Microbial current production from Streptococcus mutans correlates with biofilm metabolic activity

Once pathogens form a biofilm, they become more tolerant to drugs and quicker to recover from physical removal than planktonic cells. Because such robustness of a biofilm is associated with the active metabolism of its constituent microbes, establishment of a direct assay quantifying biofilm's metabolic activity is important for developing antibiofilm substrates and techniques. Current production capability via extracellular electron transport (EET) was recently found in Gram-positive pathogens, which we hypothesized to correlate with the metabolic activity of their biofilm. Here, we identified current production from the biofilm of oral pathogen Streptococcus mutans that enables the electrochemical assessments of their metabolic activity in situ which conventionally require gene insertion for a fluorescent protein expression. Single-potential amperometry (SA) showed that S. mutans produced an anodic current and formed a biofilm within 8 h on a +0.4 V electrode vs a standard hydrogen electrode (SHE) in the presence of the electron donor glucose. Current production was significantly decreased by the addition of a metabolic inhibitor Triclosan. Furthermore, the anabolic activity of a single cell using high-resolution mass spectroscopy revealed that higher current production resulted in a higher metabolic fixation of an atomically labeled nitrogen ¹⁵N. These results demonstrate that current production in S. mutans reflects its metabolic activity. Given electrochemical impedance spectroscopy (EIS) helps quantifying the bacterial cell adhesion on the electrode, combination of EIS and SA could be a novel assay for EET capable pathogens for quantifying their time-dependent metabolic activity, cellular electrode coverage and physiological response to antibiofilm compounds.