Biological Approaches for Disease Control in Aquaculture: Advantages, Limitations and Challenges

Streptococcus iniae in aquaculture: a review of pathogenesis, virulence, and antibiotic resistance

One of the main challenges in aquaculture is pathogenic bacterial control. Streptococcus iniae stands out for its ability to cause high mortality rates in populations of commercially important fish populations and its recent recognition as an emerging zoonotic pathogen. The rise in identifying over 80 strains some displaying antibiotic resistance coupled with the emerging occurrence of infections in marine mammal species and wild fish underscores the urgent need of understanding pathogenesis, virulence and drug resistance mechanisms of this bacterium. This understanding is crucial to ensure effective control strategies. In this context, the present review conducts a bibliometric analysis to examine research trends related to S. iniae, extending into the mechanisms of infection, virulence, drug resistance and control strategies, whose relevance is highlighted on vaccines and probiotics to strengthen the host immune system. Despite the advances in this field, the need for developing more efficient identification methods is evident, since they constitute the basis for accurate diagnosis and treatment.

An ex vivo Approach in European Seabass Leucocytes Supports the in vitro Regulation by Postbiotics of Aip56 Gene Expression of Photobacterium damselae subsp. piscicida

Shewanella putrefaciens Pdp11 (SpPdp11) is a probiotic strain assayed in aquaculture; however, its postbiotic potential is unknown. Postbiotics are bacterial metabolites, including extracellular products (ECPs) that improve host physiology and immunity. Their production and composition can be affected by different factors such as the growing conditions of the probiotics. Photobacterium damselae subsp. piscicida strain Lg 41/01 (Phdp) is one of the most important pathogens in marine aquaculture. The major virulent factor of this bacterium is the exotoxin aip56, responsible for inducing apoptosis of fish leucocytes. Viable SpPdp11 cells have been reported to increase resistance to challenges with Phdp. This work aimed to evaluate the effect of two ECPs, T2348-ECP and FM1548-ECP, obtained from SpPdp11 grown under different culture conditions that previously demonstrated to exert different degradative and non-cytotoxic activities, as well as the effect on pathogens biofilm formation. These SpPdp11-ECPs were then analyzed by their effect on the viability, phagocytosis, respiratory burst and apoptogenic activity against European sea bass leucocytes infected or not with Phdp supernatant. Both ECPs, T2348-ECP and FM1548-ECP, were not cytotoxic against leucocytes and significantly reduced their apoptosis. Phagocytosis and respiratory burst of leucocytes were significantly reduced by incubation with Phdp supernatant, and not influenced by incubation with T2348-ECP or FM1548-ECP. However, both activities were significantly increased after leucocyte incubation with combined T2348-ECP and FM1548-ECP with Phdp supernatant, compared to those incubated only with Phdp supernatant. Finally, both T2348-ECP and FM1548-ECP significantly reduced the relative in vitro expression of the Phdp aip56 encoding gene.

Molecular characterization of Aeromonas hydrophila detected in Channa marulius and Sperata sarwari sampled from rivers of Punjab in Pakistan

Aeromonas hydrophila is one of the major pathogenic bacteria responsible for causing severe outbreaks at fish farms and is also a major global public health concern. This bacterium harbors many virulence genes. The current study was designed to evaluate the antidrug and virulence potential of A. hydrophila by amplifying its antimicrobial resistance and virulence genes using PCR and examining their effects on fish tissues and organs. A total of 960 fish samples of Channa marulius and Sperata sarwari were collected from four sites of the rivers of the Punjab, Pakistan. A. hydrophila isolates were subjected to biochemical identification and detection of virulence and antimicrobial resistance (AMR) genes by PCR. We retrieved 181 (6.46%) A. hydrophila isolates from C. marulius and 177 (6.25%) isolates from S. sarwari. Amplification through PCR revealed the incidence of virulence genes in 95.7% of isolates in C. marulius and 94.4% in S. sarwari. Similarly, amplification through PCR also revealed occurrence of AMR genes in 87.1% of isolates in C. marulius and 83.9% in S. sarwari. Histopathological examination revealed congestion (5.2%) and hepatocyte necrosis (4.6%) in liver, lamellar fusion (3.3%) and the presence of bacterial colonies (3.7%) in gills, fin erosion (6%), and the presence of biofilms (3.5%) in tail fins of infected fish. Phylogenetic tree analysis of 16S rRNA and gyrB gene of A. hydrophila revealed 100% and 97% similarity, respectively, with 16S rRNA gene and gyrB of A. hydrophila isolated in previous studies. The results of antimicrobial susceptibility testing showed that all isolates demonstrated resistance to sulfamethoxazole, ampicillin, neomycin, and norfloxacin, while susceptibility to gentamicin, chloramphenicol, and tetracycline, and intermediate resistance was observed against cefotaxime. The results concluded that examined fish samples were markedly contaminated with virulent and multidrug strains of A. hydrophila which may be of a potential health risk. The study emphasizes the responsible antimicrobial use in aquaculture and the urgent need for effective strategies to control the spread of virulence and antimicrobial resistance genes in A. hydrophila.

Evaluation of the control efficacy of antagonistic bacteria from V-Ti magnetite mine tailings on kiwifruit brown spots in pot and field experiments

Seemingly barren heavy-metal-polluted vanadium (V) and titanium (Ti) magnetite mine tailings contain various functional microbes, yet it is unclear whether this includes microbial resources relevant to the biological control of plant diseases. Kiwifruit brown leaf spot disease, caused by Corynespora cassiicola, can seriously reduce kiwifruit yield. To discover effective control measures for kiwifruit leaf spot, 18 bacteria strains among 136 tailing-isolated bacteria from V-Ti magnetite mine tailings were identified as inhibiting C. cassiicola by the confrontation plate method, indicating that antagonistic bacteria surviving in the V-Ti magnetite mine tailings were present at a low level. The 18 antagonistic strains could be divided into two BOX-A1R clusters. The 13 representative strains that were selected for phylogenetic tree construction based on their 16S rRNA sequences belonged to the Bacillus genus. Five predominant strains exhibited different toxin-production times and intensities, with four of them initiating toxin production at 32 h. Among them, Bacillus sp. KT-10 displayed the highest bacteriostatic rate (100%), with a 37.5% growth inhibition rate and an antagonistic band of 3.2 cm against C. cassiicola. Bacillus sp. KT10 also showed a significant inhibitory effect against the expansion speed of kiwifruit brown spots in the pot. The relative control effect was 78.48 and 83.89% at 7 days after the first and last spraying of KT-10 dilution, respectively, confirming a good effect of KT-10 on kiwifruit brown leaf spots in the field. This study demonstrated for the first time that there are some antagonistic bacteria to pathogenic C. cassiicola in V-Ti magnetite mine tailings, and Bacillus sp. KT10 was found to have a good control effect on kiwifruit brown leaf spots in pots and fields, which provided an effective biological control measurement for kiwifruit brown leaf spots.

Intensified perfusion culture (IPC) reduced recombinant protein fragmentation

Mammalian cells remain the mainstay of biological production host. In industry, cultivating and harvest strategies are sorted in batch mode (e.g., batch, fed-batch, concentrated fed-batch and intensified fed-batch) and continuous mode (e.g., perfusion). To retrieve greater productivity and better product quality, especially for the sensitive products prone to fragmentation, culture modes with various modifications are innovated (e.g., intensified perfusion culture [IPC]). In our study, we demonstrated that the fragmentation of Fc-fusion product (Molecule A) is time-dependent in traditional fed-batch (TFB) culture. The fragmentation proportion increased from 3.8% to 12.4% for Clone A, 0.8% to 1.7% for Clone B and 0.9% to 2.0% for Clone C from Day 10 to Day 14. By applying a novel bioprocess, IPC, which allows continuous feeding of the fresh medium and constant removal of the spent medium without bleeding cells to maintain a defined constant viable cell density, the fragmentation was reduced to 0.3% while the productivity was increased from 2.96 g/L to 15.51 g/L for Clone A. To validate whether the fragmentation reduction is product-sensitive, plasmids carrying the DNA sequences of two other Fc-fusion molecules (Molecule B and Molecule C) were transfected into the host. The results showed consistent fragmentation reducing effect by using IPC. Furthermore, the cultivation scale was expanded to 50 L and 1000 L. A minimum fragmentation level below 0.1% was observed for Molecule C. Our study revealed the capability of IPC in reducing Fc-fusion protein fragmentation and the reproducibility when scaling up while maintaining high productivity.

JunD functions as a transcription factor of IL-10 to regulate bacterial infectious inflammation in grass carp (Ctenopharyngodon idella)

IL-10 is a key anti-inflammatory mediator ensuring the protection of a host from excessive inflammation in response to pathogen infections, whose transcription or expression levels are tightly linked to the onset and progression of infectious diseases. An AP-1 family member called CiJunD was shown to be a transcription factor of IL-10 in grass carp (Ctenopharyngodon idella) in the current study. CiJunD protein harbored the conserved Jun and bZIP domains. Mutant experiments demonstrated that CiJunD bound to three specific sites on IL-10 promoter, i.e., 5'-ATTATTCATA-3', 5'-AGATGAGACATCT-3', and 5'-ATTATTCATC-3', mainly relying on the bZIP domain, and initiated IL-10 transcription. Expression data from the grass carp spleen infected by Aeromonas hydrophila and lipopolysaccharide (LPS) challenged spleen leukocytes indicated that the expressions of CiJunD and IL-10 were positively correlated, while the expression of pro-inflammatory cytokines, such as IL-1β, IL-6, IL-8, IFN-γ, and TNF-α, showed an overall downward trend when CiJunD and IL-10 peaked. The ability of CiJunD to down-regulate the production of pro-inflammatory cytokines and up-regulate the expression of IL-10, both with and without LPS stimulation, was confirmed by overexpression experiments. Meanwhile, the subcellular fractionation assay revealed that the nuclear translocation of CiJunD was significantly enhanced after the LPS challenge. Moreover, in vivo administration of grass carp with Oxamflatin, a potent agonist of JunD activity, could promote IL-10 but suppress the expression of pro-inflammatory cytokines. Intriguingly, tissue inflammation lesions and the survival rates of grass carp infected with A. hydrophila were also significantly improved by Oxamflatin administration. This work sheds light on the regulation mechanism by JunD of IL-10 expression and bacterial infectious inflammation for the first time, and it may present a viable method for preventing infectious diseases in fish by regulating IL-10 expression and inflammatory response.

