Aeromonas salmonicida: updates on an old acquaintance

Disrupting quorum sensing as a strategy to inhibit bacterial virulence in human, animal, and plant pathogens

The development of sustainable alternatives to conventional antimicrobials is needed to address bacterial virulence while avoiding selecting resistant strains in a variety of fields, including human, animal, and plant health. Quorum sensing (QS), a bacterial communication system involved in noxious bacterial phenotypes such as virulence, motility, and biofilm formation, is of utmost interest. In this study, we harnessed the potential of the lactonase SsoPox to disrupt QS of human, fish, and plant pathogens. Lactonase treatment significantly alters phenotypes including biofilm formation, motility, and infection capacity. In plant pathogens, SsoPox decreased the production of plant cell wall degrading enzymes in Pectobacterium carotovorum and reduced the maceration of onions infected by Burkholderia glumae. In human pathogens, lactonase treatment significantly reduced biofilm formation in Acinetobacter baumannii, Burkholderia cepacia, and Pseudomonas aeruginosa, with the cytotoxicity of the latter being reduced by SsoPox treatment. In fish pathogens, lactonase treatment inhibited biofilm formation and bioluminescence in Vibrio harveyi and affected QS regulation in Aeromonas salmonicida. QS inhibition can thus be used to largely impact the virulence of bacterial pathogens and would constitute a global and sustainable approach for public, crop, and livestock health in line with the expectations of the One Health initiative.

Genomics of Re-Emergent Aeromonas salmonicida in Atlantic Salmon Outbreaks

Furunculosis, caused by Aeromonas salmonicida, poses a significant threat to both salmonid and non-salmonid fish in diverse aquatic environments. This study explores the genomic intricacies of re-emergent A. salmonicida outbreaks in Atlantic salmon (Salmo salar). Previous clinical cases have exhibited pathological characteristics, such as periorbital hemorrhages and gastrointestinal abnormalities. Genomic sequencing of three Chilean isolates (ASA04, ASA05, and CIBA_5017) and 25 previously described genomes determined the pan-genome, phylogenomics, insertion sequences, and restriction-modification systems. Unique gene families have contributed to an improved understanding of the psychrophilic and mesophilic clades, while phylogenomic analysis has been used to identify mesophilic and psychrophilic strains, thereby further differentiating between typical and atypical psychrophilic isolates. Diverse insertion sequences and restriction-modification patterns have highlighted genomic structural differences, and virulence factor predictions can emphasize exotoxin disparities, especially between psychrophilic and mesophilic strains. Thus, a novel plasmid was characterized which emphasized the role of plasmids in virulence and antibiotic resistance. The analysis of antibiotic resistance factors revealed resistance against various drug classes in Chilean strains. Overall, this study elucidates the genomic dynamics of re-emergent A. salmonicida and provides novel insights into their virulence, antibiotic resistance, and population structure.

Transcriptome analysis of turbot (Scophthalmus maximus) kidney responses to inactivated bivalent vaccine against Aeromonas salmonicida and Edwardsiella tarda

Turbot (Scophthalmus maximus) is a commercially important marine flatfish for global aquaculture. With intensive farming, turbot production is limited by several diseases, in which Aeromonas salmonicida and Edwardsiella tarda are two main causative agents. Vaccination is an effective and safe alternative to disease prevention compared to antibiotic treatment. In the previous study, we developed an inactivated bivalent vaccine against A. salmonicida and E. tarda with relative percent survival (RPS) of 77.1 %. To understand the protection mechanism in molecular basis of the inactivated bivalent vaccine against A. salmonicida and E. tarda, we use RNA-seq to analyze the transcriptomic profile of the kidney tissue after immunization. A total of 391,721,176 clean reads were generated in nine libraries by RNA-seq, and 96.35 % of the clean reads were mapped to the reference genome of S. maximus. 1458 (866 upregulated and 592 downregulated) and 2220 (1131 upregulated and 1089 downregulated) differentially expressed genes (DEGs) were obtained at 2 and 4 weeks post-vaccination, respectively. The DEGs were enriched in several important immune-related GO terms, including cytokine activity, immune response, and defense response. In addition, the analysis of several immune-related genes showed upregulation and downregulation, including pattern recognition receptors, complement system, cytokines, chemokines and immune cell surface markers. Eight DEGs (ccr10, calr, casr, mybpha, cd28, thr18, cd20a.3 and c5) were randomly selected for qRT-PCR analysis, which confirmed the validity of the RNA-seq. Our results provide valuable insight into the immune mechanism of inactivated bivalent vaccine against A. salmonicida and E. tarda in Scophthalmus maximus.

Detection of Aeromonas salmonicida subsp. salmonicida infection in zebrafish by labelling bacteria with GFP and a fluorescent probe based on the siderophore amonabactin

