Fate and Proliferation of Vancomycin Resistance Genes in Two Typical Pharmaceutical Wastewater Treatment Plants

Because of the diversity of PWWTPs, many physical/chemical/biological water parameters may affect the proliferation and fate of ARGs in PWWTPs. Redundancy analysis (RDA) was performed to confirm the relationship between the relative abundance of ARG subtypes and various environmental factors (Figure 3). PH, COD, TOC, and vancomycin (VCM) were considered. The redundancy analysis indicated that the vancomycin concentration was positively related to the abundance of ARGs (vanA and vanB). In PWWTPs, high concentrations of antibiotic residues loaded in influents appeared to be unavoidable, and they might promote the propagation and persistence of relevant typical ARGs. The concentrations of antibiotics in each process in the two PWWTPs are shown in Table S2. Antibiotics were not able to inhibit the microbiological activities in a sewage treatment plant, since the minimum inhibitory concentrations (MICs) of some specific bacteria in wastewater were two- to tenfold higher than the antibiotic residues detected in the present study [25,34]. However, wastewater contained extremely high levels of antibiotics in PWWTPs, which were higher than MICs in many cases. For example, Qiu et al. [15] obtained the MIC of VCM for some typical sensitive bacteria from the European Committee on Antimicrobial Susceptibility Testing (EUCAST), when the highest, moderate, and lowest MICs were 23.76, 0.50, and 0.13 μg/mL. Takashi et al. [17] also found that two enterococcal strains were resistant to a high concentration of vancomycin (>128 μg/mL) via the MIC test. Furthermore, Zhang et al. [35] found the level of corresponding ARGs more often relies on the existence and concentration of antibiotics when chromosomal mutations occur in bacteria. Our previous study also observed that ARGs, which tend to be located in chromosomes, might be much more likely to show positive correlations with antibiotic concentrations [27]. A recent study observed that vancomycin resistance genes tended to be chromosomally encoded by metagenomic assembly and binning approaches [36], which might be attributed to the regulation of a two-component system consisting of a histidine kinase and a response regulator localized on a chromosomal operon [37,38]. Consistent with our findings, it was observed that the proliferation of vanA generally occurred in the early steps of the treatment processes. Similarly, the proliferation of vanB generally occurred in the biological units of the treatment processes. This observation indicated that ARGs normally increased during the initial stages of the treatment processes, which may be explained by the selective pressure of high vancomycin concentrations.

COD and TOC correlated with ARGs through redundancy analysis, which is in keeping with the findings of a report by [39], who observed that most ARB and ARG amplification phenomena were positively correlated with the COD of raw sewage. This finding may be attributed to COD, which changes the microbial community structure [18]. Most bacteria were expected to adhere to suspended solids or colloids in wastewater, causing reductions in ARB/ARGs to be positively related to COD to a certain extent.