Joint antibacterial activity of soil-adsorbed antibiotics trimethoprim and sulfamethazine.

Trimethoprim (TMP) and sulfamethazine (SMZ) are two antibiotics that are often administered in combination. We investigated the sorption and desorption behaviors of TMP and SMZ individually as single solute and in combination as co-solute in three representative soils, and evaluated joint antibacterial activity of the soil-adsorbed antibiotics to a reference strain Escherichia coli ATCC 25922. Comparative sorption tests showed that co-solute sorption of TMP and SMZ was not considerably different from their single sorption. Soil-adsorbed TMP was found to effectively inhibit the growth of E. coli at environmentally relevant concentrations in all three soils, and moreover co-presence of SMZ enhanced the antibacterial effects on bacteria both in its dissolved form and soil-adsorbed form. Overall, the results from this study suggest that various soil-adsorbed antibiotic residues could play a joint influencing role in soil bacterial community activity.