Ionic Liquid Facilitates the Conjugative Transfer of Antibiotic Resistance Genes Mediated by Plasmid RP4

Bacterial Concentrations and Water Turbulence Influence the Importance of Conjugation Versus Phage-Mediated Antibiotic Resistance Gene Transfer in Suspended Growth Systems

Despite the abundance of phage-borne antibiotic resistance genes (ARGs) in the environment, the frequency of ARG propagation via phage-mediated transduction (relative to via conjugation) is poorly understood. We investigated the influence of bacterial concentration and water turbulence level [quantified as Reynold's number (Re)] in suspended growth systems on the frequency of ARG transfer by two mechanisms: delivery by a lysogenic phage (phage λ carrying gentamycin-resistance gene, genR) and conjugation mediated by the self-transmissible plasmid RP4. Using Escherichia coli (E. coli) as the recipient, phage delivery had a comparable frequency (1.2 ± 0.9 × 10^-6) to that of conjugation (1.1 ± 0.9 × 10^-6) in suspensions with low cell concentration (10⁴ CFU/mL) and moderate turbulence (Re = 5 × 10⁴). Turbulence affected cell (or phage)-to-cell contact rates and detachment (due to shear force), and thus, it affected the relative importance of conjugation versus phage delivery. At 10⁷ CFU/mL, no significant difference was observed between the frequencies of ARG transfer by the two mechanisms under quiescent water conditions (2.8 ± 0.3 × 10^-5 for conjugation vs 2.2 ± 0.5 × 10^-5 for phage delivery, p = 0.19) or when Re reached 5 × 10⁵ (3.4 ± 1.5 × 10^-5 for conjugation vs 2.9 ± 1.0 × 10^-5 for phage delivery, p = 0.52). Transcriptomic analysis of genes related to conjugation and phage delivery and simulation of cell (or phage)-to-cell collisions at different Re values corroborate that the importance of phage delivery relative to conjugation increases under either quiescent or turbulent conditions. This finding challenges the prevailing view that conjugation is the dominant ARG transfer mechanism and underscores the need to consider and mitigate potential ARG dissemination via transduction.

Paclitaxel and its derivative facilitate the transmission of plasmid-mediated antibiotic resistance genes through conjugative transfer

The rapid dissemination of antibiotic resistance by horizontal gene transfer (HGT) renders the global resistance crisis more tense and urgent as few effective antimicrobials are available to combat multidrug-resistant (MDR) pathogens at present. Conjugation is one of the most dominant and representative pathways of HGT. Antibiotic residue in environment is recognized as an important accelerator for conjugal transfer, whereas the roles of non-antibiotic pharmaceuticals in this process are not fully understood. Here we found that environmentally relevant concentrations of paclitaxel as well as its derivative docetaxel, two commonly used anticancer drugs, remarkably facilitated the conjugative transfer of resistance plasmids carrying multiple antibiotic resistance genes (ARGs). The underlying mechanisms accounting for the enhanced conjugation were investigated by detecting the activity of RpoS regulon, membrane permeability, SOS response and gene expression of conjugative transfer systems. Our results showed that paclitaxel induced a series of cellular responses, including up-regulation of rpoS expression, activated SOS response, increased cell membrane permeability, enhanced plasmid replication and mating pilus formation. Collectively, our data provide new insight on the roles of paclitaxel and its derivative in promoting the conjugal transfer of ARGs, highlighting the importance of good antimicrobial stewardship.

Dissolved biochar eliminates the effect of Cu(II) on the transfer of antibiotic resistance genes between bacteria

The proliferation of antibiotic resistance genes (ARGs) has posed significant risks to human and environmental health. Research has confirmed that Cu(II) could accelerate the conjugative transfer of ARGs between bacteria. This study found that adding dissolved biochar effectively weakened or eliminated the Cu(II)-facilitated efficient transfer of ARGs. The efficiency of conjugative transfer was promoted after treatment with Cu(II) (0.05 mg/L) or dissolved biochar at a pyrolysis temperature of 300 °C. When exposed to the combination of Cu(II) and dissolved biochar, the transfer frequency was significantly reduced; this occurred regardless of the Cu(II) concentration or pyrolysis temperature of dissolved biochar. In particular, when the Cu(II) concentration exceeded 0.5 mg/L, the transfer efficiency was entirely inhibited. Gene expression analysis indicated that different treatments affect transfer efficiency by regulating the expression of three global regulatory genes: korA, korB, and trbA. Among them, humic acid repressed the expression of these genes; however, Cu(II) formed complex with the humic acid-like components, gradually weakening the inhibitive effect of these components. The promotion of low molecule organic matters dominated, resulting in a dynamic decline in the transfer efficiency. This study provides a new environmental contaminant treatment approach to eliminate the heavy metal-facilitated transfer of ARGs between bacteria.

Prochloraz alone or in combination with nano-CuO promotes the conjugative transfer of antibiotic resistance genes between Escherichia coli in pure water

Conjugative plasmid transfer is a major contributor to the spread of antibiotic resistance genes (ARGs). However, the role of conventional fungicides on conjugative plasmid transfer has been neglected. Based on the condition that the increasing use of the combination of nano- and conventional fungicides will lead to combined contamination, the effects of a conventional fungicide prochloraz alone or in combination with nano-CuO on the conjugation of plasmid RP4 between Escherichia coli in phosphate-buffered saline were investigated in this study. The results demonstrated that 50 µg/L prochloraz alone significantly increased the conjugative transfer by 1.82 folds. The combination of 100 µg/L nano-CuO and prochloraz at 5, 50, and 500 µg/L significantly increased the conjugation by 2.56, 3.61, and 2.13 folds, respectively. The promotion of conjugative transfer of ARGs mediated by fungicides is mainly attributed to (i) the increased cell membrane permeability, (ii) the increased cell adhesion via enhancing the synthesis of polysaccharides in extracellular polymeric substances, and (iii) the up-regulation of the genes relevant to conjugation, oxidative stress, SOS response, outer membrane, polysaccharide export, intercellular adhesion, and ATP synthesis. Our findings provide evidence for the contribution of fungicides to ARGs transfer, which is significant to control the risk of ARGs dissemination.

Biochar effectively inhibits the horizontal transfer of antibiotic resistance genes via transformation

The rapid spread of antibiotic resistance genes (ARGs) has posed a risk to human health. Here, the effects of biochar (BC) on the horizontal transfer of ARG-carrying plasmids to Escherichia coli via transformation were systematically investigated. BC could significantly inhibit the transformation of ARGs and the inhibition degree increased with pyrolysis temperature. Rice straw-derived BC showed a stronger inhibitory effect on the transformation of ARGs than that of peanut shell-derived BC from the same pyrolysis temperature. The inhibitory effect of BC from low pyrolysis temperature (300 ℃) was mainly caused by BC dissolutions, while it was mainly attributed to BC solids for high pyrolysis temperature (700 ℃) BC. BC dissolutions could induce intramolecular condensation and even agglomeration of plasmids, hindering their transformation into competent bacteria. The cell membrane permeability was slightly decreased in BC dissolutions, which might also contribute to the inhibitory effect. Plasmid can be adsorbed by BC solids and the adsorption increased with BC pyrolysis temperature. Meanwhile, BC-adsorbed plasmid could hardly be transformed into E. coli. BC solids could also deactivate E. coli and thereby inhibit their uptake of ARGs. These findings provide a way using BC to limit the spread of ARGs in the environment.

Effects and relevant mechanisms of non-antibiotic factors on the horizontal transfer of antibiotic resistance genes in water environments: A review

Antibiotic resistance has created obstacles in the treatment of infectious diseases with antibiotics. The horizontal transfer of antibiotic resistance genes (ARGs) can exacerbate the dissemination of antibiotic resistance in water environments. In addition to antibiotic selective pressure, multiple non-antibiotic factors can affect the horizontal transfer of ARGs. Herein, we seek to comprehensively review the effects and relevant mechanisms of non-antibiotic factors on the horizontal transfer of ARGs in water environments, especially contaminants from human activities and water treatment processes. Four pathways have been identified to accomplish horizontal gene transfer (HGT), i.e., conjugation, transformation, transduction, and vesiduction. Changes in conjugative frequencies by non-antibiotic factors are mainly related to their concentrations, which conform to hormesis. Relevant mechanisms involve the alteration in cell membrane permeability, reactive oxygen species, SOS response, pilus, and mRNA expression of relevant genes. Transformation induced by extracellular DNA may be more vulnerable to non-antibiotic factors than other pathways. Except bacteriophage infection, the effects of non-antibiotic factors on transduction exhibit many similarities with that of conjugation. Given the secretion of membrane vesicles stimulated by non-antibiotic factors, their effects on vesiduction can be inferred. Furthermore, contaminants from human activities at sub-inhibitory or environmentally relevant concentrations usually promote HGT, resulting in further dissemination of antibiotic resistance. The horizontal transfer of ARGs is difficult to be inhibited by individual water treatment processes (e.g., chlorination, UV treatment, and photocatalysis) unless they attain sufficient intensity. Accordingly, the synergistic application containing two or more water treatment processes is recommended. Overall, we believe this review can elucidate the significance for risk assessments of contaminants from human activities and provide insights into the development of environment-friendly and cost-efficient water treatment processes to inhibit the horizontal transfer of ARGs.

