Novel Transposon Tn6433 Variants Accelerate the Dissemination of tet(E) in Aeromonas in an Aerobic Biofilm Reactor under Oxytetracycline Stresses

Horizontal gene transfer plays a crucial role in the development of antibiotic resistance in an antibiotic-free shrimp farming system

Antibiotic selective pressure in aquaculture systems often results in the antibiotic resistance genes (ARGs) proliferation. Nonetheless, a paucity of data exists concerning the mechanisms of ARGs development in aquaculture systems without the influences of antibiotics. This study utilized metagenomic approaches to elucidate the dynamics and transfer mechanisms of ARGs throughout the aquaculture of Pacific white shrimp. A marked change in the resistome was observed throughout the aquaculture without antibiotics. The total ARGs relative abundance increased from 0.05 to 0.33 by day 90 of cultivation, with even higher in mixed wastewater (0.44). Both bacterial communities and mobile genetic elements play pivotal roles in the development of ARGs. Metagenome-assembled genomes showed enrichment of environmentally intrinsic ARGs on chromosomes including macB and mdtK. The plasmid-mediated horizontal transfer was recognized as a principal factor contributing to the rise of ARGs, particularly for tetG and floR, and this led to an escalation of resistance risk, peaking at a risks core of 35.43 on day 90. This study demonstrates that horizontal gene transfer plays a crucial role in ARGs development without antibiotic pressure, which can provide a theoretical foundation for controlling ARGs proliferation in aquaculture systems.

One Health-based management for sustainably mitigating tetracycline-resistant Aeromonas hydrophila-induced health risk

Aeromonas hydrophila has ability to spread tetracycline resistance (tetR) under stresses of oxytetracycline (OTC), one of the most important antibiotics in aquaculture industry. Even though environmental reservoir of Aeromonas allows it to be at interfaces across One Health components, a robust modelling framework for rigorously assessing health risks is currently lacking. We proposed a One Health-based approach and leveraged recent advances in quantitative microbial risk assessment appraised by available dataset to interpret interactions at the human-animal-environment interfaces in various exposure scenarios. The dose-response models were constructed considering the effects on mortality for aquaculture species and tetR genes transfer for humans. A scenario-specific risk assessment on pond species-associated A. hydrophila infection and human gut-associated tetR genes transfer was examined. Risk-based control strategies were involved to test their effectiveness. We showed that farmed shrimp exposed to tetracycline-resistant A. hydrophila in OTC-contaminated water experienced higher infection risk (relative risk: 1.25-1.34). The tetR genes transfer risk for farmers in shrimp ponds (∼2 × 10-4) and swimmers in coastal areas (∼4 × 10-6) during autumn exceeded acceptable risk (10-6). This cautionary finding underscores the importance of accounting for monitoring, assessing, and mitigating occupational health hazards among workers in shrimp farming sectors within future One Health-based strategies for managing water infection risks. We recommend that OTC emission rate together with A. hydrophila concentration should be reduced by up to 70-99% to protect human, farmed shrimp, and environmental health. Our predictive framework can be adopted for other systems and be used as a "risk detector" for assessing tetR-related health risks that invoke potential risk management on addressing sustainable mitigation on offsetting residual OTC emission and tetR genes spread in a species-human-environmental health system.

In vitro assessment of the bacterial stress response and resistance evolution during multidrug-resistant bacterial invasion of the Xenopus tropicalis intestinal tract under typical stresses

The intestinal microbiome might be both a sink and source of resistance genes (RGs). To investigate the impact of environmental stress on the disturbance of exogenous multidrug-resistant bacteria (mARB) within the indigenous microbiome and proliferation of RGs, an intestinal conjugative system was established to simulate the invasion of mARB into the intestinal microbiota in vitro. Oxytetracycline (OTC) and heavy metals (Zn, Cu, Pb), commonly encountered in aquaculture, were selected as typical stresses for investigation. Adenosine 5'-triphosphate (ATP), hydroxyl radical (OH·-) and extracellular polymeric substance (EPS) were measured to investigate their influence on the acceptance of RGs by intestinal bacteria. The results showed that the transfer and diffusion of RGs under typical combined stressors were greater than those under a single stressor. Combined effect of OTC and heavy metals (Zn, Cu) significantly increased the activity and extracellular EPS content of bacteria in the intestinal conjugative system, increasing intI3 and RG abundance. OTC induced a notable inhibitory response in Citrobacter and exerted the proportion of Citrobacter and Carnobacterium in microbiota. The introduction of stressors stimulates the proliferation and dissemination of RGs within the intestinal environment. These results enhance our comprehension of the typical stresses effect on the RGs dispersal in the intestine.

