Effects of levofloxacin exposure on sequencing batch reactor (SBR) behavior and microbial community changes

Highlights: • LVX resistance/tolerance bacteria have lived in the sludge of actual WWTPs. • At least 50% of the influent LVX still remained in the effluent. • Bacteria related to N removal were affected by LVX exposure thus deteriorated. • AOB recovery led to better NH₄-N oxidation efficiencies at the end of exposure. • Acinetobacter sp. and Chryseobacterium sp. could be one of the sources for ARB. -- Abstract: The adaptation mechanisms of bacterial community for nitrogen removal performance exposed to fluctuated levels of levofloxacin (LVX) during wastewater treatment in SBRs were investigated. Although LVX is completely synthetic, the results of minimum inhibitory concentration (MIC, 32 mg-LVX/L) and minimum bactericidal concentration (MBC, 512 mg-LVX/L) of the sampled sludge showed that the LVX resistance/tolerance for bacterial growth has already existed in the actual wastewater treatment plants (WWTPs). The key bacteria, i.e. Nitrosomonas sp. (ammonia-oxidizing bacteria), Nitrospira sp. (nitrite-oxidizing bacteria) and Thauera sp. (the predominant denitrifiers), decreased with LVX exposure, and the recovery of biological process in the reactor was disturbed due to LVX exposure. However, after stopping exposure their population was quickly increased and thus the performance was recovered. The results of the non-metric multidimensional scaling and microbial community by sequencing showed the LVX concentration was a crucial factor to the change of bacterial communities and controlled the quantitative evolution of the communities in our systems. This effect was more pronounced as the LVX concentration was higher. The results suggested the removal of residual antibiotics to accomplish under no effect concentration before biological treatment is important to suppress emerging and increasing of the antibiotic resistant bacteria in WWTPs.