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Antimicrobial resistance (AMR) is a growing threat to human and animal health. However, in aquatic animals—the fastest growing food animal sector globally—AMR trends are seldom documented, particularly in Asia, which contributes two-thirds of global food fish production. Here, we present a systematic review and meta-analysis of 749 point prevalence surveys reporting antibiotic-resistant bacteria from aquatic food animals in Asia, extracted from 343 articles published in 2000–2019. We find concerning levels of resistance to medically important antimicrobials in foodborne pathogens. In aquaculture, the percentage of antimicrobial compounds per survey with resistance exceeding 50% (P50) plateaued at 33% [95% confidence interval (CI) 28 to 37%] between 2000 and 2018. In fisheries, P50 decreased from 52% [95% CI 39 to 65%] to 22% [95% CI 14 to 30%]. We map AMR at 10-kilometer resolution, finding resistance hotspots along Asia’s major river systems and coastal waters of China and India. Regions benefitting most from future surveillance efforts are eastern China and India. Scaling up surveillance to strengthen epidemiological evidence on AMR and inform aquaculture and fisheries interventions is needed to mitigate the impact of AMR globally. Trends in antimicrobial resistance (AMR) in aquatic food animals are seldom documented, particularly in Asia. Here, Schar et al. review 749 point prevalence surveys, describing AMR trends in Asian aquaculture and fisheries over two decades, and identifying resistance hotspots as well as regions that would benefit most from future surveillance efforts.

The mucosal surfaces of fish harbour microbial communities that can act as the first-line of defense against pathogens. Infectious diseases are one of the main constraints to aquaculture growth leading to huge economic losses. Despite their negative impacts on microbial diversity and overall fish health, antibiotics are still the method of choice to treat many such diseases. Here, we use 16 rRNA V4 metataxonomics to study over a 6 week period the dynamics of the gill and skin microbiomes of farmed seabass before, during and after a natural disease outbreak and subsequent antibiotic treatment with oxytetracycline. Photobacterium damselae was identified as the most probable causative agent of disease. Both infection and antibiotic treatment caused significant, although asymmetrical, changes in the microbiome composition of the gills and skin. The most dramatic changes in microbial taxonomic abundance occurred between healthy and diseased fish. Disease led to a decrease in the bacterial core diversity in the skin, whereas in the gills there was both an increase and a shift in core diversity. Oxytetracycline caused a decrease in core diversity in the gill and an increase in the skin. Severe loss of core diversity in fish mucosae demonstrates the disruptive impact of disease and antibiotic treatment on the microbial communities of healthy fish.

The salmon louse Lepeophtheirus salmonis has been a substantial obstacle in Norwegian farming of Atlantic salmon for decades. With a limited selection of available medicines and frequent delousing treatments, resistance has emerged among salmon lice. Surveillance of salmon louse sensitivity has been in place since 2013, and consumption of medicines has been recorded since the early 80's. The peak year for salmon lice treatments was 2015, when 5.7 times as many tonnes of salmonids were treated compared to harvested. In recent years, non-medicinal methods of delousing farmed fish have been introduced to the industry. By utilizing data on the annual consumption of medicines, annual frequency of medicinal and non-medicinal treatments, the aim of the current study was to describe the causative factors behind salmon lice sensitivity in the years 2000-2019, measured through toxicity tests-bioassays. The sensitivity data from 2000-2012 demonstrate the early emergence of resistance in salmon lice along the Norwegian coast. Reduced sensitivity towards azamethiphos, deltamethrin and emamectin benzoate was evident from 2009, 2009 and 2007, respectively. The annual variation in medicine consumption and frequency of medicinal treatments correlated well with the evolution in salmon louse sensitivity. The patterns are similar, with a relatively small response delay from the decline in the consumption of medicines in Norway (2016 and onward) to the decline in measured resistance among salmon louse (2017 and onward). 2017 was the first year in which non-medicinal treatments outnumbered medicinal delousing treatments as well as the peak year in numbers of cleanerfish deployed. This study highlights the significance of avoiding heavy reliance on a few substance groups to combat ectoparasites, this can be a potent catalyst for resistance evolution. Further, it demonstrates the importance of transparency in the global industry, which enables the industry to learn from poor choices in the past.