Zebrafish (Danio rerio) is an excellent model to study bacterial infections in fish and their treatment. We used zebrafish as a model of infection for Aeromonas salmonicida subsp. salmonicida (hereinafter A. salmonicida), the causative agent of fish furunculosis. The infection process of A. salmonicida was studied by immersion of zebrafish larvae in 2 different doses of the bacteria and the fish mortality was monitored for three days. The bacterium caused a high mortality (65 %) in zebrafish larvae only when they were exposed to a high bacterial concentration (107 bacterial cells/mL). To evaluate the use of fluorescence microscopy to follow A. salmonicida infection in vivo, two different fluorescent strains generated by labeling an A. salmonicida strain with either, the green fluorescent protein (GFP), or with a previously reported siderophore amonabactin-sulforhodamine B conjugate (AMB-SRB), were used. The distribution of both labeled bacterial strains in the larvae tissues was evaluated by conventional and confocal fluorescence microscopy. The fluorescent signal showed a greater intensity with the GFP-labeled bacteria, so it could be observed using conventional fluorescence microscopy. Since the AMB-SRB labeled bacteria showed a weaker signal, the larvae were imaged using a laser scanning confocal microscope after 48 h of exposure to the bacteria. Both fluorescent signals were mainly observed in the larvae digestive tract, suggesting that this is the main colonization route of zebrafish for waterborne A. salmonicida. This is the first report of the use of a siderophore-fluorophore conjugate to study a bacterial infection in fish. The use of a siderophore-fluorophore conjugate has the advantage that it is a specific marker and that does not require genetic manipulation of the bacteria.

Bacterial diseases in marine fish species: current trends and future prospects in disease management

The fisheries sub-sector of aquaculture-i.e., the pisciculture industry, contributes significantly to a country's economy, employing a sizable proportion of the population. It also makes important contributions to household food security because the current demand for animal protein cannot be fulfilled by harvesting wild fish from riverines, lakes, dams, and oceans. For good pond management techniques and sustaining fish health, the fisherfolk, and the industry require well-established regulatory structures, efficient disease management strategies, and other extended services. In rearing marine fish, infections resulting from disease outbreaks are a weighty concern because they can cause considerable economic loss due to morbidity and mortality. Consequently, to find effective solutions for the prevention and control of the major diseases limiting fish production in aquaculture, multidisciplinary studies on the traits of potential fish pathogens, the biology of the fish as hosts, and an adequate understanding of the global environmental factors are fundamental. This review highlights the various bacterial diseases and their causative pathogens prevalent in the pisciculture industry and the current solutions while emphasising marine fish species. Given that preexisting methods are known to have several disadvantages, other sustainable alternatives like antimicrobial peptides, synthetic peptides, probiotics, and medicinal treatments have emerged to be an enormous potential solution to these challenges.

An Outbreak of Aeromonas salmonicida in Juvenile Siberian Sturgeons (Acipenser baerii)

Aeromonas salmonicida is one of the major threats to world aquaculture, causing fish furunculosis and high mortality rates in cultured fish, particularly salmonids. Although Aeromonas spp. is a thoroughly studied pathogen, little is known regarding aeromoniasis in sturgeons. After a mortality outbreak, four juvenile sturgeons (Acipenser baerii) were submitted for autopsy and tissue samples were collected for histopathological and microbiological studies. The external examination revealed size heterogenicity, skin hyperpigmentation and reduced body condition of sturgeons. Within the abdominal cavity, mild hepatomegaly and splenomegaly were observed, as well as generalized organic congestion. Histology revealed severe multifocal haemorrhagic and ulcerative dermatitis, mainly localized in the dorsal and latero-ventral areas of fish. The histological study also showed moderate to severe inflammation of gills and organic lesions compatible with septicaemia. Bacterial isolates were identified as Aeromonas salmonicida subsp. salmonicida using MALDI-TOF MS and PCR. Overall, the lesions first described here are consistent with those previously reported in other cultured fish species and contribute to a better understanding of the pathogenesis of Aeromonas salmonicida subsp. salmonicida in the Siberian sturgeon, aside from providing new diagnostic tools for bacterial diseases impacting the fast-growing industry of caviar.

Characterization of atypical pathogenic Aeromonas salmonicida isolated from a diseased Siberian sturgeon (Acipenser baerii)

Acipenser baerii (Siberian sturgeon) is native to Kazakhstan and is currently endangered and is listed within the first class of protected animals. Sturgeon aquaculture is becoming an important tool for the recovery of this endangered species. Nonetheless, diseases involving typical symptoms of skin ulceration and systemic bacterial hemorrhagic septicemia have occurred in cultured A. baerii on a fish farm located in Western Kazakhstan. In this study, an infectious strain of bacteria isolated from an ulcer of diseased A. baerii was identified as Aeromonas salmonicida (strain AB001). This identification involved analyses of 16S rRNA, gyrB, rpoD, and flaA genes' sequences. Even though strain AB001 belongs to A. salmonicida, it exhibited noticeable mobility and growth at temperatures of ≥37 °C. Profiling of virulence genes uncovered the presence of seven such genes related to pathogenicity. Antibiotic sensitivity testing showed that the strain is sensitive to aminoglycosides, amphenicols, nitrofurans, quinolones, and tetracyclines. Half-lethal doses (LD₅₀) of strain AB001 for Oreochromis mossambicus and A. baerii were determined: respectively 1.7 × 10⁸ and 7.2 × 10⁷ colony-forming units per mL. The experimentally induced infection revealed that strain AB001 causes considerable histological lesions in O. mossambicus, including tissue degeneration, necrosis, and hemorrhages of varied severity.

