Antimicrobial resistance in aquaculture: Occurrence and strategies in Southeast Asia

Isolation, characterization, and application of a novel Vibrio parahaemolyticus bacteriophage from retail shrimp in Sarawak, Malaysia

Shrimp farming, a highly profitable sector in global aquaculture, has seen remarkable growth in recent years. This increasing demand and the expansion of farming operations, including in Sarawak, Malaysia, highlight the sector's potential. However, the industry faces significant challenges, particularly the prevalence of vibriosis, a bacterial infection caused by Vibrio species. Contamination of food products has also increased the risk of vibriosis in humans. The widespread use of antibiotics to combat this disease has led to the rapid emergence of antimicrobial resistance (AMR) bacteria. This study specifically focuses on the isolation and characterization of phage EniLVP02, a novel bacteriophage with the potential to combat V. parahaemolyticus infections. EniLVP02 was successfully isolated from shrimp purchased at a retail market and exhibited strong lytic activity against V. parahaemolyticus strains. Structural analysis categorized EniLVP02 within the Straboviridae family, belonging to the class Caudoviricetes. The phage displayed a narrow host range and lytic nature only towards V. parahaemolyticus strains isolated from the Telaga Air shrimp farm. Phage EniLVP02 exhibited long latent period of 120 min and large burst size of 144 phages per infected cells. Stability studies revealed EniLVP02's resilience across various pH (pH 4.0-9.0) and temperature (28 °C-65 °C) conditions, particularly at physiological temperatures. Comparative genome analyses indicated its distinct evolutionary relationship and low homology with other Vibriophages, suggesting its novelty. EniLVP02 demonstrated significant potential in biofilm prevention and destruction, with absorbance (OD600 nm) reduction from 0.592 ± 0.055 to 0.204± 0.016 and from 0.843± 0.003 to 0.174± 0.026 respectively. Moreover, in the treatment of V. parahaemolyticus-contaminated shrimp meat, EniLVP02 effectively inhibit bacterial concentrations by 75.2 % at room temperature and 16.2 % at 4 °C after 24 h. Genomic sequencing revealed low similarity between EniLVP02 with other phages, suggesting its novelty. Importantly, the absence of lysogeny-related, antibiotic resistance, and virulence genes in its genome supports EniLVP02's safety for therapeutic use. This study underscores the importance of exploring phages from retail food products for therapeutic applications and highlights the promising attributes of phage EniLVP02 in combating V. parahaemolyticus infections in aquaculture. Further investigations on its compatibility with other phages and application in diverse food matrices are warranted to assess its full potential.

Antibiotic erythromycin in fish: Pharmacokinetics, effects, and health risks

Erythromycin is a macrolide antibiotic commonly utilized in veterinary medicine and aquaculture. It functions by binding to the 50S subunit of 70S ribosomes, inhibiting protein synthesis and effectively treating numerous bacterial diseases. Due to the extensive use of erythromycin, it has been detected in various aquatic systems in recent years. Multiple studies have reported the occurrence of erythromycin resistance and its adverse effects on diverse aquatic organisms. Consequently, potential environmental health risks associated with erythromycin have garnered increasing attention. As an integral component of aquatic ecosystems, fish have been the subject of numerous reports regarding the bioaccumulation and adverse effects of erythromycin; however, these data have not been collated and interpreted. This report provides a comprehensive overview of the environmental fate of erythromycin, detection methods, pharmacokinetics, and impacts on fish. In addition to the therapeutic benefits against pathogens, acute or chronic exposure of fish to erythromycin at concentrations ranging from μg/L to mg/L disrupts the primary defense, antioxidant, and xenobiotic metabolism systems, leading to oxidative stress, cellular structural damage, and metabolic disorders, manifesting as cytotoxicity, organ toxicity, neurotoxicity, developmental toxicity, and reproductive toxicity. However, further in-depth studies are warranted to evaluate the therapeutic efficacy at relatively high levels, particularly when considering pathogens with developed resistance to erythromycin, as well as the long-term effects of erythromycin exposure at environmentally relevant concentrations in fish, thereby better assessing the health risks posed by erythromycin to fish and their consumers humans.

