Urban wastewater treatment plants as hotspots for antibiotic resistant bacteria and genes spread into the environment: a review

Urban wastewater treatment plants (UWTPs) are among the main sources of antibiotics' release into the environment. The occurrence of antibiotics may promote the selection of antibiotic resistance genes (ARGs) and antibiotic resistant bacteria (ARB), which shade health risks to humans and animals. In this paper the fate of ARB and ARGs in UWTPs, focusing on different processes/technologies (i.e., biological processes, advanced treatment technologies and disinfection), was critically reviewed. The mechanisms by which biological processes influence the development/selection of ARB and ARGs transfer are still poorly understood. Advanced treatment technologies and disinfection process are regarded as a major tool to control the spread of ARB into the environment. In spite of intense efforts made over the last years to bring solutions to control antibiotic resistance spread in the environment, there are still important gaps to fill in. In particular, it is important to: (i) improve risk assessment studies in order to allow accurate estimates about the maximal abundance of ARB in UWTPs effluents that would not pose risks for human and environmental health; (ii) understand the factors and mechanisms that drive antibiotic resistance maintenance and selection in wastewater habitats. The final objective is to implement wastewater treatment technologies capable of assuring the production of UWTPs effluents with an acceptable level of ARB.

Microplastic is an abundant and distinct microbial habitat in an urban river

Recent research has documented microplastic particles (< 5 mm in diameter) in ocean habitats worldwide and in the Laurentian Great Lakes. Microplastic interacts with biota, including microorganisms, in these habitats, raising concerns about its ecological effects. Rivers may transport microplastic to marine habitats and the Great Lakes, but data on microplastic in rivers is limited. In a highly urbanized river in Chicago, Illinois, USA, we measured concentrations of microplastic that met or exceeded those measured in oceans and the Great Lakes, and we demonstrated that wastewater treatment plant effluent was a point source of microplastic. Results from high-throughput sequencing showed that bacterial assemblages colonizing microplastic within the river were less diverse and were significantly different in taxonomic composition compared to those from the water column and suspended organic matter. Several taxa that include plastic decomposing organisms and pathogens were more abundant on microplastic. These results demonstrate that microplastic in rivers are a distinct microbial habitat and may be a novel vector for the downstream transport of unique bacterial assemblages. In addition, this study suggests that urban rivers are an overlooked and potentially significant component of the global microplastic life cycle.

Phthalate esters in the environment: A critical review of their occurrence, biodegradation, and removal during wastewater treatment processes

Phthalate esters are one of the most frequently detected persistent organic pollutants in the environment. A better understanding of their occurrence and degradation in the environment and during wastewater treatment processes will facilitate the development of strategies to reduce these pollutants and to bioremediate contaminated freshwater and soil. Phthalate esters occur at measurable levels in different environments worldwide. For example, the concentrations of dimethyl phthalate (DMP) in atmospheric particulate matter, fresh water and sediments, soil, and landfills are N.D.-10.4 ng/m(3), N.D.-31.7 μg/L, N.D.-316 μg/kg dry weight, and N.D.-200 μg/kg dry weight, N.D.-43.27 μg/L, respectively. Bis(2-ethylhexyl) phthalate (DEHP) and di-n-butyl phthalate (DBP) are primary phthalate ester pollutants. Urbanization has increased the discharge of phthalate esters to atmospheric and aquatic environments, and the use of agricultural plastics has exacerbated soil contamination by phthalate esters in rural areas. Aerobic biodegradation is the primary manner of phthalate ester mineralization in the environment, and this process has been widely studied. Phthalate esters can be removed during wastewater treatment processes. The combination of different wastewater treatment technologies showed greater efficiency in the removal of phthalate esters than individual treatment steps, such as the combination of anaerobic wastewater treatment with a membrane bioreactor would increase the efficiency of phthalate ester removal from 65%-71% to 95%-97%. This review provides a useful framework to identify future research objectives to achieve the mineralization and elimination of phthalate esters in the environment.

