Fig. 2.

The working principle of the α-HL nanopore-based detection of bacteria at single-particle level. The negative electric potential on the trans side of a lipid membrane containing a single α-HL nanopore drives negatively charged bacteria into the nanopore. Within a spherical symmetry formalism, the absolute value of the electrophoretic force (F _elp) acting on the bacteria is proportional to the negative electric charge on bacterial surface (Q) and the electric field (E) measured in bulk at a radial distance (r) from the pore, which emanates from the applied transmembrane potential (ΔV) [44]. In the inset formula, d and l represent the pore’s diameter and length. The bacteria-nanopore collisions at the lumen entrance of the α-HL determine brief obstructions of the nanopore’s permeating pathway, seen as reversible blockades of the ionic current, further measured with a sensitive current amplifier. The idealized trace inset allows us to define the main characteristics which are subsequently used to quantify the single bacteria-nanopore interactions, namely the extent of the ionic current blockade (ΔI _block = I _{blocked pore} − I _{free pore}), average time of intervals measured between successive blockade events (τ _on) and average time spent by the nanopore in the blocked state, following its interaction with a bacterium (τ _off). Note that bacteria and the α-HL protein are not drawn to scale