The combination of ciprofloxacin and dialkyldimethyl ammonium compound synergistically proliferated intracellular resistance genes in nitrifying system.

Antibiotics and quaternary ammonium compounds (QACs) usually co-exist in wastewater treatment plants. Hence, three sequencing batch reactors were established and named as R1, R2 and R3, to investigate the effects of individual and combined exposure of different concentrations of ciprofloxacin (CIP) (0.2, 1.0 and 2.0 mg/L) and dialkyldimethyl ammonium compound (DADMAC) (0.4, 2.0 and 4.0 mg/L) on the performance, microbial community structures and resistance genes (RGs) in nitrifying system during 150 days. Results showed that CIP had a slight effect on ammonia oxidation activity, while 2.0 and 4.0 mg/L DADAMAC could obviously inhibit it, and the combination of CIP and DADMAC had a synergistic inhibitory effect. Besides, both CIP and DADMAC caused partial nitrification, and the order of nitrite accumulation rate was ranked as R3 > R2 > R1. The combination of CIP and DADMAC had an antagonistic effect on the increase of sludge particle size and α-Helix/(ß-Sheet + Random coil) was lowest in R3 (0.40). The combination of CIP and DADMAC synergistically stimulated most intracellular RGs in sludge, and the relative abundances of target RGs (e.g., qacEdelta1-01, qacH-01 and qnrS) at the end of operation in R3 were increased by 4.61-18.19 folds compared with those in CK, which were 1.34-5.57 folds higher than the R1 and R2. Moreover, the combination of CIP and DADMAC also promoted the transfer of RGs from sludge to water and enriched more potential hosts of RGs, further promoting the spread of RGs in nitrifying system. Thus, the combined pollution of CIP and DADMAC in wastewaters should attract more attentions.

The spread of different resistance genes fractions in nitrification system under chronic exposure to varying alkyl chain length benzalkyl dimethylammonium compounds.

Benzalkyl dimethylammonium compounds (BACs) are generally applied as surfactants and disinfectants. In this study, the nitrification systems were exposed to different alkyl chain lengths (C12-C16) and different levels of BACs (0-5 mg/L), respectively, totally 120 days and to explore the chronic effect of BACs on resistance genes (RGs). RGs were classified into four fractions based on activated sludge properties. Ammonia oxidation performance were not significantly affected by BACs, whereas BACs increased the absolute abundance of most intracellular RGs in sludge (si-RGs). Under the exposure of BACs, extracellular RGs in water (we-RGs) showed a decrease trend and si-RGs tended to be converted to we-RGs. Tightly bound-Tyrosine side chain was significantly correlated with most we-RGs, and we-intI1 might contribute to the propagation of RGs. Therefore, the risk of transmission of different fractions of RGs in the nitrification system under the stress of BACs should be taken seriously.