A global multinational survey of cefotaxime-resistant coliforms in urban wastewater treatment plants

The World Health Organization Global Action Plan recommends integrated surveillance programs as crucial strategies for monitoring antibiotic resistance. Although several national surveillance programs are in place for clinical and veterinary settings, no such schemes exist for monitoring antibiotic-resistant bacteria in the environment. In this transnational study, we developed, validated, and tested a low-cost surveillance and easy to implement approach to evaluate antibiotic resistance in wastewater treatment plants (WWTPs) by targeting cefotaxime-resistant (CTX-R) coliforms as indicators. The rationale for this approach was: i) coliform quantification methods are internationally accepted as indicators of fecal contamination in recreational waters and are therefore routinely applied in analytical labs; ii) CTX-R coliforms are clinically relevant, associated with extended-spectrum β-lactamases (ESBLs), and are rare in pristine environments. We analyzed 57 WWTPs in 22 countries across Europe, Asia, Africa, Australia, and North America. CTX-R coliforms were ubiquitous in raw sewage and their relative abundance varied significantly (<0.1% to 38.3%), being positively correlated (p < 0.001) with regional atmospheric temperatures. Although most WWTPs removed large proportions of CTX-R coliforms, loads over 10³ colony-forming units per mL were occasionally observed in final effluents. We demonstrate that CTX-R coliform monitoring is a feasible and affordable approach to assess wastewater antibiotic resistance status.

Antibiotic resistance genes in treated wastewater and in the receiving water bodies: A pan-European survey of urban settings

There is increasing public concern regarding the fate of antibiotic resistance genes (ARGs) during wastewater treatment, their persistence during the treatment process and their potential impacts on the receiving water bodies. In this study, we used quantitative PCR (qPCR) to determine the abundance of nine ARGs and a class 1 integron associated integrase gene in 16 wastewater treatment plant (WWTP) effluents from ten different European countries. In order to assess the impact on the receiving water bodies, gene abundances in the latter were also analysed. Six out of the nine ARGs analysed were detected in all effluent and river water samples. Among the quantified genes, intI1 and sul1 were the most abundant. Our results demonstrate that European WWTP contribute to the enrichment of the resistome in the receiving water bodies with the particular impact being dependent on the effluent load and local hydrological conditions. The ARGs concentrations in WWTP effluents were found to be inversely correlated to the number of implemented biological treatment steps, indicating a possible option for WWTP management. Furthermore, this study has identified bla_OXA-58 as a possible resistance gene for future studies investigating the impact of WWTPs on their receiving water.

High Throughput Analysis of Integron Gene Cassettes in Wastewater Environments

Integrons are extensively targeted as a proxy for anthropogenic impact in the environment. We developed a novel high-throughput amplicon sequencing pipeline that enables characterization of thousands of integron gene cassette-associated reads, and applied it to acquire a comprehensive overview of gene cassette composition in effluents from wastewater treatment facilities across Europe. Between 38 100 and 172 995 reads per-sample were generated and functionally characterized by screening against nr, SEED, ARDB and β-lactamase databases. Over 75% of the reads were characterized as hypothetical, but thousands were associated with toxin-antitoxin systems, DNA repair, cell membrane function, detoxification and aminoglycoside and β-lactam resistance. Among the reads characterized as β-lactamases, the carbapenemase bla_OXA was dominant in most of the effluents, except for Cyprus and Israel where bla_GES was also abundant. Quantitative PCR assessment of bla_OXA and bla_GES genes in the European effluents revealed similar trends to those displayed in the integron amplicon sequencing pipeline described above, corroborating the robustness of this method and suggesting that these integron-associated genes may be excellent targets for source tracking of effluents in downstream environments. Further application of the above analyses revealed several order-of-magnitude reductions in effluent-associated β-lactamase genes in effluent-saturated soils, suggesting marginal persistence in the soil microbiome.

Sulfide-induced nitrate reduction in the sludge of an anaerobic digester of a zero-discharge recirculating mariculture system

The anaerobic digester is a vital component in a zero-discharge mariculture system as therein most of the organic matter is mineralized and nitrogen-containing compounds are converted to gaseous N(2). Although denitrification is a major respiratory process in this nitrate-rich treatment stage, also sulfate respiration takes place and may cause undesirable high sulfide concentrations in the effluent water. To examine the effect of sulfide on nitrate reduction, in situ depth profiles of inorganic nitrogen and sulfur compounds were determined. Additionally, nitrate reduction was examined as a function of ambient sulfide concentrations in sludge collected from different locations in the anaerobic reactor. Depth profiles showed high concentrations of nitrate and low concentrations of sulfide and ammonia in the aqueous layer of the reactor. A sharp decrease of nitrate and an increase in sulfide and ammonia concentrations was measured at the water-sludge interface. Nitrate reduction was highest in this interface zone with rates of up to 8.05+/-0.57 micromol NO(3)(-)h(-1)g((sludge))(-1). Addition of sulfide increased the nitrate reduction rate at all sludge depths, pointing to the important role of autotrophic denitrification in the anaerobic reactor. Dissimilatory nitrate reduction to ammonia (DNRA) was found to be low in all sludge layers but was enhanced when sludge was incubated at high sulfide concentrations. Although nitrate reduction rates increased as a result of sulfide addition to sludge samples, no differences in nitrate reduction rates were observed between the samples incubated with different initial sulfide concentrations. This as opposed to sulfide oxidation rates, which followed Michaelis-Menten enzymatic kinetics. Partial oxidation of sulfide to elemental sulfur instead of a complete oxidation to sulfate, could explain the observed patterns of nitrate reduction and sulfide oxidation in sludge incubated with different initial sulfide concentrations.