Plasmonic imaging of protein interactions with single bacterial cells

Quantifying the interactions of bacteria with external ligands is fundamental to the understanding of pathogenesis, antibiotic resistance, immune evasion, and mechanism of antimicrobial action. Due to inherent cell-to-cell heterogeneity in a microbial population, each bacterium interacts differently with its environment. This large variability is washed out in bulk assays, and there is a need of techniques that can quantify interactions of bacteria with ligands at the single bacterium level. In this work, we present a label-free and real-time plasmonic imaging technique to measure the binding kinetics of ligand interactions with single bacteria, and perform statistical analysis of the heterogeneity. Using the technique, we have studied interactions of antibodies with single Escherichia coli O157:H7 cells and demonstrated a capability of determining the binding kinetic constants of single live bacteria with ligands, and quantify heterogeneity in a microbial population. •Applied high resolution SPR (SPR microscopy) to determine binding interactions of a protein with a single bacterial cell.•Achieved a detection limit of a single microbial cell; orders of magnitude higher compared to conventional SPR approaches.•New tool quantifies cell-to-cell variations in kinetics constants (e.g. KD).•Discovered large inherent heterogeneity in a microbial population.•Label-free, real-time, quantitative, high-resolution optical biosensor.