Antimicrobial and antioxidant activity of encapsulated tea polyphenols in chitosan/alginate-coated zein nanoparticles: a possible supplement against fish pathogens in aquaculture

Regulation of antibiotic use in aquaculture calls for the emergence of more sustainable alternative treatments. Tea polyphenols (GTE), particularly epigallocatechin gallate (EGCG), have various biological activities. However, tea polyphenols are susceptible to degradation. In this work, EGCG and GTE were encapsulated in zein nanoparticles (ZNP) stabilized with alginate (ALG) and chitosan (CS) to reduce the degradation effect. ALG-coated ZNP and ALG/CS-coated ZNP encapsulating EGCG or GTE were obtained with a hydrodynamic size of less than 300 nm, an absolute ζ-potential value >30 mV, and an encapsulation efficiency greater than 75%. The antioxidant capacity of the encapsulated substances, although lower than that of the free ones, maintained high levels. On the other hand, the evaluation of antimicrobial activity showed greater efficiency in terms of growth inhibition for ALG/CS-ZNP formulations, with average overall values of around 60%, reaching an inhibition of more than 90% for Photobacterium damselae. These results support encapsulation as a good strategy for tea polyphenols, as it allows maintaining significant levels of antioxidant activity and increasing the potential for antimicrobial activity, in addition to increasing protection against sources of degradation.

Effects of extended dietary supplementation with Santalum album essential oil on hemato-biochemical changes, innate immune response, antioxidant status, and expression of related gene in Nile tilapia (Oreochromis niloticus)

The effects of long-term dietary supplementation with sandalwood (Santalum album L.) essential oil (SEO) was investigated on hemato-biochemical biomarkers, immune status, antioxidant capacity, and resistance against Staphylococcus aureus in Nile tilapia (Oreochromis niloticus). Five groups (with four replicates) of O. niloticus (12.60 ± 0.20 g) were fed diets supplemented with SEO at doses of 0, 0.5, 1.0, 2.0, and 4.0 mL/kg diet for 60 days. Results indicated a substantial increase in blood protein levels and lower serum cholesterol, cortisol, glucose, urea, creatinine levels and, transaminase activities of fish fed a 2.0-mL SEO/kg diet. Serum lysozyme activity, nitric oxide, complement-3 levels, and phagocytic activity were significantly improved in O. niloticus after 60 days of feeding SEO-supplemented diets. Dietary SEO at level of 2.0-mL SEO/kg diet increased the activities of SOD, CAT, and GPx, and decreased MDA levels in liver homogenate. In addition, dietary 2.0-mL SEO/kg diet significantly upregulated antioxidant genes expression (CAT, SOD, GPx, GST, and GSR) with downregulation of apoptotic genes (HSP70, TLR2, caspase-3, and PCNA) in the liver. Furthermore, SEO-enriched diets significantly down-regulated pro-inflammatory (TNF-α, IL-1β, and IL-8) and up-regulated anti-inflammatory cytokine genes (TFG-β and IL-10) in the spleen. Moreover, SEO fortification increased the relative percentage of survival against S. aureus challenge and regulated immune-antioxidant genes in the spleen after the challenge. Overall, the results revealed that long-term using SEO might strengthen the physiological performance, hepatic oxidant/antioxidant balance, innate immune response, and resistance of O. niloticus against bacterial infections.

Prediction of the potential distribution area of Spodoptera frugiperda and its parasitic wasp, Trichogramma pretiosum

Spodoptera frugiperda is a migratory agricultural pest with fast-spreading speed, long migration distance, and wide host range, which seriously threatens the safety of economic crops. To predict the trends of S. frugiperda and its parasitoid wasp Trichogramma pretiosum in their habitats under current and future climatic conditions, based on MaxEnt model and geographic distribution data of their historical occurrence, we project the feasibility of introducing T. pretiosum to control S. frugiperda by evaluating on their potential global distribution. The results show that, under the current greenhouse gas concentration, the potential distribution area of S. frugiperda is concentrated in 50° N-30° S, with a total area of 1.74 × 106  km2 , and the potential distribution area of T. pretiosum in the whole world is 2.91 × 106  km2 . The suitable areas of T. pretiosum cover almost all the suitable areas of S. frugiperda, which indicates that T. pretiosum can be introduced to control S. frugiperda. The results of this study can provide a theoretical basis for the monitoring and early warning of S. frugiperda and the use of T. pretiosum to control S. frugiperda.

Sweet orange peel waste as a feed additive in growth promoting and protective effect against Aeromonas hydrophila of juvenile bagrid catfish Mystus nemurus

Sweet orange Citrus sinensis peel is a phytobiotic agricultural waste with bioactive compounds that have potential functional properties as a growth promoter and immune stimulator. This study aims to evaluate the dietary effects of sweet orange peel (SOP) as a feed additive on growth enhancement of juvenile bagrid catfish Mystus nemurus and their disease resistance ability against Aeromonas hydrophila infection. Four experimental diets were formulated to contain 0 (SOP0, control), 4 (SOP4), 8 (SOP8) and 12 g/kg (SOP12) SOP. After 90 d of the feeding experiment, improvement in weight gain, specific growth rate, feed conversion ratio, and protein efficiency ratio were observed in the fish fed with SOP4. While fish survival was not significantly affected, hepatosomatic and viscerosomatic indices were significantly higher in fish fed with SOP12. Muscle protein was higher in fish fed with SOP4, SOP8, and SOP12 than in control but muscle lipids showed an opposite trend. A 14-d post-challenge test against A. hydrophila revealed no significant effect on the fish survival. Nevertheless, fish fed SOP4 encountered delayed bacterial infection compared to other treatments and fish fed with SOP0 and SOP4 performed numerically better survival. Infected fish showed skin depigmentation, haemorrhagic signs at the abdomen and anus, internal bleeding, and stomach and intestine enlargement. In conclusion, SOP4 could be recommended as a growth promoter while slightly delaying A. hydrophila infection in M. nemurus.

Effects of Antibiotic Residues on Fish Gut Microbiome Dysbiosis and Mucosal Barrier-Related Pathogen Susceptibility in Zebrafish Experimental Model

The symbiotic community of microorganisms in the gut plays an important role in the health of the host. While many previous studies have been performed on the interactions between the gut microbiome and the host in mammals, studies in fish are still lacking. In this study, we investigated changes in the intestinal microbiome and pathogen susceptibility of zebrafish (Danio rerio) following chronic antibiotics exposure. The chronic antibiotics exposure assay was performed on zebrafish for 30 days using oxytetracycline (Otc), sulfamethoxazole/trimethoprim (Smx/Tmp), or erythromycin (Ery), which are antibiotics widely used in the aquaculture industry. The microbiome analysis indicated that Fusobacteria, Proteobacteria, Firmicutes, and Bacteroidetes were the dominant phyla in the gut microbiome of the zebrafish used in this study. However, in Smx/Tmp-treated zebrafish, the compositions of Fusobacteria and Proteobacteria were changed significantly, and in Ery-treated zebrafish, the compositions of Proteobacteria and Firmicutes were altered significantly. Although alpha diversity analysis showed that there was no significant difference in the richness, beta diversity analysis revealed a community imbalance in the gut microbiome of all chronically antibiotics-exposed zebrafish. Intriguingly, in zebrafish with dysbiosis in the gut microbiome, the pathogen susceptibility to Edwardsiella piscicida, a representative Gram-negative fish pathogen, was reduced. Gut microbiome imbalance resulted in a higher count of goblet cells in intestinal tissue and an upregulation of genes related to the intestinal mucosal barrier. In addition, as innate immunity was enhanced by the increased mucosal barrier, immune and stress-related gene expression in the intestinal tissue was downregulated. In this study, we provide new insight into the effect of gut microbiome dysbiosis on pathogen susceptibility.

Isolation of Lactic Acid Bacteria (LAB) from Salmonids for Potential Use as Probiotics: In Vitro Assays and Toxicity Assessment of Salmo trutta Embryonated Eggs

This research investigates the potential of lactic acid bacteria (LAB) from freshwater salmonids as prospective probiotics for application in aquaculture. LAB and pathogenic bacteria were obtained from mucus and tissues of Oncorhynchus mykiss and Salmo trutta from fish farms in northeast Spain that had not used antibiotics for the six months preceding the study. Isolates were identified using Gram staining and sequencing of 16S rRNA and ITS-1. To assess the safety of the LAB, antibiotic susceptibility tests (ASTs) against 23 antimicrobials were performed. In vitro antagonism assays were conducted to evaluate the inhibitory effects of living LAB using the agar diffusion test method and their metabolites using the agar well diffusion method. The assays targeted six specific pathogens: Aeromonas salmonicida subsp. salmonicida, Carnobacterium maltaromaticum, Vagococcus salmoninarum, Yersinia ruckeri, Lactococcus garvieae, and the marine pathogen Vibrio jasicida. Additionally, a toxicity assay was conducted on embryonic eggs of S. trutta. The ASTs on probiotic LAB candidates revealed varied responses to antimicrobials, but no resistance to oxytetracycline or florfenicol, which are two antibiotics commonly used in aquaculture, was detected. The in vitro assays indicate that LAB exhibit antagonistic effects against pathogens, primarily when directly stimulated by their presence. In applications involving embryonic eggs or larvae, certain live strains of LAB were found to have adverse effects, with some isolates resulting in higher mortality rates compared to the control group or other isolates. Furthermore, the potential pathogenicity of certain LAB strains, typically considered safe in salmonids, warrants deeper investigation.