Antimicrobial and antioxidant properties of chitosan and its derivatives and their applications: A review

In this era, there is a global concern in the use of bioactive molecules such as chitosan in the field of antimicrobial and antioxidant benefits. Because of its biodegradability, biological compatibility, antimicrobial, antioxidants activity, and high safety, chitosan could be used in a large number of applications. It could exist in many forms, such as fibers, gels, films, sponges, nanoparticles, and beads. The different biological activities of chitosan and its products are extensively investigated to broaden the application fields in several areas. Chitosan's natural properties depend strongly on water and other solvent solubility. Consequently, the chitosan oligosaccharides with a low polymerization degree are getting significant attention in the pharmaceutical and medical applications because they have lower viscosity and higher water solubility than chitosan. The objective of this review article is to put the antioxidant and antimicrobial properties of chitosan and its derivatives under the spotlight. The impacts of chitosan on physicochemical parameters like molecular weight and deacetylation degree on its bioactivities are also identified. Additionally, other applications of chitosan and its derivatives, including wound healing products, wastewater treatment, and cosmetics, have also been highlighted.

Metabolic network analysis reveals microbial community interactions in anammox granules

Microbial communities mediating anaerobic ammonium oxidation (anammox) represent one of the most energy-efficient environmental biotechnologies for nitrogen removal from wastewater. However, little is known about the functional role heterotrophic bacteria play in anammox granules. Here, we use genome-centric metagenomics to recover 17 draft genomes of anammox and heterotrophic bacteria from a laboratory-scale anammox bioreactor. We combine metabolic network reconstruction with metatranscriptomics to examine the gene expression of anammox and heterotrophic bacteria and to identify their potential interactions. We find that Chlorobi-affiliated bacteria may be highly active protein degraders, catabolizing extracellular peptides while recycling nitrate to nitrite. Other heterotrophs may also contribute to scavenging of detritus and peptides produced by anammox bacteria, and potentially use alternative electron donors, such as H2, acetate and formate. Our findings improve the understanding of metabolic activities and interactions between anammox and heterotrophic bacteria and offer the first transcriptional insights on ecosystem function in anammox granules.

Prevalence and proliferation of antibiotic resistance genes in two municipal wastewater treatment plants

The propagation of antibiotic resistance genes (ARGs) is an emerging health concern worldwide. Thus, it is important to understand and mitigate their occurrence in different systems. In this study, 30 ARGs that confer resistance to tetracyclines, sulfonamides, quinolones or macrolides were detected in two activated sludge wastewater treatment plants (WWTPs) in northern China. Bacteria harboring ARGs persisted through all treatment units, and survived disinfection by chlorination in greater percentages than total Bacteria (assessed by 16S rRNA genes). Although the absolute abundances of ARGs were reduced from the raw influent to the effluent by 89.0%-99.8%, considerable ARG levels [(1.0 ± 0.2) × 10(3) to (9.5 ± 1.8) × 10(5) copies/mL)] were found in WWTP effluent samples. ARGs were concentrated in the waste sludge (through settling of bacteria and sludge dewatering) at (1.5 ± 2.3) × 10(9) to (2.2 ± 2.8) × 10(11) copies/g dry weight. Twelve ARGs (tetA, tetB, tetE, tetG, tetH, tetS, tetT, tetX, sul1, sul2, qnrB, ermC) were discharged through the dewatered sludge and plant effluent at higher rates than influent values, indicating overall proliferation of resistant bacteria. Significant antibiotic concentrations (2%-50% of raw influent concentrations) remained throughout all treatment units. This apparently contributed selective pressure for ARG replication since the relative abundance of resistant bacteria (assessed by ARG/16S rRNA gene ratios) was significantly correlated to the corresponding effluent antibiotic concentrations. Similarly, the concentrations of various heavy metals (which induce a similar bacterial resistance mechanism as antibiotics - efflux pumps) were also correlated to the enrichment of some ARGs. Thus, curtailing the release of antibiotics and heavy metals to sewage systems (or enhancing their removal in pre-treatment units) may alleviate their selective pressure and mitigate ARG proliferation in WWTPs.

Detection and fate of antibiotic resistant bacteria in wastewater treatment plants: a review

Antibiotics are among the most successful group of pharmaceuticals used for human and veterinary therapy. However, large amounts of antibiotics are released into municipal wastewater due to incomplete metabolism in humans or due to disposal of unused antibiotics, which finally find their ways into different natural environmental compartments. The emergence and rapid spread of antibiotic resistant bacteria (ARB) has led to an increasing concern about the potential environmental and public health risks. ARB and antibiotic resistant genes (ARGs) have been detected extensively in wastewater samples. Available data show significantly higher proportion of antibiotic resistant bacteria contained in raw and treated wastewater relative to surface water. According to these studies, the conditions in wastewater treatment plants (WWTPs) are favourable for the proliferation of ARB. Moreover, another concern with regards to the presence of ARB and ARGs is their effective removal from sewage. This review gives an overview of the available data on the occurrence of ARB and ARGs and their fate in WWTPs, on the biological methods dealing with the detection of bacterial populations and their resistance genes, and highlights areas in need for further research studies.

