"Cocktail" of Xenobiotics at Human Relevant Levels Reshapes the Gut Bacterial Metabolome in a Species-Specific Manner

Toxicity Assessment of Environmental Liquid Crystal Monomers: A Bacteriological Investigation on Escherichia coli and Staphylococcus epidermidis

The widespread existence of liquid crystal monomers (LCMs) in various environmental matrices has been demonstrated, yet studies on the toxicological effects of LCMs are considerably scarce and are urgently needed to be conducted to assess the adverse impacts on ecology and human health. Here, we conducted a bacteriological study on two representative human commensal bacteria, Escherichia coli (E. coli) and Staphylococcus epidermidis (S. epidermidis), to investigate the effect of LCMs at human-relevant dosage and maximum environmental concentration on growth, metabolome, enzymatic activity, and mRNA expression. Microbial growth results exhibited that the highest inhibition ratio of LCMs on S. epidermidis reached 33.6% in our set concentration range, while the corresponding data on E. coli was only 14.3%. Additionally, LCMs showed more dose-dependent toxicity to S. epidermidis rather than E. coli. A novel in vivo solid-phase microextraction (SPME) fiber was applied to capture the in vivo metabolites of microorganisms. In vivo metabolomic analyses revealed that dysregulated fatty acid metabolism-related products of both bacteria accounted for >50% of the total number of differential substances, and the results also showed the species-specific and concentration-dependent metabolic dysregulation in LCM-exposed bacteria. The determination of enzymatic activity and mRNA relative expression levels related to oxidative stress confirmed our speculation that the adverse effects were related to the oxidative metabolism of fatty acids. This study complements the gaps in toxicity data for LCMs against bacteria and provides a new and important insight regarding metabolic dysregulation induced by environmental LCMs in human commensal bacteria.

Magnesium lithospermate B ameliorates diabetic nephropathy by suppressing the uremic toxin formation mediated by gut microbiota

Diabetic nephropathy (DN) is a major cause of renal failure and urgently necessitates new therapeutic strategies. Magnesium lithospermate B (MLB) showed a good protective effect on kidney injure by oral administration, despite its extremely low bioavailability. The current study aimed to investigate its gut microbiota-targeted mechanism to explain the paradoxical properties of pharmacodynamics and pharmacokinetics. Here we show that MLB alleviated DN by recovering the dysfunction of gut microbiota and their associated metabolites in colon content, such as short-chain fatty acids and amino acids. Moreover, MLB significantly decreased uremic toxin levels in plasma, especially the p-cresyl sulfate. We further discovered that MLB could affect the metabolism of p-cresyl sulfate by suppressing the formation of its intestinal precursors, i.e. the microbiota-mediated conversion from 4-hydroxyphenylacetate to p-cresol. In addition, the inhibition effects of MLB were confirmed. MLB and its metabolite danshensu exhibited inhibitory effects on p-cresol formation mediated by three strains belonging to the genus Clostridium, Bifidobacterium, and Fusobacterium, respectively. Meanwhile, MLB decreased the levels of p-cresyl sulfate in plasma and p-cresol in feces caused by rectal administration of tyrosine in mice. To summarize, the results indicated that MLB ameliorated DN through modulating gut microbiota-associated p-cresyl sulfate metabolism. Together, this study provides new insights on the microbiota-targeted mechanism of MLB in intervening DN and a new strategy in lowering plasma uremic toxins by blocking the formation of their precursors in intestine.

In vivo environmental metabolomic profiling via a novel microextraction fiber unravels sublethal effects of environmental norfloxacin in gut bacteria

Emerging contaminants (ECs), especially antibiotics, have significantly polluted the environment and threaten the living circumstance of organisms. Environmental metabolomic has emerged to investigate the sublethal effects of ECs. However, lacking noninvasive and real-time sample pretreatment techniques restricts its development in environmental toxicology. Hence, in this study, a real-time and in vivo untargeted analytical technique towards microbial endogenous metabolites was developed via a novel composite solid-phase microextraction (SPME) fiber of ZIF-67 and polystyrene to realize the high-coverage capture of living gut microbial metabolites. To reveal the exposure risks of typical antibiotic - norfloxacin (NFX) to gut bacteria, four representative bacteria were exposed to NFX at environmentally relevant levels. Using the proposed SPME fiber, 70 metabolites were identified to obtain an apparent metabolic separation feature between control and NFX-treated (10 ng/mL) microbial groups, which revealed that the low environmental relevant concentration of NFX would affect normal metabolism of gut bacteria. Additionally, NFX exhibited species-specific toxic effects on microbial growth, especially Escherichia coli displaying a distinct dose-dependent trend. Antioxidative enzymatic activities results demonstrated that beneficial bacteria maintained the state of oxidative stress while symbiotic bacteria suffered from oxidative stress injury under NFX contamination, further corroborating its impact on human intestinal health. This study highlights the suitability of in vivo SPME in the field of metabolite extraction and simultaneously possesses a brilliant application foreground in the environmental metabolomics.

