Diversity and Distribution of Arsenic-Related Genes Along a Pollution Gradient in a River Affected by Acid Mine Drainage. - Related Documents




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868701.0000Diversity and Distribution of Arsenic-Related Genes Along a Pollution Gradient in a River Affected by Acid Mine Drainage. Some microorganisms have the capacity to interact with arsenic through resistance or metabolic processes. Their activities contribute to the fate of arsenic in contaminated ecosystems. To investigate the genetic potential involved in these interactions in a zone of confluence between a pristine river and an arsenic-rich acid mine drainage, we explored the diversity of marker genes for arsenic resistance (arsB, acr3.1, acr3.2), methylation (arsM), and respiration (arrA) in waters characterized by contrasted concentrations of metallic elements (including arsenic) and pH. While arsB-carrying bacteria were representative of pristine waters, Acr3 proteins may confer to generalist bacteria the capacity to cope with an increase of contamination. arsM showed an unexpected wide distribution, suggesting biomethylation may impact arsenic fate in contaminated aquatic ecosystems. arrA gene survey suggested that only specialist microorganisms (adapted to moderately or extremely contaminated environments) have the capacity to respire arsenate. Their distribution, modulated by water chemistry, attested the specialist nature of the arsenate respirers. This is the first report of the impact of an acid mine drainage on the diversity and distribution of arsenic (As)-related genes in river waters. The fate of arsenic in this ecosystem is probably under the influence of the abundance and activity of specific microbial populations involved in different As biotransformations.201626603631
751810.9996Deciphering the toxic effects of metals in gold mining area: Microbial community tolerance mechanism and change of antibiotic resistance genes. Mine tailing dumps represent significant threats to ecological environments due to the presence of toxic substances. The present work investigated the relationship among microbial activity, the community, antibiotic resistance genes (ARGs) and trace metals in soil surrounding gold mine tailings. Using microbial metabolic activity and high-throughput sequencing analysis, we found the trace metals Cd and Hg could be main factors influencing the microbial community. According to bacterial co-occurrence pattern analysis, the effects of total cadmium and total mercury on bacterial diversity are potentially mediated by influencing bacteria community in the keystone module II. Additionally, most of metal-resistant bacteria belong to Actinobacteria and Proteobacteria, and the metal tolerance suggested to be linked with various functions including replication, recombination and repair, as well as inorganic ion transport and metabolism based on PICRUSt2 analysis. We also found that metals generated by mining activity may trigger the co-selection of antibiotic resistance in the phyla Actinobacteria and Proteobacteria due to co-resistance or cross resistance. Additionally, PLS-PM analysis revealed that metals could indirectly affect ARGs by influencing bacterial diversity in gold mining areas.202032678731
610320.9996Culture-dependent study of arsenic-reducing bacteria in deep aquatic sediments of Bengal Delta. Biogeochemical release of soil-bound arsenic (As) governs mobilization of the toxic metalloid into the groundwater. The present study has examined As(V)-reduction ability of bacteria from anoxic aquatic sediments that might contribute to arsenic mobilization in the Bengal Delta. Arsenic-reducing bacteria from deep layers of pond sediment were enriched and isolated in anaerobic environments and As(V) reduction was assessed in culture medium. The pond sediment enrichments harboured As(V)-reducing bacteria belonging to the phyla Firmicutes and Proteobacteria with dominance of Paraclostridium benzoelyticum and P. bifermentans. Among total 17 isolates, the respiratory reductase genes were not detected by the most common primers and only 3 strains had arsenic reductase ArsC gene suggesting involvement of resistance and some unknown mechanisms in As(V) reduction. Presence of high levels of organic matter, As, and As-reducing bacteria might make deep aquatic sediments a hot spot of As mobilization and aquifer contamination.202134482463
751730.9995Bacterial Heavy-Metal and Antibiotic Resistance Genes in a Copper Tailing Dam Area in Northern China. Heavy metal resistance genes (MRGs) and antibiotic resistance genes (ARGs) in bacteria can respond to the inducement of heavy metals. However, the co-occurrence of MRGs and ARGs in the long-term heavy metal contaminated area is still poorly understood. Here, we investigated the relationship between the abundance of soil bacteria MRGs, ARGs and heavy metal pollution in a copper tailing dam area of northern China. We found that arsC and ereA genes coding for resistance mechanisms to arsenic and to macrolides, respectively, are the most abundant MRG and ARG in the study area. The abundance of MRGs is positively correlated with cadmium (Cd) concentration, and this indicates the importance of Cd in the selection of MRGs. The network analysis results show that sulII and MRGs co-occur and copB occur with ARGs, which suggests that MRGs and ARGs can be co-selected in the soil contaminated by heavy metal. The network analysis also reveals the co-occurrence of Cd and MRGs, and thus heavy metal with a high 'toxic-response' factor can be used as the indicator of MRGs. This study improves the understanding of the relationship between bacterial resistance and multi-metal contamination, and underlies the exploration of the adaptive mechanism of microbes in the multi-metal contaminated environment.201931481945