Background The constant increase of aquaculture production and wealthy seafood consumption has forced the industry to explore alternative and more sustainable raw aquafeed materials, and plant ingredients have been used to replace marine feedstuffs in many farmed fish. The objective of the present study was to assess whether plant-based diets can induce changes in the intestinal mucus proteome, gut autochthonous microbiota and disease susceptibility of fish, and whether these changes could be reversed by the addition of sodium butyrate to the diets. Three different trials were performed using the teleostean gilthead sea bream ( Sparus aurata ) as model. In a first preliminary short-term trial, fish were fed with the additive (0.8%) supplementing a basal diet with low vegetable inclusion (D1) and then challenged with a bacteria to detect possible effects on survival. In a second trial, fish were fed with diets with greater vegetable inclusion levels (D2, D3) and the long-term effect of sodium butyrate at a lower dose (0.4%) added to D3 (D4 diet) was tested on the intestinal proteome and microbiome. In a third trial, the long-term effectiveness of sodium butyrate (D4) to prevent disease outcome after an intestinal parasite ( Enteromyxum leei ) challenge was tested. Results The results showed that opposed forces were driven by dietary plant ingredients and sodium butyrate supplementation in fish diet. On the one hand, vegetable diets induced high parasite infection levels that provoked drops in growth performance, decreased intestinal microbiota diversity, induced the dominance of the Photobacterium genus, as well as altered the gut mucosal proteome suggesting detrimental effects on intestinal function. On the other hand, butyrate addition slightly decreased cumulative mortality after bacterial challenge, avoided growth retardation in parasitized fish, increased intestinal microbiota diversity with a higher representation of butyrate-producing bacteria and reversed most vegetable diet-induced changes in the gut proteome. Conclusions This integrative work gives insights on the pleiotropic effects of a dietary additive on the restoration of intestinal homeostasis and disease resilience, using a multifaceted approach.

This study was divided into two parts. The first part involved the isolation, and detection of the prevalence and antimicrobial resistance profile of Aeromonas hydrophila, Pseudomonas aeruginosa , and Vibrio species from Nile tilapia fish and marine aquatic water. One hundred freshly dead Nile tilapia fish were collected from freshwater aquaculture fish farms located in Al-Abbassah district, Sharkia Governorate, and 100 samples of marine aquatic water were collected from fish farms in Port Said. The second part of the study focused on determining the in vitro inhibitory effect of dual-combination of AgNPs-H2O2 on bacterial growth and its down regulatory effect on crucial virulence factors using RT-PCR. The highest levels of A. hydrophila and P. aeruginosa were detected in 43%, and 34% of Nile tilapia fish samples, respectively. Meanwhile, the highest level of Vibrio species was found in 37% of marine water samples. Additionally, most of the isolated A. hydrophila, P. aeruginosa and Vibrio species exhibited a multi-drug resistance profile. The MIC and MBC results indicated a bactericidal effect of AgNPs-H2O2. Furthermore, a transcriptional modulation effect of AgNPs-H2O2 on the virulence-associated genes resulted in a significant down-regulation of aerA, exoU, and trh genes in A. hydrophila, P. aeruginosa, and Vibrio spp ., respectively. The findings of this study suggest the effectiveness of AgNPs-H2O2 against drug resistant pathogens related to aquaculture.