Dose-response metabolomics and pathway sensitivity to map molecular cartography of bisphenol A exposure

In the toxicological regime, the toxicological endpoint and its dose-response relationship are two of the most prominent characters in conducting a risk assessment for chemical exposure. Systems biological methods have been used to comprehensively characterize the impact of toxicants on the biochemical pathways. However, the majority of the current studies are only based on single-dose, and limited information can be extrapolated to other doses from these experiments, regardless of the sensitivity of each endpoint. This study aims to understand the dose-response metabolite dysregulation pattern and metabolite sensitivity at the system-biological level. Here, we applied bisphenol A (BPA), an endocrine-disrupting chemical (EDC), as the model chemical. We first employed the global metabolomics method to characterize the metabolome of breast cancer cells (MCF-7) upon exposure to different doses (0, 20, 50, and 100 µM) of BPA. The dysregulated features with a clear dose-response relationship were also effectively picked up with an R-package named TOXcms. Overall, most metabolites were dysregulated by showing a significant dose-dependent behaviour. The results suggested that BPA exposure greatly perturbed purine metabolism and pyrimidine metabolism. Interestingly, most metabolites within the purine metabolism were described as a biphasic dose-response relationship. With the established dose-response relationship, we were able to fully map the metabolite cartography of BPA exposure within a wide range of concentrations and observe some unique patterns. Furthermore, an effective concentration of certain fold changes (e.g., EC₊₁₀ means the dose at which metabolite is 10% upregulated) and metabolite sensitivity were defined and introduced to this dose-response omics information. The result showed that the purine metabolism pathway is the most venerable target of BPA, which can be a potential endogenous biomarker for its exposure. Overall, this study applied the dose-response metabolomics method to fully understand the biochemical pathway disruption of BPA treatment at different doses. Both dose-response omics strategy and metabolite sensitivity analysis can be further considered and emphasized in future chemical risk assessments.

A review of environmental metabolism disrupting chemicals and effect biomarkers associating disease risks: Where exposomics meets metabolomics

Humans are exposed to an ever-increasing number of environmental toxicants, some of which have gradually been elucidated to be important risk factors for metabolic diseases, such as diabetes and obesity. These metabolism-sensitive diseases typically occur when key metabolic and signaling pathways were disrupted, which can be influenced by the exposure to contaminants such as endocrine disrupting chemicals (EDCs), along with genetic and lifestyle factors. This promotes the concept and research on environmental metabolism disrupting chemicals (MDCs). In addition, identifying endogenous biochemical markers of effect linked to disease states is becoming an important tool to screen the biological targets following environmental contaminant exposure, as well as to provide an overview of toxicity risk assessment. As such, the current review aims to contribute to the further understanding of exposome and human health and disease by characterizing environmental exposure and effect metabolic biomarkers. We summarized MDC-associated metabolic biomarkers in laboratory animal and human cohort studies using high throughput targeted and nontargeted metabolomics techniques. Contaminants including heavy metals and organohalogen compounds, especially EDCs, have been repetitively associated with metabolic disorders, whereas emerging contaminants such as perfluoroalkyl substances and microplastics have also been found to disrupt metabolism. In addition, we found major limitations in the effective identification of metabolic biomarkers especially in human studies, toxicological research on the mixed effect of environmental exposure has also been insufficient compared to the research on single chemicals. Thus, it is timely to call for research efforts dedicated to the study of combined effect and metabolic alterations for the better assessment of exposomic toxicology and health risks. Moreover, advanced computational and prediction tools, further validation of metabolic biomarkers, as well as systematic and integrative investigations are also needed in order to reliably identify novel biomarkers and elucidate toxicity mechanisms, and to further utilize exposome and metabolome profiling in public health and safety management.