Antibiotic resistance genes identified in wastewater treatment plant systems - A review

The intensive use of antibiotics for human, veterinary and agricultural purposes, results in their continuous release into the environment. Together with antibiotics, antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) are introduced into wastewater. Wastewater treatment plants (WWTPs) are believed to be probable hotspots for antibiotic resistance dissemination in the environment as they offer convenient conditions for ARB proliferation as well as for horizontal transfer of ARGs among different microorganisms. In fact, genes conferring resistance to all classes of antibiotics together with mobile genetic elements (MGEs) like plasmids, transposons, bacteriophages, integrons are detected in WWTPs in different countries. It seems that WWTPs with conventional treatment processes are capable of significant reduction of ARB but are not efficient in ARG removal. Implementation of advanced wastewater cleaning processes in addition to a conventional wastewater treatment is an important step to protect the aquatic environment. Growing interest in presence and fate of ARB and ARGs in WWTP systems resulted in the fact that knowledge in this area has increased staggeringly in the past few years. The main aim of the article is to collect and organize available data on ARGs, that are commonly detected in raw sewage, treated wastewater or activated sludge. Resistance to the antibiotics usually used in antibacterial therapy belonging to main classes like beta-lactams, macrolides, quinolones, sulfonamides, trimethoprim and tetracyclines was taken into account. The presence of multidrug efflux genes is also included in this paper. The occurrence of antibiotics may promote the selection of ARB and ARGs. As it is important to discuss the problem considering all aspects that influence it, the levels of antibiotics detected in influent and effluent of WWTPs were also presented.

Evaluation of toxicity and biodegradability of choline chloride based deep eutectic solvents

Deep eutectic solvents (DESs) have been dramatically expanding in popularity as a new generation of environmentally friendly solvents with possible applications in various industrial fields, but their ecological footprint has not yet been thoroughly investigated. In the present study, three choline chloride-based DESs with glucose, glycerol and oxalic acid as hydrogen bond donors were evaluated for in vitro toxicity using fish and human cell line, phytotoxicity using wheat and biodegradability using wastewater microorganisms through closed bottle test. Obtained in vitro toxicity data on cell lines indicate that choline chloride: glucose and choline chloride:glycerol possess low cytotoxicity (EC50>10 mM for both cell lines) while choline chloride:oxalic acid possess moderate cytotoxicity (EC50 value 1.64 mM and 4.19 mM for fish and human cell line, respectively). Results on phytotoxicity imply that tested DESs are non-toxic with seed germination EC50 values higher than 5000 mg L(-1). All tested DESs were classified as'readily biodegradable' based on their high levels of mineralization (68-96%). These findings indicate that DESs have a green profile and a good prospect for a wider use in the field of green technologies.

Antibiotic resistance in European wastewater treatment plants mirrors the pattern of clinical antibiotic resistance prevalence

Integrated antibiotic resistance (AR) surveillance is one of the objectives of the World Health Organization global action plan on antimicrobial resistance. Urban wastewater treatment plants (UWTPs) are among the most important receptors and sources of environmental AR. On the basis of the consistent observation of an increasing north-to-south clinical AR prevalence in Europe, this study compared the influent and final effluent of 12 UWTPs located in seven countries (Portugal, Spain, Ireland, Cyprus, Germany, Finland, and Norway). Using highly parallel quantitative polymerase chain reaction, we analyzed 229 resistance genes and 25 mobile genetic elements. This first trans-Europe surveillance showed that UWTP AR profiles mirror the AR gradient observed in clinics. Antibiotic use, environmental temperature, and UWTP size were important factors related with resistance persistence and spread in the environment. These results highlight the need to implement regular surveillance and control measures, which may need to be appropriate for the geographic regions.

Co-occurrence of integrase 1, antibiotic and heavy metal resistance genes in municipal wastewater treatment plants