Polyinfection in Fish Aeromoniasis: A Study of Co-Isolated Aeromonas Species in Aeromonas veronii Outbreaks

We studied the phenotypic and genomic characteristics related to the virulence and antibiotic resistance of two Aeromonas strains, which were co-isolated before an outbreak of Aeromonas veronii among diseased seabass on Agathonisi Island, Greece, in April 2015. The first strain, AG2.13.2, is a potentially pathogenic mesophilic variant of Aeromonas salmonicida, and the second, AG2.13.5, corresponds to an Aeromonas rivipollensis related to A. rivipollensis KN-Mc-11N1 with an ANI value of 97.32%. AG2.13.2 lacks the type III secretion system just like other mesophilic strains of A. salmonicida. This characteristic has been associated with lower virulence. However, the genome of AG2.13.2 contains other important virulence factors such as type II and type VI secretion systems, and toxins such as rtxA, aerolysin aer/act, and different types of hemolysins. The strain also carries several genes associated with antibiotic resistance such as the tetE efflux pump, and exhibits resistance to tetracycline, ampicillin, and oxolinic acid. In an in vivo challenge test with gilthead seabream larvae, the A. veronii bv sobria strain AG5.28.6 exhibited the highest virulence among all tested strains. Conversely, both A. salmonicida and A. rivipollensis showed minimal virulence when administered alone. Interestingly, when A. veronii bv sobria AG5.28.6 was co-administered with A. rivipollensis, the larvae survival probability increased compared to those exposed to A. veronii bv sobria AG5.28.6 alone. This finding indicates an antagonistic interaction between A. veronii bv sobria AG5.28.6 and A. rivipollensis AG2.13.5. The co-administration of A. veronii bv sobria AG5.28.6 with Aeromonas salmonicida did not yield distinct survival probabilities. Our results validate that the primary pathogen responsible for European seabass aeromoniasis is Aeromonas veronii bv sobria.

Genomic Characterization of Aeromonas veronii Provides Insights into Taxonomic Assignment and Reveals Widespread Virulence and Resistance Genes throughout the World

Aeromonas veronii is a Gram-negative bacterial species that causes disease in fish and is nowadays increasingly recurrent in enteric infections of humans. This study was performed to characterize newly sequenced isolates by comparing them with complete genomes deposited at the NCBI (National Center for Biotechnology Information). Nine isolates from fish, environments, and humans from the São Francisco Valley (Petrolina, Pernambuco, Brazil) were sequenced and compared with complete genomes available in public databases to gain insight into taxonomic assignment and to better understand virulence and resistance profiles of this species within the One Health context. One local genome and four NCBI genomes were misidentified as A. veronii. A total of 239 virulence genes were identified in the local genomes, with most encoding adhesion, motility, and secretion systems. In total, 60 genes involved with resistance to 22 classes of antibiotics were identified in the genomes, including mcr-7 and cphA. The results suggest that the use of methods such as ANI is essential to avoid misclassification of the genomes. The virulence content of A. veronii from local isolates is similar to those complete genomes deposited at the NCBI. Genes encoding colistin resistance are widespread in the species, requiring greater attention for surveillance systems.

Emergence of Aeromonas veronii strain co-harboring blaKPC-2, mcr-3.17, and tmexC3.2-tmexD3.3-toprJ1b cluster from hospital sewage in China

Introduction

The raise of multi-drug resistant bacteria involving carbapenem, colistin, or tigecycline resistance constitutes a threat to public health, which partly results from the transmission of corresponding mobile resistance genes, such as blaKPC and blaNDM for carbapenem, mcr for colistin, and tmexCD-toprJ gene cluster for tigecycline. Herein, we described the emergence of an Aeromonas veronii strain HD6454 co-harboring blaKPC-2, mcr-3.17, and tmexC3.2-tmexD3.3-toprJ1b gene cluster from hospital sewage.

Methods

Whole genome sequencing (WGS) was used to determine the genome sequence of HD6454, and the detailed genomic analysis of genetic elements or regions carrying key antimicrobial resistance genes (ARGs) from HD6454 were performed. Cloning experiment was conducted to confirm the function of key ARGs in mediating antimicrobial resistance. Conjugation experiment was conducted to determine the mobility of the plasmid.