Feeding state greatly modulates the effect of xenobiotics on gut microbiome metabolism: A case study of tetracycline

The human gut microbiome is crucial in modulating host health mostly through bacterial metabolites. Chemical exposure is typical external stress which alters its composition and functionality. To date, very few studies have investigated the effect of feeding state on chemical-induced gut microbial metabolic dysregulations. Here, we set up an in vitro human gut microbiome and incorporated a metabolomics approach to investigate the effect of tetracycline (TET) at multiple doses (i.e., 10, 1, and 0.01 mg/L) on gut microbiome under the fed and fasted states. Overall, the metabolome was highly responsive at the fed state with 62 metabolites dysregulated while only 14 were altered at the fasted state under 10 mg/L (clinical TET dose). As expected, nutrient consumption was significantly inhibited under clinical TET dose at the fed state accumulating nutrients such as glutamate and leucine. Interestingly, at the fed state, TET could increase the synthesis of indole and phenyl derivatives including indole-3-aldehyde and hydrocinnamate, while inhibiting indoxyl, tryptamine, and vitamin B production, all of which have host health implications. Furthermore, metabolites like indoxyl and xanthurenic acid were still responsive at 0.01 mg/L (dietary TET dose). Collectively, results demonstrated that the feeding state greatly modulates the chemical-induced gut microbial metabolic alterations.

Combined effect of microplastics and DDT on microbial growth: A bacteriological and metabolomics investigation in Escherichia coli

Microplastics (MPs) can adsorb toxic chemicals in biological or environmental matrixes and thus influence their behavior and availability. In order to investigate how the combined pollution of MPs and toxic organic chemical influence microbial growth and metabolism, Escherichia coli (E. coli) was grown in a complex, well-defined media and treated with polystyrene microplastics (PS MPs) and dichloro-diphenyl-tricgloroethane (DDT) at human relevant concentration levels. In vivo metabolites captured by a novel solid phase microextraction (SPME) probe, were used to reflect the metabolic dysregulation of E. coli under different pollution stresses. Results showed that the toxic effect of DDT displayed a distinct dose-dependent phenomenon while the existence of PS decreased the growth and metabolic interference effect of DDT on E. coli. Adsorption results revealed a mechanism that PS weakened the adverse impact of DDT by decreasing its free concentration in the treated culture media. Tricarboxylic acid (TCA) cycle related enzymes activities and antioxidant defense related substances of E. coli also proved the mechanism. The current study is believed to broaden our understanding of the ecotoxicity of MPs with toxic organic chemicals on microorganism.

Analysis of global human gut metagenomes shows that metabolic resilience potential for short-chain fatty acid production is strongly influenced by lifestyle

High taxonomic diversity in non-industrial human gut microbiomes is often interpreted as beneficial; however, it is unclear if taxonomic diversity engenders ecological resilience (i.e. community stability and metabolic continuity). We estimate resilience through genus and species-level richness, phylogenetic diversity, and evenness in short-chain fatty acid (SCFA) production among a global gut metagenome panel of 12 populations (n = 451) representing industrial and non-industrial lifestyles, including novel metagenomic data from Burkina Faso (n = 90). We observe significantly higher genus-level resilience in non-industrial populations, while SCFA production in industrial populations is driven by a few phylogenetically closely related species (belonging to Bacteroides and Clostridium), meaning industrial microbiomes have low resilience potential. Additionally, database bias obfuscates resilience estimates, as we were 2-5 times more likely to identify SCFA-encoding species in industrial microbiomes compared to non-industrial. Overall, we find high phylogenetic diversity, richness, and evenness of bacteria encoding SCFAs in non-industrial gut microbiomes, signaling high potential for resilience in SCFA production, despite database biases that limit metagenomic analysis of non-industrial populations.

Metabolomic and Transcriptomic Analysis of MCF-7 Cells Exposed to 23 Chemicals at Human-Relevant Levels: Estimation of Individual Chemical Contribution to Effects

BACKGROUND

Humans are constantly being exposed to various xenobiotics at relatively low concentrations. To date, limited evidence is available to ascertain whether a complex xenobiotic mixture at human-relevant levels causes any health effect. Moreover, there is no effective method to pinpoint the contribution of each chemical toward such an effect.

OBJECTIVES

This study aims to understand the responses of cells to a mixture containing 23 xenobiotics at human-relevant levels and develop a feasible method to decipher the chemical(s) that contribute significantly to the observed effect.