Inactivation of two oyster pathogens by photocatalysis and monitoring of changes in the microbiota of seawater: A case study on Ostreid herpes virus 1 μVar and Vibrio harveyi

The pollution of seawater by both biotic (bacteria, viruses) and abiotic contaminants (biocides, pharmaceutical residues) frequently leads to economic losses in aquaculture activities mostly mortality events caused by microbial infection. Advanced Oxidation Processes (AOPs) such as heterogeneous photocatalysis allow the removal of all organic contaminants present in water and therefore could reduce production losses in land-based farms. Oysters in land-based farms such as hatcheries and nurseries suffer from a large number of mortality events, resulting in significant losses. If photocatalysis has been widely studied for the decontamination, its application for disinfection is still overlooked, especially on seawater for viruses. We therefore studied seawater disinfection using the photocatalysis (UV365/TiO2) method in the context of Pacific oyster mortality syndrome (POMS). POMS has been defined as a polymicrobial disease involving an initial viral infection with Ostreid Herpes Virus 1, accompanied by multiple bacterial infections. We investigated the impact of treatment on Vibrio harveyi, a unique opportunistic pathogenic bacterium, and on a complex microbial community reflecting a natural POMS event. Viral inactivation was monitored using experimental infections to determine whether viral particles were still infectious after. Changes in the total bacterial community in seawater were studied by comparing UV365/TiO2 treatment with UV365-irradiated seawater and untreated seawater. In the case of OsHV-1, a 2-h photocatalytic treatment prevents POMS disease and oyster mortality. The same treatment also inactivates 80% of viable Vibrio harveyi culture (c.a. 1.5 log). Since OsHV-1 and Vibrio harveyi are effectively inactivated without long-term destabilization of the total bacterial microbiota in the seawater, photocatalysis appears to be a relevant alternative for disinfecting seawater in land-based oyster beds.

Dietary supplementation of Polygonum chinense improves the immunity of Asian seabass, Lates calcarifer (Bloch, 1790) against Vibrio harveyi infection

Aquaculture plays a significant role in the overall fish production in Malaysia, contributing a substantial quantity of food-fish amounting to roughly 573,683 tonnes with an estimated economic value of US$860 million in 2022. However, diseases have become a significant limitation for aquaculture production. Therefore, herbal immunostimulant has been considered a natural and practical approach of preventing disease infection in fish. The ability of Polygonum chinense extract (PCE) on haemato-biochemistry parameters, immunomodulatory properties, and disease resistance of Lates calcarifer (Asian seabass) under Vibrio harveyi challenge was evaluated in this study, with a focus on dose-response associations and variability over various exposure durations (0-, 7- and 14-day post-infection). A total of 480 Asian seabass (9.5 ± 0.2 g) were distributed in 12 aquaria and fed four diets supplemented with 0 (control), 2, 5 and 10 g/kg diet for 60 days before being challenged with V. harveyi. Dietary PCE significantly improved (P < 0.05) survival, with the dose of 10 g/kg showing the highest survival rate (90 %) when compared to the control (60 %). Additionally, hematological (red and white blood cell counts, hemoglobulin, packed cell volume, and mean corpuscular volume) and immunological (activities of lysozyme, phagocytic activity and respiratory burst, and serum total immunoglobulin) properties were significantly increased (P < 0.05) in comparison to the control group. In contrast, serum aspartate aminotransferase and alanine aminotransferase levels, as well as glucose level were significantly reduced (P < 0.05) in PCE-fed fish compared to the control group. Conclusively, the current study discovered that supplementing fish feed with P. chinense extract improves fish haemato-biochemical profile, immunocompetence and disease resistance to V. harveyi infection.

Health benefits of butyrate and its producing bacterium, Clostridium butyricum, on aquatic animals

Aquaculture plays an important role in contributing to global food security and nutrition; thus, the intensification and diversification of aquaculture are increasingly considered. However, paralleling the development of the industrial scale in aquaculture, the occurrence of diseases is always an important issue that causes great losses in economics. The finding of approaches that not only improve culture production but also reduce the impact of diseases in cultured animals is crucially essential. Previously, several studies have addressed the potential application of feed additives, such as prebiotics, probiotics, synbiotics, and microbial-derived metabolites (including short-chain fatty acids-SCFAs), in aquaculture. In this review, we provide an update focusing on the health benefits of dietary supplementation with a type of SCFAs, butyrate, and its producer, Clostridium butyricum, including their effects on growth, feed utilization, body composition, intestinal structure and function, antioxidant activity, immune response, and tolerance against stress and infection in aquatic animals. The outcomes of this study may indicate more benefits of the use of C. butyricum than that of butyrate (and its forms). This review provides general knowledge of the efficacy of butyrate and C. butyricum in aquaculture.

Effects of the combination of chitosan and Acinetobacter KU011TH on the growth and health performances and disease resistance of juvenile hybrid catfish (Clarias gariepinus × C. macrocephalus)

Aquatic animal health management has become a crucial component in the goal of increasing catfish aquaculture productivity. Additionally, hybrid catfish (Clarias gariepinus × C. macrocephalus) has been promoted as a highly profitable freshwater fish in Asia. Interestingly, the crucial diseases induced by Aeromonas hydrophila have been reported to greatly impede catfish production. To overcome this challenge, the aim was to investigate the effects of the oral administration of potentially synbiotic chitosan (CH) and Acinetobacter KU011TH (AK) on the growth performance, immunological responses, and disease resistance of hybrid catfish against A. hydrophila. The control group was fed a basal diet (A), the diet fed to treatment group B was supplemented with 20 mL of CH/kg diet (B), and the experimental feed fed to groups C-D was mixed with 1 × 108, 1 × 109 and 1 × 1010 CFU/mL AK coated with 20 mL of CH/kg diet. Five different groups of juvenile hybrid catfish were continuously fed the 5 formulated feeds for 4 weeks. The results revealed that all tested feeds did not significantly enhance the hybrid catfish's average daily gain, specific growth rate, feed conversion ratio, hematocrit and erythrocyte counts. Interestingly, the application of CH and AK significantly increased the leukocyte counts, respiratory burst, lysozyme activity, alternative complement pathway hemolytic activity, and bactericidal activity (P < 0.05). The expression levels of the immune-related genes in the whole blood, head kidney, and spleen were significantly increased after CH-AK application (P < 0.05), but this finding was not observed in the liver (P > 0.05). Additionally, after 14 days of A. hydrophila peritoneal injection, the fish in group C showed significantly higher survival rates of approximately 70.0 % compared with the control fish in groups B, D, and E (52.5 %, 40.0 %, 45.0 %, and 45.0 %, respectively) (P < 0.05). These results collectively suggest that short-term application of the diet fed to group C effectively boosted the immune responses and disease resistance of hybrid catfish against A. hydrophila.

Abamectin induced brain and liver toxicity in carp: The healing potential of silybin and potential molecular mechanisms

Abamectin (ABM) abuse contaminated aquatic environment and posed a potential threat to fish health as well as public safety. Silybin (SIL), a flavonoid, has been widely used as a novel feed additive to promote fish health. This research was to explore the potential antagonistic mechanism between ABM and SIL on brain and liver toxicity was investigated in common carp. Sixty carp were divided into four groups at random: the Control group, the SIL group, the ABM group, and ABM + SIL group. This experiment lasted for 30 d. According to behavioral observation, the detection of levels of acetylcholinesterase (AchE), iron, and mRNA expression levels of blood-brain barrier (BBB) related tight junction proteins (ZO-1, Claudin7, Occludin, MMP2, MMP9, and MMP13) in brain tissues, it was found that SIL relieved neurobehavioral disorders caused by ABM-induced BBB destruction in carp. H&E staining showed SIL mitigated nerve injury and liver injury caused by ABM. Oil Red O staining and liver-related parameters showed that SIL alleviated hepatotoxicity and lipid metabolism disorder caused by ABM exposure. Furthermore, this work also explored the specific molecular mechanism of SIL in liver protection and neuroprotection. It was shown that SIL lowered ROS levels in liver and brain tissues via the GSK-3β/TSC2/TOR pathway. Simultaneously, SIL inhibited NF-κB signaling pathway and played an anti-inflammatory role. In conclusion, we believed that SIL supplementation has a protective effect on the brain and liver by regulating oxidative stress and inflammation.

Effects of dietary dihydromyricetin on growth performance, antioxidant capacity, immune response and intestinal microbiota of shrimp (Litopenaeus vannamei)

A 56-day culture trial was conducted to evaluate the effects of dietary dihydromyricetin (DMY) on growth performance, antioxidant capacity, immune response and intestinal microbiota of shrimp (Litopenaeus vannamei). 840 healthy shrimp (1.60 ± 0.21 g) in total were fed with four different levels of DMY diets at 0 (Control), 100 (D1), 200 (D2), and 300 (D3) mg/kg, respectively. Samples were collected after the culture trial, and then, a 7-day challenge experiment against Vibrio parahaemolyticus was conducted. The results demonstrated that DMY significantly enhanced the activity of protease, amylase and lipase as well as the expression of lipid and protein transport-related genes (P < 0.05). The results of plasma lipid parameters indicated that DMY reduced lipid deposition, manifested by significantly (P < 0.05) decreased plasma total cholesterol (T-CHO), triglyceride (TG) and low-density lipoprotein cholesterol (LDL-C). The expression of genes involved in fatty acid β-oxidation and triglyceride catabolism was significantly up-regulated (P < 0.05), and genes involved in triglyceride synthesis were significantly down-regulated in DMY groups when compared to control group (P < 0.05). Moreover, dietary DMY also significantly (P < 0.05) increased the total antioxidant capacity (T-AOC), antioxidant enzymes activity and glutathione (GSH) content of shrimp, and a significant increase of total hemocytes count (THC), phagocytic rate (PR), antibacterial activity (AA) and bacteriolytic activity (BA) was observed in DMY groups (P < 0.05). The addition of DMY to the diet significantly augmented immune response by up-regulating the expression of genes related to toll-like receptors (Toll) signaling pathway, immune deficiency (IMD) signaling pathway and intestinal mucin. Furthermore, dietary DMY could modulate the composition and abundance of intestinal microbiota. In conclusion, DMY showed promising potential as a functional feed additive for shrimp to improve the growth performance and physiological health.

