Concentrations, distribution, and key influencing factors of antibiotic resistance genes and bacterial community in water and reared fish tissues in a typical tilapia farm in South China.

Although previous studies have investigated the occurrence of antibiotic resistance genes (ARGs) in aquaculture, few have monitored the concentrations and propagation of ARGs in biological tissues or investigated the key factors influencing their spread in aquaculture. This study investigated the concentration, propagation, and distribution of ARGs and bacterial communities in water sources, pond water, and tilapia tissues, and their key influencing factors, in a typical tilapia farm. ErmF, sul1, and sul2 were the dominant ARGs with high concentrations. The total concentrations of ARGs (TCAs) in tilapia tissues decreased in the following order: stomach > scales > intestine > gills (P < 0.05). Redundancy analysis and multiple linear regression revealed that suspended solids (SS) and chemical oxygen demand (COD) were positively correlated with the dominant ARGs ermF sul2, and the TCAs (P < 0.05); additionally, Chloroflexi and Bacteroidetes in tilapia aquaculture water were positively correlated with the dominant ARGs ermF and sul2, as well as the TCAs (P < 0.05). This study suggests that SS and COD were the key factors driving the distribution and spread of ARGs in tilapia aquaculture water. Additionally, Chloroflexi and Bacteroidetes were the key bacterial flora affecting the propagation of ARGs in tilapia aquaculture systems.

Temporal variations, distribution, and dissemination of antibiotic resistance genes and changes of bacterial communities in a biofloc-based zero-water-exchange mariculture system.

Antibiotic resistance genes (ARGs) have obtained an increasing number of global concerns for the severe risks they pose to food safety and public health. Studies have investigated the concentrations and distribution of ARGs in the environment. However, the distribution and dissemination of ARGs, the bacterial communities, and the key influencing factors during the entire rearing period in the biofloc-based zero-water-exchange mariculture system (BBZWEMS) remain unclear. The current study investigated the concentrations, temporal variations, distribution, and dissemination of ARGs, the changes in the bacterial communities, as well as the key influencing factors during the rearing period in the BBZWEMS. Sul1 and sul2 were dominant ARGs. Total concentrations of ARGs followed a trend of decrease in pond water, while they followed a trend of increase in source water, biofloc, and shrimp gut. Total concentrations of targeted ARGs in the water source were higher than those in the pond water and biofloc samples for each corresponding rearing stage by 2.25-122.97-fold (p < 0.05). The bacterial communities in biofloc and pond water did not change much, while they changed considerably in the shrimp gut samples during the rearing period. Pearson correlation, redundancy analysis, and multivariable linear regression analysis showed that suspended substances and Planctomycetes were positively correlated with the concentrations of ARGs (p < 0.05). The current study indicates that the water source may be a critical source of ARGs, and that suspended substances is a key factor influencing the distribution and dissemination of ARGs in the BBZWEMS. Early intervention measures on ARGs in water sources should be implemented to aid in the prevention and control of resistance genes in aquaculture industry, and reduce the potential risks of ARGs to public health and food safety.

Metagenomic analysis of the abundances, diversity, and distribution of antibiotic resistance genes and their potential bacterial hosts in two types of shrimp-rearing farms in South China.

Antibiotic resistance genes (ARGs) are attracting increasing concern worldwide. Many previous studies have investigated the occurrence and concentrations of ARGs in aquaculture. However, the sources of ARGs and the links with their potential bacterial hosts have not yet been explored. This study investigated the abundances and diversity of ARGs in two types of shrimp farms in South China through metagenomic sequencing. In total, 14 ARG types were detected. Tetracycline was the dominant ARG type. The abundances of ARGs in samples decreased in the order of duck feces > water source > sediment > shrimp gut > pond water. The samples from the duck-shrimp integrated farm contained 1.29-3.81-fold more abundant ARGs than those from the shrimp monoculture farm (p < 0.05). Several ARGs, that were most predominant in the duck feces samples, were also the most predominant in the shrimp gut samples from the duck-shrimp integrated farm. Redundancy analysis indicated that the abundances and distribution of ARGs formed three clusters: duck feces, water samples, and sediment and shrimp gut samples. The dominant genera in duck feces known as human pathogenic bacteria were potential hosts of ARGs, and were also dominant in the shrimp gut samples in the duck-shrimp integrated farm. Additionally, the abundances of dominant genera in the shrimp gut samples of the duck-shrimp integrated farm were 1.74-35.07-fold higher than those in the shrimp monoculture farm (p < 0.01). The duck-shrimp integrated farm had 3.36-fold and 4.94-fold higher abundances of ARGs and mobile genetic elements in the shrimp gut samples than those from the shrimp monoculture farm, respectively (p < 0.05). The results indicate that duck feces may be a crucial source of diverse and abundant ARGs spreading to reared shrimps in duck-shrimp integrated farms, posing a severe risk to public health.