The impact of human activities on the spread and on the persistence of antibiotic resistances in the environment is still far from being understood. The natural background of resistances is influenced by human activities, and the wastewater treatment plants (WWTPs) are among the main sources of the release of antibiotic resistance into the environment. The various treatments of WWTPs provide a number of different environmental conditions potentially favoring the selection of antibiotic resistance genes (ARGs) and thereby their well-documented spread in the environment. Although the distribution of different ARGs in WWTPs has been deeply investigated, very little is known on the ecology and on the molecular mechanisms underlying the selection of specific ARGs. This study investigates the fate of diverse ARGs, heavy metal resistance genes (HMRGs) and of a mobile element (the class I integron) in three WWTPs. Abundances of the different genetic markers were correlated to each other and their relation to biotic and abiotic factors (total organic carbon, total nitrogen, prokaryotic cell abundance and its relative distribution in single cells and aggregates) influencing the microbial communities in the different treatment phases in three WWTPs, were investigated. Water samples were analyzed for the abundance of six ARGs (tetA, sulII, blaTEM, blaCTXM,ermB, and qnrS), two HMRGs (czcA and arsB), and of the class I integron (int1). The measured variables clustered in two well-defined groups, the first including tetA, ermB, qnrS and the different biotic and abiotic factors, and a second group around the genes sulII, czcA, arsB and int1. Moreover, the dynamics of sulII, HMRGs, and int1 correlated strongly. Our results suggest a potentially crucial role of HMRGs in the spread, mediated by mobile elements, of some ARGs, i.e. sulII. The possibility of a relation between heavy metal contamination and the spread of ARGs in WWTPs calls for further research to clarify the mechanisms of co-selection and their ecology, in order to implement the removal efficiency of the applied treatments.

Spectroscopic characterization of extracellular polymeric substances from a mixed culture dominated by ammonia-oxidizing bacteria

Extracellular polymeric substances (EPS) of aerobic (AerAOB) and anaerobic ammonium-oxidizing bacteria (AnAOB) are expected to have a significant impact on the performance of autotrophic nitrogen removal in engineered systems. However, there are a few investigations of the EPS of AerAOB and AnAOB, and the results are contradictory. In this study, photometric measurements indicated that the EPS of AerAOB- (31.74 ± 1.48 mg/g-VSS, volatile suspended solids) and AnAOB-enriched cultures (30.12 ± 1.52 mg/g-VSS) contained more polysaccharides than did conventional activated sludge from a municipal wastewater treatment facility (10.76 ± 0.83 mg/g-VSS). In addition, the EPS of the AnAOB-enriched culture was dominated by proteins, leading to a considerably higher protein/polysaccharide ratio (2.64 ± 0.12) than those of the AerAOB-enriched culture (0.56 ± 0.03) and conventional activated sludge (1.96 ± 0.09). Characterization using Fourier transform infrared spectroscopy (FTIR) revealed the dominance of amide bands and/or polysaccharide-associated bands in the EPS of AnAOB and AerAOB. These results corroborate the data from the photometric measurements. In addition, the EPS of AnAOB (23.1% ± 1.2%) and AerAOB (21.9% ± 1.1%) had a higher portion of α-helices, which is the key protein secondary structure that determines flocculation or cell aggregation, in the amide I band than that of activated sludge (16.7% ± 0.8%). X-ray photoelectron spectroscopy (XPS) characterization also revealed significantly different functionalities among the EPS of the three mixed cultures; e.g., O-(C,H), which indicates the presence of polysaccharides, was richer in the EPS of AerAOB, whereas protonated amines, which are commonly found in amino acids and amino sugars, accounted for a large portion of the EPS of AnAOB. The results of this study can potentially expand our knowledge of the microbial aggregates responsible for autotrophic nitrogen removal.

Ultraviolet disinfection of antibiotic resistant bacteria and their antibiotic resistance genes in water and wastewater

Disinfection of wastewater treatment plant effluent may be an important barrier for limiting the spread of antibiotic-resistant bacteria (ARBs) and antibiotic resistance genes (ARGs). While ideally disinfection should destroy ARGs, to prevent horizontal gene transfer to downstream bacteria, little is known about the effect of conventional water disinfection technologies on ARGs. This study examined the potential of UV disinfection to damage four ARGs, mec(A), van(A), tet(A), and amp(C), both in extracellular form and present within a host ARBs: methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium (VRE), Escherichia coli SMS-3-5, and Pseudomonas aeruginosa 01, respectively. An extended amplicon-length quantitative polymerase chain reaction assay was developed to enhance capture of ARG damage events and also to normalize to an equivalent length of target DNA (∼1000 bp) for comparison. It was found that the two Gram-positive ARBs (MRSA and VRE) were more resistant to UV disinfection than the two Gram-negative ARBs (E. coli and P. aeruginosa). The two Gram-positive organisms also possessed smaller total genome sizes, which could also have reduced their susceptibility to UV because of fewer potential pyrimidine dimer targets. An effect of cell type on damage to ARGs was only observed in VRE and P. aeruginosa, the latter potentially because of extracellular polymeric substances. In general, damage of ARGs required much greater UV doses (200-400 mJ/cm² for 3- to 4-log reduction) than ARB inactivation (10-20 mJ/cm² for 4- to 5-log reduction). The proportion of amplifiable ARGs following UV treatment exhibited a strong negative correlation with the number of adjacent thymines (Pearson r < -0.9; p < 0.0001). ARBs surviving UV treatment were negatively correlated with total genome size (Pearson r < -0.9; p < 0.0001) and adjacent cytosines (Pearson r < -0.88; p < 0.0001) but positively correlated with adjacent thymines (Pearson r > 0.85; p < 0.0001). This suggests that formation of thymine dimers is not the sole mechanism of ARB inactivation. Overall, the results indicate that UV is limited in its potential to damage ARGs and other disinfection technologies should be explored.

