Abstract
Wastewater treatment plants (WWTPs) are an effective barrier in the protection of human and environment health around the world, although WWTPs also are suggested to be selectors and-or reservoirs of antibiotic resistance genes (ARGs) before entering the environment. The dogma about WWTPs as “ARG selectors” presumes that biotreatment compartments (e.g., activated sludge; AS) are single densely populated ecosystems with elevated horizontal gene transfer. However, recent work has suggested WWTP biotreatment compartments may be different than previously believed relative to antibiotic resistance (AR) fate, and other process factors, such as bacterial separation and specific waste sources, may be key to ARGs released to the environment. Here we combined 16S rRNA metagenomic sequencing and high-throughput qPCR to characterise microbial communities and ARGs across a wastewater network in Spain that includes both community (i.e., non-clinical urban) and hospital sources. Contrary to expectations, ARGs found in downstream receiving waters were not dominated by AS biosolids (RAS), but more resembled raw wastewater sources. In fact, ARGs and microbial communities in liquid-phase WWTP effluents and RAS were significantly different (Bray–Curtis dissimilarity index = 0.66 ± 0.11), with a consequential fraction of influent ARGs and organisms passing directly through the WWTP with limited association with RAS. Instead, ARGs and organisms in the RAS may be more defined by biosolids separation and biophysical traits, such as flocculation, rather than ARG carriage. This explains why RAS has significantly lower ARG richness (47 ± 4 ARGs) than liquid-phase effluents (104 ± 5 ARGs), and downstream water column (135 ± 4 ARGs) and river sediments (120 ± 5 ARGs) (Tukey's test, p < 0.001). These data suggest RAS and liquid-phase WWTP effluents may reflect two parallel ecosystems with potentially limited ARG exchange. As such, ARG mitigation in WWTPs should more focus on removing bacterial hosts from the liquid phase, AR source reduction, and possibly disinfection to reduce ARG releases to the environment.
Highlights
Antibiotics historically have been among the most effective classes of therapeutic drugs used in the treatment of infectiousM
Rarefaction curves for Operational Taxonomic Unit (OTU) showed different bacterial community diversities across sampling sites, which were confirmed when evaluating a-diversity metrics, including Richness, Shannon and Simpson indices (Figure S-1, Tables Se4). These indices indicate that raw wastewater-associated samples have significantly lower diversity compared with upstream river samples, WWTP liquid effluent, and downstream river samples (p-value < 0.05)
Bacterial diversity was greater in non-wastewater samples, presumably due to more rare taxa, which is supported by rank abundance distributions (Figure S-2)
Summary
Antibiotics historically have been among the most effective classes of therapeutic drugs used in the treatment of infectiousM. Quintela-Baluja et al / Water Research 162 (2019) 347e357 compartments and microbial communities This has implications to human health owing to possible horizontal gene transfer (HGT) between environmental bacteria and human pathogens, impacting the potential evolution and selection of new AR phenotypes. Examples include the gut and faeces of the original individual; the sewer line that carries wastewater to the WWTP; each unit operation within the WWTP; and different receiving water compartments (e.g., water column and sediments). Each of these ecosystems has different antibiotic/chemical exposures, microbial cell densities and diversity, levels of mixing, and metahabitat conditions; all of which potentially influence resident ARGs, their hosts, and HGT within the overall network
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
