Total resistance of native bacteria as an indicator of changes in the water environment

This study analyzes changes in the total (intrinsic and acquired) resistance of autochthonous bacteria in a river which is a receiver of treated wastewater. In the analyzed samples, tetracycline contamination levels were low and characteristic of surface water bodies. An increase in the populations of tetracycline-resistant and fluoroquinolone-resistant microorganisms was noted in downstream river water samples in comparison with upstream river water samples, but the above trend was not observed in bacteria resistant to macrolides and beta-lactams. The counts of doxycycline-resistant bacteria (DOXR) were significantly correlated with doxycycline levels. The minimum inhibitory concentrations (MICs) for doxycycline in DOXR isolates were higher in downstream river water than in upstream river water samples. The discharge of treated wastewater had no effect on the multi-drug resistance of oxytetracycline-resistant and doxycycline-resistant isolates. The results of the experiment indicate that the presence of doxycycline-resistant bacteria is a robust indicator of anthropogenic stress in river water. (C) 2012 Elsevier Ltd. All rights reserved.