Results

The results showed that this strain belonged to a novel sequence type (ST) variant ST1016, and carried 18 important ARGs. Among them, the blaKPC-2 was carried by non-self-transmissible IncP-6 plasmid, while tmexC3.2-tmexD3.3-toprJ1b gene cluster and mcr-3.17 were carried by integrative and mobilizable element (IME) or IME-related region in chromosome. The mcr-3.17, mcr-3.6, and mcr-3-like3 genes were further inferred to originate from IMEs of Aeromonas species. Additionally, for the first time, the mcr-3.17 was confirmed to confer low-level resistance to colistin under inducible expression, while tmexC3.2-tmexD3.3-toprJ1b gene cluster was confirmed to confer low-level resistance to tigecycline.

Discussion

This is the first report of a strain co-harboring blaKPC-2, mcr-3.17, and tmexC3.2-tmexD3.3-toprJ1b gene cluster. Although the resistance and/or mobility of these ARGs are limited in this strain, the emergence of this multiple important ARGs-carrying strain deserves further attention.

Deciphering the antibiotic resistome and microbial community in municipal wastewater treatment plants at different elevations in eastern and western China

Antibiotic resistance genes (ARGs) as emerging environmental contaminants pose severe global risks to public health and ecosystems. Municipal wastewater treatment plants (WWTPs) are crucial transmitters for the dissemination and propagation of ARGs into receiving water bodies via mobile genetic elements (MGEs). However, the comprehensive and deep deciphering of the diversity, abundance, and potential hosts of ARGs in two distinct altitudinal WWTPs is scarce. In this work, we revealed the elevational distribution characteristics of the resistance genes and microbial community of six WWTPs from two distinct geographical zones: a low-elevation (LE) region (Shandong, 10-22 m above sea level) and a high-elevation (HE) region (Gansu, 1,520-1,708 m above sea level). Significant elevational variations in the diversity and relative abundance of resistance genes were observed. Wastewater treatment could significantly reduce the concentrations of ARGs and MGEs by about 1-2 and 2-3 orders of magnitude, respectively. However, above 69.95% of resistance genes were enriched in effluent. In particular, 24 ARG subtype, 3 MGE subtypes, and 59 bacterial genera were persistent in all samples. More potential hosts for ARGs in LE region and more abundant human gut microbiota in HE region were identified. This work provides helpful information for controlling the spread of ARGs for their management and assessment, thereby mitigating the risks of ARGs in WWTPs.

Salt tolerance evolution facilitates antibiotic resistome in soil microbiota: Evidences from dissemination evaluation, hosts identification and co-occurrence exploration

Salinity is considered as one of the vital factors affecting the profiles of antibiotic resistance genes (ARGs) in soils, whereby its roles in shaping the antibiotic resistome were still poorly understood. Here, metagenomic analysis was conducted to track the ARGs distributions and dissemination in soils during salt accumulation and desalinization processes. Neutral-salt accumulation for 45 and 90 days significantly increased the relative abundances of ARGs and mobile genetic elements (MGEs) carrying antibiotic resistance contigs (ARCs). The ARGs within antibiotic efflux and target protection families primarily carried by Streptomyces, Nocardioides, Rhodanobacter and Monashia were largely enriched by salinity. The ARGs subtypes of the resistance-nodulation-division (RND) family, ATP-binding cassette (ABC) family, rRNA methyltransferase and other efflux were closely associated with MGEs, contributing to the enrichment of ARGs. Moreover, the ARGs subtypes and transposons were genetically linked with the salt-tolerance mechanisms of organic osmolyte transporters and K⁺ uptake proteins on the same ARC, demonstrating the coselection of ARGs and halotolerant genes. Furthermore, the antibiotic resistome could recover to a normal state after the prolonged incubation by alleviating salt stress. Nevertheless, the acquisition of ARGs by opportunistic pathogens after salt treatment was increased, serving to prioritize further efforts on the health risks correlated with resistance propagation and human exposure in saline soils.

