Concentration and reduction efficiency of vancomycin-resistant heterotrophic bacteria and vanA and vanB genes in each wastewater treatment unit processes

OBJECTIVES

This study elucidated the distribution and fate of vancomycin (VCM)-resistant heterotrophic bacteria (HTB) and resistance genes, vanA and vanB, during each treatment unit process of a wastewater treatment plant (WWTP).

METHODS

Several bacterial counts as well as copy numbers of vanA and vanB genes were determined in each wastewater and sludge. In addition, HTB strains isolated from wastewater and sludge were analyzed for VCM susceptibility. Then, the fate and reduction ratios of each bacterial counts, copy numbers of vanA and vanB genes, and the existence ratio of VCM-resistant HTB strains in the wastewater treatment unit process were evaluated.

RESULTS

VCM-resistant HTB were detected in all wastewater and sludge samples, and their existence ratio decreased along the treatment (92.9% in influent wastewater to 39.4% in chlorinated water). Notably, most of the HTB isolated from the influent wastewater were resistant to 8.0 µg/mL of VCM, strongly suggesting that a significant amount of ARB is flowing into the WWTP from urban areas through the sewerage system. The vanA and vanB genes were also detected in all wastewater and sludge, with high copy numbers (10²-10⁴ copies/mL) even in chlorinated water samples.

CONCLUSIONS

The results revealed that residual VCM-resistant HTB, and resistance genes, which could not be completely removed, were ubiquitously released into the aquatic environment. Furthermore, a high existence ratio of VCM-resistant HTB and high copy numbers of resistance genes were also detected in the sludge, indicating that they are constantly circulating in the wastewater treatment system via the returned sludge.