Melatonin prevents conjugative transfer of plasmid-mediated antibiotic resistance genes by disrupting proton motive force

The widespread dissemination of antibiotic resistance genes (ARGs) is a serious problem and constitutes a threat for public health. Plasmid-mediated conjugative transfer of ARGs is recognized as one of the most important pathways accounting for this global crisis. Inhibiting the conjugative transfer of resistant gene-bearing plasmids provides a feasible strategy to prevent the spread of antibiotic resistance. Here we found that melatonin, a neurohormone secreted from pineal gland, substantially inhibited the horizontal transfer of RP4-7 plasmid in a dose-dependent manner. Furthermore, melatonin could also suppress the conjugal frequency of different types of clinical plasmids that carrying colistin resistance gene mcr-1 rather than blaNDM or tet(X) genes. Next, we investigated the mechanisms underlying the inhibitory effect of melatonin on conjugation. As a result, we showed that the addition of melatonin markedly reduced bacterial membrane permeability and inhibited the oxidative stress. In line with these observations, the conjugative transfer-related genes were regulated accordingly. Most importantly, we uncovered that melatonin disrupted bacterial proton motive force (PMF), which is an essential bacterial energy metabolism substance and is important for conjugative process. Collectively, these results provide implications that some non-antibiotics such as melatonin are effective inhibitors of transmission of ARGs and raise a promising strategy to confront the increasing resistant infections.

Phenolic compounds promote the horizontal transfer of antibiotic resistance genes in activated sludge

Phenolic compounds are common organic pollutants in wastewater. During the wastewater treatment process, these compounds may influence the microbial community structure and functions. However, the impact of the phenolic compounds in the wastewater treatment plants on the horizontal transfer of antibiotic resistance genes (ARGs) has not been well assessed. In this study, we investigated the horizontal transfer of ARGs under the stress of phenolic compounds. The results showed that in pure culture bacteria system, p-nitrophenol (PNP), p-aminophenol (PAP) and phenol (PhOH) (10-100 mg/L) can significantly increase the horizontal transfer frequency of ARGs by 2.2-4.6, 3.6-9.4 and 1.9-9.0 fold, respectively. And, the RP4 plasmid transfer from Escherichia coli HB101 (E. coli HB101) to the bacteria in activated sludge increased obviously under the stress of phenolic compounds. Further investigation revealed that the PNP and PhOH at the concentration of 10-100 mg/L increased the production of reactive oxygen species and the permeability of cell membrane in the donor and recipient, which could be the causes of horizontal transfer of RP4 plasmid. In addition, it was also found that PNP, PAP and PhOH stress inhibit the expression of the global regulatory genes korB and trbA in the RP4 plasmid, and increase the expression level of the traF gene, thereby promoting the conjugative transfer of the RP4 plasmid. Taken together, these results improved our understanding of the horizontal transfer of ARGs under the stress of phenolic compounds and provided basic information for management of the systems that treat wastewater containing phenolic compounds.

Rainfall facilitates the transmission and proliferation of antibiotic resistance genes from ambient air to soil

Antibiotic resistance is common in bacterial communities and appears to be correlated with human activities. However, the source of antibiotic resistance genes (ARGs) and antibiotic-resistant bacteria (ARB) in remote regions remains unclear. In this study, we examined the abundance of ARGs in fine particulate matter (PM_2.5) as a carrier throughout the rainfall process (4 mm rain/h) to observe the effects of rainfall on the transmission of ARGs. The results suggested that rainwater served as a reservoir that facilitated the spread of ARGs and that wind and particulate matter (PM) concentrations might be meteorological parameters that influence the distribution of ARGs in rainwater. In addition, soil microcosm experiments were performed to investigate the influence of rainfall on antibiotic resistance in soils with different environmental backgrounds. Rainwater facilitated the proliferation of ARGs and mobile genetic elements (MGEs) from ambient air to soil, and this influence was more obvious in heavy metal-contaminated soil. This is the first study to investigate the routes by which rainfall acts as a mobile reservoir to facilitate the transmission and proliferation of ARGs, and the results indicate the potential source of ARGs in remote regions where humans rarely interfere.

Individual and combined applications of biochar and pyroligneous acid mitigate dissemination of antibiotic resistance genes in agricultural soil

Remediation of agricultural soils polluted with antibiotic resistance genes (ARGs) is important for protecting food safety and human health. However, the feasibility of co-application of biochar and pyroligneous acid, two multifunctional soil amendments, for mitigating dissemination of soil ARGs is unknown. Thus, a woody biochar (BC450) and its by-product, pyroligneous acid (PA450) simultaneously produced at 450 °C from blended wood wastes, were used to compare their individual and combined effects on soil ARG abundance using a 65-day pot experiment planted with leafy vegetable Brassica chinensis L. The individual and combined applications of PA450 and BC450 significantly reduced the absolute abundance of ARGs by 65.7-81.4% and 47.5-72.9% in the corresponding rhizosphere and bulk soil. However, the co-application showed little synergistic effect, probably due to the counteractive effect of BC450 on the PA450-mitigated soil ARG proliferation, resulted from the promoted soil bacterial growth and/or adsorption of antimicrobial components of PA450 by BC450. The decreased abundances of mobile genetic element intI1 and Tn916/1545 in the PA450 treatments demonstrated the potential of PA450 for weakening horizontal gene transfer (HGT). Furthermore, weakened HGT by individual PA450, lowered availability of heavy metals by individual BC450, and different bacterial community (e.g., reduced ARGs bacterial host) together with improved soil properties from co-application of PA450 and BC450 all contributed to the reduced ARG level. This study highlighted the feasibility of co-applications of biochar and pyroligneous acid amendment for mitigating soil ARG pollution. These findings provide important information for developing eco-friendly technologies using biochar and pyroligneous acid in remediating ARG-contaminated soils.

Antifungal and antiaflatoxigenic assessment of new Cu(II)-pq complexes against Aspergillus parasiticus, in dark conditions and under visible irradiation

The issue of food contamination by fungi and aflatoxins; constitutes a serious concern not only for human/animal health but also for agriculture and the economy. Aflatoxins are secondary metabolites produced by certain filamentous fungi and contaminate a variety of foodstuffs. In this context, control of fungal growth and aflatoxin contamination appears to be important. The present study aimed to investigate new Cu(I) and Cu(II)-quinoxaline complexes, namely [Cu(2,2´-pq)(NO3)](NO3) (1), [Cu(2,2´-pq)2(NO3)](NO3)·6H2O (2) and [Cu(2,2΄-pq)2](BF4) (3), where 2,2´-pq is 2-(2'-pyridyl quinoxaline), as antifungal agents against Aspergillus parasiticus. All complexes, the ligand and the starting material Cu(NO3)2-3H2O, regardless of the concentration used, caused inhibition of A. parasiticus growth ranged from 8.52 to 33.33%. The fungal growth inhibition was triggered when irradiation in visible (λ > 400 nm) was continuously applied (range 18.36-57.20%). The highest inhibitory activity was exhibited by the complex [Cu(2,2´-pq)2(NO3)](NO3)·6H2O and for this reason, it was selected to be studied for its ability to suppress aflatoxin B1 produced by A. parasiticus. AFB1 production after the irradiation process was found to be suppressed by 25% compared to AFB1 produced in dark conditions.

Effect of dissolved biochar on the transfer of antibiotic resistance genes between bacteria

The spread of antibiotic resistance genes (ARGs) is a global environmental issue. Dissolved biochar is more likely to contact bacteria in water, producing ecological risks. This study explored the effects of dissolved biochar on ARGs transfer in bacteria. Conjugative transfer efficiency was significantly different following treatment with different types of dissolved biochar. Typically, humic acid-like substance in dissolved biochar can significantly improve the transfer efficiency of ARGs between bacteria. When the concentration of dissolved biochar was ≤10 mg biochar/mL, humic acid-like substance substantially promoted ARGs transfer. An increase in dissolved biochar concentration weakened the ARGs transfer from humic acid-like substance. The inhibitory effects of small-molecule matters dominated, decreasing conjugative transfer frequency. At a concentration of 100 mg biochar/mL, the conjugative transfer efficiency of all treatments was lower than that of control. Compared with corn straw dissolved biochar, there were more transconjugants in pine sawdust dissolved biochar. Following treatment with 10 mg biochar/mL pine sawdust dissolved biochar, the number of transconjugants was at its maximum; approximately 7.3 folds higher than the control. We also explored mechanisms by which dissolved biochar impacts conjugative transfer. Due to the complex composition of dissolved biochar, its effects on the expression of conjugative transfer-related genes were also dynamic. This study investigates the ecological risk of biochar and guides its scientific application.