METHODS

We characterized the metabolome and transcriptome of breast cancer cells (MCF-7) before and after exposure to the mixture at human-relevant levels; preexposure levels were derived from existing large-scale biomonitoring data. A high-throughput metabolomics-based "leave-one-out" method was proposed to understand the relative contribution of each component by comparing the metabolome with and without the particular chemical in the mixture.

RESULTS

The metabolomic analysis suggested that the mixture altered metabolites associated with cell proliferation and oxidative stress. For the transcriptomes, gene ontology terms and pathways including "cell cycle," "cell proliferation," and "cell division" were significantly altered after mixture exposure. The mixture altered genes associated with pathways such as "genotoxicity" and "nuclear factor erythroid 2-related factor 2 (Nrf2)." Through joint pathways analysis, metabolites and genes were observed to be well-aligned in pyrimidine and purine metabolisms. The leave-one-out results showed that many chemicals made their contributions to specific metabolic pathways. The overall metabolome pattern of the absence of 2,4-dihyroxybenzophenone (DHB) or bisphenol A (BPA) showed great resemblance to controls, suggesting their higher relative contribution to the observed effect.

DISCUSSION

The omics results showed that exposure to the mixture at human-relevant levels can induce significant in vitro cellular changes. Also, the leave one out method offers an effective approach for deconvoluting the effects of the mixture. https://doi.org/10.1289/EHP6641.

Long-term oxytetracycline exposure potentially alters brain thyroid hormone and serotonin homeostasis in zebrafish

The impact of oxytetracycline (OTC) exposure in water on the fish still remains unclear. We hypothesized OTC exposure could alter fish gut microbiome and affect thyroid hormone and serotonin homeostasis in the brain via "chemical-gut-brain" axis. Here, ∼2-month-old juvenile zebrafish (Danio rerio) was exposed to two concentrations of OTC (1 and 100 μg/L) for one month until adulthood. Thyroxine-associated gene analysis in the brain revealed that deiodinase 2 (DIO2), deiodinase 3 (DIO3), and thyroid hormone receptor beta (THRβ) expression was significantly decreased. Quantification of thyroid hormones showed a decrease in triiodothyronine (T3) under OTC treatment, which agrees with reduced activity of DIO2. For the serotonin (5-HT) synthesis, the expression of tryptophan hydroxylase (TPH2) was 41 % and 9.3 % of the control group for 1 and 100 μg/L OTC exposed groups; respectively. The intestinal 16S rRNA analysis revealed an increased abundance of Fusobacteria and Proteobacteria, while Actinobacteria was decreased significantly. The altered microbial balance between Proteobacteria and Firmicutes have been previously reported to affect nutrient uptakes such as zinc, which can potentially reduce the activity of DIO2. In summary, this study suggests that long-term OTC exposure not only alters gut microbiome but also changes thyroid hormone and serotonin homeostasis.

Metabolic Signatures of the Exposome-Quantifying the Impact of Exposure to Environmental Chemicals on Human Health

Human health and well-being are intricately linked to environmental quality. Environmental exposures can have lifelong consequences. In particular, exposures during the vulnerable fetal or early development period can affect structure, physiology and metabolism, causing potential adverse, often permanent, health effects at any point in life. External exposures, such as the “chemical exposome” (exposures to environmental chemicals), affect the host’s metabolism and immune system, which, in turn, mediate the risk of various diseases. Linking such exposures to adverse outcomes, via intermediate phenotypes such as the metabolome, is one of the central themes of exposome research. Much progress has been made in this line of research, including addressing some key challenges such as analytical coverage of the exposome and metabolome, as well as the integration of heterogeneous, multi-omics data. There is strong evidence that chemical exposures have a marked impact on the metabolome, associating with specific disease risks. Herein, we review recent progress in the field of exposome research as related to human health as well as selected metabolic and autoimmune diseases, with specific emphasis on the impacts of chemical exposures on the host metabolome.

