Physicochemical characterization of natural ionic Microreservoirs: Bacillus subtilis dormant spores

The kinetics of proton exchange between dormant spores and aqueous environment was examined by time-resolved micropotentiometry, the method we recently introduced for hydrogel particles of micro- and nanometer diameter (J. Phys. Chem. B 2006, 110, 15107). In this work, the method was applied to the suspensions of dormant Bacillus subtilis spore of different concentrations to show that proton uptake kinetics was a multistep process involving a number of successively approximately 10-fold slower steps of proton penetration into the bulk and their binding to the ionizable groups within different layers of a spore structure. By analyzing the proton equilibrium binding to ionizable groups inside a spore, it was shown that each Bacillus subtilis spore behaves like almost infinite ionic reservoir capable of accumulating billions of protons (N approximately 2 x 10(10) per spore). The obtained pK(a) value of 4.7 for the spores studied is the first quantitative indication on carboxyl groups as the major ionizable groups fixed in a spore matrix. In general, proton equilibrium binding within the spore matrix obeys the fundamental law of the Langmuir isotherm. The proton binding to the ionizable groups slows down the free proton diffusion within a spore, but this effect is substantially weakened by increasing the initial concentration of protons added. On the basis of the diffusion time analysis, it was found that the effective diffusion coefficient for hydrogen ions within the spore core can be up to 3 orders of magnitude lower than that within the coats and cortex. We speculate that the spore inner membrane which separates core from cortex and coats in a dormant spore is a major permeability barrier for protons to penetrate into a lockbox of the genetic information (core).