Antibiotic resistance, antimicrobial residues and bacterial community composition in urban wastewater

This study was based on the hypothesis that the occurrence of antimicrobial residues and antibiotic resistant bacteria in the sewage could be correlated with the structure and composition of the bacterial community and the antibiotic resistance loads of the final effluent. Raw and treated wastewater composite samples were collected from an urban treatment plant over 14 sampling dates. Samples were characterized for the i) occurrence of tetracyclines, penicillins, sulfonamides, quinolones, triclosan, arsenic, cadmium, lead, chromium and mercury; ii) antibiotic resistance percentages for tetracycline, sulfamethoxazole, ciprofloxacin and amoxicillin and iii) 16S rRNA gene-DGGE patterns. The data of corresponding samples, taking into account the hydraulic residence time, was analyzed using multivariate analysis. Variations on the bacterial community structure of the final effluent were significantly correlated with the occurrence of tetracyclines, penicillins, sulfonamides, quinolones and triclosan in the raw inflow. Members of the class Epsilonproteobacteria presented positive correlations with those antimicrobials, whereas negative correlations were observed with Beta and Gammaproteobacteria and Firmicutes. Antibiotic resistance percentages presented different trends of variation in heterotrophs/enterobacteria and in enterococci, varied over time and after wastewater treatment. Antibiotic resistance was positively correlated with the occurrence of tetracyclines residues and high temperature. A relationship between antibiotic residues, bacterial community structure and composition and antibiotic resistance is demonstrated. Further studies, involving more wastewater treatment plants may help to elucidate this complex relationship.

Antibiotic resistance in wastewater treatment plants: Tackling the black box

Wastewater is among the most important reservoirs of antibiotic resistance in urban environments. The abundance of carbon sources and other nutrients, a variety of possible electron acceptors such as oxygen or nitrate, the presence of particles onto which bacteria can adsorb, or a fairly stable pH and temperature are examples of conditions favouring the remarkable diversity of microorganisms in this peculiar habitat. The wastewater microbiome brings together bacteria of environmental, human and animal origins, many harbouring antibiotic resistance genes (ARGs). Although numerous factors contribute, mostly in a complex interplay, for shaping this microbiome, the effect of specific potential selective pressures such as antimicrobial residues or metals, is supposedly determinant to dictate the fate of antibiotic resistant bacteria (ARB) and ARGs during wastewater treatment. This paper aims to enrich the discussion on the ecology of ARB&ARGs in urban wastewater treatment plants (UWTPs), intending to serve as a guide for wastewater engineers or other professionals, who may be interested in studying or optimizing the wastewater treatment for the removal of ARB&ARGs. Fitting this aim, the paper overviews and discusses: i) aspects of the complexity of the wastewater system and/or treatment that may affect the fate of ARB&ARGs; ii) methods that can be used to explore the resistome, meaning the whole ARB&ARGs, in wastewater habitats; and iii) some frequently asked questions for which are proposed addressing modes. The paper aims at contributing to explore how ARB&ARGs behave in UWTPs having in mind that each plant is a unique system that will probably need a specific procedure to maximize ARB&ARGs removal.

Bacterial community compositions of coking wastewater treatment plants in steel industry revealed by Illumina high-throughput sequencing

In this study, Illumina high-throughput sequencing was used to reveal the community structures of nine coking wastewater treatment plants (CWWTPs) in China for the first time. The sludge systems exhibited a similar community composition at each taxonomic level. Compared to previous studies, some of the core genera in municipal wastewater treatment plants such as Zoogloea, Prosthecobacter and Gp6 were detected as minor species. Thiobacillus (20.83%), Comamonas (6.58%), Thauera (4.02%), Azoarcus (7.78%) and Rhodoplanes (1.42%) were the dominant genera shared by at least six CWWTPs. The percentages of autotrophic ammonia-oxidizing bacteria and nitrite-oxidizing bacteria were unexpectedly low, which were verified by both real-time PCR and fluorescence in situ hybridization analyses. Hierarchical clustering and canonical correspondence analysis indicated that operation mode, flow rate and temperature might be the key factors in community formation. This study provides new insights into our understanding of microbial community compositions and structures of CWWTPs.