751540.9995Bacterial metal resistance genes and metal bioavailability in contaminated sediments. In bacteria a metal may be defined as bioavailable if it crosses the cytoplasmic membrane to reach the cytoplasm. Once inside the cell, specific metal resistance systems may be triggered. In this research, specific metal resistance genes were used to estimate metal bioavailability in sediment microbial communities. Gene levels were measured by quantitative PCR and correlated to metals in sediments using five different protocols to estimate dissolved, particle-adsorbed and occluded metals. The best correlations were obtained with czcA (a Cd/Zn/Co efflux pump) and Cd/Zn adsorbed or occluded in particles. Only adsorbed Co was correlated to czcA levels. We concluded that the measurement of czcA gene levels by quantitative PCR is a promising tool which may complement the classical approaches used to estimate Cd/Zn/Co bioavailability in sediment compartments.201424662000
610750.9995Metagenomic and genomic analysis of heavy metal-tolerant and -resistant bacteria in resource islands in a semi-arid zone of the Colombian Caribbean. Bacteria from resource islands can adapt to different extreme conditions in semi-arid regions. We aimed to determine the potential resistance and tolerance to heavy metals from the bacterial community under the canopy of three resource islands in a semi-arid zone of the Colombian Caribbean. Total DNA was extracted from soil and through a metagenomics approach, we identified genes related to heavy metal tolerance and resistance under the influence of drought and humidity conditions, as well as the presence or absence of vegetation. We characterized the genomes of bacterial isolates cultivated in the presence of four heavy metals. The abundances of genes related to heavy metal resistance and tolerance were favored by soil moisture and the presence of vegetation. We observed a high abundance of resistance genes (60.4%) for Cu, Zn, and Ni, while 39.6% represented tolerance. These genes positively correlated with clay and silt content, and negatively correlated with sand content. Resistance and tolerance were associated with detoxification mechanisms involving oxidoreductase enzymes, metalloproteases, and hydrolases, as well as transmembrane proteins involved in metal transport such as efflux pumps and ion transmembrane transporters. The Bacillus velezensis C3-3 and Cytobacillus gottheilii T106 isolates showed resistance to 5 mM of Cd, Co, Mn, and Ni through detoxification genes associated with ABC pumps, metal transport proteins, ion antiporter proteins, and import systems, among others. Overall, these findings highlight the potential of bacteria from resource islands in bioremediation processes of soils contaminated with heavy metals.202438127234
768660.9995Bacterial tolerances to metals and antibiotics in metal-contaminated and reference streams. Anthropogenic-derived sources of selection are typically implicated as mechanisms for maintaining antibiotic resistance in the environment. Here we report an additional mechanism for maintaining antibiotic resistance in the environment through bacterial exposure to metals. Using a culture-independent approach, bacteria sampled along a gradient of metal contamination were more tolerant of antibiotics and metals compared to bacteria from a reference site. This evidence supports the hypothesis that metal contamination directly selects for metal tolerant bacteria while co-selecting for antibiotic tolerant bacteria. Additionally, to assess how antibiotic and metal tolerance may be transported through a stream network, we studied antibiotic and metal tolerance patterns over three months in bacteria collected from multiple stream microhabitats including the water column, biofilm, sediment and Corbicula fluminea (Asiatic clam) digestive tracts. Sediment bacteria were the most tolerant to antibiotics and metals, while bacteria from Corbicula were the least tolerant. Differences between microhabitats may be important for identifying reservoirs of resistance and for predicting how these genes are transferred and transported in metal-contaminated streams. Temporal dynamics were not directly correlated to a suite of physicochemical parameters, suggesting that tolerance patterns within microhabitats are linked to a complex interaction of the physicochemical characteristics of the stream.200617064270