Inhibition of quorum sensing, biofilm formation, and virulence-related characteristics in shrimp pathogenic Vibrio campbellii by rambutan (Nephelium lappaceum L. cv. Rong Rian) peel extract

Vibrio campbellii is a significant pathogen in shrimp aquaculture, causing luminous vibriosis and leading to considerable declines in productivity and quality. The rapid emergence of multi-drug and detergent-resistant strains presents a major challenge in controlling this pathogen. This study investigates the inhibitory effects of rambutan peel extract (RPE) on quorum sensing (QS) systems, biofilm formation, and virulence-related traits in V. campbellii. The minimum inhibitory concentration (MIC) of RPE was found to be 2048 μg/ml for the pathogenic strain V. campbellii HY01 and 1024 μg/ml for the non-pathogenic QS reporter strains. Sub-inhibitory concentrations significantly reduced bioluminescence in V. campbellii, indicating interference with QS systems, particularly harveyi autoinducer-1 (HAI-1) and autoinducer-2 (AI-2). RPE disrupted autoinducer detection, down-regulated the expression of QS sensor genes, inhibited phosphorylation, and affected the transcription of QS regulator AphA. Additionally, RPE reduced biofilm formation, swimming motility, caseinase production, and virulence gene expression in the shrimp pathogenic strain HY01. These findings demonstrate the strong anti-QS activity of RPE against V. campbellii by targeting QS systems, phosphorylation pathways, and the master QS regulator. The study highlights the potential of RPE as a sustainable approach to control luminous vibriosis, offering a promising strategy for managing disease outbreaks and improving shrimp health in aquaculture.

Investigating the impact of organic matter on Vibrio parahaemolyticus inactivation in aquaculture water by UV-LED system

Ultraviolet (UV) irradiation becomes a promising technology in inactivating pathogenic microbes, but the compositional change of organics and its consequence of inactivation need further study in raw water during UV light-emitting diode (UV-LED) irradiation. Herein, the bench-scale study aimed at evaluating the effect of organic fractions isolated from shrimp-farming water on the inactivation efficiency of Vibrio parahaemolyticus using UV-LED process at wavelengths of 265 nm, 280 nm, and combined wavelengths. The lowest required UV fluence (4.06 mJ/cm2) for 3-log inactivation was attained with UV-LED 280 nm. After UV irradiation the changes in elemental compositions of organic compounds, based on H/C and O/C ratios, were small. This is probably due to low UV exposure and UV resistant structure of organic constituents, predominantly lipid-like compounds. Contrarily, fluorescent spectroscopic analysis that showed degradation of protein-like substances by UV irradiation. In addition, the significant declines in the number of chemical formulas in organic compounds were identified through non-target screening using orbitrap mass spectrometry, suggesting degradation and amalgamation into new compounds. The presence of organic compounds did not profoundly affect inactivation efficiency at applying a minimum required fluence or greater. This study highlights the potential of UV-LED irradiation, particularly at 280 nm, for efficient inactivation of V. parahaemolyticus and subsequent molecular structure alteration of organic matter after UV irradiation.

Attenuating Vibrio harveyi Virulence Through Quorum Sensing Interference Using Piperine: An In Vitro and In Silico Approach

Aquaculture diseases caused by pathogens such as Vibrio spp., amplified by the emergence of antibiotic resistance, threaten the aquaculture industry. Due to its critical role in regulating the expression of genes related to antibiotic resistance and virulence, quorum sensing (QS) was proved to be an ideal target in disease control. In this study, we investigated the effects of the alkaloid piperine on the QS system of a fish pathogenic Vibrio harveyi strain. In vitro assays showed that piperine inhibits biofilm formation of V. harveyi without affecting their growth. Moreover, piperine specifically reduced QS activity in V. harveyi, as evident in the inhibition of biofilm and bioluminescence, likely through the AI-2 pathway. Molecular docking simulations showed significant binding energies between piperine and QS proteins-LuxP, LuxQ, LuxR and LuxS-revealing competitive inhibitory effects against LuxP, LuxR, and LuxS, and non-competitive interactions with LuxQ. This study demonstrated the effects of piperine against V. harveyi and elucidated its mechanism of action against V. harveyi QS, implying its potential application in aquaculture systems.