Minimizing Risks of Antimicrobial Resistance Development in the Environment from a Public One Health Perspective

Antimicrobial resistance (AMR) is a globally recognized crisis with meaningful engagement across humans, animals, and the environment as in the One Health approach. The environment is the potential source, reservoir, and transmission route of AMR, and it plays a key role in AMR development from the One Health perspective. Animal farming, hospitals, and the pharmaceutical industry are identified as the main emission sources in the environment. Minimizing emissions and determining antimicrobial emission limits are priorities in the containment of environmental AMR development. From the perspectives of environmental management and environmental engineering, some important actions to minimize risks of AMR development are summarized, including the recent progress in enhanced hydrolysis pre-treatment technology to control the development of antibiotic resistance genes (ARGs) during biological wastewater treatment. It is desirable to establish a holistic framework to coordinate international actions on the containment of environmental AMR development. To establish a community with a shared future for humanity, China should and could play an important role in international cooperation to cope with AMR challenges.

Heavy metals and microbiome are negligible drivers than mobile genetic elements in determining particle-attached and free-living resistomes in the Yellow River

Suspended particles in water can shelter both microorganisms and contaminants. However, the emerging pollutants antibiotic resistance genes (ARGs) in free-living (FL) or particle-attached (PA) bacteria in aquatic environments are less explored. In this study, we compared the free-living and particle-attached ARGs during four seasons in the Yellow River using high-throughput quantitative PCR techniques and 16S rRNA gene sequencing. Our results demonstrated that both the free-living water and particles were dominated by tetracycline and beta-lactamase resistance genes. The PA-ARGs had a higher absolute abundance than FL-ARGs in the Yellow River, regardless of the season. Both PA-ARGs and FL-ARGs had the highest absolute abundance and diversity during winter. Mobile genetic elements (MGEs) were the dominant driver for both size-fractionated ARGs. However, the microbiome had less influence on PA-ARG profiles than the FL-ARG profiles, while the effects of the heavy metals on ARGs were negligible. The community assembly of both FL-ARG and PA-ARG can be explained by neutral processes. Several opportunistic pathogens (e.g., Escherichia coli) associated with human health exhibited a higher relative abundance in the particles than during a free-living lifestyle. Parts of these pathogens were potential ARG hosts. As such, it is important to monitor the ARGs and opportunistic pathogens from size-fractionated bacteria and develop targeted strategies to manage ARG dissemination and opportunistic pathogens to ensure public health.

A Risk-Based Approach for Managing Aquaculture Used Oxytetracycline-Induced TetR in Surface Water Across Taiwan Regions

Oxytetracycline (OTC), one of the most important antibiotics in aquaculture industry, has been linked to emergence of antibiotic resistant genes in the aquatic environment. Given rapid growth of the aquaculture industry and unregulated use of antibiotics, it is necessary to implement measures to mitigate the impact of antibiotic resistance risk on environmental and human health. However, there is a lack of quantitative models to properly assess risk of antibiotic resistance associated with environmentally relevant antibiotic residues. To address this issue, here we developed a computational framework to assess antibiotic resistance risk posed by low-concentration OTC in aquaculture ponds and rivers across Taiwan regions. To this end, estimated amount of aquaculture used OTC as a crucial input parameter was incorporated into a multimedia fugacity model to predict environmental concentrations of OTC in surface water/sediment. A pharmacodynamic-based dose–response model was used to characterize the OTC concentration–antibiotic resistance relationships. The risk of antibiotic resistance selection in an aquatic environment could be assessed based on a probabilistic risk model. We also established a control measure model to manage the risks of substantial OTC-induced antibiotic resistance impacts. We found that OTC residues were likely to pose a high risk of tetracycline resistance (tetR) genes selection in aquaculture ponds among all the study basins, whereas risk of tetR genes selection in rivers experienced a variably changing fashion. We also showed that it was extremely difficult to moderate the tetR genes selection rates to less than 10% increase in aquaculture ponds situated at northeastern river basins in that the minimum reductions on OTC emission rates during spring, summer, and autumn were greater than 90%. On the other hand, water concentrations of OTC during spring and summer in southwestern rivers should be prioritized to be severely limited by reducing 67 and 25% of OTC emission rate, respectively. Overall, incorporating a computational fugacity model into a risk assessment framework can identify relative higher risk regions to provide the risk-based control strategies for public health decision-making and development of robust quantitative methods to zero-in on environment with high risk of tetR genes selection in relation to aquaculture-used pharmaceutical residues.