Exploiting extracellular polymeric substances (EPS) controlling strategies for performance enhancement of biological wastewater treatments: An overview

Extracellular polymeric substances (EPS) are present both outside of the cells and in the interior of microbial aggregates, and account for a main component in microbial aggregates. EPS can influence the properties and functions of microbial aggregates in biological wastewater treatment systems, and specifically EPS are involved in biofilm formation and stability, sludge behaviors as well as sequencing batch reactors (SBRs) granulation whereas they are also responsible for membrane fouling in membrane bioreactors (MBRs). EPS exhibit dual roles in biological wastewater treatments, and hence the control of available EPS can be expected to lead to changes in microbial aggregate properties, thereby improving system performance. In this review, current updated knowledge with regard to EPS basics including their formation mechanisms, important properties, key component functions as well as sub-fraction differentiation is given. EPS roles in biological wastewater treatments are also briefly summarized. Special emphasis is laid on EPS controlling strategies which would have the great potential in promoting microbial aggregates performance and in alleviating membrane fouling, including limitation strategies (inhibition of quorum sensing (QS) systems, regulation of environmental conditions, enzymatic degradation of key components, energy uncoupling etc.) and elevation strategies (enhancement of QS systems, addition of exogenous agents etc.). Those strategies have been confirmed to be feasible and promising to enhance system performance, and they would be a research niche that deserves further study.

Prevalence of antibiotic resistance genes and bacterial community composition in a river influenced by a wastewater treatment plant

Antibiotic resistance represents a global health problem, requiring better understanding of the ecology of antibiotic resistance genes (ARGs), their selection and their spread in the environment. Antibiotics are constantly released to the environment through wastewater treatment plant (WWTP) effluents. We investigated, therefore, the effect of these discharges on the prevalence of ARGs and bacterial community composition in biofilm and sediment samples of a receiving river. We used culture-independent approaches such as quantitative PCR to determine the prevalence of eleven ARGs and 16S rRNA gene-based pyrosequencing to examine the composition of bacterial communities. Concentration of antibiotics in WWTP influent and effluent were also determined. ARGs such as qnrS, bla_TEM, bla_CTX-M, bla_SHV, erm(B), sul(I), sul(II), tet(O) and tet(W) were detected in all biofilm and sediment samples analyzed. Moreover, we observed a significant increase in the relative abundance of ARGs in biofilm samples collected downstream of the WWTP discharge. We also found significant differences with respect to community structure and composition between upstream and downstream samples. Therefore, our results indicate that WWTP discharges may contribute to the spread of ARGs into the environment and may also impact on the bacterial communities of the receiving river.

Detection of nitrifiers and evaluation of partial nitrification for wastewater treatment: A review

Partial nitrification has gained broad interests in the biological nitrogen removal (BNR) from wastewater, since it alleviates carbon limitation issues and acts as a shortcut nitrogen removal system combined with anaerobic ammonium oxidation (Anammox) process. The occurrence and maintenance of partial nitrification relies on various conditions, which favor ammonium oxidizing bacteria (AOB) but inhibit or limit nitrite oxidizing bacteria (NOB). The studies of the AOB and NOB activities have been conducted by state-of-the-art molecular techniques, such as Polymerase Chain Reaction (PCR), Quantitative PCR, denaturing gradient gel electrophoresis (DGGE), Fluorescence in situ hybridization (FISH) technique, Terminal Restriction Fragment Length Polymorphism (T-RFLP), Live/Dead BacLight, and quinone profile. Furthermore, control strategies for obtaining partial nitrification are mainly focused on the pH, temperature, dissolved oxygen concentration, real-time aeration control, sludge retention time, substrate concentration, alternating anoxic and aerobic operation, inhibitor and ultrasonic treatment. Existing problems and further perspectives for the scale-up of partial nitrification are also proposed and suggested.