Bioassay and Whole-Genome Analysis of Bacillus velezensis FIO1408, a Biocontrol Agent Against Pathogenic Bacteria in Aquaculture

Bacterial disease is one of the most critical problems in aquaculture. Probiotics represent a promising biological approach to control bacterial disease because it is effective against pathogens and environmentally friendly. This study assessed the antagonistic activities of a bacterial strain FIO1408 isolated from deep-sea water against many pathogenic bacteria in aquaculture, including Listonella anguillarum, Vibrio parahaemolyticus, Vibrio alginolyticus, Aeromonas hydrophila, Edwardsiella anguillarum, Edwardsiella tarda, and Edwardsiella piscicida. The complete genome of strain FIO1408 consisted of a circular chromosome of 4,137,639 bp and two plasmids of 16,439 bp and 24,472 bp. Phylogenetic analysis showed strain FIO1408 clustered with Bacillus velezensis strains. 12 genes/gene clusters responsible for the synthesis of secondary metabolites were identified in the FIO1408 genome, including three lipopeptides, three polyketides, three bacteriocins, one siderophore, one dipeptide, and one unknown type. Also identified were 273 unique orthologous genes primarily involved in phage resistance, protein hydrolysis, environmental survivability, and genetic stability compared to B. velezensis KACC 13105, B. velezensis FZB42T, and B. velezensis NRRL B-41580. The principal safety of FIO1408 was demonstrated by genetic analyses and feeding trials. These findings will contribute to studies on the biocontrol mechanisms of B. velezensis FIO1408 and facilitate its application as a potent biological control agent against bacterial pathogens in aquaculture.

Biofouling in Membrane Bioreactors-Mitigation and Current Status: a Review

Biological fouling as termed biofouling is caused by varied living organisms and is difficult to eliminate from the environment thus becoming a major issue during membrane bioreactors. Biofouling in membrane bioreactors (MBRs) is a crucial problem in increasing liquid pressure due to reduced pore diameter, clogging of the membrane pores, and alteration of the chemical composition of the water which greatly limits the growth of MBRs. Thus, membrane biofouling and/or microbial biofilms is a hot research topic to improve the market competitiveness of the MBR technology. Though several antibiofouling strategies (addition of bioflocculant or sponge into MBRs) came to light, biological approaches are sustainable and more practicable. Among the biological approaches, quorum sensing-based biofouling control (so-called quorum quenching) is an interesting and promising tool in combating biofouling issues in the MBRs. Several review articles have been published in the area of membrane biofouling and mitigation approaches. However, there is no single source of information about biofouling and/or biofilm formation in different environmental settings and respective problems, antibiofilm strategies and current status, quorum quenching, and its futurity. Thus, the objectives of the present review were to provide latest insights on mechanism of membrane biofouling, quorum sensing molecules, biofilm-associated problems in different environmental setting and antibiofilm strategies, special emphasis on quorum quenching, and its futurity in the biofilm/biofouling control. We believe that these insights greatly help in the better understanding of biofouling and aid in the development of sustainable antibiofouling strategies.

Integration of microbiome and Koch's postulates to reveal multiple bacterial pathogens of whitish muscle syndrome in mud crab, Scylla paramamosain

BACKGROUND

For more than a century, the Koch's postulates have been the golden rule for determining the causative agents in diseases. However, in cases of multiple pathogens-one disease, in which different pathogens can cause the same disease, the selection of microorganisms that regress infection is hard when Koch's postulates are applied. Microbiome approaches can obtain relatively complete information about disease-related microorganisms and can guide the selection of target microorganisms for regression infection. In the present study, whitish muscle syndrome (WMS) of Scylla paramamosain, which has typical symptoms with whitish muscle and blackened hemolymph was used as an example to establish a new research strategy that integrates microbiome approaches and Koch's postulates to determinate causative agents of multiple pathogens-one disease.

RESULTS

Microbiome results revealed that Aeromonas, Acinetobacter, Shewanella, Chryseomicrobium, Exiguobacterium, Vibrio and Flavobacterium, and Kurtzmaniella in hemolymph were bacterial and fungal indicators for WMS. A total of 23 bacteria and 14 fungi were isolated from hemolymph and muscle tissues, and among the bacteria, Shewanella chilikensis, S. xiamenensis, Vibrio alginolyticus, S. putrefaciens, V. fluvialis, and V. parahaemolyticus were present in hemolymph and/or muscle tissues in each WMS crab, and the last three species were also present in three Healthy crabs. The target bacteria and fungi were further screened to regression infections based on two criteria: whether they belonged to the indicator genera for WMS, whether they were isolated from both hemolymph and muscle tissues in most WMS crabs. Only S. chilikensis, S. putrefaciens, S. xiamenensis, V. alginolyticus, V. fluvialis, and V. parahaemolyticus met both two criteria. The six bacteria that met both two criteria and six fungi and another bacterium that unmatched any of two criteria were used to perform regression infection experiments based on Koch's postulates. S. chilikensis, S. putrefaciens, S. xiamenensis, V. alginolyticus, V. fluvialis, and V. parahaemolyticus met both two criteria, and the results indicate that they cause WMS in crabs independently.

CONCLUSIONS

This study fully demonstrated that our research strategy that integrates the microbiome and Koch's postulates can maximize the ability to catch pathogens in one net for the situation of multiple pathogens-one disease. Video Abstract.

Long-Term Application of a Synbiotic Chitosan and Acinetobacter KU011TH Mixture on the Growth Performance, Health Status, and Disease Resistance of Hybrid Catfish (Clarias gariepinus × C. macrocephalus) during Winter

The effects of potential synbiotic chitosan and Acinetobacter KU011TH mixture on growth performance, immune response, and A. hydrophila resistance were investigated for the first time. The control group was fed a basal diet (A), and group B was given the formula B diet that was supplemented with chitosan at 20 mL/kg diet via top dressing. The other synbiotic groups, C, D, and E, were top-dressed with the target probiotics at 1 × 10⁸, 1 × 10⁹, and 1 × 10¹⁰ CFU/kg diet, respectively, and coated with the same concentration of chitosan. Fish were continuously fed the five different feeds for 16 weeks during winter. At the end of the trial, the growth parameters of the test groups did not significantly differ from those of the control (p > 0.05). All the symbiotic-chitosan treatments significantly increased various hematological and serum immune parameters. Moreover, the expression levels of immune-related genes were strongly elevated in the head kidney and spleen, whereas upregulated expression was observed in the liver and whole blood (p < 0.05). Survival analysis indicated that fish in groups B and C showed significantly higher survival (84.33 ± 2.21 and 79.50 ± 6.34%) than those in groups A, D and E (55.33 ± 8.82%-74.00 ± 6.50) (p < 0.05) after injection with A. hydrophila for 14 days.

Effects of dietary supplementation of Gracilaria lemaneiformis-derived sulfated polysaccharides on the growth, antioxidant capacity, and innate immunity of rabbitfish (Siganus canaliculatus)

The dietary supplementation of red seaweed-derived polysaccharides has been shown to be beneficial to fish and shellfish aquaculture. However, the function of red seaweed (Gracilaria lemaneiformis)-extracted polysaccharide (GLP) on the health status of rabbitfish (Siganus canaliculatus) is still unknown. This study explored the influences of GLP on growth performance, antioxidant activity, and immunity of rabbitfish. Herein, the fish were fed commercial pelleted feed incorporated with the diverse amount of GLP: 0 (control), 0.10 (GLP0.10), and 0.15 g kg^-1 (GLP0.15) for 60 days. The results demonstrated that dietary GLP0.15 significantly elevated FBW and WG, while feed utilization efficiency improved (reduced feed conversion ratio and increased protein efficiency ratio) upon GLP0.10 treatment, regarding the control (P < 0.05). Also, dietary administration of GLP0.15 suggestively improved the serum acid phosphatase and lysozyme activity as well as hepatic total antioxidant capacity, catalase, and superoxide dismutase activity. In contrast, GLP0.15decreased the serum alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase, and malonaldehyde activity when compared to the control (P<0.05). Moreover, the lipase (36.08 and 16.46 U/mgprot in GLP0.10 and GLP0.15, respectively) and amylase (0.43 and 0.23 U/mgprot in GLP0.10 and GLP0.15, respectively) activity recorded the maximum values than the control (8.61 and 0.13 U/mgprot, respectively).Further, the intestinal morphometry was developed (such as increased villus length, width, and area) in the fish fed with a GLP-supplemented diet compared to the control. The KEGG pathway analysis unveiled that several differentially expressed genes (DEGs) in control vs. GLP0.10 and control vs. GLP0.15 were associated with metabolic or immune-associated pathways like antigen processing and presentation, phagosome, complement and coagulation cascades, and platelet activation. The DEGs, namely C3, f5, fgb, MHC1, and cfb, were evaluated in control vs. GLP0.10 and C3 and MHC1 in control vs. GLP0.15, suggesting their possible contributions to GLP-regulated immunity. Additionally, the cumulative mortality of rabbitfish after the Vibrio parahaemolyticus challenge was lower in both GLP0.10 (8.88%) and GLP0.15 (11.11%) than in control (33.33%) (P<0.05). Thus, these findings direct the potential use of GLP as an immunostimulant and growth promoter in rabbitfish aquaculture.