Toxicity of the microcystin-producing cyanobacteria Microcystis aeruginosa to shrimp Litopenaeus vannamei.

Microcystis aeruginosa is reported to cause cyanobacterial blooms in shrimp breeding ponds, which can result in significant shrimp mortality. However, the toxic effects of M. aeruginosa on Litopenaeus vannamei are still not completely understood. In this paper, the toxicity of M. aeruginosa cells to L. vannamei was examined, and the toxic components in the cells were analyzed through high-pressure liquid chromatography (HLPC). In addition, the immune response of shrimp to the M. aeruginosa cell extract was assessed by measuring the activity of immune-related enzymes, as well as the transcription of the relevant genes. The results showed that M. aeruginosa cells, extract and cell-free cultured medium resulted in a 100%, 98.3%, and 1.7% mortality rate in shrimp, respectively. HPLC analysis results revealed the presence of microcystin-LR (MC-LR) at a concentration of 190.40 mg/kg of cells. In addition, the activity and gene transcription of two immune related enzymes, SOD and LZM, were both significantly reduced in shrimp hepatopancreas (p < 0.05) after injection with extract. However, reduced glutathione (GSH) content was slightly increased, but the ratio of GSH to GSSG decreased. The transcription of gst gene function as detoxification, was significantly downregulated (p < 0.05). The results demonstrated that M. aeruginosa cell extract was highly toxic to L. vannamei, and exerted a negative effect on shrimp immunity including reduction of antioxidant capacity, antibacterial activity and detoxification activity, due to toxins including microcystin-LR.

Contamination of antibiotic resistance genes (ARGs) in a typical marine aquaculture farm: source tracking of ARGs in reared aquatic organisms.

Although the prevalence and concentrations of antibiotic resistance genes (ARGs) in aquaculture is receiving increasing scientific interest, there is little understanding of the direct sources and dissemination pathways of ARGs in marine aquaculture-reared organisms. This study investigated the dynamics of ARGs and the bacterial community throughout the rearing period in a typical marine aquaculture farm in South China. The results demonstrated that sul1 and qnrD were predominant in the sediment, and qnrD and qnrA were predominant in the intestinal tracts of shrimps. Network analysis showed that the chemical oxygen demand, total organic carbon, dissolved organic carbon, suspended solids, and total phosphorus were positively correlated with the predominant ARGs. The results of the network and source tracking analyses indicate that environmental factors and the bacterial community may drive the dissemination of ARGs dissemination in the environment and in shrimp reared by marine aquaculture, and sediment is the most direct and important medium in this dissemination. These results aid in improving our understanding of the sources, level, and dissemination of ARGs in marine aquaculture.

Uptake, Elimination, and Biotransformation Potential of a Progestagen (Cyproterone Acetate) in Tilapia Exposed at an Environmental Concentration.

Although the distribution of progestagens in aquatic environments has been widely reported, details on their uptake, elimination, and biotransformation in fish have received little attention. This study investigated the uptake, elimination, and biotransformation potential of a progestagen, cyproterone acetate (CPTA), in Nile tilapia ( Oreochromis niloticus) exposed to an environmentally relevant concentration under semistatic regimes. CPTA in tilapia tissues followed a similar pattern, reaching a concentration plateau within 4 days of exposure, and dropping to below limits of quantitation within 4 days of elimination. The calculated steady-state bioconcentration factors suggest a low bioconcentration potential of CPTA in juvenile tilapia. Results of enzymatic hydrolysis treatments revealed that no conjugates of CPTA were present in tissues, but conjugated biotransformation products of CPTA were found in bile, liver, and muscle. Most CPTA entered tissues and then was biotransformed into seven different products by phase I and phase II metabolism. The concentrations of endogenous cortisol were significantly influenced by CPTA in plasma and liver during the uptake period. These findings suggest that biotransformation products of CPTA should be considered for the assessment of the bioconcentration potential and ecological effects of progestagens.