A high-throughput method to characterize the gut bacteria growth upon engineered nanomaterial treatment

Human are increasingly exposed to various types of engineered nanomaterials (ENMs) via dietary ingestion of nano-enabled food products, but these ENMs' impact on the gut bacteria health is still poorly understood. Current efforts in understanding the impact of these ENMs are hampered by their optical interferences in conventional quantification and viability assays, such as optical density and whole cell fluorescence staining assays. Therefore, there is a need to develop a more reliable bacteria quantification method in the presence of ENMs to effectively screen the potential adverse effects arising from the exposure of increasing ENMs on human gut microbiome. In this study, we developed a DNA-based quantification (DBQ) method in a 96-well plate format. Post-spiking method was used to correct the interference from ENMs on the reading. We showed the applicability of this method for several types of ENMs, i.e., cellulose nanofiber (CNF), graphene oxide (GO), silicon dioxide (SiO2), and chitosan, both in pure bacterial culture and in vitro human gut microbiome community. The detection limit for the highest dosing of CNF, GO, SiO2, and chitosan ENMs was approximately 0.18, 0.19, 0.05, and 0.24 as OD600, respectively. The method was also validated by a dose response experiment of E. coli with chitosan in the course of 8 hr. We believe that this method has great potential to be used in screening the effect of ENMs on the growth of gut bacteria or any other in vitro models and normalization for metabolites or proteins analysis.

Impact of Mixture Effects between Emerging Organic Contaminants on Cytotoxicity: A Systems Biological Understanding of Synergism between Tris(1,3-dichloro-2-propyl)phosphate and Triphenyl Phosphate

Humans are exposed to many xenobiotics simultaneously, but little is known about the toxic effects based on chemical-chemical interactions. This study aims at evaluating the binary interactions between 13 common environmental organic compounds (resulting in 78 pairs) by observing their cytotoxicity on HepG2 cells. Among all of the tested pairs, the combination of flame-retardant triphenyl phosphate (TPP) and tris(1,3-dichloro-2-propyl)phosphate (TDCPP) exhibited one of the most significant synergistic effects. We further characterized the transcriptome and metabolome after combined exposure to TPP and TDCPP and individual exposure. The results suggested that the coexposure caused many more changes in gene expressions and cellular activities. The transcriptome data showed that the coexposure triggered significant pathway changes including "cholesterol biosynthesis" and "ATF6-Alpha activated chaperone genes", together with distinct gene ontology (GO) terms such as the "negative regulation of the ERK1 and ERK2 cascade". Additionally, coexposure enhanced the biological activity of liver X receptors and nuclear factor erythroid 2-related factor 2 (Nrf2). The metabolome data showed that coexposure significantly elevated oxidative stress and affected the purine and pyrimidine metabolism. Overall, this study showed that interactions, which may enhance or suppress the biological processes, are common among environmental chemicals, although their environmental relevance should be studied in the future.

Long-term exposure to TET increases body weight of juvenile zebrafish as indicated in host metabolism and gut microbiome

The application of tetracycline (TET) is very common in medical treatment, fisheries, and animal husbandry, resulting in its frequent detection with abundant concentrations in the aquatic environment. Though the effects of TET on zebrafish (Danio rerio) at embryonic and larval stages have been reported, there is very limited information on the possible long-term effect on aquatic fishes at the juvenile stage, especially at environmentally relevant levels. In this study, we have exposed juvenile zebrafish to two levels of TET at 1 and 100 µg/L for one month until their adulthood. The result showed that both levels of TET can significantly increase the body weight of the zebrafish, while there is no change in the body length. TET exposure also affected the liver microstructure by lipid vacuoles generation and global lipidomics analysis revealed a significant upregulation in hepatic triglyceride (TAG) levels. The metabolomics analysis showed great dysregulations in hepatic metabolic pathways including linoleic acid metabolism, tyrosine metabolism, and methionine metabolism, which are known to be linked with increased body weight gain through hepatic lipid accumulation. The hepatic gene expression involved in lipid transport (e.g., apoa4 and fabp11) and lipogenic factors (e.g., ppar) have been significantly upregulated in the livers of TET exposed zebrafish. Interestingly, the 16 rRNA gene sequence-based zebrafish gut microbial community analysis revealed an enhanced community diversity and altered microbial community composition upon TET exposure. To our knowledge, this is the first study showing that TET exposure can increase the body weight in juvenile zebrafish and the study on the ecotoxicity of antibiotic occurrences in the aquatic system can be further warranted.

