Characterizing Heterogeneous Bacterial Surface Functional Groups Using Discrete Affinity Spectra for Proton Binding

Abstract

Here we report results from a quantitative investigation of the types and densities of proton binding sites on a bacterial surface, Bacillus subtilis, from replicate acid−base titrations on bacteria at two ionic strengths (0.025 and 0.1 M). In contrast to the surface complexation modeling (SCM) approach developed and widely used for mineral, e.g., iron oxides, and more recently bacterial surfaces; we fit the data using the linear programming method (LPM). Our results using LPM indicate five discrete binding sites occurring on the surface of B. subtilis likely corresponding to carboxylic sites at low pKa values, phosphoric sites at near-neutral pKa values, and amine sites at high pKa values. Replicate titrations on subsamples from the same bacterial population indicated less variability than has been suggested for bacterial surfaces. Both the pKa and site density values were found to be dependent on ionic strength. Comparing the pKa values determined here with LPM for B. subtilis to those determined indepe...