Probiotics and Immunostimulant modulate intestinal flora diversity in Reeves pond tortoise (Mauremys reevesii) and effects of Clostridium butyricum on its spleen transcriptome

In this study, we investigated the effects of Clostridium butyricum (group A), Bacillus subtilis (group B), and the immune enhancer algal β-1,3 glucan (group C) on the intestinal flora of Mauremys reevesii and the effects of C. butyricum on the transcriptome of M. reevesii splenic immune tissues. M. reevesii were assigened to four groups, each containing three replicates from 18 samples. Juvenile turtles with an initial weight of 106.35 ± 0.03 g were fed a basic diet containing no probiotics (control group D), or a basic diet containing the A, B, or C supplement. After the turtles had been fed for 60, 90, and 120 d of the experimental period, high-throughput sequencing of the 16S rRNA gene revealed no significant difference in alpha diversity among the four groups at 60 days of feeding (P > 0.05), and at 90 days, the alpha diversity in group A was significantly different (P < 0.05), with an increase of 26.62% in the Shannon index and a decrease of 83.33% in the Simpson index; at 120 d, the alpha diversity (Shannon index)showed a decreasing trend in order for groups A, B, and C, At the phylum level, the abundance of Bacteroidetes, Proteobacteria, and Fusobacteria in group A increased significantly with increasing feeding time (P < 0.05)，At the genus level, the abundance of Ruminococcaceae and Anaerotruncus in group A increased significantly compared with that in the other three groups (P < 0.05). Transcriptome analysis showed that 384 genes were differentially expressed in the spleen of M. reevesii, 195 genes were upregulated and 189 genes were downregulated, and C. butyricum TF201120 regulated the hematopoietic cell lineage signaling pathway in the spleen of M. reevesii (P < 0.05). The regulation of several identified immune-related genes was confirmed by qPCR, These results showed that C. butyricum, B. subtilis and the immune enhancer algal β-1,3 glucan can improve the intestinal flora of M. reevesii, with C. butyricum TF20 being the most effective and significantly enhancing the immunity of M. reevesii.

The genetic variability and evolution of red-spotted grouper nervous necrosis virus quasispecies can be associated with its virulence

Nervous necrosis virus, NNV, is a neurotropic virus that causes viral nervous necrosis disease in a wide range of fish species, including European sea bass (Dicentrarchus labrax). NNV has a bisegmented (+) ssRNA genome consisting of RNA1, which encodes the RNA polymerase, and RNA2, encoding the capsid protein. The most prevalent NNV species in sea bass is red-spotted grouper nervous necrosis virus (RGNNV), causing high mortality in larvae and juveniles. Reverse genetics studies have associated amino acid 270 of the RGNNV capsid protein with RGNNV virulence in sea bass. NNV infection generates quasispecies and reassortants able to adapt to various selective pressures, such as host immune response or switching between host species. To better understand the variability of RGNNV populations and their association with RGNNV virulence, sea bass specimens were infected with two RGNNV recombinant viruses, a wild-type, rDl956, highly virulent to sea bass, and a single-mutant virus, Mut270Dl965, less virulent to this host. Both viral genome segments were quantified in brain by RT-qPCR, and genetic variability of whole-genome quasispecies was studied by Next Generation Sequencing (NGS). Copies of RNA1 and RNA2 in brains of fish infected with the low virulent virus were 1,000-fold lower than those in brains of fish infected with the virulent virus. In addition, differences between the two experimental groups in the Ts/Tv ratio, recombination frequency and genetic heterogeneity of the mutant spectra in the RNA2 segment were found. These results show that the entire quasispecies of a bisegmented RNA virus changes as a consequence of a single point mutation in the consensus sequence of one of its segments. Sea bream (Sparus aurata) is an asymptomatic carrier for RGNNV, thus rDl965 is considered a low-virulence isolate in this species. To assess whether the quasispecies characteristics of rDl965 were conserved in another host showing different susceptibility, juvenile sea bream were infected with rDl965 and analyzed as above described. Interestingly, both viral load and genetic variability of rDl965 in seabream were similar to those of Mut270Dl965 in sea bass. This result suggests that the genetic variability and evolution of RGNNV mutant spectra may be associated with its virulence.

Effects of dietary walnut (Juglans regia) leaves extract on immunity, gene expression responses, and disease resistance in Oreochromis niloticus

The dietary effects of walnut leaf extract (WLE) on the growth, immunity, and resistance of Oreochromis niloticus to bacterial infection have been investigated. Five diets were prepared with various WLE doses of 0, 250, 500, 750, and 1000 mg/kg, termed as Con (control), WLE250, WLE500, WLE750, and WLE1000, respectively. Fish (11.67 ± 0.21 g) were fed these diets for 60 days and then challenged with Plesiomonas shigelloides. Before the challenge, it was observed that dietary WLE did not significantly affect the growth, blood proteins (globulin, albumin, and total protein), and liver function enzymes (ALT and AST) activities. The WLE250 group significantly increased serum SOD and CAT activities more than other groups. The serum immunological indices (lysozyme and myeloperoxidase activities) and hematological parameters (phagocytic activity %, phagocytic index, respiratory burst activity, and potential activity) were significantly increased in the WLE groups compared with the Con group. The expression of IgM heavy chain, IL-1β, and IL-8 genes were significantly upregulated in all WLE-supplemented groups in comparison with the Con group. The fish survival rates (SR; %) post challenge in the Con, WLE250, WLE500, WLE750 and WLE1000 groups were 40.0%, 49.3%, 86.7%, 73.3%, and 70.7%, respectively. The Kaplan-Meier survivorship curves illustrated that the highest SR% was found in the WLE500 group (86.7%) amongst the other groups. Accordingly, we can suggest that feeding O. niloticus with a diet supplied with WLE at a dose rate of 500 mg/kg over 60 days could enrich haemato-immune responses and increase the fish survival against the challenge with P. shigelloides. These results recommend using WLE as a herbal dietary supplement to substitute antibiotic use in aquafeed.

Relationship of environmental factors in pond water and dynamic changes of gut microbes of sea bass Lateolabrax japonicus

The effect of structure of gut microbes on the health of host has attracted increasing attention. Sea bass Lateolabrax japonicus is an important farmed fish in China. The relationship of the dynamic changes of intestinal bacterial communities in L. japonicus and the cultural water environment is very important for healthy culture. Here, the diversity and abundance of the gut microbial communities of L. japonicus were evaluated during the culture using 16S rRNA Illumina sequencing. Both the opportunistic pathogens Aeromonas (1.68%), Vibrio (1.59%), and Acinetobacter (1.22%); and the potential probiotics Lactobacillus (2.27%), Bacillus (1.16%), and Lactococcus (0.37%) were distributed in the gut of L. japonicus. The increasing concentration of nitrogen of water environments with the increase of culture time significantly correlated with shifts in the microbial community structure: 40.04% of gut microbial changes due to nitrogen concentration. Higher concentrations of nitrogen showed a significantly negative correlation with intestinal probiotics in L. japonicus. The results indicate that the abundance of intestinal bacteria of L. japonicus is mainly driven by the changes of environmental factors (e.g., nitrogen), and it’s very important that the linking environmental parameters with bacterial data of guts could be used as an early warning indicator in L. japonicus heath culture.

Design of Lactococcus lactis Strains Producing Garvicin A and/or Garvicin Q, Either Alone or Together with Nisin A or Nisin Z and High Antimicrobial Activity against Lactococcus garvieae

Lactococcus garvieae is a main ichthyopathogen in rainbow trout (Oncorhynchus mykiss, Walbaum) farming, although bacteriocinogenic L. garvieae with antimicrobial activity against virulent strains of this species have also been identified. Some of the bacteriocins characterized, such as garvicin A (GarA) and garvicin Q (GarQ), may show potential for the control of the virulent L. garvieae in food, feed and other biotechnological applications. In this study, we report on the design of Lactococcus lactis strains that produce the bacteriocins GarA and/or GarQ, either alone or together with nisin A (NisA) or nisin Z (NisZ). Synthetic genes encoding the signal peptide of the lactococcal protein Usp45 (SP_usp45), fused to mature GarA (lgnA) and/or mature GarQ (garQ) and their associated immunity genes (lgnI and garI, respectively), were cloned into the protein expression vectors pMG36c, which contains the P₃₂ constitutive promoter, and pNZ8048c, which contains the inducible P_nisA promoter. The transformation of recombinant vectors into lactococcal cells allowed for the production of GarA and/or GarQ by L. lactis subsp. cremoris NZ9000 and their co-production with NisA by Lactococcus lactis subsp. lactis DPC5598 and L. lactis subsp. lactis BB24. The strains L. lactis subsp. cremoris WA2-67 (pJFQI), a producer of GarQ and NisZ, and L. lactis subsp. cremoris WA2-67 (pJFQIAI), a producer of GarA, GarQ and NisZ, demonstrated the highest antimicrobial activity (5.1- to 10.7-fold and 17.3- to 68.2-fold, respectively) against virulent L. garvieae strains.