Genetically engineered microbial remediation of soils co-contaminated by heavy metals and polycyclic aromatic hydrocarbons: Advances and ecological risk assessment

Soils contaminated with heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) have been becoming a worldwide concerned environmental problem because of threatening public healthy via food chain exposure. Thus soils polluted by HMs and PAHs need to be remediated urgently. Physical and chemical remediation methods usually have some disadvantages, e.g., cost-expensiveness and incomplete removal, easily causing secondary pollution, which are hence not environmental-friendly. Conventional microbial approaches are mostly used to treat a single contaminant in soils and lack high efficiency and specificity for combined contaminants. Genetically engineered microorganisms (GEMs) have emerged as a desired requirement of higher bioremediation efficiency for soils polluted with HMs and PAHs and environmental sustainability, which can provide a more eco-friendly and cost-effective strategy in comparison with some conventional techniques. This review comments the recent advances about successful bioremediation techniques and approaches for soil contaminated with HMs and/or PAHs by GEMs, and discusses some challenges in the simultaneous removal of HMs and PAHs from soil by designing multi-functional genetic engineering microorganisms (MFGEMs), such as improvement of higher efficiency, strict environmental conditions, and possible ecological risks. Also, the modern biotechnological techniques and approaches in improving the ability of microbial enzymes to effectively degrade combined contaminants at a faster rate are introduced, such as reasonable gene editing, metabolic pathway modification, and protoplast fusion. Although MFGEMs are more potent than the native microbes and can quickly adapt to combined contaminants in soils, the ecological risk of MFGEMs needs to be evaluated under a regulatory, safety, or costs benefit-driving system in a way of stratified regulation. Nevertheless, the innovation of genetic engineering to produce MFGEMs should be inspired for the welfare of successful bioremediation for soils contaminated with HMs and PAHs but it must be supervised by the public, authorities, and laws.

Effects of emerging pollutants on the occurrence and transfer of antibiotic resistance genes: A review

The emergence and spread of antibiotic resistance genes (ARGs) have become major concerns for both public health and environmental ecosystems. Emerging pollutants (EPs) that accumulate in environmental compartments also pose a potential risk for the enrichment of ARGs in indigenous microorganisms. This paper presents a comprehensive review of the effects and intrinsic mechanisms of EPs, including microplastics, engineered nanomaterials, disinfection byproducts, pharmaceuticals, and personal care products, on the occurrence and dissemination of ARGs. State-of-the-art methods for identifying culture-independent ARG-host interactions and monitoring horizontal gene transfer (HGT) processes in real-time are first reviewed. The contributions of EPs to the abundance and diversity of ARGs are then summarized. Finally, we discussed the underlying mechanisms related to the regulation of HGT, increased mutagenesis, and the evolution of microbial communities. Further details of three HGT (i.e., conjugation, transformation, and transduction) frequency patterns in response to various EPs are also examined. This review contemplates and reassesses the risks of ARG evolution posed by the manufacture and application of EPs.

High prevalence of unstable antibiotic heteroresistance in cyanobacteria causes resistance underestimation

Both cyanobacterial bloom and antibiotic resistance have aggravated worldwide and posed a global threat to public health in recent years. Cyanobacteria can exhibit discrepancy between their resistance genotype and susceptible phenotype due to antibiotic heteroresistance, which leads to difficulties in unambiguously classifying cyanobacterial strains as susceptible or resistant. Here we profiled the prevalence and mechanisms of antibiotic heteroresistance in cyanobacterial strains isolated from 50 sites across four eutrophicated lakes in China. Among 300 cyanobacterial isolates tested against 19 different antibiotics, over 90% of cyanobacterial isolates exhibited HR to multiple antibiotics and 19.5% of isolate/antibiotic interactions classified as susceptible by traditional minimum inhibitory concentration (MIC) estimates were designated heteroresistant. Over 97% of these monoclonal HR cases were unstable, with an increased resistance of subpopulations due to amplification of known resistance genes with high fitness cost. Wide-type cyanobacterial isolates of Synechococcus, Synechocystis, Anabaena and Microcystis aeruginosa exposed to sub-MIC level of four antibiotics evolved high-level resistance with little fitness cost, resulting in stable polyclonal HR. Both stable polyclonal HR and unstable monoclonal HR observed in different cyanobacterial strains can be promoted under environmental levels of antibiotic pressure. The highly prevalent and unstable monoclonal HR with the potential for susceptibility misclassification highlighted underestimation of cyanobacteria-derived antibiotic resistance. Cost-effective strategies should be developed to identify heteroresistance in cyanobacteria and to avoid false positive or negative results in traditional susceptibility testing.

Identification of CO2 induces oxidative stress to change bacterial surface properties

The surface properties of bacteria play an essential role in their abilities to perform transmembrane communication, adherence, immobilization, flocculation, etc. However, the responsiveness of bacterial surfaces to elevated atmospheric CO₂ remains unknown. In this study, using the model bacteria, Paracoccus denitrificans, the effect of CO₂ on the primary bacterial surface properties, specifically hydrophobicity and surface charge, has been explored. We found that hydrophilicity and negative surface charge both rose in conjunction with increased atmospheric CO₂ concentrations. Studies of the potential mechanisms involved have illustrated that elevated CO₂ significantly increases the production of polysaccharides in extracellular polymeric substances (EPS). Various hydrophilic groups and negative charges in these polysaccharides prompt hydrophilicity and surface charge variations in bacteria. Further research has identified that elevations in CO₂ result in the accumulation of reactive species, specifically reactive nitrogen species (RNS). In this study, it was found that RNS damaged the permeability of bacterial membranes by inducing lipid peroxidation and then caused the leakage of intracellular substrate, which ultimately led to an increase in EPS polysaccharides. Our findings suggest that changes in bacterial surface properties due to atmospheric CO₂ elevation, as well as the reactions these trigger, merit widespread attention.

Co-effect of cadmium and iron oxide nanoparticles on plasmid-mediated conjugative transfer of antibiotic resistance genes

Conjunctive transfer of antibiotic resistance genes (ARGs) among bacteria driven by plasmids facilitated the evolution and spread of antibiotic resistance. Heavy metal exposure accelerated the plasmid-mediated conjunctive transfer of ARGs. Nanomaterials are well-known adsorbents for heavy metals removal, with the capability of combatting resistant bacteria/facilitating conjunctive transfer of ARGs. However, co-effect of heavy metals and nanomaterials on plasmid-mediated conjunctive transfer of ARGs was still unknown. In this study, we investigated the effect of the simultaneous exposure of Cd²⁺ and nano Fe₂O₃ on conjugative transfer of plasmid RP4 from Pseudomonas putida KT2442 to water microbial community. The permeability of bacterial cell membranes, antioxidant enzyme activities and conjugation gene expression were also investigated. The results suggested that the combination of Cd²⁺ and high concentration nano Fe₂O₃ (10 mg/L and 100 mg/L) significantly increased conjugative transfer frequencies of RP4 plasmid (p < 0.05). The most transconjugants were detected in the treatment of co-exposure to Cd²⁺ and nano Fe₂O₃, the majority of which were identified to be human pathogens. The mechanisms of the exacerbated conjugative transfer of ARGs were involved in the enhancement of cell membrane permeability, antioxidant enzyme activities, and mRNA expression levels of the conjugation genes by the co-effect of Cd²⁺ and nano Fe₂O₃. This study confirmed that the simultaneous exposure to Cd²⁺and nano Fe₂O₃ exerted a synergetic co-effect on plasmid-mediated conjunctive transfer of ARGs, emphasizing that the co-effect of nanomaterials and heavy metals should be prudently evaluated when combating antibiotic resistance.

Nonnutritive sweeteners can promote the dissemination of antibiotic resistance through conjugative gene transfer

Antimicrobial resistance (AMR) poses a worldwide threat to human health and biosecurity. The spread of antibiotic resistance genes (ARGs) via conjugative plasmid transfer is a major contributor to the evolution of this resistance. Although permitted as safe food additives, compounds such as saccharine, sucralose, aspartame, and acesulfame potassium that are commonly used as nonnutritive sweeteners have recently been associated with shifts in the gut microbiota similar to those caused by antibiotics. As antibiotics can promote the spread of antibiotic resistance genes (ARGs), we hypothesize that these nonnutritive sweeteners could have a similar effect. Here, we demonstrate for the first time that saccharine, sucralose, aspartame, and acesulfame potassium could promote plasmid-mediated conjugative transfer in three established conjugation models between the same and different phylogenetic strains. The real-time dynamic conjugation process was visualized at the single-cell level. Bacteria exposed to the tested compounds exhibited increased reactive oxygen species (ROS) production, the SOS response, and gene transfer. In addition, cell membrane permeability increased in both parental bacteria under exposure to the tested compounds. The expression of genes involved in ROS detoxification, the SOS response, and cell membrane permeability was significantly upregulated under sweetener treatment. In conclusion, exposure to nonnutritive sweeteners enhances conjugation in bacteria. Our findings provide insight into AMR spread and indicate the potential risk associated with the presence of nonnutritive sweeteners.