674270.9994Influence of epiphytic bacteria on arsenic metabolism in Hydrilla verticillata. Microbial assemblages such as biofilms around aquatic plants play a major role in arsenic (As) cycling, which has often been overlooked in previous studies. In this study, arsenite (As(III))-oxidizing, arsenate (As(V))-reducing and As(III)-methylating bacteria were found to coexist in the phyllosphere of Hydrilla verticillata, and their relative activities were shown to determine As speciation, accumulation and efflux. When exposed to As(III), As(III) oxidation was not observed in treatment H(III)-B, whereas treatment H(III)+B showed a significant As(III) oxidation ability, thereby indicating that epiphytic bacteria displayed a substantial As(III) oxidation ability. When exposed to As(V), the medium only contained 5.89% As(III) after 48 h of treatment H(V)-B, while an As(III) content of 86.72% was observed after treatment H(V)+B, thereby indicating that the elevated As(III) in the medium probably originated from As(V) reduction by epiphytic bacteria. Our data also indicated that oxidizing bacteria decreased the As accumulation (by approximately 64.44% compared with that of treatment H(III)-B) in plants, while reducing bacteria played a critical role in increasing As accumulation (by approximately 3.31-fold compared with that of treatment H(V)-B) in plants. Regardless of whether As(III) or As(V) was supplied, As(III) was dominant in the plant tissue (over 75%). Furthermore, the presence of epiphytic bacteria enhanced As efflux by approximately 9-fold. Metagenomic analysis revealed highly diverse As metabolism genes in epiphytic bacterial community, particularly those related to energetic metabolism (aioAB), and As resistance (arsABCR, acr3, arsM). Phylogenetic analysis of As metabolism genes revealed evidence of both vertical inheritance and horizontal gene transfer, which might have contributed to the evolution of the As metabolism genes. Taken together, our research suggested that the diversity of As metabolism genes in epiphytic bacterial community is associated with aquatic submerged macrophytes which may play an important role in As biogeochemistry in aquatic environments.202032114122
869380.9994An Overview of Bacteria-Mediated Heavy Metal Bioremediation Strategies. Contamination-free groundwater is considered a good source of potable water. Even in the twenty-first century, over 90 percent of the population is reliant on groundwater resources for their lives. Groundwater influences the economical state, industrial development, ecological system, and agricultural and global health conditions worldwide. However, different natural and artificial processes are gradually polluting groundwater and drinking water systems throughout the world. Toxic metalloids are one of the major sources that pollute the water system. In this review work, we have collected and analyzed information on metal-resistant bacteria along with their genetic information and remediation mechanisms of twenty different metal ions [arsenic (As), mercury (Hg), lead (Pb), chromium (Cr), iron (Fe), copper (Cu), cadmium (Cd), palladium (Pd), zinc (Zn), cobalt (Co), antimony (Sb), gold (Au), silver (Ag), platinum (Pt), selenium (Se), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), and uranium (U)]. We have surveyed the scientific information available on bacteria-mediated bioremediation of various metals and presented the data with responsible genes and proteins that contribute to bioremediation, bioaccumulation, and biosorption mechanisms. Knowledge of the genes responsible and self-defense mechanisms of diverse metal-resistance bacteria would help us to engineer processes involving multi-metal-resistant bacteria that may reduce metal toxicity in the environment.202437410353
747190.9994Impact of fluoroquinolone and heavy metal pollution on antibiotic resistance maintenance in aquatic ecosystems. BACKGROUND: Freshwater pollution with compounds used during anthropogenic activities could be a major driver of antibiotic resistance emergence and dissemination in environmental settings. Fluoroquinolones and heavy metals are two widely used aquatic pollutants that show a high stability in the environment and have well-known effects on antibiotic resistance selection. However, the impact of these compounds on antibiotic resistance maintenance in aquatic ecosystems remains unknown. In this study, we used a microcosm approach to determine the persistence of two fluoroquinolones (ciprofloxacin, ofloxacin) and two heavy metals (copper and zinc) in the Rhône river over 27 days. In addition, we established links between antibiotic and metal pollution, alone and in combination, and the composition of freshwater bacterial communities, the selection of specific members and the selection and maintenance of antibiotic and metal resistance genes (ARGs and MRGs) using a metagenomics approach. RESULTS: Whereas ofloxacin was detected at higher levels in freshwater after 27 days, copper had the strongest influence on bacterial communities and antibiotic and metal resistance gene selection. In addition, heavy metal exposure selected for some ARG-harboring bacteria that contained MRGs. Our research shows a heavy metal-driven transient co-selection for fluoroquinolone resistance in an aquatic ecosystem that could be largely explained by the short-term selection of Pseudomonas subpopulations harboring both fluoroquinolone efflux pumps and copper resistance genes. CONCLUSION: This research highlights the complexity and compound-specificity of dose-response relationships in freshwater ecosystems and provides new insights into the medium-term community structure modifications induced by overall sub-inhibitory levels of antibiotic and heavy metal pollution that may lead to the selection and maintenance of antibiotic resistance in low-impacted ecosystems exposed to multiple pollutants.202540426239