Flexible silver-metal-organic framework probe for highly sensitive and visual detection of tetracycline hydrochloride in freshwater fish

Tetracycline hydrochloride (TCH) is commonly used in aquaculture to prevent bacterial infections, renders the detection of its residues highly crucial for ensuring the safety of aquatic products. In this study, an Ag-based metal-organic framework, Ag-dcp (H3dcp = 3-(3,5-dicarboxyphenyl)-6-carboxypyridine), was synthesized through hydrothermal method. The Ag-dcp framework offers environmental advantages, including a green synthesis process and potential for sustainable, eco-friendly applications. The uncoordinated carboxyl groups in Ag-dcp provide potential interaction sites for TCH, while the rhombic channels in the flexible structure of Ag-dcp that match the size of TCH effectively facilitate host-guest interactions. The Ag-dcp enables ratiometric fluorescence detection with high sensitivity via intramolecular charge transfer (ICT) triggered by TCH-induced ether bond rotation. The probe demonstrates a fluorescence color changes from blue to orange, achieving a detection limit as low as 0.14 μM and offering an ultrafast response time of just 10 s. Principal component analysis (PCA) was employed to effectively distinguish TCH from other tetracycline analogs (minocycline hydrochloride (MH) and doxycycline hydrochloride (DOX)), thereby enhancing the selectivity of the Ag-dcp probe. The fluorescence probe demonstrated performance comparable to HPLC in detecting TCH in freshwater fish. The detection strategy proposed here, based on the MOF structure, offers valuable insights for designing MOF materials and their specialized applications.

Evaluation of public health impact risks associated with bacterial antimicrobial resistome in tropical coastal environments

This study investigated antimicrobial resistance genes (ARGs) and microbial communities in Singapore's tropical coastal environments, comparing natural marine waters with aquaculture sites using metagenomic analysis. Results show consistently low ARG levels in natural ecosystems, whereas aquaculture sites exhibit elevated ARG concentrations, particularly those relevant to human pathogens, with considerably temporal fluctuations likely driven by human activities and seasonal factors. Additionally, aquaculture sites were found to host mobile genetic elements (MGEs) that may facilitate ARG spread, identifying aquaculture as a key reservoir of resistance genes with potential public health implications. These findings underscore distinct prevalence of ARG between natural and managed marine environments and highlight the need for continued surveillance to monitor ARG dynamics in coastal areas. By elucidating the potential for ARGs to transfer from aquaculture environments to the human food chain, this research emphasizes the importance of sustainable aquaculture practices and proactive ARG management strategies to mitigate antimicrobial resistance risks posed by tropical coastal ecosystem.

High risk of Vibrio pathogen and antibiotic resistance transfer in live seafood wet markets of Shantou, China

The global demand for seafood necessitates robust food safety practices, particularly within traditional wet markets. This study investigated the microbiomes of live Japanese mantis shrimp (JMS) and their associated environments (water and biofilm) in local wet markets to assess the risk of pathogen and antibiotic resistance gene (ARG) transfer. Metagenomic analysis showed a significant link between microbiome composition and the type of sample (shrimp, biofilm, and water). While several known human pathogens were associated with shrimp samples, water and biofilm samples exhibited higher abundances of ARGs, suggesting a high risk of pathogen and ARG transfer from the market environment. Notably, this study focused on the diversity and characterization of poorly understood Vibrio species associated with JMS. The prevalence of β-lactam resistance genes in Vibrio isolates, combined with a comparative genomic analysis of several species, highlights this concern. Our study emphasizes the need to improve hygiene practices in wet markets to reduce foodborne illness risks and address antibiotic resistance. This work represents, to our knowledge, the first comparative genomic analysis of Vibrio species in the context of JMS and wet market seafood safety.

