Application of microorganisms as surface modifiers in flocculation has generated a great deal of interest in recent times. The surface properties such as zeta-potential and hydrophobicity of minerals and microorganisms play a major role in determining the adsorption of microorganisms onto the minerals and hence the efficiency of flocculation. The utility of microorganisms, including Escherichia coli (wild-type and genetically modified strain Sip), Arthrobacter nicotianae, Bacillus licheniformis, and Pseudomonas maltophilia, has been evaluated by measuring their zeta-potentials and carrying out adsorption and flocculation experiments. Of the tested microorganisms, adsorption of E. coli strain Sip significantly modified the quartz surface. The zeta-potential of the quartz became highly positive at acidic pH, and its IEP (isoelectric point) was shifted from pH <2 to pH 4.3. Moreover, the settling velocity of the bio-treated quartz reached its maximum value at this pH. The number of cells adsorbed onto quartz was low at pH above the IEP due to the identical surface charges of the mineral and bacterial cells. This led to a repulsive force between the mineral particles and bacterial cells, which hindered the adsorption process. For all of the studied strains, the settling velocity of bio-treated quartz was high at their respective IEPs. An interaction model of microorganisms is proposed to explain the flocculation behavior of bio-treated quartz. Potential energies were calculated using the DLVO theory, and the results were found to be in good agreement with the flocculation tests. The settling velocity of the bio-treated quartz was maximized at pH close to the IEP of microbial cells. Interaction between cells is identified as the most likely cause of flocculation of bio-treated quartz. (C) 2011 Elsevier B.V. All rights reserved.