Seven antibiotics including norfloxacin (NOR) were tested via disk susceptibility test on E. coli
culture isolated from the MLSS of the two types of lab-scale sequencing batch reactors (SBR), a
common SBR and an SBR with rnicrofiltration membrane (SB-MBR) for treatment of synthetic
municipal wastewater. The same experiment treating the NOR-added wastewater to examine the
possible induction of resistance to itself and the other antibiotics. The MLSS from Bangl
culture (TISTR780) were spiked daily into both reactors. The reactors were continuously operated
under 2hr/2hr of aeration/non-aeration cycle and resistances to antibiotics of E. coli in MLSS were
monitored. When NOR was not added, the SB-MBR showed lower percentages of resistant E. coli
than the SBR did to amoxicillin/clavulanic acid, amikacin, nalidixic acid, tetracycline and
chloramphenicol. Oppositely, the SB-MBR treating the NOR-added wastewater appeared to
promote resistances of E. coli to nalidixic acid, sulfamethoxasole and tetracycline probably due to a
long SRT and low DO compared to that of SBR. Although its mechanism should be analyzed with
molecular techniques in further studies, this NOR-induced expression of resistance resulted in a
higher occurrence of multidrug resistant E. coli in the SB-MBR than that in the SBR.
