TESTING THE AFFINITY OF MONOVALENT CATIONS TO ADSORB TO THE SURFACE OF BACILLUS SUBTILIS

Virtually all experimental investigations of bacterial adsorption have been conducted in the presence of a monovalent salt electrolyte in order to buffer ionic strength. These studies implicitly assume that the monovalent ions present in the electrolyte are inert to adsorption onto the bacterial cell wall, and thermodynamic stability constants for bacterial surface complexes are calculated based on this assumption. However, if monovalent cation adsorption does occur to a significant extent, then these stability constant values would be inaccurate.

In this study, we measured the adsorption of Li, Rb, and Cs onto the Gram-positive bacteria Bacillus subtilis in the presence of a NaClO₄ electrolyte as a function of pH and electrolyte concentration in order to determine if monovalent cations adsorb to the bacterial surface and if they compete substantially with other cations for cell wall adsorption sites. We measured adsorption over the pH range 5 – 9, in solutions with buffered ionic strength from 10^-3.6 to 10^-1 M NaClO₄. Under these conditions, adsorption was relatively pH independent, but exhibited a strong relationship with ionic strength, with adsorption increasing significantly with decreasing ionic strength.

Our experimental results suggest that monovalent adsorption onto bacterial cell wall functional groups occurs, albeit to a significantly lower extent than does adsorption of higher charged cations. We use the experimental measurements to derive thermodynamic stability constants for the bacterial surface complexes that involve the monovalent cations studied, and we use these stability constants to estimate the importance of monovalent bacterial surface complexes under realistic conditions for bacteria-bearing geologic systems.