Investigating the effects of municipal and hospital wastewaters on horizontal gene transfer

Horizontal gene transfer (HGT) plays an important role in the dissemination of antibiotic resistance genes. In sewer systems, human-associated and environmental bacteria are mixed together and exposed to many substances known to increase HGT, including various antibacterial compounds. In wastewaters, those substances are most often detected below concentrations known to induce HGT individually. Still, it is possible that such wastewaters induce HGT, for example via mixture effects. Here, a panel of antibiotics, biocides and other pharmaceuticals was measured in filter-sterilized municipal and hospital wastewater samples from Gothenburg, Sweden. The effects on HGT of the chemical mixtures in these samples were investigated by exposing a complex bacterial donor community together with a GFP-tagged E. coli recipient strain. Recipients that captured sulfonamide resistance-conferring mobile genetic elements (MGEs) from the bacterial community were enumerated and characterized by replicon typing, antibiotic susceptibility testing and long read sequencing. While exposure to municipal wastewater did not result in any detectable change in HGT rates, exposure to hospital wastewater was associated with an increase in the proportion of recipients that acquired sulfonamide resistance but also a drastic decrease in the total number of recipients. Although, concentrations were generally higher in hospital than municipal wastewater, none of the measured substances could individually explain the observed effects of hospital wastewater. The great majority of the MGEs captured were IncN plasmids, and resistance to several antibiotics was co-transferred in most cases. Taken together, the data show no evidence that chemicals present in the studied municipal wastewater induce HGT. Still, the increased relative abundance of transconjugants after exposure to hospital wastewater could have implications for the risks of both emergence and transmission of resistant bacteria.

Increased expression of antibiotic-resistance genes in biofilm communities upon exposure to cetyltrimethylammonium bromide (CTAB) and other stress conditions

Quaternary ammonium compounds (QAC, e.g., cetyltrimethylammonium bromide, (CTAB)) are widely used as surfactants and disinfectants. QAC already are commonly found in wastewaters, and their concentration could increase, since QAC are recommended to inactivate the SARS-CoV-2 (COVID-19) virus. Exposure of bacteria to QAC can lead to proliferation of antibiotic resistance genes (ARG). In particular, O₂-based membrane biofilm reactors (O₂-MBfRs) achieved excellent CTAB biodegradation, but ARG increased in their biofilms. Here, we applied meta-transcriptomic analyses to assess the impacts of CTAB exposure and operating conditions on microbial community's composition and ARG expression in the O₂-MBfRs. Two opportunistic pathogens, Pseudomonas aeruginosa and Stenotrophomonas maltophilia, dominated the microbial communities and were associated with the presence of ARG. Operating conditions that imposed stress on the biofilms, i.e., limited supplies of O₂ and nitrogen or a high loading of CTAB, led to large increases in ARG expression, particularly for genes conferring antibiotic-target protection. Important within the efflux pumps was the Resistance-Nodulation-Division (RND) family, which may have been active in exporting CTAB from cells. Oxidative stress appeared to be the key factor that triggered ARG proliferation by selecting intrinsically resistant species and accentuating the expression of ARG. Our findings suggest that means to mitigate the spread of ARG, such as shown here in a O₂-based membrane biofilm reactor, need to consider the impacts of stressors, including QAC exposure and stressful operating conditions.

In Vitro Assessment of Antimicrobial Resistance Dissemination Dynamics during Multidrug-Resistant-Bacterium Invasion Events by Using a Continuous-Culture Device

Antimicrobial-resistant pathogens display significant public health threats by causing difficulties in clinical treatment of bacterial infection. Antimicrobial resistance (AMR) is transmissible between bacteria, significantly increasing the appearance of antimicrobial-resistant pathogens and aggravating the AMR problem. In this work, the dissemination dynamics of AMR from invading multidrug-resistant (MDR) Escherichia coli to a community of pathogenic Salmonella enterica was investigated using a continuous-culture device, and the behaviors of dissemination dynamics under different levels of antibiotic stress were investigated. Three MDR E. coli invasion events were analyzed in this work: MDR E. coli-S. enterica cocolonization, MDR E. coli invasion after antibiotic treatment of S. enterica, and MDR E. coli invasion before antibiotic treatment of S. enterica It was found that both horizontal gene transfer (HGT) and vertical gene transfer (VGT) play significant roles in AMR dissemination, although different processes contribute differently under different circumstances, that environmental levels of antibiotics promote AMR dissemination by enhancing HGT rather than leading to selective advantage for resistant bacteria, and that early invasion of MDR E. coli completely and quickly sabotages the effectiveness of antibiotic treatment. These findings contribute to understanding the drivers of AMR dissemination under different antibiotic stresses, the detrimental impact of environmental tetracycline contamination, and the danger of nosocomial presence and dissemination of MDR nonpathogens.IMPORTANCE Antimicrobial resistance poses a grave threat to public health and reduces the effectiveness of antimicrobial drugs in treating bacterial infections. Antimicrobial resistance is transmissible, either by horizontal gene transfer between bacteria or by vertical gene transfer following inheritance of genetic traits. The dissemination dynamics and behaviors of this threat, however, have not been rigorously investigated. In this work, with a continuous-culture device, we studied antimicrobial resistance dissemination processes by simulating antimicrobial-resistant Escherichia coli invasion to a pathogenic Salmonella enterica community. Using this novel tool, we provide evidence on the drivers of antimicrobial resistance dissemination, on the detrimental impact of environmental antibiotic contamination, and on the danger of antimicrobial resistance in hospitals, even if what harbors the antimicrobial resistance is not a pathogen. This work furthers our understanding of antimicrobial resistance and its dissemination between bacteria and of antibiotic therapy, our most powerful tool against bacterial infection.

Ionic liquids: prospects for nucleic acid handling and delivery

Operations with nucleic acids are among the main means of studying the mechanisms of gene function and developing novel methods of molecular medicine and gene therapy. These endeavours usually imply the necessity of nucleic acid storage and delivery into eukaryotic cells. In spite of diversity of the existing dedicated techniques, all of them have their limitations. Thus, a recent notion of using ionic liquids in manipulations of nucleic acids has been attracting significant attention lately. Due to their unique physicochemical properties, in particular, their micro-structuring impact and tunability, ionic liquids are currently applied as solvents and stabilizing media in chemical synthesis, electrochemistry, biotechnology, and other areas. Here, we review the current knowledge on interactions between nucleic acids and ionic liquids and discuss potential advantages of applying the latter in delivery of the former into eukaryotic cells.

Copper Ions Facilitate the Conjugative Transfer of SXT/R391 Integrative and Conjugative Element Across Bacterial Genera

Copper can persist stably in the environment for prolonged periods. Except for inducing antibiotic resistance in bacteria, copper ions (Cu²⁺) can facilitate the horizontal transfer of plasmid DNA. However, whether and how Cu²⁺ can accelerate the conjugative transfer of SXT/R391 integrative and conjugative element (ICE) is still largely unknown. In this study, Proteus mirabilis ChSC1905, harboring an SXT/R391 ICE that carried 21 antibiotic resistance genes (ARGs), was used as a donor, and Escherichia coli EC600 was used as a recipient. Cu²⁺, at subinhibitory and environmentally relevant concentrations (1–10 μmol/L), significantly accelerated the conjugative transfer of SXT/R391 ICE across bacterial genera (from P. mirabilis to E. coli) (p < 0.05). The combined analyses of phenotypic tests and genome-wide sequencing indicated that reactive oxygen species (ROS) production and cell membrane permeability were critical in the enhanced conjugative transfer of SXT/R391 ICE. Furthermore, the expression of genes related to cell adhesion and ATP synthesis was also significantly upregulated on exposure to Cu²⁺ at a concentration of 5 μmol/L. This study clarified the potential mechanisms of Cu²⁺ to promote the conjugative transfer of SXT/R391 ICE, revealing the potential risk imposed by Cu²⁺ on the horizontal transfer of SXT/R391 ICE-mediated ARGs.

Will a Non-antibiotic Metalloid Enhance the Spread of Antibiotic Resistance Genes: The Selenate Story

The rapid emergence of antibiotic resistance genes (ARGs) has become an increasingly serious threat to public health. Previous studies illustrate the antibiotic-like effect of many substances. However, whether and how commonly used or existing non-antibiotic metalloids (e.g., selenate) would enhance ARG spread remains poorly known. Here, we tracked the long-term operation of a bioreactor continuously fed with selenate for more than 1000 days. Metagenomic sequencing identified 191 different ARGs, of which the total abundance increased significantly after the amendment of selenate. Network analyses showed that ARGs resisting multiple drugs had very similar co-occurrence patterns, implying a potentially larger health risk. Host classification not only indicated multidrug-resistant species but also distinguished the mechanism of ARG enrichment for vertical transfer and horizontal gene transfer. Genome reconstruction of an ARG host suggested that selenate and its bioreduction product selenite could stimulate the overproduction of intracellular reactive oxygen species, which was confirmed by the direct measurement. Bacterial membrane permeability, type IV pilus formation, and DNA repair and recombination were also enhanced, together facilitating the horizontal acquirement of ARGs. Overall, this study for the first time highlights the ARG emergence and dissemination induced by a non-antibiotic metalloid and identifies ARG as a factor to consider in selenate bioremediation.

