Mechanistic Assessment of Metabolic Interaction between Single Oral Commensal Cells by Scanning Electrochemical Microscopy

The human oral microbiome heavily influences the status of oral and systemic diseases through different microbial compositions and complex signaling between microbes. Recent evidence suggests that investigation of interactions between oral microbes can be utilized to understand how stable communities are maintained and how they may preserve health. Herein, we investigate two highly abundant species in the human supragingival plaque, Streptococcus mitis and Corynebacterium matruchotii, to elucidate their real-time chemical communication in commensal harmony. Specifically, we apply nanoscale scanning electrochemical microscopy (SECM) using a submicropipet-supported interface between two immiscible electrolyte solutions as an SECM probe not only to image the permeability of S. mitis and C. matruchotii membranes to tetraethylammonium (TEA⁺) probe ions but also to real-time visualize the metabolic interaction between two microbes via lactate production/consumption at a single-cell level. The metabolic relationship between two strains is quantitatively assessed by determining (1) the passive permeability of both bacterial membranes of 2.4 × 10^–4 cm/s to the free diffusion of TEA⁺, (2) 0.5 mM of the lactate concentration produced by a single S. mitis strain at a rate of 2.7 × 10^–4 cm/s, and (3) a lactate oxidation rate ≥5.0 × 10⁶ s^–1 by an individual C. matruchotii strain. Significantly, this study, for the first time, describes a mechanism of in situ metabolic interaction between oral commensals at the single-cell level through quantitative analysis, which supports the observed in vivo spatial arrangements of these microbes.