Anammox-based technologies for nitrogen removal: Advances in process start-up and remaining issues

Nitrogen removal from wastewater via anaerobic ammonium oxidation (anammox)-based process has been recognized as efficient, cost-effective and low energy alternative to the conventional nitrification and denitrification processes. To date, more than one hundred full-scale anammox plants have been installed and operated for treatment of NH4(+)-rich wastewater streams around the world, and the number is increasing rapidly. Since the discovery of anammox process, extensive researches have been done to develop various anammox-based technologies. However, there are still some challenges in practical application of anammox-based treatment process at full-scale, e.g., longer start-up period, limited application to mainstream municipal wastewater and poor effluent water quality. This paper aimed to summarize recent status of application of anammox process and researches on technological development for solving these remaining problems. In addition, an integrated system of anammox-based process and microbial fuel cell is proposed for sustainable and energy-positive wastewater treatment.

Distinguishing effects of ultraviolet exposure and chlorination on the horizontal transfer of antibiotic resistance genes in municipal wastewater

Growing attention has been paid to the dissemination of antibiotic resistance genes (ARGs) in wastewater microbial communities; however, the disinfection processes, as microbial control technologies, have not been evaluated for their impacts on ARGs transfer. In this study, the effects of ultraviolet (UV) disinfection and chlorination on the frequency of ARGs transfer have been explored based on the conjugative transfer model between Gram-negative strains of E. coli. The results indicated that UV disinfection and chlorination exhibit distinct influences on the conjugative transfer. Low UV doses (up to 8 mJ/cm2) had little influence on the frequency of conjugative transfer, and UV exposure only decreased the bacterial number but did not change the cell permeability. By comparison, low chlorine doses (up to 40 mg Cl min/L) significantly promoted the frequency of conjugative transfer by 2-5-fold. The generated chloramine stimulated the bacteria and improved the cell permeability. More pilus were induced on the surface of conjugative cells, which acted as pathways for ARGs transfer. The frequency of ARG transfers was greatly suppressed by high doses of UV (>10 mJ/cm2) or chlorine (>80 mg Cl min/L).

Chlorine disinfection increases both intracellular and extracellular antibiotic resistance genes in a full-scale wastewater treatment plant

The emergence and spread of antibiotic resistance has posed a major threat to both human health and environmental ecosystem. Although the disinfection has been proved to be efficient to control the occurrence of pathogens, little effort is dedicated to revealing potential impacts of disinfection on transmission of antibiotic resistance genes (ARGs), particularly for free-living ARGs in final disinfected effluent of urban wastewater treatment plants (UWWTP). Here, we investigated the effects of chlorine disinfection on the occurrence and concentration of both extracellular ARGs (eARGs) and intracellular ARGs (iARGs) in a full-scale UWWTP over a year. We reported that the concentrations of both eARGs and iARGs would be increased by the disinfection with chlorine dioxide (ClO₂). Specifically, chlorination preferentially increased the abundances of eARGs against macrolide (ermB), tetracycline (tetA, tetB and tetC), sulfonamide (sul1, sul2 and sul3), β-lactam (ampC), aminoglycosides (aph(2')-Id), rifampicin (katG) and vancomycin (vanA) up to 3.8 folds. Similarly, the abundances of iARGs were also increased up to 7.8 folds after chlorination. In terms of correlation analyses, the abundance of Escherichia coli before chlorination showed a strong positive correlation with the total eARG concentration, while lower temperature and higher ammonium concentration were assumed to be associated with the concentration of iARGs. This study suggests the chlorine disinfection could increase the abundances of both iARGs and eARGs, thereby posing risk of the dissemination of antibiotic resistance in environments.

Heterogeneous Fenton catalysts: A review of recent advances

Heterogeneous Fenton catalysts are emerging as excellent materials for applications related to water purification. In this review, recent trends in the synthesis and application of heterogeneous Fenton catalysts for the abatement of organic pollutants and disinfection of microorganisms are discussed. It is noted that as the complexity of cell wall increases, the resistance level towards various disinfectants increases and it requires either harsh conditions or longer exposure time for the complete disinfection. In case of viruses, enveloped viruses (e.g. SARS-CoV-2) are found to be more susceptible to disinfectants than the non-enveloped viruses. The introduction of plasmonic materials with the Fenton catalysts broadens the visible light absorption efficiency of the hybrid material, and incorporation of semiconductor material improves the rate of regeneration of Fe(II) from Fe(III). A special emphasis is given to the use of Fenton catalysts for antibacterial applications. Composite materials of magnetite and ferrites remain a champion in this area because of their easy separation and reuse, owing to their magnetic properties. Iron minerals supported on clay materials, perovskites, carbon materials, zeolites and metal-organic frameworks (MOFs) dramatically increase the catalytic degradation rate of contaminants by providing high surface area, good mechanical stability, and improved electron transfer. Moreover, insights to the zero-valent iron and its capacity to remove a wide range of organic pollutants, heavy metals and bacterial contamination are also discussed. Real world applications and the role of natural organic matter are summarised. Parameter optimisation (e.g. light source, dosage of catalyst, concentration of H2O2 etc.), sustainable models for the reusability or recyclability of the catalyst and the theoretical understanding and mechanistic aspects of the photo-Fenton process are also explained. Additionally, this review summarises the opportunities and future directions of research in the heterogeneous Fenton catalysis.