Magnesium modulates phospholipid metabolism to promote bacterial phenotypic resistance to antibiotics

Non-inheritable antibiotic or phenotypic resistance ensures bacterial survival during antibiotic treatment. However, exogenous factors promoting phenotypic resistance are poorly defined. Here, we demonstrate that Vibrio alginolyticus are recalcitrant to killing by a broad spectrum of antibiotics under high magnesium. Functional metabolomics demonstrated that magnesium modulates fatty acid biosynthesis by increasing saturated fatty acid biosynthesis while decreasing unsaturated fatty acid production. Exogenous supplementation of unsaturated and saturated fatty acids increased and decreased bacterial susceptibility to antibiotics, respectively, confirming the role of fatty acids in antibiotic resistance. Functional lipidomics revealed that glycerophospholipid metabolism is the major metabolic pathway remodeled by magnesium, where phosphatidylethanolamine biosynthesis is reduced and phosphatidylglycerol production is increased. This process alters membrane composition, increasing membrane polarization, and decreasing permeability and fluidity, thereby reducing antibiotic uptake by V. alginolyticus. These findings suggest the presence of a previously unrecognized metabolic mechanism by which bacteria escape antibiotic killing through the use of an environmental factor.

Biotransformation of chloramphenicol by enriched bacterial consortia and the newly isolated bacterial strain Bordetella sp. C3: Detoxifying biotransformation pathway and its potential application in agriculture

Limited sources of consortia/pure cultures that degrade chloramphenicol (CAP) and the incomplete biodegradation profiles of CAP hinder the remediation of CAP pollution. In this study, two CAP-degrading consortia (designated as CM and PM) were obtained after long-term acclimation, and Alcaligenaceae and Enterobacteriaceae enriched in CM and PM, respectively. Notably, Bordetella sp. C3, a new isolate belonging to the family Alcaligenaceae, was isolated from CM and capable of degrading 85.7 % 10 mg/L CAP at 30 ℃ and pH 7 in 10 d. The biotransformation of CAP by Bordetella sp. C3 was proposed as a detoxification process, including a novel initial degradation pathway: dechlorination of CAP into AP. Strain C3 can also function as a plant growth-promoting bacterium that solubilizes inorganic phosphate and produces siderophores and indole-3-acetic acid (IAA). This study expands our knowledge of the migration and transformation pathways of CAP and microbial community profiles during acclimatization.

Antimicrobial resistance pattern of Escherichia coli isolated from imported frozen shrimp in Saudi Arabia

Contamination of seafood products with multi-drug-resistant (MDR) bacteria is considered to be a potential source for the spread of MDR bacteria in communities. However, little is known about the extent of the contamination of seafood, in particular shrimp, with MDR bacteria in Saudi Arabia. In this study, imported frozen shrimp in retail markets were examined for the antimicrobial susceptibility patterns of Escherichia coli. Escherichia coli was isolated from 40 frozen shrimp samples; a total of 25 and 15 shrimp samples were imported from China and Vietnam, respectively. Of the 40 examined frozen shrimp samples, 30 tested positive for E. coli, resulting in an overall isolation rate of 75%, with a total of 180 isolates being identified. The largest number of positive samples for E. coli isolates (n = 140) were found in 22 out of 25 samples from frozen shrimp imported from China. Additionally, eight out of 15 samples from frozen shrimp imported from Vietnam were positive for E. coli, leading to the recovery of 40 isolates. Overall, the susceptibilities among E. coli isolates were observed against 99.4% for amikacin, augmentin and kanamycin, 95% for cefoxitin and 92.7% for ceftazidime and nitrofurantoin. High percentage of the isolates exhibited resistance to cephalothin (174/180, 96.6%), ampicillin (167/180, 92.7%), Cephalexin (163/180. 90.5%), piperacillin (156/180, 86.6%), ceftriaxone (123/180, 68.3%), nalidixic acid (95/180, 52.7%), trimethoprim sulphamethoxazole (90/180, 50%), and tetracycline (88/180, 48.8%). Additionally, the MDR (resistant to ≥3 different class of antimicrobials) rate among E. coli isolates was 94.4% (170/180). A relatively high intermediate resistance of 60% (108/180) was exhibited for aztreonam. Notably, E. coli showed 71 different antibiotic resistance patterns with the multiple antibiotic resistant (MAR) index ranging from 0.04 to 0.66 and 89.4% of E. coli isolates recorded very significant MAR indexes above the range of 0.2. In this study, we recorded highest antimicrobial co-resistance patterns of 162 (90%) isolates between cephalothin and ampicillin, followed by 158 (87.7%) isolates between cephalothin and cephalexin. Furthermore, and interestingly, the segregation of antimicrobial resistance patterns based on the source of country origin of frozen shrimp revealed few inter-country resistant patterns found interconnecting and this influenced 44 (24.4%) isolates overlapping between isolates origin from frozen shrimp imported from China and Vietnam. This study documents the antimicrobial resistance in E. coli isolated from imported frozen shrimp and the presence of MDR E. coli in shrimp consuming communities, which may pose a risk to public health and the transfer of resistant bacteria to the food chain and environment.