Abundance and removal of antibiotic resistance genes (ARGs) in the rearing environments of intensive shrimp aquaculture in South China.

Although research regarding antibiotic resistance genes (ARGs) in aquaculture environments has gained increasing scientific interest, further studies are required to understand the abundances and removal mechanisms of ARGs during the entire rearing period of shrimp aquaculture. Thus, in this study, abundances, distributions and removal rates of ARGs in different environmental compartments of intensive shrimp farms in South China were investigated during the entire rearing period. The results indicated that sul1 and cmlA were the predominant ARGs in the water and sediment samples. Additionally, the total abundance of ARGs was higher in shrimp pond water than in the source water and farm effluent. Moreover, sediment samples indicated significantly higher ARG abundances than water samples from the shrimp ponds (P < 0.05). Environmental factors were found to significantly affect the distribution of ARGs in shrimp rearing environments. Furthermore, stable ponds aided the removal of ARGs from shrimp pond water. This study accounted for temporal variations in ARG abundances as well as removal of ARGs in different environmental compartments during the entire shrimp rearing period. However, additional research is required to optimize the water treatment process for removal of ARGs from the aquaculture.

Dissemination of antibiotic resistance genes in representative broiler feedlots environments: identification of indicator ARGs and correlations with environmental variables.

Livestock operations are known to harbor elevated levels of antibiotic resistance genes (ARGs) that may pose a threat to public health. Broiler feedlots may represent an important source of ARGs in the environment. However, the prevalence and dissemination mechanisms of various types of ARGs in the environment of broiler feedlots have not previously been identified. We examined the occurrence, abundance and variation of ARGs conferring resistance to chloramphenicols, sulfonamides and tetracyclines in the environments of two representative types of broiler feedlots (free range and indoor) by quantitative PCR, and assessed their dissemination mechanisms. The results showed the prevalence of various types of ARGs in the environmental samples of the broiler feedlots including manure/litter, soil, sediment, and water samples, with the first report of five chloramphenicol resistance genes (cmlA, floR, fexA, cfr, and fexB) in broiler feedlots. Overall, chloramphenicol resistance genes and sulfonamides sul genes were more abundant than tetracyclines tet genes. The ARG abundances in the samples from indoor boiler feedlots were generally different to the free range feedlots, suggesting the importance of feeding operations in ARG dissemination. Pearson correlation analysis showed significant correlations between ARGs and mobile genetic element genes (int1 and int2), and between the different classes of ARGs themselves, revealing the roles of horizontal gene transfer and coselection for ARG dissemination in the environment. Further regression analysis revealed that fexA, sul1 and tetW could be reliable indicator genes to surrogate anthropogenic sources of ARGs in boiler feedlots (correlations of fexA, sul1 and tetW to all ARGs: R = 0.95, 0.96 and 0.86, p < 0.01). Meanwhile, significant correlations were also identified between indicator ARGs and their corresponding antibiotics. In addition, some ARGs were significantly correlated with typical metals (e.g., Cu, Zn, and As with fexA, fexB, cfr, sul1, tetW, tetO, tetS: R = 0.52-0.71) and some environmental parameters (e.g., TOC, TN, TP, NH3-N with fexA, fexB, cfr, sul1, tetW, tetO, tetQ, tetS: R = 0.53-0.87) (p < 0.01). Further redundancy analysis demonstrated that the distribution and transportation of ARGs from the boiler feedlots to the receiving environments were correlated with environmental variables. The findings highlight the contribution of some chemicals such as antibiotics and metals to the development of ARGs in broiler feedlots environments; and the observed ARG dissemination mechanism in the broiler feedlots facilitates the development of effective mitigation measures.

Impact of Organic Carbons Addition on the Enrichment Culture of Nitrifying Biofloc from Aquaculture Water: Process, Efficiency, and Microbial Community.