Exploring the mechanism of a novel recirculating aquaculture system based on water quality parameters and bacterial communities

To mitigate the serious environmental problems caused by aquaculture wastewater discharge, the development of improved aquaculture systems with more self-purification capacity and less environmental impact has become essential. A novel recirculating aquaculture system (RAS) with ecological ponds was introduced. However, the mechanism of how the ecological ponds decompose the nutrients from the residual feed and excrement of fish is still unclear. Therefore, we designed a seven-week field experiment to explore the dynamic of water quality and the shift of bacterial communities during the initial stage of the RAS ecosystem to maintain the stability of the system. According to the result, the dissolved oxygen concentration maintained at 5.63 to 10.22 mg·L^-1 in aquaculture water, and electrical conductivity increased by 100% to over 800 μs·cm^-1. High-throughput sequencing showed that the abundance and diversity of the bacterial communities in sediment samples were significantly higher than in water samples, and the Proteobacteria, Bacteroidetes, Cyanobacteria, and Actinobacteria were dominant phyla in all samples. The relative abundance of ammonia-oxidizing archaea, ammonia-oxidizing bacteria, and nitrite-oxidizing bacteria was lower than 0.10, but rising. Redundancy analysis suggested that TOC, EC, NO₃^-, and NO₂^- were the most important factors shaping the bacterial communities in aquaculture water. Our studies assessed the RAS with ecological ponds for the first time based on water quality parameters and bacterial communities, which indicates that decomposition capacity is insufficient but improved at the initial stage of the operation, and potential risks like eutrophication require attention.

Tocopherol-assisted magnetic Ag-Fe3O4-TiO2 nanocomposite for photocatalytic bacterial-inactivation with elucidation of mechanism and its hazardous level assessment with zebrafish model

In recent years, many endeavours have been prompted with photocatalytic nanomaterials by the need to eradicate pathogenic microorganisms from water bodies. Herein, a tocopherol-assisted Ag-Fe₃O₄-TiO₂ nanocomposite (TAFTN) was synthesized for photocatalytic bacterial inactivation. The prepared TAFTN became active under sunlight due to its narrowed bandgap, inactivating the bacterial contaminants via photo-induced ROS stress. The ROS radicals destroy bacteria by creating oxidative stress, which damages the cell membrane and cellular components such as nucleic acids and proteins. For the first time, the nano-LC-MS/MS-based quantitative proteomics reveals that the disrupted proteins are involved in a variety of cellular functions; the most of these are involved in the metabolic pathway, eventually leading to bacterial death during TAFTN-photocatalysis under sunlight. Furthermore, the toxicity analysis confirmed that the inactivated bacteria seemed to have no detrimental impact on zebrafish model, showing that the disinfected water via TAFTN-photocatalysis is enormously safe. Furthermore, the TAFTN-photocatalysis successfully killed the bacterial cells in natural seawater, indicating the consistent photocatalytic efficacy when recycled repeatedly. The results of this work demonstrate that the produced nanocomposite might be a powerful recyclable and sunlight-active photocatalyst for environmental water treatment.

Allium-Derived Compound Propyl Propane Thiosulfonate (PTSO) Reduces Vibrio Populations and Increases Body Weight of European Seabass (Dicentrarchus labrax) Juveniles

The global demand for fish products is continuously increasing as the population grows, and aquaculture plays an important role in supplying this demand. However, industrial antibiotic misuse has contributed to the spread of antimicrobial resistance among pathogenic bacteria, therefore, several antibiotic alternatives have been proposed. In this study, we have analyzed the effects of Allium-derived propyl propane thiosulfonate (PTSO) in European seabass juveniles' growth and performance. These effects were tested by measuring the body weight and analyzing the gut microbiome of fish after 89 days of feeding trial. The relative abundance of potentially pathogenic Vibrio in the foregut and hindgut of supplemented fish decreased, while Pseudomonas and Kocuria increased significantly. Alpha diversity indices significantly decreased in both gut regions of fish fed with Allium-derived PTSO supplemented diet, as well as between bacterial community composition. These results may indicate a positive effect of the supplementation in the diet with Allium-derived PTSO, reducing potentially pathogenic Vibrio and increasing body weight at the end of the experiment (89 days). However, this supplementation with Allium-derived PTSO produces changes in the diversity and composition of microbial communities, so further experiments would be necessary to explore bacterial community composition and health relationship.

Effects of two host-associated probiotics Bacillus mojavensis B191 and Bacillus subtilis MRS11 on growth performance, intestinal morphology, expression of immune-related genes and disease resistance of Nile tilapia (Oreochromis niloticus) against Streptococcusiniae

The intensification and diversification of production systems have increased the incidence of diseases, which are usually treated with antibiotics. However, its use should be restricted due to the increasing prevalence of antibiotic-resistant bacteria. Probiotics represent therefore an alternative environmentally friendly strategy for improving growth and disease resistance in aquaculture. Considering that host-derived probiotics may offer greater advantages than those from other environments in terms of safety and efficacy, two potential host-associated probiotic strains (Bacillus mojavensis B191 and Bacillus subtilis MRS11) were used in the present study, which were previously isolated from intestinal mucus of Nile tilapia (Oreochromis niloticus). This study was conducted to assess the effects of dietary administration of two Bacillus strains on growth performance, intestinal morphology, immunity, and disease resistance of Nile tilapia. A total of 375 fish were randomly divided into five groups in triplicate. Nile tilapia were fed a basal diet (control group) or a basal diet supplemented with Bacillus mojavensis B191 (BM) or Bacillus subtilis MRS11 (BS) spores at different concentrations of 1 × 10⁶ (BM6 and BS6, respectively) and 1 × 10⁸ (BM8 and BS8, respectively) CFU/g of feed for 60 days. Moreover, the survival rate of tilapia upon challenge with Streptococcus iniae was determined following the feeding trial. After the feeding trial, the growth performances were significantly improved in all probiotic-fed groups, with the BS8 group being the highest. Light and electron microscopy observations revealed elevated goblet cells, intestinal villus length (except BM8), microvilli length, microvilli density, and perimeter ratio increase in the intestine of all probiotic-fed groups compared with the control group. Regarding the expression analysis, HSP70 gene was only up-regulated in the BM8 group and a general trend of up-regulation of some immune-related cytokines (TGF-β, IL-10, TNF-α and IL-1β) was observed in all probiotic-fed groups. Likewise, the best protection against Streptococcus iniae was observed in the BS8 group, followed by BS6, BM6 and BM8 groups. Altogether, dietary probiotic supplementation with BS8 and BM6 may improve growth performance, intestinal morphology, immunity, and disease resistance in Nile tilapia.

Signature selection forces and evolutionary divergence of immune-survival genes compared between two important shrimp species

In recent years, shrimp aquaculture industry had grown significantly to become the major source of global shrimp production. Despite that, shrimp aquaculture production was impeded by various shrimp diseases over the past decades. Interestingly, different shrimp species demonstrated variable levels of immune strength and survival (immune-survival) ability towards different diseases, especially the much stronger immune-survival ability shown by the ancient shrimp species, Macrobrachium rosenbergii compared to other shrimp species. In this study, two important shrimp species, M. rosenbergii and Penaeus monodon (disease tolerant strain) (uninfected control and VpAHPND-infected) were compared to uncover the potential underlying genetic factors. The shrimp species were sampled, followed by RNA extraction and cDNA conversion. Five important immune-survival genes (C-type Lectin, HMGB, STAT, ALF3, and ATPase 8/6) were selected for PCR, sequencing, and subsequent genetics analysis. The overall genetic analyses conducted, including Analysis of Molecular Variance (AMOVA) and population differentiation, showed significant genetic differentiation (p<0.05) between different genes of M. rosenbergii and P. monodon. There was greater genetic divergence identified between HMGB subgroups of P. monodon (uninfected control and VpAHPND-infected) compared to other genes. Besides that, based on neutrality tests conducted, purifying selection was determined to be the main evolutionary driving force of M. rosenbergii and P. monodon with stronger purifying selection exhibited in M. rosenbergii genes. Potential balancing selection was identified for VpAHPND-infected HMGB subgroup whereas directional selection was detected for HMGB (both species) and ATPase 8/6 (only P. monodon) genes. The divergence times between M. rosenbergii and P. monodon genes were estimated through Bayesian molecular clock analysis, which were 438.6 mya (C-type Lectin), 1885.4 mya (HMGB), 432.6 mya (STAT), 448.1 mya (ALF3), and 426.4 mya (ATPase 8/6) respectively. In conclusion, important selection forces and evolutionary divergence information of immune-survival genes between M. rosenbergii and P. monodon were successfully identified.

Effects of dietary Ginkgo biloba leaf extract on growth performance, immunity and environmental stress tolerance of Penaeus vannamei

Ginkgo biloba leaf extract (GBE) has been extensively used in the treatment of diseases due to its anti-inflammatory, antioxidant, and immunomodulatory effects. In aquaculture, GBE is widely used as a feed additive, which is important to enhance the immunity of aquatic animals. The current study evaluated the effects of adding GBE to the diet of Penaeus vannamei (P. vannamei) under intensive aquaculture. The GBE0 (control group), GBE1, GBE2, and GBE4 groups were fed a commercial feed supplemented with 0.0, 1.0, 2.0, and 4.0 g/kg GBE for 21 days, respectively. The results showed that dietary GBE could alleviate hepatopancreas tissue damage and improve the survival rate of shrimp, and dietary 2 g/kg GBE could significantly increase the total hemocyte count (THC), the hemocyanin content, the antioxidant gene's expression, and the activity of their encoded enzymes in P. vannamei. Furthermore, transcriptome data revealed that immunity-related genes were upregulated in the GBE2 group compared with the GBE0 group after 21 days of culture. Drug metabolism-cytochrome P450, sphingolipid metabolism, linoleic acid metabolism, glycerolipid metabolism, fat digestion and protein digestion and absorption pathways were significantly enriched, according to KEGG results. Surprisingly, all of the above KEGG-enriched pathways were significantly upregulated. These findings demonstrated that supplementing P. vannamei with 2 g/kg GBE improved its environmental adaptability by improving immunity, lipid metabolism, and detoxification. In this study, a comprehensive evaluation of the effects of dietary GBE on the intensive aquaculture of P. vannamei was conducted to provide a reference for the healthy culture of P. vannamei.