A multi-pronged approach to assessing antimicrobial resistance risks in coastal waters and aquaculture systems

Antimicrobial resistance (AMR) is a global challenge that has impacted aquaculture and surrounding marine environments. In this study, a year-long monitoring program was implemented to evaluate AMR in two different aquaculture settings (i.e., open cage farming, recirculating aquaculture system (RAS)) and surrounding marine environment within a tropical coastal region. The objectives of this study are to (i) investigate the prevalence and co-occurrence of antibiotic-resistant bacteria (ARB), antibiotic resistance genes (ARGs), antibiotics (AB) and various associated chemical compounds at these study sites; (ii) explore the contributing factors to development and propagation of AMR in the coastal environment; and (iii) assess the AMR risks from different perspectives based on the three AMR determinants (i.e., ARB, ARGs and AB). Key findings revealed a distinct pattern of AMR across the different aquaculture settings, notably a higher prevalence of antibiotic-resistant Vibrio at RAS outfalls, suggesting a potential accumulation of microorganisms within the treatment system. Despite the relative uniform distribution of ARGs across marine sites, specific genes such as qepA, blaCTX-M and bacA, were found to be abundant in fish samples, especially from the RAS. Variations in chemical contaminant prevalence across sites highlighted possible anthropogenic impacts. Moreover, environmental and seasonal variations were found to significantly influence the distribution of ARGs and chemical compounds in the coastal waters. Hierarchical cluster analysis that was based on ARGs, chemical compounds and environmental data, categorized the sites into three distinct clusters which reflected strong association with location, seasonality and aquaculture activities. The observed weak correlations between ARGs and chemical compounds imply that low environmental concentrations may be insufficient for resistance selection. A comprehensive risk assessment using methodologies such as the multiple antibiotic resistance (MAR) index, comparative AMR risk index (CAMRI) and Risk quotient (RQ) underscored the complexity of AMR risks. This research significantly contributes to the understanding of AMR dynamics in natural aquatic systems and provides valuable insights for managing and mitigating AMR risks in coastal environments.

Effects of passion fruit peel (Passiflora edulis) pectin and red yeast (Sporodiobolus pararoseus) cells on growth, immunity, intestinal morphology, gene expression, and gut microbiota in Nile tilapia (Oreochromis niloticus)