In this study, we developed a rapid and effective method for enriching the culture of nitrifying bioflocs (NBF) from aquacultural brackish water. The self-designed mixotrophic mediums with a single or mixed addition of sodium acetate, sodium citrate, and sucrose were used to investigate the enrichment process and nitrification efficiency of NBF in small-scale reactors. The results showed that NBF with an MLVSSs from 1170.4 mg L-1 to 2588.0 mg L-1 were successfully enriched in a period of less than 16 days. The citrate group performed the fastest enrichment time of 10 days, while the sucrose group had the highest biomass of 2588.0 ± 384.7 mg L-1. In situ testing showed that the highest nitrification efficiency was achieved in the citrate group, with an ammonia oxidation rate of 1.45 ± 0.34 mg N L-1 h-1, a net nitrification rate of 2.02 ± 0.20 mg N L-1 h-1, and a specific nitrification rate of 0.72 ± 0.14 mg N g-1 h-1. Metagenomic sequencing revealed that Nitrosomonas (0.0~1.0%) and Nitrobacter (10.1~26.5%) were dominant genera for AOB and NOB, respectively, both of which had the highest relative abundances in the citrate group. Linear regression analysis further demonstrated significantly positive linear relations between nitrification efficiencies and nitrifying bacterial genera and gene abundance in NBF. The results of this study provide an efficient enrichment culture method of NBF for the operation of biofloc technology aquaculture systems, which will further promote its wide application in modern intensive aquaculture.

Antibiotic resistance and genetic diversity of Escherichia coli isolates from traditional and integrated aquaculture in South China.

This study investigated antibiotic resistance profiles including antibiotic resistance frequencies, resistance genes and resistance patterns in Escherichia coli strains isolated from traditional and integrated aquaculture systems in South China by using antibiotic susceptibility testing and real time polymerase chain reaction (PCR) technique. The E. coli isolates were found to be resistant to at least one antibiotic among 12 antibiotics. Higher resistance frequencies to ampicillin, sulfamethoxazole, trimethoprime, streptomycin and tetracycline were found compared to the rest antibiotics. Among the 10 tetracycline resistance genes detected in the resistant isolates, the most prevalent tetracycline resistance genes were tetA, tetW and tetB with the frequency of 69.7%, 63.5% and 21.9%, respectively. Three sulfonamide resistance genes were detected in these resistant isolates, with their detection frequencies in the following order: sul2 (55.3%) > sul3 (28.2%) > sul1 (6.2%). Four resistance genes mainly encoding extended-spectrum ß-lactamases (ESBLs) were detected in these resistant isolates, with the detection frequencies of blaTEM (28.4%) > blaOXA (9.7%) > blaCTX (9.3%) > blaCARB (5.2%) > blaSHV (0.0%). It was found that the integrated aquaculture system exhibited generally higher prevalence of antibiotic resistance than the traditional aquaculture system. An integrated aquaculture system could facilitate development of bacterial resistance and spread of the antibiotic resistance genes, and consequently become an important reservoir of resistance genes.

Effect of bacterial community succession on environmental factors during litter decomposition of the seaweed Gracilaria lemaneiformis.

In large-scale seaweed farming, an understanding of the decomposition process plays a pivotal role in optimizing cultivation practices by considering the influence of the bacterial community. Therefore, we assessed the bacterial community structure and its influence on environmental factors during Gracilaria lemaneiformis decomposition, utilizing both microcosms and in-situ simulations. The decomposition rates in the microcosms and in situ simulations reached 79 % within 180 days and 81 % within 50 days, respectively In the microcosms, the dissolved oxygen content decreased from 5.3 to 0.4 mg/L, while the concentrations of total organic carbon, nitrogen, and phosphorus in the water increased by 165 %, 1636 %, and 2360 %, respectively. The common dominant bacteria included Proteobacteria, Planctomycetes, Firmicutes, Bacteroidetes, and Spirochaetae. Planctomycetes and Firmicutes were positively correlated with the total organic carbon, nitrogen, and phosphorus concentrations. Planctomycetes species played significant roles during the decomposition process. The overall findings of this study could inform more sustainable seaweed cultivation practices.

Determination of biocides in different environmental matrices by use of ultra-high-performance liquid chromatography-tandem mass spectrometry.

