Bacterial community characteristics under long-term antibiotic selection pressures

To investigate bacterial community characteristics under long-term antibiotic selection pressures, water samples from the upstream and the downstream sections of two rivers individually receiving the treated penicillin G and oxytetracycline production wastewater, as well as the anaerobic and the aerobic effluent of the penicillin G production wastewater treatment plant, were taken and analyzed. Antibiotic resistance ratios of bacterial communities in water samples were estimated by culture-based analysis. The majority of bacterial colonies (approximately 55%-70%) in both downstream rivers and the aerobic effluent showed resistance to 80 μg/ml of antibiotics tested, while the resistance ratios were less than 10% and 5% respectively for both upstream rivers. Six 16S rRNA gene clone libraries were constructed with 355 sequences and 215 OTUs totally obtained representing 465 clones. The antibiotic stresses seemed not reduce the diversities of bacterial communities in antibiotic containing water samples compared to those in the two reference upstream rivers. Bacterial groups present in the two reference upstream rivers were common residents in freshwater ecosystems, with the dominant groups as the phyla Proteobacteria including Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria, as well as Actinobacteria and Bacteroidetes. The phyla Proteobacteria and Firmicutes were dominant in all antibiotic containing water samples, with the clones belonged to Deltaproteobacteria and Epsilonproteobacteria significantly abundant, as well as Gram-positive low GC bacteria in the classes Clostridia and Bacilli. It thus seemed that Deltaproteobacteria, Epsilonproteobacteria, Clostridia and Bacilli might be specifically associated with antibiotic containing environments.