6737100.9994Microbial-mediated conversion of soil organic carbon co-regulates the evolution of antibiotic resistance. The influence of organic carbon on the proliferation of antibiotic resistance genes (ARGs) in the soil has been widely documented. However, it is unclear how soil organic carbon (SOC) interacts with the evolution of antibiotic resistance in bacteria. Here, we examined the variations in ARGs abundance during SOC mineralization and explored the microbiological mechanisms and key metabolic pathways involved in their coevolution. The results showed that the SOC mineralization rate was closely correlated with ARGs abundance (p < 0.05). High organic carbon (OC) mineralization was conducive to the occurrence of multidrug resistance genes. For example, multidrug_transporter and mexB increased 2.26 and 7.83 times from the initial level. The competitor (stress) evolutionary strategy model revealed that higher OC inputs drive environmental microorganisms to evolve from stress tolerant to high resistance and strong adaptation. Meta-genomic and transcriptomic analyses revealed that the conversion process of pyruvate to acetyl-CoA to acetate was the critical metabolic pathway for the co-regulation of antibiotic resistance. Gene deletion validation trials have demonstrated that the key functional genes (ackA and pta) involved in this process can modulate the development of vancomycin and multidrug resistance. This outcome provides a preliminary framework for microbial mechanisms that target the co-regulation of microbial OC conversion and the evolution of antibiotic resistance.202438688217
8688110.9994Adaptation of intertidal biofilm communities is driven by metal ion and oxidative stresses. Marine organisms in intertidal zones are subjected to periodical fluctuations and wave activities. To understand how microbes in intertidal biofilms adapt to the stresses, the microbial metagenomes of biofilms from intertidal and subtidal zones were compared. The genes responsible for resistance to metal ion and oxidative stresses were enriched in both 6-day and 12-day intertidal biofilms, including genes associated with secondary metabolism, inorganic ion transport and metabolism, signal transduction and extracellular polymeric substance metabolism. In addition, these genes were more enriched in 12-day than 6-day intertidal biofilms. We hypothesize that a complex signaling network is used for stress tolerance and propose a model illustrating the relationships between these functions and environmental metal ion concentrations and oxidative stresses. These findings show that bacteria use diverse mechanisms to adapt to intertidal zones and indicate that the community structures of intertidal biofilms are modulated by metal ion and oxidative stresses.201324212283
8519120.9994Effects of Antibiotic Resistance Genes and Antibiotics on the Transport and Deposition Behaviors of Bacteria in Porous Media. Antibiotics present in the natural environment would induce the generation of antibiotic-resistant bacteria (ARB), causing great environmental risks. The effects of antibiotic resistance genes (ARGs) and antibiotics on bacterial transport/deposition in porous media yet are unclear. By using E. coli without ARGs as antibiotic-susceptible bacteria (ASB) and their corresponding isogenic mutants with ARGs in plasmids as ARB, the effects of ARGs and antibiotics on bacterial transport in porous media were examined under different conditions (1-4 m/d flow rates and 5-100 mM NaCl solutions). The transport behaviors of ARB were comparable with those of ASB under antibiotic-free conditions, indicating that ARGs present within cells had negligible influence on bacterial transport in antibiotic-free solutions. Interestingly, antibiotics (5-1000 μg/L gentamicin) present in solutions increased the transport of both ARB and ASB with more significant enhancement for ASB. This changed bacterial transport induced by antibiotics held true in solution with humic acid, in river water and groundwater samples. Antibiotics enhanced the transport of ARB and ASB in porous media via different mechanisms (ARB: competition of deposition sites; ASB: enhanced motility and chemotaxis effects). Clearly, since ASB are likely to escape sites containing antibiotics, these locations are more likely to accumulate ARB and their environmental risks would increase.202337406198
