Chemical equilibrium modeling techniques for the analysis of high-resolution bacterial metal sorption data

An ion selective electrode was used to monitor binding of Cd2+ on two bacte ria, Bacillus subtilis (Gram+) and Escherichia coli (Gram-), as a function of increasing pH. A competitive Langmuir sorption isotherm was used in conj unction with a linear programming method (LPM) or FITEQL to fit experimenta l data. Results obtained with simulated data showed that LPM is less sensit ive than FITEQL to variations in sorption data. Application of the LPM to e xperimental data found three discrete metal binding sites on B. subtilis an d E. coli with -log equilibrium constant (pK(S)) values of -0.80 +/- 0.20, 0.63 +/- 0.09, and 2.35 +/- 0.10, and -0.60 +/- 0.10, 0.25 +/- 0.19, and 1. 93 +/- 0.17, respectively, at a constant ionic strength, I = 0.1 M (KNO3)Th e corresponding site densities were 0.09 +/- 0.01, 0.07 +/- 0.01, and 0.07 +/- 0.01, and 0.01 +/- 0.00(2), 0.02 +/- 0.01, and 0.04 +/- 0.01 mu mol of Cd2+/mg of B. subtilis or E. coli. From FITEQL, pK(S) values of -1.18 +/- 0 .15, 0.40 +/- 0.11, and 2.31 +/- 0.32 for B. subtilis and -1.46 +/- 0.34, 0 .20 +/- 0.12, and 1.87 +/- 0.12 for E. coli were recovered with site densit ies of 0.10 +/- 0.07, 0.07 +/- 0.06, and 0.06 +/- 0.02, and 0.02 +/- 0.005, 0.02 +/- 000(4), and 0.04 +/- 0.04 mu mol of Cd2+/Mg of B. subtilis or E. coli, respectively. Total site densities of 0.22 +/- 0.02 and 0.06 0.01 mu mol/mg were obtained by LPM for B. subtilis and E. coli, whereas FITEQL yie lded values of 0.23 +/- 0.02 and 0.08 +/- 0.07 mu mol/mg. Both LPM and FITE QL produced feasible results, but LPM was less sensitive to error and did n ot require an a priori assumption of the number of binding sites. (C) 2001 Academic Press.