The prevalence of ampicillin-resistant opportunistic pathogenic bacteria undergoing selective stress of heavy metal pollutants in the Xiangjiang River, China

The emergence of clinically relevant β-lactam-resistant bacteria poses a serious threat to human health and presents a major challenge for medical treatment. How opportunistic pathogenic bacteria acquire antibiotic resistance and the prevalence of antibiotic-resistant opportunistic pathogenic bacteria in the environment are still unclear. In this study, we further confirmed that the selective pressure of heavy metals contributes to the increase in ampicillin-resistant opportunistic pathogens in the Xiangjiang River. Four ampicillin-resistant opportunistic pathogenic bacteria (Pseudomonas monteilii, Aeromonas hydrophila, Acinetobacter baumannii, and Staphylococcus epidermidis) were isolated on Luria-Bertani (LB) agar plates and identified by 16S rRNA sequencing. The abundance of these opportunistic pathogenic bacteria significantly increased in the sites downstream of the Xiangjiang River that were heavily influenced by metal mining activities. A microcosm experiment showed that the abundance of β-lactam resistance genes carried by opportunistic pathogenic bacteria in the heavy metal (Cu²⁺ and Zn²⁺) treatment group was 2-10 times higher than that in the control. Moreover, heavy metals (Cu²⁺ and Zn²⁺) significantly increased the horizontal transfer of plasmids in pathogenic bacteria. Of particular interest is that heavy metals facilitated the horizontal transfer of conjugative plasmids, which may lead to the prevalence of multidrug-resistant pathogenic bacteria in the Xiangjiang River.

Cadmium enhances conjugative plasmid transfer to a fresh water microbial community

Co-selection of antibiotic resistance genes (ARGs) by heavy metals might facilitate the spread of ARGs in the environments. Cadmium contamination is ubiquitous, while, it remains unknown the extent to which cadmium (Cd²⁺) impact plasmid-mediated transfer of ARGs in aquatic bacterial communities. In the present study, we found that Cd²⁺ amendment at sub-inhibitory concentration significantly increased conjugation frequency of RP4 plasmid from Pseudomonas putida KT2442 to a fresh water microbial community by liquid mating method. Cd²⁺ treatment (1-100 mg/L) significantly increased the cell membrane permeability and antioxidant activities of conjugation mixtures. Amendments of 10 and 100 mg/L Cd²⁺ significantly enhanced the mRNA expression levels of mating pair formation gene (trbBp) and the DNA transfer and replication gene (trfAp) due to the repression of regulatory genes (korA, korB and trbA). Phylogenetic analysis of transconjugants indicated that Proteobacteria was the dominant recipients and high concentration of Cd²⁺ treatment resulted in expanded recipient taxa. This study suggested that sub-inhibitory Cd²⁺ contamination would facilitate plasmid conjugation and contributed to the maintenance and spread of plasmid associated ARGs, and highlighted the urgent need for effective remediation of Cd²⁺ in aquatic environments.

Critical roles of cyanobacteria as reservoir and source for antibiotic resistance genes

The widespread occurrence of antibiotic resistance genes (ARGs) throughout aquatic environments has raised global concerns for public health, but understanding of the emergence and propagation of ARGs in diverse environmental media remains limited. This study investigated the occurrence and spatio-temporal patterns of six classes of ARGs in cyanobacteria isolated from Taihu Lake. Tetracycline and sulfonamide resistance genes were identified as dominant ARGs. The abundance of ARGs in cyanobacteria was significantly higher in the bloom period than in the non-bloom period. The contribution and persistence of ARGs were higher in extracellular DNA (eDNA) than in intracellular DNA (iDNA) from cyanobacteria. Cyanobacteria-associated eDNA carrying ARGs was more stable at lower temperature. The relative abundances of ARGs in Microcystis and Synechococcus, the dominant genera of cyanobacterial blooms in Taihu Lake, were significantly higher than those in other cyanobacterial strains. The conjugative transfer efficiency for bacterial assimilation of ARGs in cyanobacteria was facilitated by increasing temperature and cyanobacterial cell concentration. Our results demonstrated that cyanobacteria could act as a significant reservoir and source for the acquisition and dissemination of ARGs in aquatic environments, hence the definition of negative ecological effects of cyanobacterial blooms was expanded.

New parameters for the quantitative assessment of the proliferation of antibiotic resistance genes dynamic in the environment and its application: A case of sulfonamides and sulfonamide resistance genes

Antibiotic resistance genes (ARGs) have been widely detected around the world and are generally viewed as emerging pollutants with environmental persistence. The proliferation of ARGs can be easily promoted by antibiotics. However, the dynamics of ARGs in the environment are still unable to be quantified using a single parameter, which is vital to evaluating the ability of ARGs to spread by antibiotics and effectively controlling ARGs. A new parameter, termed the relative area ratio of sample to control (ΔA^R), was developed based on the quantitative features determined by ARG-time curves in soils contaminated with sulfonamides (SAs) and verified by quantitative structure-activity relationships (QSARs) models. The results showed that ΔA^R can not only be used to accurately quantify the characteristics of SAs resistance genes (Suls) over time but also be applied to reveal the relationships between the proliferation of Suls and important factors (i.e., concentrations and chemical structures). Moreover, the ΔA^R-based QSARs model indicated that bioavailability and the frequency of conjugative transfer, rather than the ability of induced mutations in bacteria, tend to be key processes of the characteristics of the proliferation of Suls. Therefore, ΔA^R is a useful parameter to perform environmental risk assessments of ARG proliferation in the environment.

Heavy metal copper accelerates the conjugative transfer of antibiotic resistance genes in freshwater microcosms

Recent studies have consistently demonstrated increasing abundances of antibiotic resistance genes (ARGs) in the absence of antibiotic use. There is a large amount of quantitative data that has correlated the elevated ARGs levels with the concentrations of heavy metals in environments with anthropogenic impact. However, the mechanisms by which heavy metals facilitate the proliferation and horizontal gene transfer of ARGs among environmental bacteria were still unknown. This study validated effects of four typical heavy metals (Cu, Cd, Pb, Zn) on the plasmid RP4 mediated conjugative transfer of ARGs in freshwater microcosms. The results suggested that the typical heavy metals including Cu, Pb and Zn would promote conjugative transfer of the plasmid RP4, and Cu (5.0 μg/L) had the greatest ability to increase conjugative transfer by 16-fold higher than the control groups. In conjugative transfer microcosms, the species of each cultivable transconjugant were isolated, and their minimum inhibitory concentrations (MICs) were assessed via antibiotic susceptibility testing. The mechanism of the increased conjugative transfer of Cu was that Cu induced cell damage and the reduced conjugative transfer of Cd was that Cd increased the content of extracellular polymers substances (EPS). This study confirms that heavy metal Cu facilitates the conjugative transfer of environmental-mediated plasmid RP4 by cell damage effect, therefore accelerating the transmission and proliferation of ARGs.

Preservatives accelerate the horizontal transfer of plasmid-mediated antimicrobial resistance genes via differential mechanisms

Increasing concentrations of preservatives have been detected in environments due to the overuse and misuse of preservatives in food and personal care products. Recent studies have relied heavily on the toxicity, biodegradability, and fate of preservatives in the environment. However, the biological effects of preservatives on antimicrobial resistance, which poses great threats to public health worldwide, are largely unknown. This study investigated three preservatives for their ability and mechanisms of promoting horizontal transfer of antimicrobial resistance genes (ARGs). The results demonstrated that these preservatives (sodium nitrite, sodium benzoate, and triclocarbon), under daily-use concentrations, led to concentration-dependent increases in conjugative transfer by 1.24-2.63, 6.79-7.05, and 2.17-4.31 folds compared with the control group. Even these three preservatives had different patterns on generating intracellular reactive oxidative species (ROS) and reactive nitrogen species (RNS), all of them could stimulate radical-induced RpoS regulon and SOS response, increase cell membrane permeability, and regulate conjugative transfer-related genes, subsequently promoting horizontal transfer of ARGs. The present results expanded the understanding of biological effects induced by preservatives, and provided mechanistic insight into the preservatives-induced resistance. This study also opens an intriguing question on the roles of emerging contaminants including preservatives in the emerging and spread of ARGs in various environments.