Efficient detection and assessment of human exposure to trace antibiotic residues in drinking water

Human exposure to antibiotic residues in drinking water has not been well evaluated. This study is the first attempt to simultaneously and efficiently identify and quantify 92 antibiotic residues in filtered tap water (multistage filtration at the tap) (n = 36) collected from 10 areas of a large city in southern China, 10 Chinese brands of bottled/barreled water (n = 30) and six foreign brands of bottled water (n = 18) obtained from the Chinese market. The average and median concentrations of all the detected antibiotic compounds was 182 and 92 ng/L in filtered tap water, 180 and 105 ng/L in Chinese brands of bottled/barreled water, and 666 and 146 ng/L in foreign brands of bottled water, respectively. A total of 58 antibiotics were detected in the filtered tap water, and 45 and 36 antibiotics were detected in the Chinese and foreign brands of bottled water, respectively. More types of antibiotics were detected in Chinese brands of bottled water than in the other bottled waters. In addition, Chinese waters had high roxithromycin concentrations, while the foreign brands of bottled water had high concentrations of dicloxacillin. The average and median values of the estimated overall daily intake of all the detected antibiotics were 4.3 and 2.3 ng/kg/day when only filtered tap water was drunk, 4.0 and 2.5 ng/kg/day when Chinese brands of bottled water was drunk, and 16.0 and 4.9 ng/kg/day when foreign brands of bottled water was drunk. Further study is needed to develop a more comprehensive estimation of human exposure to antibiotic residues in the environment and a more in-depth understanding of the potential hazard of ingested antibiotic residues to the human microbiome.

Low-dose tetracycline exposure alters gut bacterial metabolism and host-immune response: "Personalized" effect?

The human gut microbiome (GM) in healthy people is chronically exposed to tetracycline (TET) via environmental exposure and dietary uptake. However, limited information is available on its effect on the GM metabolome and effect on the host, especially at the dietary exposure level. Here, we investigated how TET at both sub-pharmaceutical and dietary exposure levels affects the metabolome and the secretome-induced host immune response by studying several representative gut bacteria. Interestingly, the metabolome showed a highly species-specific pattern with a distinct dose-response relationship. B. fragilis was highly sensitive to TET and vitamin, nucleotide, and amino acid metabolism pathways were the most vulnerable metabolic pathways at dietary exposure level. For key metabolite short chain fatty acids, TET significantly induced the synthesis of butyrate in B. fragilis, rather than C. sporogenes and E. coli. Furthermore, TET induced the release of lipopolysaccharides (LPS) in E. coli and enhanced the immune response; however, there was no obvious effect on B. fragilis. Interestingly, the overall immune response modulation with TET exposure relied on the ratio between E. coli and B. fragilis, possibly due to the neutralization of active LPS from E. coli by the LPS from B. fragilis. Overall, our results showed that the effect of TET from environmental exposure on the host health would be highly dependent on the GM composition, especially for the gut bacterial metabolome and secretome induced immune response.

Human health risk assessment of antibiotic resistance associated with antibiotic residues in the environment: A review

The extensive use of antibiotics leading to the rapid spread of antibiotic resistance poses high health risks to humans, but to date there is still lack of a quantitative model to properly assess the risks. Concerns over the health risk of antibiotic residues in the environment are mainly (1) the potential hazard of ingested antibiotic residues in the environment altering the human microbiome and promoting emergence and selection for bacteria resistance inhabiting the human body, and (2) the potential hazard of creating a selection pressure on environmental microbiome and leading to reservoirs of antibiotic resistance in the environment. We provide a holistic view of health risk assessment of antibiotic resistance associated with antibiotic residues in the environment in contrast with that of the antibiotic resistant bacteria and discuss the main knowledge gaps and the future research that should be prioritized to achieve the quantitative risk assessment. We examined and summarized the available data and information on the four core elements of antibiotic resistance associated with antibiotic residues in the environment: hazard identification, exposure assessment, dose-response assessment, and risk characterization. The data required to characterize the risks of antibiotic residues in the environment is severely limited. The main future research needs have been identified to enable better assessments of antibiotic resistance associated with antibiotic residues in the environment: (1) establishment of a standardized monitoring guide of antibiotic residues and antibiotic resistance in the environment, (2) derivation of the relationship between antibiotic levels and pathogenic antibiotic-resistance development in different settings, and (3) establishment of the dose-response relationship between pathogenic antibiotic resistant bacteria and various infection diseases. After identification of key risk determinant parameters, we propose a conceptual framework of human health risk assessments of antibiotic residues in the environment. CAPSULE: A holistic view of human health risk assessment of antibiotic residues in the environment was provided.