Emergence of multidrug-resistant Acinetobacter baumannii under fluctuating levofloxacin concentration and its control by chlorine and UV disinfection

Although wastewater treatment plants are considered hotspots for the spread of antibiotic-resistant bacteria (ARB) and antibiotic-resistant genes (ARGs), few studies have explored the mechanisms underlying the emer-gence of multidrug-resistance. Using four sequencing batch reactors under fluctuating concentrations of levo-floxacin (LVX), we isolated 440 isolates, which were identified based on the sequence of the 16 S rRNA gene and growth at 37 oC, focusing on potential human pathogens. We then evaluated the multidrug-resistance of 62% of these isolates by determining the minimum inhibition concentration (MIC) of erythromycin, tetracycline, chloramphenicol, and LVX in the emergence of ARB. The average MIC was increased more than 100-fold in the first exposure, whereas did not show the same response after re-exposure due to different LVX concentrations between the 2 conditions. We identified 33 Acinetobacter baumannii isolates that emerged only during re -exposure, especially in R-2 and R-3, in which the LVX concentration was changed from 4 and 16 mg/L, respectively, to 128 mg/L. Moreover, we assessed the disinfection efficiency of chlorine/UV treatment against pathogenic ARB. The growth of multidrug-resistant A. baumannii isolates was decreased by 0.9-7.4 log10 following disinfection treatment with > 0.5 mg /L Cl2 or > 30 min UV irradiation.