Occurrence of Aeromonas Species in the Cutaneous Mucus of Barbour’s Seahorses (Hippocampus barbouri) as Revealed by High-Throughput Sequencing

Although several studies have described the bacterial community composition associated with marine fish, there is limited information related to seahorses. Moreover, previous studies have demonstrated that the skin microbiota is useful for determining health status and common disorders in the host. This study, therefore, aimed to explore the skin bacterial community composition in Barbour’s seahorse (Hippocampus barbouri) using high-throughput sequencing of 16S ribosomal RNA genes. Water and sediment samples from the surrounding environment were also analyzed for comparative purposes. The results revealed that sequences affiliated with the Shewanellaceae family were dominant in the skin of female Barbour’s seahorses and sediment samples, whereas sequences affiliated with the Bacillaceae family were dominant in the skin of male Barbour’s seahorses. Interestingly, sequences affiliated with the Aeromonas genus were found in the skin of Barbour’s seahorses, whose abundance was slightly similar between the female and male specimens. Further comparative analysis showed that the presence of Aeromonas species in the skin of Barbour’s seahorses was strongly influenced by the surrounding sediment. Given that some Aeromonas species are known to be important pathogens in humans and fish, these results may be used for further research on the dependency of the skin microbial composition on the environment as well as determine whether the presence of Aeromonas and other detected species has implications on seahorse health.

Effects of Resource Availability and Antibiotic Residues on Intestinal Antibiotic Resistance in Bellamya aeruginosa

Widespread and inappropriate use of antibiotics has been shown to increase the spread of antibiotics and antimicrobial resistance genes (ARGs) in aquatic environments and organisms. Antibiotic use for the treatment of human and animal diseases is increasing continuously globally. However, the effects of legal antibiotic concentrations on benthic consumers in freshwater environments remain unclear. In the present study, we tested the growth response of Bellamya aeruginosa to florfenicol (FF) for 84 days under high and low concentrations of sediment organic matter (carbon [C] and nitrogen [N]). We characterized FF and sediment organic matter impact on the bacterial community, ARGs, and metabolic pathways in the intestine using metagenomic sequencing and analysis. The high concentrations of organic matter in the sediment impacted the growth, intestinal bacterial community, intestinal ARGs, and microbiome metabolic pathways of B. aeruginosa. B. aeruginosa growth increased significantly following exposure to high organic matter content sediment. Proteobacteria, at the phylum level, and Aeromonas at the genus level, were enriched in the intestines. In particular, fragments of four opportunistic pathogens enriched in the intestine of high organic matter content sediment groups, Aeromonas hydrophila, Aeromonas caviae, Aeromonas veronii, and Aeromonas salmonicida, carried 14 ARGs. The metabolic pathways of the B. aeruginosa intestine microbiome were activated and showed a significant positive correlation with sediment organic matter concentrations. In addition, genetic information processing and metabolic functions may be inhibited by the combined exposure to sediment C, N, and FF. The findings of the present study suggest that antibiotic resistance dissemination from benthic animals to the upper trophic levels in freshwater lakes should be studied further.

Cytotoxicity and Antimicrobial Resistance of Aeromonas Strains Isolated from Fresh Produce and Irrigation Water

The genus Aeromonas has received constant attention in different areas, from aquaculture and veterinary medicine to food safety, where more and more frequent isolates are occurring with increased resistance to antibiotics. The present paper studied the interaction of Aeromonas strains isolated from fresh produce and water with different eukaryotic cell types with the aim of better understanding the cytotoxic capacity of these strains. To study host-cell pathogen interactions in Aeromonas, we used HT-29, Vero, J774A.1, and primary mouse embryonic fibroblasts. These interactions were analyzed by confocal microscopy to determine the cytotoxicity of the strains. We also used Galleria mellonella larvae to test their pathogenicity in this experimental model. Our results demonstrated that two strains showed high cytotoxicity in epithelial cells, fibroblasts, and macrophages. Furthermore, these strains showed high virulence using the G. mellonella model. All strains used in this paper generally showed low levels of resistance to the different families of the antibiotics being tested. These results indicated that some strains of Aeromonas present in vegetables and water pose a potential health hazard, displaying very high in vitro and in vivo virulence. This pathogenic potential, and some recent concerning findings on antimicrobial resistance in Aeromonas, encourage further efforts in examining the precise significance of Aeromonas strains isolated from foods for human consumption.

The Pathogen Aeromonas salmonicida achromogenes Induces Fast Immune and Microbiota Modifications in Rainbow Trout

Environmental stressors can disrupt the relationship between the microbiota and the host and lead to the loss of its functions. Among them, bacterial infection caused by Aeromonas salmonicida, the causative agent of furunculosis, results in high mortality in salmonid aquaculture. Here, rainbow trout were exposed to A. salmonicida achromogenes and its effects on the taxonomic composition and structure of the microbiota was assessed on different epithelia (gills, skin, and caudal fin) at 6 and 72 h post-infection (hpi) using the V1-V3 region of the 16S rRNA sequencing. Moreover, the infection by the pathogen and immune gene responses were evaluated in the head kidney by qPCR. Our results suggested that α-diversity was highly diverse but predominated by a few taxa while β-diversity was affected very early by infection in the gills after 6 h, subsequently affecting the microbiota of the skin and caudal fin. A dysbiosis of the microbiota and an increase in genera known to be opportunistic pathogens (Aeromonas, Pseudomonas) were also identified. Furthermore, an increase in pro-inflammatory cytokines and virulence protein array (vapa) was observed in trout head kidney as soon as 6 hpi and remained elevated until 72 hpi, while the anti-inflammatory genes seemed repressed. This study suggests that the infection by A. salmonicida achromogenes can alter fish microbiota of gills in the few hours post-infection. This result can be useful to develop a non-invasive technique to prevent disease outbreak in aquaculture.

