Bivalent cations increase both the subpopulation of adhering bacteria and their adhesion efficiency in sand columns

The need to understand important factors affecting the spread of bacteria i n groundwater aquifers is evident for fields as diverse as drinking water s afety or environmental engineering concerned with bioremediation of pollute d sites. For example, increasing concentrations of dissolved minerals tend to increase the deposition efficiency of bacteria in porous media. As bacte ria and mineral surfaces are mostly negatively charged, this is generally a ssumed to be a consequence of the higher ionic strength, which leads to str onger shielding of the surface charges by the counterion-cloud in solution. We found Mg2+ to enhance deposition of Pseudomonas sp, strain B13 in sand columns with respect to a solution of identical ionic strength containing N a+. Hence bivalent cations are likely to affect microbial deposition specif ically, for example due to specific binding to the cell surface. Moreover, low concentrations of Pb2+ or Cu2+ reverted the surface potential of strain B13, thus providing additional evidence for this hypothesis. Recently, we showed strain B13 to split up in a well-adhering and in a nonadhering subpo pulation. In experiments conducted with Mg2+ and Na+ at various ionic stren gth, bivalent cations seemed to increase the well-adhering subpopulation as well as its adhesion efficiency.