The effect of subinhibitory concentrations of tetracycline on the spread of associated plasmid mediated resistance genes in laboratory scale sequential batch reactors to model wastewater treatment plants

The mixed communities were fed two different tetracycline concentrations (400 and 1400 µg/L) and the reactors were run and monitored for 30 days. During the experiment the tetM gene was monitored via qPCR and DNA samples were taken to monitor the effect of ampicillin on the microbial community. The relative copy number of tetM in the reactors declined in both reactors as well as in the control reactor showing that sub-MIC concentrations of antibiotics do not serve as a strong selector for resistance genes. High-throughput sequencing showed that continuous exposure to tetracycline caused a shift from a Actinobacteria to Proteobacteria in the bacterial community. The combined use of qPCR and high-throughput sequencing showed that tetracycline stimulates a diversification in the bacterial community but no high levels in resistance genes.