Bacterial Pathogenesis in Various Fish Diseases: Recent Advances and Specific Challenges in Vaccine Development

Aquaculture is a fast-growing food sector but is plagued by a plethora of bacterial pathogens that infect fish. The rearing of fish at high population densities in aquaculture facilities makes them highly susceptible to disease outbreaks, which can cause significant economic loss. Thus, immunity development in fish through vaccination against various pathogens of economically important aquaculture species has been extensively studied and has been largely accepted as a reliable method for preventing infections. Vaccination studies in aquaculture systems are strategically associated with the economically and environmentally sustainable management of aquaculture production worldwide. Historically, most licensed fish vaccines have been developed as inactivated pathogens combined with adjuvants and provided via immersion or injection. In comparison, live vaccines can simulate a whole pathogenic illness and elicit a strong immune response, making them better suited for oral or immersion-based therapy methods to control diseases. Advanced approaches in vaccine development involve targeting specific pathogenic components, including the use of recombinant genes and proteins. Vaccines produced using these techniques, some of which are currently commercially available, appear to elicit and promote higher levels of immunity than conventional fish vaccines. These technological advancements are promising for developing sustainable production processes for commercially important aquatic species. In this review, we explore the multitude of studies on fish bacterial pathogens undertaken in the last decade as well as the recent advances in vaccine development for aquaculture.

The Development of the Bacterial Community of Brown Trout (Salmo trutta) during Ontogeny

Brown trout (Salmo trutta) is an important aquaculture species in Germany, but its production faces challenges due to global warming and a high embryo mortality. Climate factors might influence the fish's bacterial community (BC) and thus increase embryo mortality. Yet, knowledge of the physiological BC during ontogeny in general is scarce. In this project, the BC of brown trout has been investigated in a period from unfertilized egg to 95 days post fertilization (dpf) using 16S rRNA gene amplicon sequencing. Developmental changes differed between early and late ontogeny and major differences in BC occurred especially during early developmental stages. Thus, analysis was conducted separately for 0 to 67 dpf and from 67 to 95 dpf. All analyzed stages were sampled in toto to avoid bias due to different sampling methods in different developmental stages. The most abundant phylum in the BC of all developmental stages was Pseudomonadota, while only two families (Comamonadaceae and Moraxellaceae) occurred in all developmental stages. The early developmental stages until 67 dpf displayed greater shifts in their BC regarding bacterial richness, microbial diversity, and taxonomic composition. Thereafter, in the fry stages, the BC seemed to stabilize and changes were moderate. In future studies, a reduction in the sampling time frames during early development, an increase in sampling numbers, and an attempt for biological reproduction in order to characterize the causes of these variations is recommended.

Exogenous polyunsaturated fatty acids (PUFAs) influence permeability, antimicrobial peptide resistance, biofilm formation and membrane phospholipid structure in an A-layer and non-A-layer strain of Aeromonas salmonicida

Aeromonas salmonicida is a Gram-negative bacterium that can infect a wide host range of fish populations, including salmonids and non-salmonids as well as freshwater and marine life. Some strains of A. salmonicida cause the disease furunculosis, which can cause lethargy, intestinal inflammation, ulcers, haemorrhaging and death. The infection is spread through fish-to-fish contact, and the presence of infection can have devastating effects on cultivated fish populations. The purpose of this study was to explore the ability of non-A-layer and A-layer A. salmonicida strains to incorporate polyunsaturated fatty acids (PUFAs) into their lipid profile and test the phenotypic effects thereof. Lipids were extracted from PUFA-exposed cultures and analysed for lipid modification by thin-layer chromatography and ultraperformance liquid chromatography-mass spectrometry, showing A. salmonicida, regardless of A-layer, capable of incorporating all seven of the PUFAs studied. Phenotypic effects were determined through the use of assays that tested for biofilm formation, membrane permeability and cyclic peptide susceptibility. Temperature-dependent effects on biofilm formation were observed, and PUFA exposure showed significant (p < .001) increases in membrane permeability as tested by the uptake of the hydrophobic compounds crystal violet and ethidium bromide. Additionally, some PUFAs elicited modest protection and vulnerability against the membrane-targeting cyclic peptides polymyxin B (PMB) and colistin. The diverse, strain-specific responses to exogenous PUFAs may allude to evolved adaptive strategies that enhance survival, persistence and virulence of non-pathogenic and pathogenic members of bacteria that oscillate between environmental and fish host niches.

14-3-3 Activated Bacterial Exotoxins AexT and ExoT Share Actin and the SH2 Domains of CRK Proteins as Targets for ADP-Ribosylation

Bacterial exotoxins with ADP-ribosyltransferase activity can be divided into distinct clades based on their domain organization. Exotoxins from several clades are known to modify actin at Arg177; but of the 14-3-3 dependent exotoxins only Aeromonas salmonicida exoenzyme T (AexT) has been reported to ADP-ribosylate actin. Given the extensive similarity among the 14-3-3 dependent exotoxins, we initiated a structural and biochemical comparison of these proteins. Structural modeling of AexT indicated a target binding site that shared homology with Pseudomonas aeruginosa Exoenzyme T (ExoT) but not with Exoenzyme S (ExoS). Biochemical analyses confirmed that the catalytic activities of both exotoxins were stimulated by agmatine, indicating that they ADP-ribosylate arginine residues in their targets. Side-by-side comparison of target protein modification showed that AexT had activity toward the SH2 domain of the Crk-like protein (CRKL), a known target for ExoT. We found that both AexT and ExoT ADP-ribosylated actin and in both cases, the modification compromised actin polymerization. Our results indicate that AexT and ExoT are functional homologs that affect cytoskeletal integrity via actin and signaling pathways to the cytoskeleton.