A sensitive and robust method using solid-phase extraction and ultrasonic extraction for preconcentration followed by ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS-MS) has been developed for determination of 19 biocides: eight azole fungicides (climbazole, clotrimazole, ketoconazole, miconazole, fluconazole, itraconazole, thiabendazole, and carbendazim), two insect repellents (N,N-diethyl-3-methylbenzamide (DEET), and icaridin (also known as picaridin)), three isothiazolinone antifouling agents (1,2-benzisothiazolinone (BIT), 2-n-octyl-4-isothiazolinone (OIT), and 4,5-dichloro-2-n-octyl-isothiazolinone (DCOIT)), four paraben preservatives (methylparaben, ethylparaben, propylparaben, and butylparaben), and two disinfectants (triclosan and triclocarban) in surface water, wastewater, sediment, sludge, and soil. Recovery of the target compounds from surface water, influent, effluent, sediment, sludge, and soil was mostly in the range 70-120%, with corresponding method quantification limits ranging from 0.01 to 0.31 ng L(-1), 0.07 to 7.48 ng L(-1), 0.01 to 3.90 ng L(-1), 0.01 to 0.45 ng g(-1), 0.01 to 6.37 ng g(-1), and 0.01 to 0.73 ng g(-1), respectively. Carbendazim, climbazole, clotrimazole, methylparaben, miconazole, triclocarban, and triclosan were detected at low ng L(-1) (or ng g(-1)) levels in surface water, sediment, and sludge-amended soil. Fifteen target compounds were found in influent samples, at concentrations ranging between 0.4 (thiabendazole) and 372 ng L(-1) (methylparaben). Fifteen target compounds were found in effluent samples, at concentrations ranging between 0.4 (thiabendazole) and 114 ng L(-1) (carbendazim). Ten target compounds were found in dewatered sludge samples, at concentrations ranging between 1.1 (DEET) and 887 ng g(-1) (triclocarban).

Use of TIE techniques to characterize industrial effluents in the Pearl River Delta region.

We investigated the acute toxicity of various industrial effluents in the Pearl River Delta region using lux bacteria, duckweed, green algae, crustaceans and zebrafish. The potential toxicants in the industrial effluents were identified and evaluated by lux bacteria bioassay and chemical analysis. The results show that green algae (Pseudokirchneriella subcapitata) and crustacean (Ceriodaphnia dubia) were more sensitive to the effluents from electronic and electroplate factories than other test species, while lux bacteria were more sensitive to all the other effluents. The toxicities of effluents from electronic and electroplate factories to the six test organisms were significantly higher than those of the other industrial effluents, and mainly caused by metals. Noticeably, organic pollutants were the main contributing factor to the toxicity of effluents from textile and dyeing plants, pulp and paper mills, fine chemical factories and municipal wastewater treatment plants.

Effect of Input C/N Ratio on Bacterial Community of Water Biofloc and Shrimp Gut in a Commercial Zero-Exchange System with Intensive Production of Penaeus vannamei.

Although increasing attention has been attracted to the study and application of biofloc technology (BFT) in aquaculture, few details have been reported about the bacterial community of biofloc and its manipulation strategy for commercial shrimp production. An 8-week trial was conducted to investigate the effects of three input C/N ratios (8:1, 12:1 and 16:1) on the bacterial community of water biofloc and shrimp gut in a commercial BFT tank system with intensive aquaculture of P. vannamei. Each C/N ratio group had three randomly assigned replicate tanks (culture water volume of 30 m3), and each tank was stocked with juvenile shrimp at a density of 300 shrimp m-3. The tank systems were operated with zero-water exchange, pH maintenance and biofloc control. During the trial, the microbial biomass and bacterial density of water biofloc showed similar variation trends, with no significant difference under respective biofloc control measures for the three C/N ratio groups. Significant changes were found in the alpha diversity, composition and relative abundance of bacterial communities across the stages of the trial, and they showed differences in water biofloc and shrimp gut among the three C/N ratio groups. Meanwhile, high similarity could be found in the composition of the bacterial community between water biofloc and shrimp gut. Additionally, nitrogen dynamics in culture water showed some differences while shrimp performance showed no significant difference among the three C/N ratio groups. Together, these results confirm that the manipulation of input C/N ratio could affect the bacterial community of both water biofloc and shrimp gut in the environment of a commercial BFT system with intensive production of P. vannamei. Moreover, there should be different operations for the nitrogen dynamics and biofloc management during shrimp production process under different C/N ratios.

Diversity, abundances and distribution of antibiotic resistance genes and virulence factors in the South China Sea revealed by metagenomic sequencing.

