One Cell at a Time: Single-Entity Impact Electrochemistry for Bacterial Counting and Functional Profiling

Single-bacterium analysis is increasingly important because population-averaged assays can obscure functional heterogeneity that governs pathogenicity, metabolic adaptation, and treatment response. Single-entity impact electrochemistry (SEIE) provides a complementary measurement paradigm by recording discrete current transients when individual cells stochastically collide with a polarized ultramicroelectrode. The resulting waveform observables (event frequency, charge, duration, and polarity) form a quantitative language that links near-electrode transport, cell-electrode contact physics, and engineered electrochemical reporting mechanisms to biological state. In this concept, we present a mechanism-guided view of SEIE for bacterial analysis, organized by increasing analytical depth: (i) bacterial counting framed explicitly as impact physics, where waveform families, electrode bias, and transport regimes determine calibration and interpretability; (ii) exogenous redox mediators as selectors that define metabolic access points and transient signatures for single-cell electron-transfer metrology; and (iii) secretion-resolved SEIE using endogenous reporters or engineered transducers to interrogate native metabolic outputs, stress responses, and interspecies interactions. We conclude by outlining key challenges and opportunities for translation, including operation in complex media, antifouling, and standardization, higher-throughput platforms, multimodal integration, and automated waveform analysis for point-of-care testing and antimicrobial susceptibility testing.