Pseudomonas aeruginosa ( P. aeruginosa ) is one of the most common ones that harm fish. P. aeruginosa has been regarded as one of the most significant threats to the fishing industry, which also affects public health. Thus, the present investigation was done in two steps; the first step was to examine the prevalence and the antibiogram of P. aeruginosa among Nile tilapia ( Oreochromas niloticus ( O. niloticus )) from aquaculture farms in Kafr El-shiekh Governorate with an emphasis on their antibiotic resistance genes ( BlaTEM, tetA, and sul1 ). The second step was to investigate the effect of levamisole as a feed supplement for tilapia fish on growth performance, immunity, serum biochemistry, and the protective effect against artificial infection with the previously isolated in the first step P. aeruginosa strain. One hundred samples were collected from morbid Nile tilapia fish in the first step. The incidence of P. aeruginosa was 14%. Susceptibility of P. aeruginosa isolates to 9 antimicrobial agents showed that about half of P. aeruginosa isolates were multidrug-resistant (MDR) to (5–6) antibiotics. All of the isolates were sensitive to amikacin, ciprofloxacin, and norfloxacin (100%), and half of them were resistant to azithromycin, amoxicillin with clavulanic, tetracycline, and sulfa with trimethoprim. P. aeruginosa isolates were confirmed diagnosed using the 16S rRNA gene, which was detected in 100% of the tested isolates, and was also evaluated for the presence of antibiotic resistance genes ( blaTEM, tetA, and sul1 ), which were 85.7%, 85.7%, and 100%, respectively. In the second step, a 2-month feeding trial was performed on 160 O. niloticus fish with a weight of 56.75 ± 3 g. Fish were randomly distributed into four groups, each at a rate of 10 fish per aquarium in four replicates, and fed on a diet containing 0.0, 500, 750, and 1000 mg levamisole/kg diet. At the end of the feeding trial, fish were challenged by pathogenic P. aeruginosa, which was isolated in the first step. The results of the in vivo trial showed that levamisole safely improved the growth and immunity of Nile tilapia without side effects on liver function.

To address the growing concern over antibiotic-resistant microbial infections in aquatic animals, we tested several promising alternative agents that have emerged as new drug candidates. Specifically, the tilapia piscidins are a group of peptides that possess antimicrobial, wound-healing, and antitumor functions. In this study, we focused on tilapia piscidin 3 (TP3) and TP4, which are peptides derived from Oreochromis niloticus, and investigated their inhibition of acute bacterial infections by infecting hybrid tilapia (Oreochromis spp.) with Vibrio vulnificus and evaluating the protective effects of pre-treating, co-treating, and post-treating fish with TP3 and TP4. In vivo experiments showed that co-treatment with V. vulnificus and TP3 (20 μg/fish) or TP4 (20 μg/fish) achieved 95.3% and 88.9% survival rates, respectively, after seven days. When we co-injected TP3 or TP4 and V. vulnificus into tilapia and then re-challenged the fish with V. vulnificus after 28 days, the tilapia exhibited survival rates of 35.6% and 42.2%, respectively. Pre-treatment with TP3 (30 μg/fish) or TP4 (20 μg/fish) for 30 minutes prior to V. vulnificus infection resulted in high survival rates of 28.9% and 37.8%, respectively, while post-treatment with TP3 (20 μg/fish or 30 μg/fish) or TP4 (20 μg/fish) 30 minutes after V. vulnificus infection yielded high survival rates of 33.3% and 48.9%. In summary, pre-treating, co-treating, and post-treating fish with TP3 or TP4 all effectively decreased the number of V. vulnificus bacteria and promoted significantly lower mortality rates in tilapia. The minimum inhibitory concentrations (MICs) of TP3 and TP4 that were effective for treating fish infected with V. vulnificus were 7.8 and 62.5 μg/ml, respectively, whereas the MICs of kanamycin and ampicillin were 31.2 and 3.91 μg/ml. The antimicrobial activity of these peptides was confirmed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), both of which showed that V. vulnificus disrupted the outer membranes of cells, resulting in the loss of cell shape and integrity. We examined whether TP3 and TP4 increased the membrane permeability of V. vulnificus by measuring the fluorescence resulting from the uptake of 1-N-phenyl-naphthylamine (NPN). Treating fish with TP3 and TP4 under different pH and temperature conditions did not significantly increase MIC values, suggesting that temperature and the acid-base environment do not affect AMP function. In addition, the qPCR results showed that TP3 and TP4 influence the expression of immune-responsive genes, including interleukin (IL)-1β, IL-6, and IL-8. In this study, we demonstrate that TP3 and TP4 show potential for development as drugs to combat fish bacterial infections in aquaculture.

