Physicochemical parameters affecting the adhesion of ciprofloxacin-resistant Escherichia coli to activated sludge

Abstract

To investigate the physicochemical conditions necessary to stably remove antibiotic-resistant bacteria (ARB) via contact with activated sludge (AS), the adhesion of ciprofloxacin (CIP)-resistant and -susceptible Escherichia coli to AS was simulated by contact tests in the laboratory. The CIP-resistant E. coli and susceptible E. coli were removed by a 3 log smaller concentration by a 5 h contact test at maximum. Considering the hydraulic retention time of a reaction tank (∼5 h) and step-feeding operation, we considered the removal rate of E. coli in the current simulated contact test to be in agreement with the actual situation where 1–2 log concentrations of E. coli were reported to be removed from an AS reaction tank. With the increase in the AS concentration and/or dissolved oxygen, the removal rate of E. coli increased. The removal rate of CIP-resistant E. coli was greater than that of susceptible E. coli under all experimental conditions. Although the mechanism by which CIP-resistant E. coli preferably adhered to AS was not clearly understood in detail, finding optimum conditions under which bacteria, including ARB, were efficiently removed by the AS process may be possible. HIGHLIGHTS Activated sludge (AS) effectively (>99%) adhered to Escherichia coli (E. coli) and ciprofloxacin (CIP)-resistant E. coli in 5 h.; The selective removal of CIP-resistant E. coli was quantitatively demonstrated as the possible outcome.; Removal rates of E. coli by adhesion increased with the AS concentrations and dissolved oxygen.; Stirring was imperative to ensure contact between E. coli and AS, but extremely high shear stress inhibited it.;

Keywords

• antibiotic resistance bacteria
• ciprofloxacin
• conventional activated sludge process
• escherichia coli
• wastewater treatment plant