Characterizing a subtropical hypereutrophic lake: From physicochemical variables to shotgun metagenomic data

Lake Cajititlán is a subtropical and endorheic lake, which is heavily impacted by nutrient pollution. Agricultural runoff and poorly treated wastewater have entered this reservoir at alarming rates during past rainy seasons, causing the cultural eutrophication of this body of water and resulting in several massive fish kill events. In this study, shotgun metagenomic sequencing was used to examine the taxonomic and functional structure of microbial communities in Lake Cajititlán during the rainy season. Several water quality features and their interactions with microbial communities were also assessed to identify the major factors affecting the water quality and biota, specifically fish species. According to current water quality regulations, most of the physicochemical variables analyzed (dissolved oxygen, pH, Secchi disk, NH4+, NO3−, blue-green algae, total phosphorus, and chlorophyll-a) were outside of the permissible limits. Planktothrix agardhii and Microcystis aeruginosa were the most abundant phytoplankton species, and the dominant bacterial genera were Pseudomonas, Streptomyces, and Flavobacterium, with Pseudomonas fluorescens, Stenotrophomonas maltophilia, and Aeromonas veronii representing the most abundant bacterial species. All of these microorganisms have been reported to be potentially harmful to fish, and the latter three (P. fluorescens, S. maltophilia, A. veronii) also contain genes associated with pathogenicity in fish mortality (fur, luxS, aer, act, aha, exu, lip, ser). Genetic evidence from the microbial communities analyzed herein reveals that anthropogenic sources of nutrients in the lake altered genes involved in nitrogen, phosphorus, sulfur, and carbon metabolism, mainly at the beginning of the rainy season. These findings suggest that abiotic factors influence the structure of the microbial communities, along with the major biogeochemical cycles of Lake Cajititlán, resulting in temporal variations and an excess of microorganisms that can thrive in high-nutrient and low-oxygen environments. After reviewing the literature, this appears to be the first study that focuses on characterizing the water quality of a subtropical hypereutrophic lake through associations between physicochemical variables and shotgun metagenomic data. In addition, there are few studies that have coupled the metabolism of aquatic ecosystems with nutrient cycles.

Development of Two Recombinase Polymerase Amplification EXO (RPA-EXO) and Lateral Flow Dipstick (RPA-LFD) Techniques for the Rapid Visual Detection of Aeromonas salmonicida

Aeromonas salmonicida is the pathogen underlying furunculosis, causing a septicemic infection that influences both salmonid and non-salmonid fish. Early diagnosis of these contagions is essential for disease surveillance and prevention, so a rapid and sensitive approach is needed. Herein, a recombinase polymerase amplification EXO (RPA-EXO) assay and RPA with a lateral flow dipstick (RPA-LFD) were produced for testing A. salmonicida. The RPA-EXO and RPA-LFD primer sets were devised based on the conserved fragment sequence of the vapA gene. Then, RPA-EXO and RPA-LFD reaction systems were established, and the reaction temperature and time were optimized. After optimization, the RPA-EXO method was capable of testing A. salmonicida within 10 min, and the RPA-LFD method could detect A. salmonicida in only 5 min. The RPA-EXO and RPA-LFD methods exhibited high specificity with no cross-reaction with other strains. To assess sensitivity, a partial vapA gene was cloned, and serial plasmid dilutions were created ranging from 1 × 10⁶ to 1 × 10^-1 copies/μL. The detection limit of RPA-EXO was 1 × 10² copies/μL, and the detection limit of RPA-LFD was 1 copy/μL. For spiked turbot tissue samples, the sensitivity detection of A. salmonicida was 1.2 × 10¹ CFU/mL and 1.2 CFU/mL by RPA-EXO and RPA-LFD, respectively. In comparative analyses of clinical samples, the diagnostic results of RPA-EXO and RPA-LFD were compared with those of the standard conventional PCR test and showed nearly 100% consistency. Therefore, our RPA-EXO and RPA-LFD assays exhibited excellent specificity and sensitivity, which provided two simple, fast and dependable methods to conduct large-scale field investigations of A. salmonicida in resource-limited settings.

Microplastics act as a carrier for wastewater-borne pathogenic bacteria in sewage

Microplastic pollution, a pressing global environmental problem, has a severe impact on both aquatic ecosystems and public health worldwide. Due to the small size of microplastics, they are able to pass through the filtration systems of municipal wastewater treatment works (WWTWs). In recent years, studies have focused on the environmental abundance and ecotoxicological effects of microplastics, but there are limited studies investigating the colonization of microplastics by bacteria, especially those pathogenic ones. In this study, we examined the colonization and composition of the bacterial communities on polyethylene microbeads after incubation in raw sewage collected from three municipal WWTWs in Hong Kong (Sha Tin Sewage Treatment Works, Stonecutters Island Sewage Treatment Works, and Shek Wu Hui Sewage Treatment Works). Scanning electron microscopy (SEM) results indicate that bacterial cells were colonized on the surfaces of the microbeads and formed biofilms after sewage incubation. Metagenomic sequencing data demonstrated an increase in bacterial diversity after 21 days of sewage incubation when compared to shorter incubation periods of 6, 11 and 16 days. Importantly, human and fish pathogens such as Arcobacter cryaerophilus, Aeromonas salmonicida, Vibrio areninigrae and Vibrio navarrensis were found in the resident bacterial communities. Taken together, our results demonstrate that microplastics could act as a carrier for wastewater-borne pathogenic bacteria in municipal sewage.