Mobile antibiotic resistome in wastewater treatment plants revealed by Nanopore metagenomic sequencing

BACKGROUND

Wastewater treatment plants (WWTPs) are recognized as hotspots for horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs). Despite our understanding of the composition and distribution of ARGs in WWTPs, the genetic location, host, and fate of ARGs remain largely unknown.

RESULTS

In this study, we combined Oxford Nanopore and Illumina metagenomics sequencing to comprehensively uncover the resistome context of influent, activated sludge, and effluent of three WWTPs and simultaneously track the hosts of the ARGs. The results showed that most of the ARGs detected in all compartments of the WWTPs were carried by plasmids. Transposons and integrons also showed higher prevalence on plasmids than on the ARG-carrying chromosome. Notably, integrative and conjugative elements (ICEs) carrying five types of ARGs were detected, and they may play an important role in facilitating the transfer of ARGs, particularly for tetracycline and macrolide-lincosamide-streptogramin (MLS). A broad spectrum of ARGs carried by plasmids (29 subtypes) and ICEs (4 subtypes) was persistent across the WWTPs. Host tracking showed a variety of antibiotic-resistant bacteria in the effluent, suggesting the high potential for their dissemination into receiving environments. Importantly, phenotype-genotype analysis confirmed the significant role of conjugative plasmids in facilitating the survival and persistence of multidrug-resistant bacteria in the WWTPs. At last, the consistency in the quantitative results for major ARGs types revealed by Nanopore and Illumina sequencing platforms demonstrated the feasibility of Nanopore sequencing for resistome quantification.

CONCLUSION

Overall, these findings substantially expand our current knowledge of resistome in WWTPs, and help establish a baseline analysis framework to study ARGs in the environment.

Abundance and persistence of antibiotic resistance genes in livestock farms: a comprehensive investigation in eastern China

Increases of antibiotic resistance genes in the environment may pose a threat to public health. The purpose of this study was to investigate the abundance and diversity of tetracycline (tet) and sulfonamide (sul) resistance genes in eight livestock farms in Hangzhou, eastern China. Ten tet genes (tetA, tetB, tetC, tetG, tetL, tetM, tetO, tetQ, tetW, and tetX), two sul genes (sulI and sulII), and one genetic element associated with mobile antibiotic resistance genes [class 1 integron (intI1)] were quantified by real-time polymerase chain reaction. No significant difference was found in the abundance of the tet and sul genes in various scales of pig, chicken, and duck farms (P>0.05). The average abundance of ribosomal protection protein genes (tetQ, tetM, tetW, and tetO) in the manure and wastewater samples was higher than most of the efflux pump genes (tetA, tetB, tetC, and tetL) and enzymatic modification gene (tetX) (P<0.05), except for efflux pump gene tetG, which was abundant and showed no difference from tetM. Most ARGs had higher relative abundance in the wastewater lagoon than in manures even after treatment. Although the three ribosomal protection protein genes (tetQ, tetW, and tetO) had higher relative abundance, numbers were reduced during the complete wastewater treatment process in pig farms (P<0.05). The relative abundance of tetG, sulI, and sulII increased after the wastewater treatment and the removal of these three genes exhibited significant positive correlations with the intI1 gene (tetG: R(2)=0.60, P<0.05; sulI: R(2)=0.72, P<0.05; sulII: R(2)=0.62, P<0.05), suggesting that intI1 may be involved in their proliferation. As for tetM and sulII genes, a highly significant difference was found in manure samples between pig farms and duck farms (P<0.001). Phylogenetic analysis showed that tetM was more diverse in duck farms than in pig farms. Additionally, sulII sequence was conserved both in pig and duck farms. This is the first comprehensive study to detail the relative abundance of specific ARGs in animal manures and agricultural wastewater treatment systems, potentially providing knowledge for managing antibiotic resistance emanating from agricultural activities.