Antibiotic resistance genes (ARGs) and virulence factors (VFs) pose considerable health risks to humans. The occurrence and abundance of several typical ARGs in the sea have been widely investigated. However, the full profiles and abundances of the antibiotic resistome and VFs in the South China Sea remain unexplored. Therefore, in this study, we investigated the full profiles of the ARGs and VFs, as well as their abundances and distribution, in the South China Sea using metagenomic approaches. In total, 140 ARG subtypes and 155 VFs were detected. The most abundant ARG was multidrug resistance gene, followed by bacitracin resistance gene. Flagella was the most abundant VF. Pearson correlation analysis revealed a strong and positive correlation between the abundances of ARGs and VFs. Redundancy analysis and co-occurrence network analysis showed that the predominant VFs were positively correlated with the predominant ARGs in the South China Sea. Nonmetric multidimensional scaling and Procrustes analyses demonstrated that the sampling sites were clustered into three compartments according to the geographical location, i.e., offshore, open sea, and reef zones. The abundances of ARGs and VFs in the offshore zone were much higher than those in the open sea and reef zones (p < 0.05). Several physico-chemical factors most closely associated with anthropogenic activities, i.e., nitrate, lead, copper, and zinc, were positively correlated with the predominant ARGs and VFs in the South China Sea. Our results suggest that the ocean is a large reservoir of diverse and abundant ARGs and VFs, which may threaten human health and seafood safety. These findings improve the understanding of the relationship between ARG dissemination and intensive anthropogenic activities and can aid in improving ocean management and seafood product safety.

Occurrence of antibiotic resistance and characterization of resistance genes and integrons in Enterobacteriaceae isolated from integrated fish farms in South China.

Antibiotics are still widely applied in animal husbandry to prevent diseases and used as feed additives to promote animal growth. This could result in antibiotic resistance to bacteria and antibiotic residues in animals. In this paper, Enterobacteriaceae isolated from four integrated fish farms in Zhongshan, South China were tested for antibiotic resistance, tetracycline resistance genes, sulfonamide resistance genes, and class 1 integrons. The Kirby-Bauer disk diffusion method and polymerase chain reaction (PCR) assays were carried out to test antibiotic susceptibility and resistance genes, respectively. Relatively high antibiotic resistance frequencies were found, especially for ampicillin (80%), tetracycline (52%), and trimethoprim (50%). Out of 203 Enterobacteriaceae isolates, 98.5% were resistant to one or more antibiotics tested. Multiple antibiotic resistance (MAR) was found highest in animal manures with a MAR index of 0.56. Tetracycline resistance genes (tet(A), tet(C)) and sulfonamide resistance genes (sul2) were detected in more than 50% of the isolates. The intI1 gene was found in 170 isolates (83.7%). Both classic and non-classic class 1 integrons were found. Four genes, aadA5, aadA22, dfr2, and dfrA17, were detected. To our knowledge, this is the first report for molecular characterization of antibiotic resistance genes in Enterobacteriaceae isolated from integrated fish farms in China and the first time that gene cassette array dfrA17-aadA5 has been detected in such fish farms. Results of this study indicated that fish farms may be a reservoir of highly diverse and abundant antibiotic resistant genes and gene cassettes. Integrons may play a key role in multiple antibiotic resistances posing potential health risks to the general public and aquaculture.

Spatial and seasonal distribution of selected antibiotics in surface waters of the Pearl Rivers, China.

The distribution and occurrence of 15 antibiotics in surface water of the Pearl River System (Liuxi River, Shijing River and Zhujiang River) and effluents of four wastewater treatment plants (WWTPs) were investigated in two sampling events representing wet season and dry season by using rapid resolution liquid chromatography-electrospray tandem mass spectrometry (RRLC-MS/MS) in positive ionization mode. Only eight antibiotics (sulfadiazine, sulfapyridine, sulfamethazine, sulfamethoxazole, trimethoprim, roxithromycin, erythromycin-H2O and norfloxacin) were detected in the water samples of the three rivers and the effluents. The detection frequencies and levels of antibiotics in the dry season were higher than those in the wet season. This could be attributed to the dilution effects in the wet season and relatively lower temperature in the dry season under which antibiotics could persist for a longer period. The levels of the detected antibiotics in different sites are generally in a decreasing order as follows: Shijing River ≥WWTP effluent ≥Zhujiang River ≥ Liuxi River. Risk assessment based on the calculated risk quotients showed that only erythromycin-H2O and roxithromycin detected in the Pearl Rivers might have adverse effects on aquatic organisms.