Proteomic analysis of carp seminal plasma provides insights into the immune response to bacterial infection of the male reproductive system

Aeromonas salmonicida is recognized as a significant bacterial pathogen in ulcerative disease of cyprinid fish. However, the mechanism of immunity to these bacteria in common carp is still not well understood, especially the immune regulation in the gonad to bacterial infection. The aims of our study were to analyze changes in the seminal plasma proteome following A. salmonicida infection in carp males. The observed pathological changes in the tissue (liver, spleen, kidney and testis) morphology and upregulation of immune-related genes (tnfa2, il6a) confirmed the successful infection challenge. Using mass spectrometry-based label-free quantitative proteomics, we identified 1402 seminal plasma proteins, and 44 proteins (20 up- and 24 downregulated) were found to be differentially abundant between infected and control males. Most differentially abundant proteins were involved in the immune response mechanisms, such as acute phase response, complement activation and coagulation, inflammation, lipid metabolism, cell-cell and cell-matrix adhesion, creatine-phosphate biosynthesis and germ cell-Sertoli cell junction signaling. Bacterial infection also caused profound changes in expression of selected genes in the testis and hematopoietic organs, which contributed to changes in seminal proteins. The altered seminal proteins and bacterial proteins in seminal plasma may serve as valuable markers of infection in the testis.

Aeromonas salmonicida intra-species divergence revealed by the various strategies displayed when grazed by Tetrahymena pyriformis

Worldwide, Aeromonas salmonicida is a major bacterial pathogen of fish in both marine and freshwater environments. Despite psychrophilic growth being common for this species, the number of characterized mesophilic strains is increasing. Thus, this species may serve as a model for the study of intraspecies lifestyle diversity. Although bacteria are preyed upon by protozoan predators, their interaction inside or outside the phagocytic pathway of the predator can provide several advantages to the bacteria. To correlate intraspecies diversity with predation outcome, we studied the fate of psychrophilic and mesophilic strains of A. salmonicida co-cultured with the ciliate Tetrahymena pyriformis. Three types of outcome were observed: digestion, resistance to phagocytosis and pathogenicity. The psychrophilic strains are fully digested by the ciliate. In contrast, the mesophilic A. salmonicida subsp. pectinolytica strain is pathogenic to the ciliate. All the other mesophilic strains display mechanisms to resist phagocytosis and/or digestion, which allow them to survive ciliate predation. In some cases, passage through the phagocytic pathway resulted in a few mesophilic A. salmonicida being packaged inside fecal pellets. This study sheds light on the great phenotypic diversity observed in the complex range of mechanisms used by A. salmonicida to confront a predator.

Bacterial and parasite co-infection in Mexican golden trout (Oncorhynchus chrysogaster) by Aeromonas bestiarum, Aeromonas sobria, Plesiomonas shigelloides and Ichthyobodo necator

Background

Bacterial infections are responsible of high economic losses in aquaculture. Mexican golden trout (Oncorhynchus chrysogaster) is a threatened native trout species that has been introduced in aquaculture both for species conservation and breeding for production and for which no studies of bacterial infections have been reported.

Case presentation

Fish from juvenile stages of Mexican golden trout showed an infectious outbreak in a farm in co-culture with rainbow trout (Oncorhynchus mykiss), showing external puntiform red lesions around the mouth and caudal pedunculus resembling furuncles by Aeromonas spp. and causing an accumulated mortality of 91%. Isolation and molecular identification of bacteria from lesions and internal organs showed the presence of Aeromonas bestiarum, Aeromonas sobria, Plesiomonas shigelloides and Ichthyobodo necator isolated from a single individual. All bacterial isolates were resistant to amoxicillin-clavulanic acid and cefazoline. P. shigelloides was resistant to third generation β-lactamics.

Conclusions

This is the first report of coinfection by Aeromonas bestiarum, Aeromonas sobria, Plesiomonas shigelloides and Ichthyobodo necator in an individual of Mexican golden trout in co-culture with rainbow trout. Resistance to β-lactams suggests the acquisition of genetic determinants from water contamination by human- or livestock-associated activities.

Outbreak in African lions of Yersinia pseudotuberculosis infection, with aberrant bacterial morphology

A concurrent outbreak of infection with Yersinia pseudotuberculosis occurred in adult captive African lions (Panthera leo). Two 17-y-old male lions and one 14-y-old female lion developed respiratory distress, lethargy, ataxia, and hyporexia. Within 3-5 d of the onset of clinical signs, one male and the female lion died and were submitted for postmortem examination. Macroscopically, the liver and spleen had multifocal-to-coalescing, semi-firm, pale-tan nodules throughout the parenchyma. The lungs were non-collapsed and marked by petechiae. Histologic examination identified lytic, necrosuppurative foci in the liver, spleen, lungs, and kidney, with abundant intralesional gram-negative coccobacilli in the male lion. Similar findings were seen in the female lion in the liver, spleen, kidney, and mesenteric lymph node; however, the intralesional bacterial colonies were more pleomorphic, comprising rod and filamentous morphologies. Aerobic bacterial culture of the liver, spleen, and lung revealed Y. pseudotuberculosis growth. The source of infection is unknown, and an epidemiologic study was performed. Sources to be considered are from the predation of rodent and/or bird reservoirs, or contaminated soil or water. Mortality associated with Y. pseudotuberculosis has been described in an African lion cub, however, to our knowledge, Y. pseudotuberculosis has not been reported in adult African lions, and this is only the second report of Y. pseudotuberculosis with aberrant bacterial morphology observed histologically.

