Quantification of vancomycin-resistant enterococci and corresponding resistance genes in a sewage treatment plant

Abstract

This study aimed to analyze vancomycin-resistant enterococci (VRE) and their resistance genes, vanA and vanB, to examine their presence in sewage treatment systems. Water samples were collected from primary sedimentation tank inlet, aeration tank, final sedimentation tank overflow outlet, and disinfection tank. Enterococcal strains were determined their vancomycin susceptibility by the minimum inhibitory concentration (MIC) test. Vancomycin-resistance genes (vanA and vanB) were quantified by real-time PCR. The sewage treatment process indeed decreased the number of most enterococci contained in the entering sewage, with a removal rate of ≥5log. The MIC test showed that two enterococcal strains resistant to a high concentration of vancomycin (>128 μg mL⁻¹). However, most of the enterococcal strains exhibited sensitivity to vancomycin, indicating that VRE were virtually absent in the sewage treatment systems. On the other hand, vancomycin-resistance genes were detected in all the sewage samples, including those collected from the chlorination disinfection tank. The highest copy numbers of vanA (1.5 × 10³ copies mL⁻¹) and vanB (1.0 × 10³ copies mL⁻¹) were detected from the water sample of effluent water and chlorinated water, respectively. Therefore, antibiotic resistance genes remain in the sewage treatment plant and might discharged into water environments such as rivers and coastal areas.

Keywords:

• Antibiotics
• fecal bacteria
• minimum inhibitory concentration
• quantitative PCR
• resistance gene
• sewage treatment system
• vancomycin-resistant enterococci

Acknowledgments

We would like to thank Yoshihiro Suzuki (University of Miyazaki) for his assistance during experiments.

Funding

This study was supported by the Kurita Water and Environment Foundation (Grant No. 13B060) and the Institute for Fermentation, Osaka.