Metagenomic-based surveillance systems for antibiotic resistance in non-clinical settings

The success of antibiotics as a therapeutic agent has led to their ineffectiveness. The continuous use and misuse in clinical and non-clinical areas have led to the emergence and spread of antibiotic-resistant bacteria and its genetic determinants. This is a multi-dimensional problem that has now become a global health crisis. Antibiotic resistance research has primarily focused on the clinical healthcare sectors while overlooking the non-clinical sectors. The increasing antibiotic usage in the environment - including animals, plants, soil, and water - are drivers of antibiotic resistance and function as a transmission route for antibiotic resistant pathogens and is a source for resistance genes. These natural compartments are interconnected with each other and humans, allowing the spread of antibiotic resistance via horizontal gene transfer between commensal and pathogenic bacteria. Identifying and understanding genetic exchange within and between natural compartments can provide insight into the transmission, dissemination, and emergence mechanisms. The development of high-throughput DNA sequencing technologies has made antibiotic resistance research more accessible and feasible. In particular, the combination of metagenomics and powerful bioinformatic tools and platforms have facilitated the identification of microbial communities and has allowed access to genomic data by bypassing the need for isolating and culturing microorganisms. This review aimed to reflect on the different sequencing techniques, metagenomic approaches, and bioinformatics tools and pipelines with their respective advantages and limitations for antibiotic resistance research. These approaches can provide insight into resistance mechanisms, the microbial population, emerging pathogens, resistance genes, and their dissemination. This information can influence policies, develop preventative measures and alleviate the burden caused by antibiotic resistance.

Isolation and Characterization of a Novel Vibrio natriegens—Infecting Phage and Its Potential Therapeutic Application in Abalone Aquaculture

Phage-based pathogen control (i.e., phage therapy) has received increasing scientific attention to reduce and prevent the emergence, transmission, and detrimental effects of antibiotic resistance. In the current study, multidrug-resistant Vibrio natriegens strain AbY-1805 was isolated and tentatively identified as a pathogen causing the death of juvenile Pacific abalones (Haliotis discus hannai Ino). In order to apply phage therapy, instead of antibiotics, to treat and control V. natriegens infections in marine aquaculture environments, a lytic phage, vB_VnaS-L3, was isolated. It could effectively infect V. natriegens AbY-1805 with a short latent period (40 min) and high burst size (~890 PFU/cell). Treatment with vB_VnaS-L3 significantly reduced the mortality of juvenile abalones and maintained abalone feeding capacity over a 40-day V. natriegens challenge experiment. Comparative genomic and phylogenetic analyses suggested that vB_VnaS-L3 was a novel marine Siphoviridae-family phage. Furthermore, vB_VnaS-L3 had a narrow host range, possibly specific to the pathogenic V. natriegens strains. It also exhibited viability at a wide range of pH, temperature, and salinity. The short latent period, large burst size, high host specificity, and broad environmental adaptation suggest that phage vB_VnaS-L3 could potentially be developed as an alternative antimicrobial for the control and prevention of marine animal infections caused by pathogenic V. natriegens.

Why vaccines fail against Piscirickettsiosis in farmed salmon and trout and how to avoid it: A review

Piscirickettsiosis is the most severe, persistent, and damaging disease that has affected the Chilean salmon industry since its origins in the 1980s. As a preventive strategy for this disease, different vaccines have been developed and used over the last 30 years. However, vaccinated salmon and trout frequently die in the sea cages and the use of antibiotics is still high demonstrating the low efficiency of the available vaccines. The reasons why the vaccines fail so often are still debated, but it could involve different extrinsic and intrinsic factors. Among the extrinsic factors, mainly associated with chronic stress, we can distinguish: 1) biotic including coinfection with sea lice, sealions attacks or harmful algal blooms; 2) abiotic including low oxygen or high temperature; and 3) farm-management factors including overcrowding or chemical delousing treatments. Among the intrinsic factors, we can distinguish: 1) fish-related factors including host's genetic variability (species, population and individual), sex or age; 2) pathogen-related factors including their variability and ability to evade host immune responses; and 3) vaccine-related factors including low immunogenicity and poor matches with the circulating pathogen strain. Based on the available evidence, in order to improve the development and the efficacy of vaccines against P. salmonis we recommend: a) Do not perform efficacy evaluations by intraperitoneal injection of pathogens because they generate an artificial protective immune response, instead cohabitation or immersion challenges must be used; b) Evaluate the diversity of pathogen strains in the field and ensure a good antigenic match with the vaccines; c) Investigate whether host genetic diversity could be improved, e.g. through selection, in favor of better and longer responses to vaccination; d) To reduce the stressful effects at the cage level, controlling the co-infection of pathogens and avoiding fish overcrowding. To date, we do not know the immunological mechanisms by which the vaccines against P. salmonis may or may not generate protection. More studies are required to identify what type of response, cellular or molecular, is required to develop effective vaccines.

Virulence Genes and In Vitro Antibiotic Profile of Photobacterium damselae Strains, Isolated from Fish Reared in Greek Aquaculture Facilities

Bacteria belonging to the species Photobacterium damselae are pathogens of cultured marine fish, causing diseases of high importance, such as Pasteurellosis. Thus, they are considered a major threat to the aquaculture sector. Despite the great importance of fish mariculture for the Greek economy, the distribution and abundance of these bacteria are not well documented in aquaculture units in Greece. Keeping this in mind, the scope of the present study was to investigate the presence, antibiotic profile, and virulence of Photobacterium bacteria originating from a representative sample of mariculture units throughout Greece. Samples were collected from diseased fish belonging to three different cultured fish species, namely Sparus aurata, Dicentrarchus labrax, and Pagrus pagrus, from both the Aegean and the Ionian Sea. Tissue samples were cultured in agar media, and bacteria were molecularly identified using both bacterial universal and species-specific primer pairs for Photobacterium spp. Additionally, the identified strains were characterized for the presence of virulence genes as well as antibiotic profiles. According to the results, the aforementioned bacteria are distributed in the Greek aquaculture units and are characterized by high pathogenicity based on the abundance of virulence genes. Furthermore, the majority of the detected strains exhibit some level of antibiotic resistance. In summary, our results indicate the need for systematic surveillance and study of their antibiotic profiles in Greek aquaculture since these bacteria constitute a major threat to the sector.

Novel Aeromonas Phage Ahy-Yong1 and Its Protective Effects against Aeromonas hydrophila in Brocade Carp (Cyprinus aka Koi)

Aeromonas hydrophila is a zoonotic pathogen and an important fish pathogen. A new lytic phage, Ahy-yong1, against multi-antibiotic-resistant pathogen A. hydrophila was isolated, identified, and tentatively used in therapy. Ahy-yong1 possesses a head of approximately 66 nm in diameter and a short tail of approximately 26 nm in length and 32 nm in width. Its complete dsDNA genome is 43,374 bp with a G + C content of 59.4%, containing 52 predicted opening reading frames (ORFs). Taxonomic analysis indicated Ahy-yong1 as a new species of the Ahphunavirus genus of the Autographiviridae family of the Caudoviricetes class. Ahy-yong1 was active only against its indicator host strain among the 35 strains tested. It is stable at 30-40 °C and at pH 2-12. Aeromonas phage Ahy-yong1 revealed an effective biofilm removal capacity and an obvious protective effect in brocade carp (Cyprinus aka Koi). The average cumulative mortality for the brocade carp in the blank groups intraperitoneally injected with PBS was 1.7% ± 2.4%;for the control groups treated with A. hydrophila (10⁸ CFU/fish) via intraperitoneal injection, it was 100.00%;and for the test group I, successively treated with A. hydrophila (10⁸ CFU/fish) and Aeromonas phage Ahy-yong1 (10⁷ PFU/fish) via intraperitoneal injection witha time interval of 2 hours, it was only 43.4% ± 4.7%. Furthermore, the cumulative mortality of the test group II, successively treated with Aeromonas phage Ahy-yong1 (10⁷ PFU/fish) and A. hydrophila (10⁸ CFU/fish), was only 20.0% ± 8.2%, and that of the test group III, simultaneously treated with Aeromonas phage Ahy-yong1 (10⁷ PFU/fish) and A. hydrophila (10⁸ CFU/fish), was only 30.0% ± 8.2%. The results demonstrated that phage Ahy-yong1 was very effective in the therapies against A. hydrophila A18, prophylaxis was more effective than rescue, and earlier treatment was better for the reduction of mortality. This study enriches knowledge about Aeromonas phages.

Cross-Host Protection of Marine Bacteria Against Macroalgal Disease

Despite an increasing awareness of disease impacts on both cultivated and native seaweed populations, the development of marine probiotics has been limited and predominately focused on farmed animals. Bleaching (loss of thallus pigmentation) is one of the most prevalent diseases observed in marine macroalgae. Endemic probiotic bacteria have been characterized to prevent bleaching disease in red macroalgae Agarophyton vermiculophyllum and Delisea pulchra; however, the extent to which probiotic strains provide cross-protection to non-endemic hosts and the influence of native microbiota remain unknown. Using A. vermiculophyllum as a model, we demonstrate that co-inoculation with the pathogen Pseudoalteromonas arctica G-MAN6 and D. pulchra probiotic strain Phaeobacter sp. BS52 or Pseudoalteromonas sp. PB2-1 reduced the disease risks compared to the pathogen only treatment. Moreover, non-endemic probiotics outperformed the endemic probiotic strain Ralstonia sp. G-NY6 in the presence of the host natural microbiota. This study highlights how the native microbiota can impact the effectiveness of marine probiotics and illustrates the potential of harnessing probiotics that can function across different hosts to mitigate the impact of emerging marine diseases.