This study explores the effects of dietary supplementation with passion fruit peel pectin (Passiflora edulis) and red yeast cell walls (Sporidiobolus pararoseus) on growth performance, immunity, intestinal morphology, gene expression, and gut microbiota of Nile tilapia (Oreochromis niloticus). Nile tilapia with an initial body weight of approximately 15 ± 0.06 g were fed four isonitrogenous (29.09-29.94%), isolipidic (3.01-4.28%), and isoenergetic (4119-4214 Cal/g) diets containing 0 g kg-1 pectin or red yeast cell walls (T1 - Control), 10 g kg-1 pectin (T2), 10 g kg-1 red yeast (T3), and a combination of 10 g kg-1 pectin and 10 g kg-1 red yeast (T4) for 8 weeks. Growth rates and immune responses were assessed at 4 and 8 weeks, while histology, relative immune and antioxidant gene expression, and gut microbiota analysis were conducted after 8 weeks of feeding. The results showed that the combined supplementation (T4) significantly enhanced growth performance metrics, including final weight, weight gain, specific growth rate, and feed conversion ratio, particularly by week 8, compared to T1, T2, and T3 (P < 0.05). Immunological assessments revealed increased lysozyme and peroxidase activities in both skin mucus and serum, with the T4 group showing the most pronounced improvements. Additionally, antioxidant and immune-related gene expression, including glutathione peroxidase (GPX), glutathione reductase (GSR), and interleukin-1 (IL1), were upregulated in the gut, while intestinal morphology exhibited improved villus height and width. Gut microbiota analysis indicated increased alpha and beta diversity, with a notable rise in beneficial phyla such as Actinobacteriota and Firmicutes in the supplemented groups. These findings suggest that the combined use of pectin and red yeast cell walls as prebiotics in aquaculture can enhance the health and growth of Nile tilapia, offering a promising alternative to traditional practices. Further research is needed to determine optimal dosages for maximizing these benefits.

Conjugal plasmid transfer in the plant rhizosphere in the One Health context

Introduction

Horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs) is one of the primary routes of antimicrobial resistance (AMR) dissemination. In the One Health context, tracking the spread of mobile genetic elements (MGEs) carrying ARGs in agri-food ecosystems is pivotal in understanding AMR diffusion and estimating potential risks for human health. So far, little attention has been devoted to plant niches; hence, this study aimed to evaluate the conjugal transfer of ARGs to the bacterial community associated with the plant rhizosphere, a hotspot for microbial abundance and activity in the soil. We simulated a source of AMR determinants that could enter the food chain via plants through irrigation.

Methods

Among the bacterial strains isolated from treated wastewater, the strain Klebsiella variicola EEF15 was selected as an ARG donor because of the relevance of Enterobacteriaceae in the AMR context and the One Health framework. The strain ability to recolonize lettuce, chosen as a model for vegetables that were consumed raw, was assessed by a rifampicin resistant mutant. K. variicola EEF15 was genetically manipulated to track the conjugal transfer of the broad host range plasmid pKJK5 containing a fluorescent marker gene to the natural rhizosphere microbiome obtained from lettuce plants. Transconjugants were sorted by fluorescent protein expression and identified through 16S rRNA gene amplicon sequencing.

Results and discussion

K. variicola EEF15 was able to colonize the lettuce rhizosphere and inhabit its leaf endosphere 7 days past bacterial administration. Fluorescence stereomicroscopy revealed plasmid transfer at a frequency of 10-3; cell sorting allowed the selection of the transconjugants. The conjugation rates and the strain's ability to colonize the plant rhizosphere and leaf endosphere make strain EEF15::lacIq-pLpp-mCherry-gmR with pKJK5::Plac::gfp an interesting candidate to study ARG spread in the agri-food ecosystem. Future studies taking advantage of additional environmental donor strains could provide a comprehensive snapshot of AMR spread in the One Health context.

Antimicrobial Resistance in Aquaculture: Risk Mitigation within the One Health Context