Inhibition of wastewater pollutants on the anammox process: A review

The anaerobic ammonium oxidation (anammox) process has been recognized as an efficient nitrogen removal technology. However, anammox bacteria are susceptible to surrounding environments and different pollutants, which limits the extensive application of the anammox process worldwide. Numerous researchers investigate the effects of various pollutants on the anammox process or bacteria, and related findings have also been reviewed with the focused on their inhibitory effects on process performance and microbial community. This review systemically summarized the recent advances in the inhibition, mechanism and recovery process of traditional and emerging pollutants on the anammox process over a decade, such as organics, metals, antibiotics, nanoparticles, etc. Generally, low-concentration pollutants exhibited a promotion on the anammox activity, while high-concentration pollutants showed inhibitory effects. The inhibitory threshold concentration of different pollutants varied. The combined effects of multipollutant also attracts more attentions, including synergistic, antagonistic and independent effects. Additionally, remaining problems and research needs are further proposed. This review provides a foundation for future research on the inhibition in anammox process, and promotes the proper operation of anammox processes treating different types of wastewaters.

Seasonal distribution of antibiotic resistance genes in the Yellow River water and tap water, and their potential transmission from water to human

The prevalence and transmission of antibiotic resistance genes (ARGs) and opportunistic pathogens in water environments can pose great threat to public health. However, the dissemination of ARGs and opportunistic pathogens from water environments to humans has been poorly explored. Here, we employed 16S rRNA gene sequencing and high-throughput quantitative PCR techniques to explore the seasonal distribution of ARGs and opportunistic pathogens in the Yellow River water (source water) and tap water, as well as their relationships with healthy humans at Lanzhou, China. Physiochemical analysis was applied to detect water quality parameters and heavy metal contents. The absolute abundance and diversity of ARGs in the Yellow River and tap water demonstrated distinct seasonal patterns. In winter, the Yellow river water had the highest ARG abundance and diversity, while tap water owned the lowest. Mobile genetic elements (MGEs) were the predominant driver of ARG profiles in both the Yellow river and tap water. Null model analysis showed that ARG assembly in the Yellow River was more influenced by stochastic processes than tap water and this was independent of seasons. Total organic carbon and arsenic contents exhibited positive correlations with many ARGs. Opportunistic pathogens Aeromonas and Pseudomonas may be potential hosts for ARGs. Approximately 80% of detected ARGs were shared between water samples and the human gut. These persistent ARGs could not be entirely eliminated through drinking water treatment processes. Thus, it is crucial to protect sources of tap water from anthropogenic pollution and improve water treatment technologies to reduce the dissemination of ARGs and ensure drinking-water biosafety for human health.

Antibiotic resistant bacteria and genes in shrimp aquaculture water: Identification and removal by ferrate(VI)

Enclosed shrimp culturing ponds are breeding environments for the spread of antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) in the aquatic environment. This paper surveyed the presence of antibiotics, ARB, and ARGs in aquaculture waters and demonstrated their removal by ferrate (Fe(VI), FeO₄^2-). Tetracyclines were the most prevalent antibiotics, followed by quinolones and β-lactam. The bacterial resistance rates to three antibiotics were ordered as follows: amoxicillin (AMX) > oxytetracycline (OTC) > enrofloxacin (ENR). Proteobacteria, Verrucomicrobia, and Bacteroidetes were the predominant phyla, while sul1 and sul2 were the predominant ARGs. sul2 was positively correlated with Proteobacteria. Water quality parameters significantly influenced the dissemination of tetracycline resistance genes in aquacultures due to high organic waste accumulation. The removal efficiency of antibiotics by Fe(VI) depended on the structural moieties of antibiotics, with phenol-containing antibiotics more thoroughly oxidized (i.e., OTC) than amine-containing (ENR and AMX) antibiotics. Greater removal of antibiotics in aquaculture waters suggested that the constituents of farming water enhances the efficacy of antibiotics removal by Fe(VI). An acidic pH environment enhanced Fe(VI) inactivation of ARB over the circumneutral pH. The presented results are intended to improve aquaculture managing practices to minimize the antibiotic proliferation in aquaculture waters and the environment.