The Early Immune Response of Lymphoid and Myeloid Head-Kidney Cells of Rainbow Trout (Oncorhynchus mykiss) Stimulated with Aeromonas salmonicida

The teleost head kidney is a highly relevant immune organ, and myeloid cells play a major role in this organ’s innate and adaptive immune responses. Because of their complexity, the early phases of the innate immune reaction of fish against bacteria are still poorly understood. In this study, naïve rainbow trout were stimulated with inactivated A. salmonicida and sampled at 12 h, 24 h and 7 d poststimulation. Cells from the head kidney were magnetically sorted with a monoclonal antibody mAB21 to obtain one (MAb21-positive) fraction enriched with myeloid cells and one (MAb21-negative) fraction enriched with lymphocytes and thrombocytes. The gene expression pattern of the resulting cell subpopulations was analysed using a panel of 43 immune-related genes. The results show an overall downregulation of the complement pathway and cytokine production at the considered time points. Some of the selected genes may be considered as parameters for diagnosing bacterial furunculosis of rainbow trout.

Antibacterial treatment of lumpfish (Cyclopterus lumpus) experimentally challenged with Vibrio anguillarum, atypical Aeromonas salmonicida and Pasteurella atlantica

Lumpfish is a novel farmed species used as cleaner fish for the removal of lice from farmed salmon. As often with new, farmed species, there are challenges with bacterial infections. The frequency of prescription of antibiotic agents to lumpfish is increasing, despite the lack of knowledge about appropriate doses, duration of treatment and application protocols for the various antibacterial agents. In the current study, we have tested the effect of medicated feed with florfenicol (FFC), oxolinic acid (OA) and flumequine (FLU) on lumpfish experimentally challenged with Vibrio anguillarum, atypical Aeromonas salmonicida and Pasteurella atlantica. We found that all three antibacterial agents efficiently treated lumpfish with vibriosis using 10 and 20 mg kg^-1  day^-1 of FFC, 25 mg kg^-1  day^-1 of OA and 25 mg kg^-1  day^-1 FLU, whereas only FFC (20 mg kg^-1  day^-1 ) had good effect on lumpfish with pasteurellosis. None of the antibacterial agents were efficient to treat lumpfish with atypical furunculosis. FFC 20 mg kg^-1  day^-1 showed promising results in the beginning of the experiment, but the mortality increased rapidly 14 days post-medication. Efficient treatment is important for the welfare of lumpfish and for reducing the risk of development of antibiotic-resistant bacteria. To our knowledge, this is the first study to establish protocols for antibacterial treatment of lumpfish.

Delivering the pain: an overview of the type III secretion system with special consideration for aquatic pathogens

Gram-negative bacteria are known to subvert eukaryotic cell physiological mechanisms using a wide array of virulence factors, among which the type three-secretion system (T3SS) is often one of the most important. The T3SS constitutes a needle-like apparatus that the bacterium uses to inject a diverse set of effector proteins directly into the cytoplasm of the host cells where they can hamper the host cellular machinery for a variety of purposes. While the structure of the T3SS is somewhat conserved and well described, effector proteins are much more diverse and specific for each pathogen. The T3SS can remodel the cytoskeleton integrity to promote intracellular invasion, as well as silence specific eukaryotic cell signals, notably to hinder or elude the immune response and cause apoptosis. This is also the case in aquatic bacterial pathogens where the T3SS can often play a central role in the establishment of disease, although it remains understudied in several species of important fish pathogens, notably in Yersinia ruckeri. In the present review, we summarise what is known of the T3SS, with a special focus on aquatic pathogens and suggest some possible avenues for research including the potential to target the T3SS for the development of new anti-virulence drugs.

Aeromonas: the multifaceted middleman in the One Health world

Aeromonas is at the interface of all the One Health components and represents an amazingly sound test case in the One Health approach, from economic loss in aquaculture tochallenges related to antibiotic-resistant bacteria selected from the environment. In human health, infections following leech therapy is an outstanding example of such One Health challenges. Aeromonads are not only ubiquitous environmental bacteria, able to rapidly colonize and cause opportunistic infections in humans and animals, they are also capable of promoting interactions and gene exchanges between the One Health components. This makes this genus a key amplifier of genetic transfer, especially of antibiotic resistance genes.

Transcriptome Analysis in the Head Kidney of Rainbow Trout (Oncorhynchus mykiss) Immunized with a Combined Vaccine of Formalin-Inactivated Aeromonas salmonicida and Vibrio anguillarum

This study aimed to identify the molecular mechanisms regulated by a combined vaccine against Aeromonas salmonicida and Vibrio anguillarum (O1 serotype). These bacteria cause furunculosis and vibriosis, respectively, and are associated with a high mortality in rainbow trout in Korea. The vaccine upregulated gene expression of TCRα, T-bet, sIgM, and mIgM, markers of an activated adaptive immune response. On days 1, 3, and 5, transcriptome analysis revealed 862 (430 up- and 432 downregulated), 492 (204 up- and 288 downregulated), and 741 (270 up- and 471 downregulated) differentially expressed genes (DEGs), respectively. Gene ontology (GO) enrichment analysis identified 377 (108 MF, 132 CC, 137 BP), 302 (60 MF, 180 CC, 62 BP), and 314 (115 MF, 129 CC, 70 BP) GOs at days 1, 3, and 5, respectively. Kyoto Encyclopedia of Genetic and Genomic enrichment analysis identified eight immune system-related pathways like cytokine-cytokine receptor interaction, NF-kappaB signaling pathway, TNF signaling pathway, NOD-like receptor signaling pathway, cytosolic DNA sensing pathway, cell adhesion molecule, complement and coagulation cascade, and antigen processing and presentation. In the analysis of the protein–protein interaction of immune-related DEGs, a total of 59, 21, and 21 interactional relationships were identified at days 1, 3, and 5, respectively, with TNF having the highest centrality at all three time points.