The application of antimicrobials in aquaculture primarily aims to prevent and treat bacterial infections in fish, but their inappropriate use may result in the emergence of zoonotic antibiotic-resistant bacteria and the subsequent transmission of resistant strains to humans via food consumption. The aquatic environment serves as a potential reservoir for resistant bacteria, providing an ideal breeding ground for development of antimicrobial resistance (AMR). The mutual inter-connection of intensive fish-farming systems with terrestrial environments, the food processing industry and human population creates pathways for the transmission of resistant bacteria, exacerbating the problem further. The aim of this study was to provide an overview of the most effective and available risk mitigation strategies to tackle AMR in aquaculture, based on the One Health (OH) concept. The stringent antimicrobial use guidelines, promoting disease control methods like enhanced farm biosecurity measures and vaccinations, alternatives to antibiotics (ABs) (prebiotics, probiotics, immunostimulants, essential oils (EOs), peptides and phage therapy), feeding practices, genetics, monitoring water quality, and improving wastewater treatment, rather than applying excessive use of antimicrobials, can effectively prevent the development of AMR and release of resistant bacteria into the environment and food. The contribution of the environment to AMR development traditionally receives less attention, and, therefore, environmental aspects should be included more prominently in OH efforts to predict, detect and prevent the risks to health. This is of particular importance for low and middle-income countries with a lack of integration of the national AMR action plans (NAPs) with the aquaculture-producing environment. Integrated control of AMR in fisheries based on the OH approach can contribute to substantial decrease in resistance, and such is the case in Asia, where in aquaculture, the percentage of antimicrobial compounds with resistance exceeding 50% (P50) decreased from 52% to 22% within the period of the previous two decades.

Contamination status, source analysis and exposure assessments of quinolone antibiotics in the south of Yancheng Coastal Wetland, China

Yancheng coastal wetland, the largest coastal wetland in the west coast of the Pacific Ocean and the margin of the Asian continent, has significant environmental, economic and social effects on local human beings. The extensive contamination and potential risk of quinolone antibiotics (QNs) on local aquaculture and human health are still not clear until now. In this study, 52 surface sediment samples were collected to investigate the contamination status and polluted sources, and evaluate ecological risks of QNs in the south of Yancheng coastal wetland. The total contents of QNs ranged from 0.33 to 21.60 ng/g dw (mean value of 4.51 ng/g dw), following the detection frequencies of QNs ranging from 19.23 to 94.23%. The highest content of QNs occurred around an aquaculture pond dominated by flumequine. The total organic carbon contents of sediment were positively correlated with sarafloxacin and lomefloxacin (p < 0.05), indicating the enhanced absorption of these QNs onto sediments. Partial QNs, such as lomefloxacin, enrofloxacin, sarafloxacin and flumequine, presented the homology features originating from the emission of medical treatment and aquaculture. There was no potential risk of QNs to human beings but a potential risk to aquatic organisms (algae > plant > invertebrate). Totally, the management and protection of Yancheng coastal wetland should be of concern with aquaculture as the important industry.

Effects of Shrimp Shell-Derived Chitosan on Growth, Immunity, Intestinal Morphology, and Gene Expression of Nile Tilapia (Oreochromis niloticus) Reared in a Biofloc System

Chitosan (CH) shows great potential as an immunostimulatory feed additive in aquaculture. This study evaluates the effects of varying dietary CH levels on the growth, immunity, intestinal morphology, and antioxidant status of Nile tilapia (Oreochromis niloticus) reared in a biofloc system. Tilapia fingerlings (mean weight 13.54 ± 0.05 g) were fed diets supplemented with 0 (CH0), 5 (CH5), 10 (CH10), 20 (CH20), and 40 (CH40) mL·kg-1 of CH for 8 weeks. Parameters were assessed after 4 and 8 weeks. Their final weight was not affected by CH supplementation, but CH at 10 mL·kg-1 significantly improved weight gain (WG) and specific growth rate (SGR) compared to the control (p < 0.05) at 8 weeks. Skin mucus lysozyme and peroxidase activities were lower in the chitosan-treated groups at weeks 4 and 8. Intestinal villi length and width were enhanced by 10 and 20 mL·kg-1 CH compared to the control. However, 40 mL·kg-1 CH caused detrimental impacts on the villi and muscular layer. CH supplementation, especially 5-10 mL·kg-1, increased liver and intestinal expressions of interleukin 1 (IL-1), interleukin 8 (IL-8), LPS-binding protein (LBP), glutathione reductase (GSR), glutathione peroxidase (GPX), and glutathione S-transferase (GST-α) compared to the control group. Overall, dietary CH at 10 mL·kg-1 can effectively promote growth, intestinal morphology, innate immunity, and antioxidant capacity in Nile tilapia fingerlings reared in biofloc systems.