Diversity and abundance of antibiotic resistance genes in rhizosphere soil and endophytes of leafy vegetables: Focusing on the effect of the vegetable species

Antibiotic resistance genes (ARGs) in the endophytes of vegetables represent a potential route of human exposure to the soil resistome. However, the effect of vegetable species on the endophytic ARG profiles is unclear, hampering our understanding of how ARGs migrate into the soil-vegetable system and their potential health risks. Here, we planted four leafy vegetables (cilantro, endive, lettuce, and pak choi), which are commonly eaten raw, and analyzed the resistomes and microbiomes in three sample types (rhizosphere soil, root, and leaf endophytes). A total of 150 ARG subtypes were detected using high-throughput quantitative PCR. Vegetable species had a significant effect on ARG diversity and abundance, and pak choi accumulated more ARGs in its associated microbiome than the other three vegetables. The bacterial community was the primary factor shaping ARG profiles and was significantly correlated with ARG subtypes. We identified aadE, tet(34), and vanSB as shared ARGs among leaves of the four vegetables; the bacterial families correlated with tet(34) and vanSB were also shared across the vegetables and belonged to Proteobacteria. This study deepens our understanding of how endophytic ARG profiles vary among different vegetables and highlights the potential health risk associated with consuming these vegetables raw.

Performance of Aerobic Denitrification by the Strain Pseudomonas balearica RAD-17 in the Presence of Antibiotics

Aerobic denitrification, one of the important nitrate metabolic pathways in biological denitrification, has been attracting increasing interest recently due to its functional advantages. In order to evaluate the effect of antibiotics on aerobic denitrification and guide practical engineering application of aerobic denitrification techniques, we evaluated the performance of aerobic denitrification by the strain Pseudomonas balearica RAD-17 in the presence of ciprofloxacin (CFX) and oxytetracycline (OTC). No significant negative impact on the performance of aerobic denitrification in the presence of CFX or OTC within the range of 50 to 300 μg L^-1 was found. Significant degradation of OTC was found within the range of 50 μg L^-1 to 300 μg L^-1 under aerobic denitrification conditions, while no degradation was found for CFX. Stimulation of cell growth occurred within the investigated range of antibiotics. Under anoxic or aerobic conditions, the addition of CFX or OTC changed the N₂O production trend. The results in the present study may play an important role in informing the use of aerobic denitrification techniques in the presence of antibiotics within environmentally relevant concentrations (<1 mg/L).

Responses of aerobic granular sludge to fluoroquinolones: Microbial community variations, and antibiotic resistance genes

In this study, aerobic granular sludge (AGS) was operated under high levels of ammonium for removing three fluoroquinolones (FQs), i.e., ciprofloxacin (CFX), ofloxacin (OFX), and norfloxacin (NFX) at 3, 300, and 900 µg/L, respectively. Two key objectives were to investigate the differential distribution of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) in sludge fractions and to evaluate correlations between ARGs and MGEs to nitrifying and denitrifying bacteria. AGS showed excellent stability under the exposure of FQs, with nitrite-oxidizing bacteria (NOB) more sensitive to FQs than ammonium-oxidizing bacteria (AOB). Specific oxygen utilization rates (SOUR) showed a reduction of 26.9% for NOB but only 4.0% of the reduced activity of AOB by 3 μg/L FQs. AGS performed better removal efficiencies for CFX and NFX than OFX, and the efficiencies increased with their elevated concentrations, except at 900 μg/L FQs. The elevated FQ concentrations led to a significant enrichment of intI1 and genus Thauera, while qnrD and qnrS showed no accumulation. Compared to nitrifiers, FQs relevant ARGs and the intI1 gene preferred to exist in denitrifiers, and the abundance of denitrifiers behaved a decreasing trend with the sludge size. Two quinoline-degrading bacteria were found in the AGS system, i.e., Alicycliphilus and Brevundimonas, possibly carrying qnrS and qnrD, respectively. Their relative abundance increased with the sludge size, which was 2.18% in sludge <0.5 mm and increased to 3.70% in sludge >2.0 mm, suggesting that the AGS may be a good choice in treating FQs-containing wastewater.

Potential dissemination mechanism of the tetC gene in Aeromonas media from the aerobic biofilm reactor under oxytetracycline stresses

The tetC gene has been found to be one of the most widely distributed tetracycline resistance (tet) genes in various environmental niches, but the detailed dissemination mechanisms are still largely unknown. In the present study, 11 tetC-containing Aeromonas media strains were isolated from an aerobic biofilm reactor under oxytetracycline stresses, and the genome of one strain was sequenced using the PacBio RSII sequencing approach to reveal the genetic environment of tetC. The tetC gene was carried by an IS26 composite transposon, named Tn6434. The tetC-carrying Tn6434 structure was detected in all of the A. media strains either in a novel plasmid pAeme2 (n=9) or other DNA molecules (n=2) by PCR screening. The NCBI database searching result shows that this structure was also present in the plasmids or chromosomes of other 13 genera, indicating the transferability of Tn6434. Inverse PCR and sequencing confirmed that Tn6434 can form a circular intermediate and is able to incorporate into a preexisting IS26 element, suggesting that Tn6434 might be responsible for the dissemination of tetC between different DNA molecules. This study will be helpful in uncovering the spread mechanism of tet genes in water environments.