7685130.9994Gut heavy metal and antibiotic resistome of humans living in the high Arctic. Contaminants, such as heavy metals (HMs), accumulate in the Arctic environment and the food web. The diet of the Indigenous Peoples of North Greenland includes locally sourced foods that are central to their nutritional, cultural, and societal health but these foods also contain high concentrations of heavy metals. While bacteria play an essential role in the metabolism of xenobiotics, there are limited studies on the impact of heavy metals on the human gut microbiome, and it is so far unknown if and how Arctic environmental contaminants impact the gut microbes of humans living in and off the Arctic environment. Using a multiomics approach including amplicon, metagenome, and metatranscriptome sequencing, we identified and assembled a near-complete (NC) genome of a mercury-resistant bacterial strain from the human gut microbiome, which expressed genes known to reduce mercury toxicity. At the overall ecological level studied through α- and β-diversity, there was no significant effect of heavy metals on the gut microbiota. Through the assembly of a high number of NC metagenome-assembled genomes (MAGs) of human gut microbes, we observed an almost complete overlap between heavy metal-resistant strains and antibiotic-resistant strains in which resistance genes were all located on the same genetic elements.202439539714
8649140.9994Antibiotic-Induced Recruitment of Specific Algae-Associated Microbiome Enhances the Adaptability of Chlorella vulgaris to Antibiotic Stress and Incidence of Antibiotic Resistance. Insights into the symbiotic relation between eukaryotic hosts and their microbiome lift the curtain on the crucial roles of microbes in host fitness, behavior, and ecology. However, it remains unclear whether and how abiotic stress shapes the microbiome and further affects host adaptability. This study first investigated the effect of antibiotic exposure on behavior across varying algae taxa at the community level. Chlorophyta, in particular Chlorella vulgaris, exhibited remarkable adaptability to antibiotic stress, leading to their dominance in phytoplankton communities. Accordingly, we isolated C. vulgaris strains and compared the growth of axenic and nonaxenic ones under antibiotic conditions. The positive roles of antibiotics in algal growth were apparent only in the presence of bacteria. Results of 16S rRNA sequencing further revealed that antibiotic challenges resulted in the recruitment of specific bacterial consortia in the phycosphere, whose functions were tightly linked to the host growth promotion and adaptability enhancement. In addition, the algal phycosphere was characterized with 47-fold higher enrichment capability of antibiotic resistance genes (ARGs) than the surrounding water. Under antibiotic stress, specific ARG profiles were recruited in C. vulgaris phycosphere, presumably driven by the specific assembly of bacterial consortia and mobile genetic elements induced by antibiotics. Moreover, the antibiotics even enhanced the dissemination potential of the bacteria carrying ARGs from the algal phycosphere to broader environmental niches. Overall, this study provides an in-depth understanding into the potential functional significance of antibiotic-mediated recruitment of specific algae-associated bacteria for algae adaptability and ARG proliferation in antibiotic-polluted waters.202337642958
8690150.9994Cellular and genetic mechanism of bacterial mercury resistance and their role in biogeochemistry and bioremediation. Mercury (Hg) is a highly toxic element that occurs at low concentrations in nature. However, various anthropogenic and natural sources contribute around 5000 to 8000 metric tons of Hg per year, rapidly deteriorating the environmental conditions. Mercury-resistant bacteria that possess the mer operon system have the potential for Hg bioremediation through volatilization from the contaminated milieus. Thus, bacterial mer operon plays a crucial role in Hg biogeochemistry and bioremediation by converting both reactive inorganic and organic forms of Hg to relatively inert, volatile, and monoatomic forms. Both the broad-spectrum and narrow-spectrum bacteria harbor many genes of mer operon with their unique definitive functions. The presence of mer genes or proteins can regulate the fate of Hg in the biogeochemical cycle in the environment. The efficiency of Hg transformation depends upon the nature and diversity of mer genes present in mercury-resistant bacteria. Additionally, the bacterial cellular mechanism of Hg resistance involves reduced Hg uptake, extracellular sequestration, and bioaccumulation. The presence of unique physiological properties in a specific group of mercury-resistant bacteria enhances their bioremediation capabilities. Many advanced biotechnological tools also can improve the bioremediation efficiency of mercury-resistant bacteria to achieve Hg bioremediation.202234464861
9632160.9993A simple model of tetracycline antibiotic resistance in the aquatic environment (with application to the Poudre River). Antibiotic resistance is a major concern, yet it is unclear what causes the relatively high densities of resistant bacteria in the anthropogenically impacted environment. There are various possible scenarios (hypotheses): (A) Input of resistant bacteria from wastewater and agricultural sources is significant, but they do not grow in the environment; (B) Input of resistant bacteria is negligible, but the resistant bacteria (exogenous or endogenous) grow due to the selection pressure of the antibiotic; (C) Exogenous bacteria transfer the resistance to the endogenous bacteria and those grow. This paper presents a simple mechanistic model of tetracycline resistance in the aquatic environment. It