Removal of antibiotic resistance genes and inactivation of antibiotic-resistant bacteria by oxidative treatments.

The persistence of antibiotics in the environment because of human activities, such as seafood cultivation, has attracted great attention as they can give rise to antibiotic resistance genes (ARGs) and antibiotic-resistant bacteria (ARB). In this study, we explored the inactivation and removal efficiencies of Escherichia coli SR1 and sul1 (plasmid-encoded ARGs), respectively, in their extracellular and intracellular forms (eARGs and iARGs) by three commonly used fishery oxidants, namely chlorine, bromine, and potassium permanganate (KMnO4), at the practical effective concentration range (0.5, 5, and 15 mg/L). Kinetics data were obtained using laboratory phosphate-buffered saline (PBS). Following the same fishery oxidation methods, the determined kinetics models were tested by studying the SR1 and sul1 disinfection efficiencies in (sterilized) pond water matrix. At concentrations of 5 and 15 mg/L, all three oxidants achieved sufficient cumulative integrated exposure (CT values) to completely inactivate SR1 and efficiently remove sul1 (up to 4.0-log). The oxidation methods were then applied to an unsterilized pond water matrix in order to study and evaluate the indigenous ARB and ARGs disinfection efficiencies in aquaculture, which reached 1.4-log and 1.0-log during treatment with fishery oxidants used in pond preparation at high concentrations before stocking (5-15 mg/L), respectively. A high chlorine concentration (15 mg/L) could efficiently remove ARGs (or iARGs) from pond water, and the iARG removal efficiency was higher than that of eARGs in pond water. The method and results of this study could aid in guiding future research and practical disinfection to control the spread of ARGs and ARB in aquaculture.

Spatiotemporal variations and source tracking of antibiotics in an ecological aquaculture farm in Southern China.

Although the occurrence and distribution of antibiotics in aquatic environments and reared organisms have been widely reported, the spatiotemporal variations and sources of antibiotics throughout the rearing period of aquaculture remain unclear. In this study, the concentrations and spatiotemporal variations of antibiotics in water sources, pond water, sediment, feed, and reared shrimp samples during three rearing periods in an ecological shrimp farm in Southern China were investigated. The water, sediment, and feed samples were found to contain twelve, nine, and four types of antibiotics, respectively, and the concentration of erythromycin-H2O was the highest among these antibiotics. No target antibiotics were detected in the reared shrimp samples from this typical shrimp farm, which employed ecological rearing with no antibiotic use throughout the rearing processes. The total concentrations of antibiotics in water source were 1.96-40.58 times higher than those in pond water. A significant decrease in the total antibiotic concentrations of the pond water was observed, while a significant increase was observed in sediment during each rearing period (p < 0.05), suggesting that antibiotics transferred from the water phase to the sediment phase in the farm. Redundancy analysis demonstrated that the chemical oxygen demand was negatively correlated with the concentration of the target antibiotics in the water samples during three rearing periods (p < 0.05). The results of calculations conducted using the concentrations of antibiotics in the water source, pond water, sediment, and feed samples detected in this study indicated that the water source was likely to be the main source of antibiotics in the rearing ponds. This study can provide a better understanding of the spatiotemporal variations and sources of antibiotics in aquaculture.

Spatial and seasonal variations of antibiotics and antibiotic resistance genes and ecological risks in the coral reef regions adjacent to two typical islands in South China Sea.

Although the occurrence of antibiotics and antibiotic resistance genes (ARGs) in aquatic environmental has been widely reported, the distribution and variations of these emerging contaminants in the coral reef regions remain unclear. This study investigated the occurrence of these contaminants, and their spatial and seasonal variations in both coral reef regions and non-coral reef regions adjacent to two typical islands in the South China Sea. Eighteen antibiotics and seven ARGs were detected in the surface water with total concentrations ranging from 43.2 to 441 ng/L, and 2.11 × 104 to 8.00 × 106 copies/L, respectively. Erythromycin-H2O was the most dominant antibiotic in all samples. QnrD was dominant in the dry season, whereas sul1, sul2, and floR were the most abundant in the wet season, indicating obvious seasonal variations. The distribution of ARGs was mainly influenced by changes in salinity caused by anthropogenic activities in wet season.