This electrical enhancement was demonstrated in an in vitro model. Streptococcus gordonii biofilms were grown for 6 days in continuous-flow reactors on one-tenth strength trypticase peptone broth. The biofilms attained a mean areal cell density of 2.4 x 10(8) c.f.u./cm2 and a thickness of approx. 19 microm. Biofilms exhibited characteristic resistance to killing by an antibiotic. When treated with 2 microg/ml gentamicin for 24 h, they exhibited a 0.84 log reduction in viable cell numbers; a 4.7 log reduction was measured in a planktonic culture. Killing of planktonic bacteria by this treatment was reduced to 1.2 log when an oxygen-scavenging enzyme was added to the medium. When a 2-mA direct current was applied during antibiotic treatment, biofilm killing increased to a 4.3 log reduction. Electrical current alone caused a 1.9 log reduction in biofilm cell counts. It is suggested that gentamicin was less effective against Strep. gordonii under anaerobic conditions than it was under aerobic conditions and that this can explain both the reduced susceptibility of the biofilm (due to oxygen depletion) and electrical enhancement of efficacy (due to oxygen generation by electrolysis).