Ionic Liquid Enriches the Antibiotic Resistome, Especially Efflux Pump Genes, Before Significantly Affecting Microbial Community Structure

An expanding list of chemicals may permeabilize bacterial cells and facilitate horizontal gene transfer (HGT), which enhances propagation of antibiotic resistance genes (ARGs) in the environment. Previous studies showed that 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF6]), an ionic liquid, can facilitate HGT of some ARGs among bacteria. However, the dynamic response of a wider range of ARGs and associated mobile genetic elements (MGEs) in different environments is unknown. Here, we used metagenomic tools to study shifts of the resistome and microbiome in both sediments and freshwater microcosms exposed to [BMIm][PF6]. Exposure for 16 h to 0.1 or 1.0 g/L significantly enriched more than 207 ARG subtypes primarily encoding efflux pumps in freshwater microcosms as well as cultivable antibiotic-resistant bacteria. This resistome enrichment was attributed to HGT facilitated by MGEs (428 plasmids, 61 integron-integrase genes, and 45 gene cassettes were enriched) as well as to HGT-related functional genes. Interestingly, resistome enrichment occurred fast (within 16 h) after [BMIm][PF6] exposure, before any significant changes in bacterial community structure. Similar ARG enrichment occurred in sediment microcosms exposed to [BMIm][PF6] for 28 d, and this longer exposure affected the microbial community structure (e.g., Proteobacteria abundance increased significantly). Overall, this study suggests that [BMIm][PF6] releases could rapidly enrich the antibiotic resistome in receiving environments by increasing HGT and fortuitously selecting for efflux pump genes, thus contributing to ARG propagation.

Fungicides enhanced the abundance of antibiotic resistance genes in greenhouse soil

Long-term substantial application of fungicides in greenhouse cultivation led to residual pollution in soils and then altered soil microbial community. However, it is unclear whether residual fungicides could affect the diversity and abundance of antibiotic resistance genes (ARGs) in greenhouse soils. Here, the dissipation of fungicides and its impact on the abundance of ARGs were determined using shotgun metagenomic sequencing in the greenhouse and mountain soils under laboratory conditions. Our results showed the greenhouse soils harbored more diverse and abundant ARGs than the mountain soils. The application of carbendazim, azoxystrobin, and chlorothalonil could increase the abundance of total ARGs in the greenhouse soils, especially for those dominant ARG subtypes including sul2, sul1, aadA, tet(L), tetA(G), and tetX2. The abundant ARGs were significantly correlated with mobile genetic elements (MGEs, e.g. intI1and R485) in the greenhouse soils but no significant relationship in the mountain soils. Meanwhile, the co-occurrence patterns of ARGs and MGEs, e.g., sul2 and R485, sul1 and transposase, were further verified via the genetic arrangement of genes on the metagenome-assembled contigs in the greenhouse soils. Additionally, host tracking analysis indicated that ARGs were mainly carried by enterobacteria in the greenhouse soils but actinomyces in the mountain soils. These findings confirmed that some fungicides might serve as the co-selectors of ARGs and elevated their abundance via MGEs-mediated horizontal gene transfer in the greenhouse soils.

Natural sphalerite nanoparticles can accelerate horizontal transfer of plasmid-mediated antibiotic-resistance genes

Minerals and microorganisms are integral parts of natural environments, and they inevitably interact. Antibiotic-resistance genes (ARGs) significantly threaten modern healthcare. However, the effects of natural minerals on ARG propagation in aquatic systems are not fully understood. The present work studied the effects of natural sphalerite (NS) nanoparticles on the horizontal transfer of ARGs from Escherichia coli DH5α (CTX) (donor) to E. coli C600 (Sm) (recipient), and from E. coli DH5α (MCR) (donor) to E. coli C600 (Sm), and their underlying mechanisms. NS particles (0.5-50 mg L^-1) induced an NS-concentration-dependent increase in conjugative transfer frequency. The underlying mechanisms associated with the facilitated ARG transfer included the production of intracellular reactive oxygen species, the SOS response, changes in bacterial cell morphology, and alteration of mRNA levels of bacterial cell membrane protein-related genes and genes associated with conjugative ARG transfer. The information herein offers new mechanistic understanding of risks of bacterial resistance resulting from NS.

Antibiotic resistance genes and microcystins in a drinking water treatment plant

Problems with antibiotic resistance genes (ARGs) and secondary pollution from microcystins (MCs), caused by cyanobacterial blooms have become significant global issues. These two pollutants co-occur in drinking water treatment plants (DWTPs), but the exact relationships between them requires further clarification. Here, a high-throughput quantitative real-time PCR and enzyme-linked immunosorbent assay were used to investigate the behavior of ARGs and MCs in a practical DWTP in the first place. After the on-site investigation, the effect of MCs on the horizontal transfer of ARGs was studied under laboratory conditions, and mechanisms explored at both cellular and molecular levels. MCs could promote the spread of ARGs, especially in relatively stationary and stable environments such as biofilms. MC-LR was the most efficient microcystin subtype promoting conjugative transfer, which was 25.13 times higher than for the control group. MCs affected the horizontal transfer of ARGs by regulating a series of gene systems involved in conjugative transfer, stimulating the formation of reactive oxygen species (ROS), and increasing cell membrane permeability. This study can provide a theoretical basis for the control of ARGs and MCs in DWTPs, which is of great significance for the scientific assessment of drinking water safety.

Comprehensive screening of quaternary ammonium surfactants and ionic liquids in wastewater effluents and lake sediments

Quaternary ammonium compounds (QACs) are widely applied as surfactants and biocides in cleaning and personal-care products. Because of incomplete removal during wastewater treatment, QACs are present in wastewater effluents, with which they are discharged into natural waters, where they accumulate in sediments. To assess the levels of QACs in aquatic environments, a liquid chromatography high-resolution mass spectrometry method using both target and suspect screening was developed. The water and sediment sample preparation, measurement, and data analysis workflow were optimized for 22 target compounds with a wide range of hydrophobicity, including ionic liquids that have potential use as solvents and QACs common in personal-care and sanitizing products. In wastewater effluents, average concentrations of all target and suspect QACs combined ranged from 0.4 μg L-1 to 6.6 μg L-1. Various homologs of benzylalkyldimethylammonium (BAC) and dialkyldimethylammonium (DADMAC) as well as the ionic liquid butylpyridinium and 15 suspect QACs were detected in at least one wastewater effluent sample. A spatial profile of sediment samples in a lake demonstrated potential inputs from both municipal wastewater effluent and agricultural sources for BACs. In sediment cores, two distinct trends of temporal QAC accumulation were observed. In lakes with large watersheds and mixed domestic and industrial wastewater sources (Lake Pepin and Duluth Harbor), peak concentrations of QACs were found at depths corresponding to deposition in the 1980s and decreases after this time are attributed to improved wastewater treatment and source control. In a smaller lake with predominantly domestic wastewater inputs (Lake Winona), concentrations of QACs increased slowly over time until today.

Both silver ions and silver nanoparticles facilitate the horizontal transfer of plasmid-mediated antibiotic resistance genes

Antibiotic resistance in bacteria is a growing threat to global human health. Horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs) is recognized as the primary contributor to antibiotic resistance dissemination. Silver nanoparticles (AgNPs) are widely used in personal care products as antimicrobial agents. While heavy metals are known to induce antibiotic resistance in bacteria, it is not known whether AgNPs in the environment can stimulate the HGT of ARGs. Here, we report that both AgNPs and ionic silver Ag⁺, at environmentally relevant and sub-lethal concentrations, facilitate the conjugative transfer of plasmid-borne ARGs across bacterial genera (from the donor Escherichia coli K-12 LE392 to the recipient Pseudomonas putida KT2440). The underlying mechanisms of the Ag⁺- or AgNPs-promoted HGT were unveiled by detecting oxidative stress and cell membrane permeability, combined with genome-wide RNA sequencing and proteomic analyses. It was found that both Ag⁺ and AgNPs exposure induced various bacterial responses that included reactive oxygen species (ROS) generation, membrane damage and the SOS response. This study exposes the potential ecological risks of environmental levels of AgNPs and Ag⁺ for promoting the spread of ARGs and highlights concerns regarding the management of nanoparticles and heavy metals.

Estuarine sediments are key hotspots of intracellular and extracellular antibiotic resistance genes: A high-throughput analysis in Haihe Estuary in China

Estuaries lie between terrestrial/freshwater and marine ecosystems, receive considerable pollutant input from land-based sources, and are considerably influenced by human activities. However, little attention has been paid to combined research on extracellular antibiotic resistance genes (eARGs) and intracellular ARGs (iARGs) in the estuarine environment. In this study, we profiled eARGs and iARGs in sediments from Haihe Estuary, China by adopting high-throughput quantitative PCR and investigated their relationship with mobile genetic elements (MGEs), the bacterial community and environmental factors. The results showed that the abundance of eARGs ranged from 9.06 × 10⁶ to 1.32 × 10⁸ copies/g and that of iARGs ranged from 3.31 × 10⁷ to 2.93 × 10⁸ copies/g, indicating that estuarine sediments were key hotspots of eARGs and iARGs. Additionally, multidrug resistance genes were both highly diverse and abundant in Haihe Estuary, especially in coastal samples. The high abundance of vancomycin and carbapenemase resistance genes may pose a potential health risk to human. Salinity altered the composition and structure of the bacterial community. Partial redundancy analysis showed that the bacterial community and MGEs appeared to be the major drivers of ARG variance in estuarine sediment. This study provides an overview of the distribution of eARG and iARG along the Haihe Estuary and draws attention to the need to control pollutants in estuary ecosystems.