A Broad-Host-Range Phage Cocktail Selectively and Effectively Eliminates Vibrio Species from Shrimp Aquaculture Environment

The protective effects of a phage cocktail composed of vB_Vc_SrVc2 and vB_Vc_SrVc9 were tested in Pacific white shrimp (Litopenaeus vannamei) postlarvae, which were originally isolated from diseased shrimps and selected due to their broad-host-range properties against several pathogenic Vibsrio species. We used culture-dependent and culture-independent approaches to explore its effect on bacterial communities associated with shrimp postlarvae. Both methods revealed that the levels of Vibrio species were significantly reduced after phage cocktail administration. Phage-treated shrimp also exhibisuppted lesser damage and higher lipid accumulation in B cells of the hepatopancreas, as revealed by histopathological examination. Taken together, this study provides clear evidence that phage therapy can selectively and effectively reduce Vibrio species, thereby providing an environmentally safe alternative to the prophylactic use of antibiotics in shrimp aquaculture.

Animal board invited review: Widespread adoption of genetic technologies is key to sustainable expansion of global aquaculture

•

The extent of application of genetic technologies to aquaculture production varies widely by species and geography.

•

Achieving a more universal application of seed derived from scientifically based breeding programmes is an important goal in order to meet increasing global demands for seafood production.

•

This article reviews the status of genetic technologies across the world’s top 10 highly produced species.

•

Opportunities and barriers to achieving broad-scale uptake of genetic technologies in global aquaculture are discussed.

•

A future outlook for potential disruptive genetic technologies and how they might affect global aquaculture production is given.

Aquaculture production comprises a diverse range of species, geographies, and farming systems. The application of genetics and breeding technologies towards improved production is highly variable, ranging from the use of wild-sourced seed through to advanced family breeding programmes augmented by genomic techniques. This technical variation exists across some of the most highly produced species globally, with several of the top ten global species by volume generally lacking well-managed breeding programmes. Given the well-documented incremental and cumulative benefits of genetic improvement on production, this is a major missed opportunity. This short review focusses on (i) the status of application of selective breeding in the world’s most produced aquaculture species, (ii) the range of genetic technologies available and the opportunities they present, and (iii) a future outlook towards realising the potential contribution of genetic technologies to aquaculture sustainability and global food security.

Addition of Chlorella sorokiniana meal in the diet of juvenile rainbow trout (Oncorhynchus mykiss): Influence on fish growth, gut histology, oxidative stress, immune response, and disease resistance against Aeromonas salmonicida

This study aimed to assess the effects of dietary addition with Chlorella sorokiniana on fish growth, gut histology, antioxidant capacity, immune response, and disease resistance in rainbow trout. Three diets with similar proximate composition and different Chlorella meal levels were formulated. The control diet, 5% Chlorella diet, and 10% Chlorella diet contained 0%, 5% Chlorella meal, and 10% Chlorella meal, respectively. Each diet was assigned to triplicate tanks containing 30 fish (165.3 ± 0.6 g) in each tank. Fish were fed experimental diets for ninety days. The results showed that the addition of 5% Chlorella in the diet significantly increased feed intake by 19.3% and weight gain rate by 17.3% (P < 0.05) without affecting feed efficiency and gut histology. Diets containing Chlorella meal significantly decreased malonaldehyde contents in the plasma after the lipopolysaccharide (LPS) challenge (P < 0.05). Dietary supplementation with Chlorella meal significantly increased lysozyme (LZM) activity levels (in the head kidney) and immunoglobulin M (IgM) (in the head kidney) and complement component 3 (C3) (in the spleen) contents before the LPS challenge, and simultaneously increased LZM activity levels (in the plasma) and C3 contents (in the plasma and head kidney) after the LPS challenge (P < 0.05). Furthermore, dietary administration of Chlorella meal significantly increased the survival rate of fish infected with Aeromonas salmonicida (P < 0.05). In conclusion, C. sorokiniana can be used to improve fish growth, antioxidant capacity, and immunity.

Use of Probiotic Bacteria and Bacteriocins as an Alternative to Antibiotics in Aquaculture

In addition to their use in human medicine, antimicrobials are also used in food animals and aquaculture, and their use can be categorized as therapeutic against bacterial infections. The use of antimicrobials in aquaculture may involve a broad environmental application that affects a wide variety of bacteria, promoting the spread of bacterial resistance genes. Probiotics and bacteriocins, antimicrobial peptides produced by some types of lactic acid bacteria (LAB), have been successfully tested in aquatic animals as alternatives to control bacterial infections. Supplementation might have beneficial impacts on the intestinal microbiota, immune response, development, and/or weight gain, without the issues associated with antibiotic use. Thus, probiotics and bacteriocins represent feasible alternatives to antibiotics. Here, we provide an update with respect to the relevance of aquaculture in the animal protein production sector, as well as the present and future challenges generated by outbreaks and antimicrobial resistance, while highlighting the potential role of probiotics and bacteriocins to address these challenges. In addition, we conducted data analysis using a simple linear regression model to determine whether a linear relationship exists between probiotic dose added to feed and three variables of interest selected, including specific growth rate, feed conversion ratio, and lysozyme activity.

Modulation of growth performance, antioxidant capacity, non-specific immunity and disease resistance in largemouth bass (Micropterus salmoides) upon compound probiotic cultures inclusion

An 8-week feeding trial was conducted to evaluate the effects of dietary supplementation of compound probiotic cultures (CPC; Bacillus subtilis, Lactobacillus plantarum and Saccharomyces cerevisiae) on the growth performance, antioxidant capacity, non-specific immunity and disease resistance of juvenile largemouth bass. Triplicate groups of largemouth bass (average weight 42.05 ± 0.02 g), with a destiny of 30 individuals per tank, were fed diets supplemented with different concentration of compound probiotic cultures (CPC) (0%, CPC (0.0); 0.5%, CPC (0.5); 1.0%, CPC (1.0); 2.0%, CPC (2.0)). After the feeding trial, tissue samples of largemouth bass were collected and the challenge test with Aeromonas hydrophila was performed. Results indicated that the CPC supplementation produced no significant difference on the growth performance, feed utilization and body composition of largemouth bass, while significantly increased the cumulative survival rate in the Aeromonas hydrophila challenge test. Meanwhile, the inclusion of CPC elevated the hepatic antioxidant capacity, and the highest activity of antioxidant enzymes, including T-AOC, CAT, GPx and T-SOD, was observed in the CPC (2.0) group. Meanwhile, the transcription of Nrf2/keap1 and antioxidant related genes, including CAT, GPx, GST, SOD1 and SOD2, was significantly elevated with the inclusion of CPC. In addition, the inclusion of CPC improved the non-specific immunity of largemouth bass. The activity of serum lysozyme was significantly elevated in the CPC (2.0) group, while the transcription of RelA and pro-inflammatory factors, including TNF-α and IL-1β, was inhibited with the inclusion of CPC. Meanwhile, related genes potentially linked to RelA, including TLR2 and p38 MAPK, were detected that their relative expression was significantly inhibited with the inclusion of CPC. The current findings indicated that the inclusion of 2% CPC improved the antioxidant capacity, non-specific immunity and disease resistance of juvenile largemouth bass, and suggested that 2% CPC as a functional additive could be applied to the diet of juvenile largemouth bass in aquaculture practice.

Application of potential probiotic strain Streptomyces sp. SH5 on anti-Aeromonas infection in zebrafish larvae

Pre-treatment of Streptomyces sp. SH5 on zebrafish lead to a significant enhancement of larvae survival upon Aeromonas hydrophila challenging. SH5 was able to colonize in zebrafish approximately at 1 × 10^2.6 cells per fish for at least seven days. The presence of SH5 strongly repelled the A. hydrophila colonization in zebrafish, and maximally, a 67.53% reduction rate was achieved. A more diversified flora was discovered in the SH5-treated zebrafish larvae at both phylum and genus levels. The expression of immune response genes of SH5-treated zebrafish, including TLR3, lysozyme and NOS2α, were enhanced at initial stage, while, that of various inflammatory stimuli genes including 1L-1β, 1L-6 and MyD88 were decreased at all tested timepoints. SH5 was shown to inhibit virulence factors production and the expression of corresponding virulence genes in A. hydrophila, suggesting its quorum sensing inhibitory potential. These results indicated favorable application perspectives of SH5 in resisting pathogenic infection in aquaculture.

Two Foreign Antimicrobial Peptides Expressed in the Chloroplast of Porphyridium purpureum Possessed Antibacterial Properties

To solve the problem of antibiotic abuse in aquaculture and to utilize the application potential of antimicrobial peptides (AMPs), a chloroplast transformation system of Porphyridium purpureum was successfully constructed for effectively expressing two exogenous AMPs. The endogenous fragments of 16S rDNA/trnA-23S rDNA were used as flanking fragments for the homologous recombination in the chloroplast genome. Two AMPs encoded by the transformation vector were controlled by the native promoter psbB in a polycistron. The plasmids were transferred into P. purpureum via particle bombardment and the transformation vectors were screened using phosphinothricin (bar), a dominant selection marker under the control of the psbA promoter. Subsequently, in the positive transformed colonies, the exogenous fragments were found to be inserted in the flanking fragments directionally as expected and two foreign AMPs were successfully obtained. Finally, two exogenous peptides with antibacterial properties were obtained from the transformed strain. The two AMPs expressed by the transformed strain were shown to have similar inhibitory effects to antibiotics by inhibition tests. This suggested that AMPs can be introduced into aquaculture using baited microalgae, providing new ideas and ways to solve a series of aquaculture diseases caused by bacteria.