includes state variables for tetracyclines, susceptible and resistant bacteria, and particulate and dissolved organic matter in the water column and sediment bed. The antibiotic partitions between freely dissolved, dissolved organic matter (DOM)-bound and solids-bound phases, and decays. Bacteria growth is limited by DOM, inhibited by the antibiotic (susceptible bacteria only) and lower due to the metabolic cost of carrying the resistance (resistant bacteria only). Resistant bacteria can transfer resistance to the susceptible bacteria (conjugation) and lose the resistance (segregation). The model is applied to the Poudre River and can reproduce the major observed (literature data) patterns of antibiotic concentration and resistance. The model suggests observed densities of resistant bacteria in the sediment bed cannot be explained by input (scenario A), but require growth (scenarios B or C).201121556198
9004170.9993Shedding light on the bacterial resistance to toxic UV filters: a comparative genomic study. UV filters are toxic to marine bacteria that dominate the marine biomass. Ecotoxicology often studies the organism response but rarely integrates the toxicity mechanisms at the molecular level. In this study, in silico comparative genomics between UV filters sensitive and resistant bacteria were conducted in order to unravel the genes responsible for a resistance phenotype. The genomes of two environmentally relevant Bacteroidetes and three Firmicutes species were compared through pairwise comparison. Larger genomes were carried by bacteria exhibiting a resistant phenotype, favoring their ability to adapt to environmental stresses. While the antitoxin and CRISPR systems were the only distinctive features in resistant Bacteroidetes, Firmicutes displayed multiple unique genes that could support the difference between sensitive and resistant phenotypes. Several genes involved in ROS response, vitamin biosynthesis, xenobiotic degradation, multidrug resistance, and lipophilic compound permeability were shown to be exclusive to resistant species. Our investigation contributes to a better understanding of UV filters resistance phenotypes, by identifying pivotal genes involved in key pathways.202134760358
169180.9993Heavy metal resistance in Cupriavidus metallidurans CH34 is governed by an intricate transcriptional network. The soil bacterium Cupriavidus metallidurans CH34 contains a high number of heavy metal resistance genes making it an interesting model organism to study microbial responses to heavy metals. In this study the transcriptional response of strain CH34 was measured when challenged to sub-lethal concentrations of various essential or toxic metals. Based on the global transcriptional responses for each challenge and the overlap in upregulated genes between different metal responses, the sixteen metals were clustered in three groups. In addition, the transcriptional response of already known metal resistance genes was assessed, and new metal response gene clusters were identified. The majority of the studied metal response loci showed similar expression profiles when cells were exposed to different metals, suggesting complex interplay at transcriptional level between the different metal responses. The pronounced redundancy of these metal resistant regions-as illustrated by the large number of paralogous genes-combined with the phylogenetic distribution of these metal response regions within either evolutionary related or other metal resistant bacteria, provides important insights on the recent evolutionary forces shaping this naturally soil-dwelling bacterium into a highly metal-resistant strain well adapted to harsh and anthropogenic environments.201121706166
7348190.9993Diversity of integron- and culture-associated antibiotic resistance genes in freshwater floc. Clinically important antibiotic resistance genes were detected in culturable bacteria and class 1 integron gene cassettes recovered from suspended floc, a significant aquatic repository for microorganisms and trace elements, across freshwater systems variably impacted by anthropogenic activities. Antibiotic resistance gene cassettes in floc total community DNA differed appreciably in number and type from genes detected in bacteria cultured from floc. The number of floc antibiotic resistance gene cassette types detected across sites was positively correlated with total (the sum of Ag, As, Cu, and Pb) trace element concentrations in aqueous solution and in a component of floc readily accessible to bacteria. In particular, concentrations of Cu and Pb in the floc component were positively correlated with floc resistance gene cassette diversity. Collectively, these results identify suspended floc as an important reservoir, distinct from bulk water and bed sediment, for antibiotic resistance in aquatic environments ranging from heavily impacted urban sites to remote areas of nature reserves and indicate that trace elements, particularly Cu and Pb, are geochemical markers of resistance diversity in this environmental reservoir. The increase in contamination of global water supplies suggests that aquatic environments will become an even more important reservoir of clinically important antibiotic resistance in the future.201222467502