Fe3O4 composited with MoS2 blocks horizontal gene transfer

In this study, we found that Fe₃O₄ promoted horizontal gene transfer (HGT), but when Fe₃O₄ was composited with MoS₂, the Fe₃O₄@MoS₂ nanocomposite interacting with bacteria significantly blocked the HGT in the conjugation system. qPCR was used to analyze the expression of genes belonging to the chromosome and plasmid in the conjugation system. Results demonstrated that Fe₃O₄@MoS₂ inhibited conjugation by promoting the expression of the global regulatory gene (trbA) and inhibiting the expression of conjugative transfer genes involved in mating pair formation (traF, trbB), DNA replication (trfA), and porins (outer membrane protein (omp) A and ompC). All of these genes are related to the permeability of the cell membrane, except for trfA. The results showed that Fe₃O₄@MoS₂ interacted with bacteria to decrease their permeability against exogenous DNA. MoS₂ may play an essential role in the HGT-inhibiting activity of Fe₃O₄@MoS₂. This study highlights the diverse biological properties of nano-materials and provides clues for nano-scientists to develop environmentally friendly materials.

MoS2 decorated nanocomposite: Fe2O3@MoS2 inhibits the conjugative transfer of antibiotic resistance genes

Nanomaterials of Al₂O₃ and TiO₂ have been proved to promote the spread of antibiotic resistance genes (ARGs) by horizontal gene transfer. In this work, we found that Fe₂O₃@MoS₂ nanocomposite inhibited the horizontal gene transfer (HGT) by inhibiting the conjugative transfer mediated by RP4-7 plasmid. To discover the mechanism of Fe₂O₃@MoS₂ inhibiting HGT, the bacterial cells were collected under the optimal mating conditions. The collected bacterial cells were used for analyzing the expression levels of genes unique to the plasmid and the bacterial chromosome in the conjugation system by qPCR. The results of genes expression demonstrated that the mechanism of Fe₂O₃@MoS₂ inhibited conjugation by promoting the expression of global regulatory gene (trbA) and inhibiting the expression of conjugative transfer genes involved in mating pair formation (traF, trbB) and DNA replication (trfA). The risk assessment of Fe₂O₃@MoS₂ showed that it had very low toxicity to organisms. The findings of this paper showed that Fe₂O₃@MoS₂, as an inhibitor of horizontal gene transfer, is an environment-friendly material.

Dissecting horizontal and vertical gene transfer of antibiotic resistance plasmid in bacterial community using microfluidics

The spread of antibiotic resistance genes (ARGs) has become an emerging threat to the global health. Although horizontal gene transfer (HGT) is regarded as one of the major pathways, more evidence has shown the significant involvement of vertical gene transfer (VGT). However, traditional cultivation-based methods cannot distinguish HGT and VGT, resulting in often contradictory conclusions. Here, single-cell microfluidics with time-lapse imaging has been successfully employed to dissect the contribution of plasmid-mediated HGT and VGT to ARG transmission in an environmental community. Using Escherichia coli with an ARG-coded plasmid pKJK5 with trimethoprim resistance as the donor, we quantified the effects of three representative antibiotics (trimethoprim, tetracycline and amoxicillin) on the ARG transfer process in an activated sludge bacterial community. It was found that HGT was influenced by the inhibitory mechanism of an antibiotic and its targets (donor, recipient alone or together), whereas VGT contributes significantly to the formation of transconjugants and consequently ARG spreading. Trimethoprim is highly resisted by the donor and transconjugants, and its presence significantly increased both the HGT and VGT rates. Although tetracycline and amoxicillin both inhibit the donor, they showed different effects on HGT rate as a result of different inhibitory mechanisms. Furthermore, we show the kinetics of HGT in a community can be described using an epidemic infection model, which in combination with quantitative measure of HGT and VGT on chip provides a promising tool to study and predict the dynamics of ARG spread in real-world communities.

Swine waste: A reservoir of high-risk blaNDM and mcr-1

Multidrug resistance associated with pigs not only affects pig production but also threatens human health by influencing the farm surrounding and contaminating the food chain. This paper focused on the occurrence and prevalence of high-risk resistance genes (using bla_NDM and mcr-1 as marker genes) in two Chinese swine farms, and investigated their fate and seasonal changes in piggery wastewater treatment systems (PWWTSs). Results revealed that bla_NDM and mcr-1 were prevalent in both confined swine farms, and even prevailed through various processing stages of PWWTSs. Moreover, the abundance of bla_NDM and mcr-1 in winter was higher than that in summer, with 0.01-1.01 logs variation in piggery wastewater. Of concern is that considerable amounts of bla_NDM and mcr-1 were present in final effluent that is applied to farmland (up to 10²-10⁴copies/mL), raising the risk of propagation to indigenous bacteria. Worse still, those pig-derived isolates harboring the bla_NDM/mcr-1 gene were confirmed to spread multidrug resistance to other bacteria, which further increased their dissemination potential in agricultural environment. This study highlights the prevalence of bla_NDM and mcr-1 in swine farms, meanwhile, also emphasizes the necessary to mitigate the release and propagation of these high-risk genes from swine farms following land fertilization and wastewater usage.

The effect and mechanism of urban fine particulate matter (PM2.5) on horizontal transfer of plasmid-mediated antimicrobial resistance genes

Fine particulate matter (PM_2.5) and antimicrobial resistance are two major threats to public health worldwide. Current air pollution studies rely heavily on the assessment of PM_2.5 chemistry and toxicity. However, whether and how PM_2.5 affects the proliferation and transfer of antimicrobial resistance genes (ARGs) in various environments has remained unanswered. This study investigated the effects and potential mechanisms of urban PM_2.5 on the horizontal transfer of ARGs between opportunistic Escherichia coli (E. coli) strains. The results showed that urban PM_2.5 samples collected from Xi'an (XA), Shanghai (SH), and Shijiazhuang (SJZ) in China induced location- and concentration-dependent promotion of conjugative transfer frequencies compared to the control group. The relevant mechanisms were also explored, including the formation of intracellular reactive oxygen species (ROS) and the subsequent induction of oxidative stress, SOS response, changes in membrane permeability, and alternations in mRNA expression of genes involved in horizontal transfer. This study highlights the effect of PM_2.5 on promoting the horizontal transfer of ARGs and elucidates the mechanism of the antimicrobial-resistance risks posed by urban PM_2.5. These findings are of great value in understanding the transmission of antimicrobial resistance in various environments and provide valuable information for re-evaluating air quality assessment practices.

CO2 promotes the conjugative transfer of multiresistance genes by facilitating cellular contact and plasmid transfer

The dissemination of antibiotic resistance genes (ARGs), especially via the plasmid-mediated conjugation, is becoming a pervasive global health threat. This study reported that this issue can be worse by CO₂, as increased CO₂ was found to facilitate the conjugative transfer of ARGs carried on plasmid RP4 by 2.4-9.0 and 1.3-3.8 fold within and across genera, respectively. Mechanistic studies revealed that CO₂ benefitted the cell-to-cell contact by increasing cell surface hydrophobicity and decreasing cell surface charge, both of which resulted in the reduced intercellular repulsion. Besides, the transcriptional expression of genes responsible for global regulator (korA, korB and trbA), plasmid transfer and replication system (trfAp), and mating pair formation system (traF and traG) were all influenced by CO₂, facilitating the mobilization and channel transfer of plasmid. Furthermore, the presence of CO₂ induced the release of intracellular Ca²⁺ and increased the transmembrane potential of recipients, which contributed to the increased proton motive force (PMF), providing more power for DNA uptake. This is the first study addressing the potential risks of increased CO₂ on the propagation of ARGs, which provides a new insight into the concerns of anthropogenic CO₂ emissions and CO₂ storage.

Copper nanoparticles and copper ions promote horizontal transfer of plasmid-mediated multi-antibiotic resistance genes across bacterial genera

The spread of antibiotic resistance has become a major concern for public health. As emerging contaminants, various metallic nanoparticles (NPs) and ionic heavy metals have been ubiquitously detected in various environments. Although previous studies have indicated NPs and ionic heavy metals could exhibit co-selection effects for antibiotic resistance, little is known about whether and how they could promote antibiotic resistance spread via horizontal gene transfer across bacterial genera. This study, we report both CuO NPs and copper ions (Cu²⁺) could stimulate the conjugative transfer of multiple-drug resistance genes. When exposing bacteria to CuO NPs or Cu²⁺ at environmental-relevant and sub-inhibitory concentrations (e.g., 1-100 μmol/L), conjugation frequencies of plasmid-encoded antibiotic resistance genes across genera (i.e., from Escherichia coli to Pseudomonas putida) were significantly enhanced (p < 0.05). The over-production of reactive oxygen species played a crucial role in promoting conjugative transfer. Genome-wide RNA and protein sequencing suggested expressional levels of genes and proteins related to oxidative stress, cell membrane permeability, and pilus generation were significantly up-regulated under CuO NPs and Cu²⁺ exposure (p < 0.05). This study provides insights in the contributions of NPs and heavy metals on the spread of antibiotic resistance.