Impact of oxytetracycline on anaerobic wastewater treatment and mitigation using enhanced hydrolysis pretreatment

In this study, two parallel-operated up-flow anaerobic sludge bed reactors, one used to treat synthetic wastewater spiked with oxytetracycline and the other used to treat the same wastewater after enhanced hydrolysis, were used to evaluate the impact of oxytetracycline on anaerobic digestion and resistance development and the efficacy of enhanced hydrolysis pretreatment on the elimination of adverse effects. The reactors were operated under a constant organic-loading rate (10 g/L/d) with increasing oxytetracycline doses (0 mg/L to 200 mg/L) over a period of 15 months. For the reactor without pretreatment, the chemical oxygen demand removal reached up to 89.5%%at oxytetracycline doses ranging from 0 mg/L to 100 mg/L, which collapsed at higher oxytetracycline doses. Miseq sequencing showed that a diverse hydrolysis/fermentation/acetogenesis bacterial community was maintained as the oxytetracycline dose was increased from 0 mg/L to 100 mg/L, while extreme dominance of Macellibacteroides (65.70%%- 71.56%) was found to occur at higher oxytetracycline doses. The total abundance of antibiotic resistance genes increased from 1.3 × 10^-1 copies per cell to 2.6 × 10^-1 copies per cell with increasing oxytetracycline dose from 0 mg/L to 5 mg/L, remained unchanged at oxytetracycline doses ranging from 25 mg/L to 100 mg/L, and then increased to 4.8 × 10^-1 copies per cell and 1.3 copies per cell at oxytetracycline doses of 150 mg/L and 200 mg/L, respectively. Multidrug resistance developed in response to oxytetracycline treatment at 200 mg/L. Poor chemical oxygen demand removal and a marked enrichment in antibiotic resistance genes was validated using a full-scale up-flow anaerobic sludge bed system fed with an influent oxytetracycline concentration of approximately 200 mg/L. For the reactor treating wastewater pretreated with enhanced hydrolysis (85 °C for 6 h), the chemical oxygen demand removal rate and antibiotic resistance genes level over the whole oxytetracycline dose range were found to be similar to those achieved with zero oxytetracycline treatment. These results demonstrated that the control of conventional pollutants and ARGs could be achieved simultaneously in the UASB reactor by employing enhanced hydrolysis pretreatment.

Antibiotic resistomes in drinking water sources across a large geographical scale: Multiple drivers and co-occurrence with opportunistic bacterial pathogens

Antibiotic resistance genes (ARGs) can survive the water treatment process. However, the prevalence patterns, key drivers, and relationships with opportunistic pathogens of the antibiotic resistome harbored in drinking water sources remain unclear. Herein, 53 drinking water samples collected across a large geographical scale in China were characterized based on ARGs, mobile genetic elements (MGEs), bacterial communities, antibiotics, and opportunistic bacterial pathogens. A total of 265 unique ARGs and MGEs were detected by high-throughput quantitative polymerase chain reaction (HT-qPCR), and 101 genes were shared among over 50% of samples. ARG abundance was higher in rivers than in reservoirs or groundwater, and ARG similarity showed a distance-decay relationship at the >4 000 km scale. Four out of the five detected opportunistic pathogens (i.e., Escherichia coli, Mycobacterium spp., Clostridium perfringens, and Bacillus cereus group) were potential hosts of ARGs. Based on multivariate statistics, our results demonstrated that the factors influencing the antibiotic resistome in drinking water sources were multiple and interactive. The bacterial community greatly contributed to ARG structure, and antibiotic concentrations and MGEs also affected ARG proliferation. The structural equation model indicated that geographical location and sample types (i.e., river, reservoir, and groundwater) had indirect effects on ARGs by changing the bacterial community and antibiotic concentration. Holistic consideration of natural and anthropogenic factors is recommended to understand antibiotic resistome variation in drinking water sources at a large geographical scale. Furthermore, large-scale diverse samples are suggested to minimize the potential influence of accident or stochasticity. Our findings provide insight into water quality risks induced by drinking water antibiotic resistomes.