation and (ii) subsequent hydroge n bonding between the carboxylic acids and oxygen atoms in the sugars. Electrostatic interactions between negatively charged groups in the b iofilm and the charged wire may also be expected to cause biofilm expa nsion when the wire was cathodic and contraction when the wire was ano dic. The consequences of the explanation of the increased susceptibili ty of biofilm cells to antibiotics in an electric field, the ''bioelec tric effect,'' are discussed.