Mitigated membrane fouling and enhanced removal of extracellular antibiotic resistance genes from wastewater effluent via an integrated pre-coagulation and microfiltration process

Antibiotic resistance genes (ARGs) have been regarded as an emerging pollutant in municipal wastewater treatment plant (WWTP) effluents due to their potential risk to human health and ecological safety when reused for landscape and irrigation. Conventional wastewater treatment processes generally fail to effectively reduce ARGs, especially extracellular ARGs (eARGs), which are persistent in the environment and play an important role in horizontal gene transfer via transformation. Herein, an integrated process of pre-coagulation and microfiltration was developed for removal of ARGs, especially eARGs, from wastewater effluent. Results show that the integrated process could effectively reduce the absolute abundances of total ARGs (tARGs) (>2.9 logs) and eARGs (>5.2 logs) from the effluent. The excellent performance could be mainly attributed to the capture of antibiotic resistant bacteria (ARB) and eARGs by pre-coagulation and co-rejection during subsequent microfiltration. Moreover, the integrated process exhibited a good performance on removing common pollutants (e.g., dissolved organic carbon and phosphate) from the effluent to improve water quality. Besides, the integrated process also greatly reduced membrane fouling compared with microfiltration. These findings suggest that the integrated process of pre-coagulation and microfiltration is a promising advanced wastewater treatment technology for ARGs (especially eARGs) removal from WWTP effluents to ensure water reuse security.

Low-level free nitrous acid efficiently inhibits the conjugative transfer of antibiotic resistance by altering intracellular ions and disabling transfer apparatus

Recently, the dissemination of antibiotic resistance genes (ARGs) via plasmid-mediated conjugation has been reported to be facilitated by a series of contaminants. This has highlighted potential challenges to the effective control of this principal mode of horizontal transfer. In the present study, we found that low levels (<0.02 mgN/L) of free nitrous acid (FNA) remarkably inhibited (over 90%) the conjugative transfer of plasmid RP4, a model broad-host-range plasmid, between Escherichia coli. The antimicrobial role of FNA at the applied dosages was firstly ruled out, since no dramatic reductions in viabilities of donor or recipient were observed. Instead, FNA appeared to reduce the available intracellular free Mg²⁺, which was confirmed to be triggered by the liberation of intracellular Fe²⁺. These alterations in intracellular Mg²⁺ and Fe²⁺ concentrations were found to significantly limit the available energy for conjugative transfer through suppression of glycolysis by decreasing the activities of glycogen phosphorylase and glyceraldehyde-3-phosphate dehydrogenase and also by diverting the glycolytic flux into the pentose phosphate pathway via activation of glucose-6-phosphate dehydrogenase towards the generation of NADPH rather than ATP. Moreover, RP4-encoding genes responsible for DNA transfer and replication (traI, traJ and trfAp), coupling (traG) and mating pair formation (traF and trbBp) were all significantly down-regulated after FNA treatment, indicating that the transfer apparatus required for plasmid processing and delivery was deactivated. By validating the inhibitory effects of FNA on conjugation in real wastewater, this study highlights a promising method for controlling the dissemination of ARGs in systems such as wastewater treatment plants.

Fate of Antibiotic Resistant Pseudomonas putida and Broad Host Range Plasmid in Natural Soil Microcosms

Plasmid conjugation is one of the dominant mechanisms of horizontal gene transfer, playing a noticeable role in the rapid spread of antibiotic resistance genes (ARGs). Broad host range plasmids are known to transfer to diverse bacteria in extracted soil bacterial communities when evaluated by filter mating incubation. However, the persistence and dissemination of broad range plasmid in natural soil has not been well studied. In this study, Pseudomonas putida with a conjugative antibiotic resistance plasmid RP4 was inoculated into a soil microcosm, the fate and persistence of P. putida and RP4 were monitored by quantitative PCR. The concentrations of P. putida and RP4 both rapidly decreased within 15-day incubation. P. putida then decayed at a significantly lower rate during subsequent incubation, however, no further decay of RP4 was observed, resulting in an elevated RP4/P. putida ratio (up to 10) after 75-day incubation, which implied potential transfer of RP4 to soil microbiota. We further sorted RP4 recipient bacteria from the soil microcosms by fluorescence-activated cell sorting. Spread of RP4 increased during 75-day microcosm operation and was estimated at around 10^-4 transconjugants per recipient at the end of incubation. Analysis of 16S rRNA gene sequences of transconjugants showed that host bacteria of RP4 were affiliated to more than 15 phyla, with increased diversity and shift in the composition of host bacteria. Proteobacteria was the most dominant phylum in the transconjugant pools. Transient transfer of RP4 to some host bacteria was observed. These results emphasize the prolonged persistence of P. putida and RP4 in natural soil microcosms, and highlight the potential risks of increased spread potential of plasmid and broader range of host bacteria in disseminating ARGs in soil.

Seasonal and spatial distribution of antibiotic resistance genes in the sediments along the Yangtze Estuary, China

Antibiotics resistance genes (ARGs) are considered as an emerging pollutant among various environments. As a sink of ARGs, a comprehensive study on the spatial and temporal distribution of ARGs in the estuarine sediments is needed. In the present study, six ARGs were determined in sediments taken along the Yangtze Estuary temporally and spatially. The sulfonamides, tetracyclines and fluoroquinolones resistance genes including sul1, sul2, tetA, tetW, aac(6')-Ib, and qnrS, were ubiquitous, and the average abundances of most ARGs showed significant seasonal differences, with relative low abundances in winter and high abundances in summer. Moreover, the relative high abundances of ARGs were found at Shidongkou (SDK) and Wusongkou (WSK), which indicated that the effluents from the wastewater treatment plant upstream and inland river discharge could influence the abundance of ARGs in sediments. The positive correlation between intI1 and sul1 implied intI1 may be related to the occurrence and propagation of sulfonamides resistance genes. Correlation analysis and redundancy discriminant analysis showed that antibiotic concentrations had no significant correlation to their corresponding ARGs, while the total extractable metal, especially the bioavailable metals, as well as other environmental factors including temperature, clay, total organic carbon and total nitrogen, could regulate the occurrence and distribution of ARGs temporally and spatially. Our findings suggested the comprehensive effects of multiple pressures on the distribution of ARGs in the sediments, providing new insight into the distribution and dissemination of ARGs in estuarine sediments, spatially and temporally.

Antidepressant fluoxetine induces multiple antibiotics resistance in Escherichia coli via ROS-mediated mutagenesis

BACKGROUND

Antibiotic resistance poses a great threat to global public health. Overuse of antibiotics is generally considered as the major factor contributing to it. However, little is known about whether non-antibiotic drugs could play potential roles in the emergence of antibiotic resistance.

OBJECTIVE

We aimed to investigate whether antidepressant fluoxetine induces multiple antibiotic resistances and reveal underlying mechanisms.

METHODOLOGY

Escherichia coli K12 was exposed to different concentrations of fluoxetine (0, 0.5, 5, 50 and 100 mg/L) and the resistant strains were isolated by plating on antibiotic containing plates. Resistant strains were randomly selected to determine the increase of minimum inhibition concentration (MIC) of multiple antibiotics. Genome-wide DNA sequencing was performed on cells cultured in lysogeny broth (LB) without any fluoxetine or antibiotics exposure. RNA sequencing and proteomic profiling of isolated mutants grown in LB with 100 mg/L fluoxetine were analyzed to reveal the underlying mechanisms.

RESULTS

Exposure of Escherichia coli to fluoxetine at 5-100 mg/L after repeated subculture in LB for 30 days promoted its mutation frequency resulting in increased resistance against the antibiotics chloramphenicol, amoxicillin and tetracycline. This increase was up to 5.0 × 10⁷ fold in a dose-time pattern. Isolated mutants with resistance to one of these antibiotics also exhibited multiple resistances against fluoroquinolone, aminoglycoside, β-lactams, tetracycline and chloramphenicol. According to global transcriptional and proteomic analyses, the AcrAB-TolC pump together with the YadG/YadH transporter, a Tsx channel and the MdtEF-TolC pump have been triggered to export the antibiotics to the exterior of the cell. Whole-genome DNA analysis of the mutants further revealed that ROS-mediated mutagenesis (e.g., deletion, insertion, and substitution) of DNA-binding transcriptional regulators (e.g., marR, rob, sdiA, cytR and crp) to up-regulate the expression of efflux pumps, may further enhance the antibiotic efflux.

CONCLUSIONS

Our findings for the first time demonstrated that the exposure to antidepressant fluoxetine induces multiple antibiotic resistance in E. coli via the ROS-mediated mutagenesis.