Edwardsiella tarda can cause fatal gastro-/extraintestinal diseases in fish and humans. Overuse of antibiotics has led to antibiotic resistance and contamination in the environment, which highlights the need to find new antimicrobial agents. In this study, the marine peptide-N6 was amidated at its C-terminus to generate N6NH2. The antibacterial activity of N6 and N6NH2 against E. tarda was evaluated in vitro and in vivo; their stability, toxicity and mode of action were also determined. Minimal inhibitory concentrations (MICs) of N6 and N6NH2 against E. tarda were 1.29–3.2 μM. Both N6 and N6NH2 killed bacteria by destroying the cell membrane of E. tarda and binding to lipopolysaccharide (LPS) and genomic DNA. In contrast with N6, N6NH2 improved the stability toward trypsin, reduced hemolysis (by 0.19% at a concentration of 256 μg/mL) and enhanced the ability to penetrate the bacterial outer and inner membrane. In the model of fish peritonitis caused by E. tarda, superior to norfloxacin, N6NH2 improved the survival rate of fish, reduced the bacterial load on the organs, alleviated the organ injury and regulated the immunity of the liver and kidney. These data suggest that the marine peptide N6NH2 may be a candidate for novel antimicrobial agents against E. tarda infections.

Saprolegniasis is one of the most dangerous fungal diseases of fish, causing significant mortality in fish hatcheries and young ones. The present study aimed to isolate and characterize the causative fungus from fingerlings of Pangasianodon hypophthalmus cultured intensively in freshwater cages in Indian reservoirs and to determine minimum inhibitory concentrations of different antifungal compounds against the fungal hyphae and zoospores. The fungal isolates grown on potato dextrose agar showed an abundance of gemmae, elongated mycelia, non-septate hyphae, primary zoospores, mature zoosporangia with numerous zoospores, cysts with bundles of long hairs and were further identified as Saprolegnia parasitica following PCR amplification and sequencing of internal transcribed spacer region. S. parasitica showed temperature-sensitive optimum growth in a narrow window of 12–24 ℃, which might drive its experimental pathogenesis as well as natural infections in the winter months. In vitro sensitivity testing established negligible inhibitory activity of fluconazole, boric acid, sodium thiosulfate, and potassium permanganate while clotrimazole arrested the spore and hyphal growths at 2 mgL -1 concentration suggesting potential of the imidazole antifungal in treating S. parasitica infection in fish. The present study will serve as the baseline information for developing therapeutic and management strategies for controlling saprolegniasis in the economically significant iridescent catfish.

Aeromonas hydrophila is an opportunistic pathogen that is highly important for freshwater fish. In the present study, two freshwater fish species Nile tilapia ( Oreochromis niloticus ) and Mullet ( Mugil cephalus ) collected from various fish farms in Kafrelsheikh Governorate, Egypt. The fish samples were examined to determine Aeromonas hydrophila presence ( A. hydrophila ). In addition, a treatment trial was conducted involving four groups of Nile tilapia fish, which treated with florfenicol (FFC) and oxytetracycline (OTC) based on the antimicrobial susceptibility test results. According to the findings, 12 (20%) A. hydrophila strains were isolated from a total of 60 collected fish samples (30 of Nile tilapia and Mullet with percentages of 30% and 10%, respectively). Based on species-specific 16 S rRNA genes, six (6) isolates were identified as A. hydrophila and carried aerolysin ( aer A) and hemolysin ( hyl A) virulence genes, with percentages of 83.3% and 50%, respectively. Whereas, the antimicrobial resistance gene results were bla TEM with percent (100%) and aadA 1 (83.3%). Histopathological changes were significantly reduced in all assessed organs (liver, spleen, kidney, and gills) in the FFC group compared to the OTC-treated group. The prevalence of virulent and multidrug-resistant A. hydrophila in aquaculture poses significant risks to fish health, economic productivity, and public health.