Salted duck eggs: the source for pathogens and antibiotic resistant bacteria

Salted duck eggs as a convenient food are very popular in China and Southeast Asia. Generally, they are produced by traditional curing methods. Here we used traditional methods to profile the bacterial community of salted duck eggs purchased from markets to systematically investigate their microbiological safety. 77 bacteria belonging to 14 genera were isolated. Bacillus related to flavor formation of salted duck eggs were the dominant genus. However, there existed some clinical pathogens which can cause food poisoning, such as Klebsiella pneumoniae, Staphylococcus aureus, and Aeromonas hydrophila. Moreover, PCA analysis showed that the composition of bacteria was related to the source and storage time rather than sampling sites. Besides, bacteria in the shell, intima and egg white of salted duck eggs were cross-linked. In addition, antimicrobial susceptibility testing indicated that resistant bacteria reached to 47.9%. And there was also no significant difference in bacterial resistance with sampling sites. Consequently, it's necessary to strengthen the food quality of salted duck eggs and improve personal dietary habit.

SUPPLEMENTARY INFORMATION

Supplementary information accompanies this paper at 10.1007/s13197-020-04962-w.

Whole-genome association study searching for QTL for Aeromonas salmonicida resistance in rainbow trout

Aeromonas salmonicida subsp. salmonicida, the causative agent of furunculosis, has extensive negative effects on wild and farmed salmonids worldwide. Vaccination induces some protection under certain conditions but disease outbreaks occur even in vaccinated fish. Therefore, alternative disease control approaches are required to ensure the sustainable expansion of rainbow trout aquaculture. Selective breeding can be applied to enhance host resistance to pathogens. The present work used genome-wide association study (GWAS) to identify quantitative trait loci (QTL) associated with A. salmonicida resistance in rainbow trout. A total 798 rainbow trout exposed to A. salmonicida by bath challenge revealed 614 susceptible and 138 resistant fish. Genotyping was conducted using the 57 K single nucleotide polymorphism (SNP) array and the GWAS was performed for survival and time to death phenotypes. We identified a QTL on chromosome 16 and located positional candidate genes in the proximity of the most significant SNPs. In addition, samples from exposed fish were examined for expression of 24 immune-relevant genes indicating a systematic immune response to the infection. The present work demonstrated that resistance to A. salmonicida is moderately heritable with oligogenic architecture. These result will be useful for the future breeding programs for improving the natural resistance of rainbow trout against furunculosis.

Marine Transcriptomics Analysis for the Identification of New Antimicrobial Peptides

Antimicrobial peptides (AMPs) participate in the immune system to avoid infection, are present in all living organisms and can be used as drugs. Fish express numerous AMP families including defensins, cathelicidins, liver-expressed antimicrobial peptides (LEAPs), histone-derived peptides, and piscidins (a fish-specific AMP family). The present study demonstrates for the first time the occurrence of several AMPs in lionfish (Pterois volitans). Using the lionfish transcriptome, we identified four transcript sequences encoding cysteine-rich AMPs and two new transcripts encoding piscidin-like peptides. These AMPs are described for the first time in a species of the Scorpaenidae family. A functional approach on new pteroicidins was carried out to determine antimicrobial sequences and potential uses, with a view to using some of these AMPs for human health or in aquaculture.

Effect of Bivalent Vaccines against Vibrio anguillarum and Aeromonas salmonicida Subspecie achromogenes on Health and Survival of Turbot

The efficacy of intraperitoneal injection of an oil-based bivalent autogenous vaccine and the commercial vaccine AlphaJect 3000 (Pharmaq AS) to prevent atypical furunculosis and vibriosis in turbot was analyzed. The effect of both vaccines on health parameters and survival of fish after challenge with V. anguillarum and A. salmonicida subsp. achromogenes was tested. The autogenous vaccine conferred high levels of protection and long-lasting immunity against both pathogens with a single dose. However, severe side effects were observed in turbot injected with this autovaccine and minor negative effects with the AlphaJect 3000 vaccine and the adjuvant Montanide or Eolane. All vaccinated fish showed remarkable antibody agglutination titers, higher than those of control fish, which were maintained 160 d after vaccination. In conclusion, the autogenous bivalent vaccine induces long-lasting protection against atypical furunculosis and vibriosis in turbot, after administration of a single dose, at the cost of high side effects in fish. Therefore, the development of new vaccines should focus on autovaccines and the use of liquid paraffin adjuvants that increase protection with reduced or no side effects.

Lethal dose and histopathological alterations induced by Aeromonas salmonicida in experimentally challenged common carp, Cyprinus carpio

Aeromonas salmonicida is the obligate pathogen of fishes having zoonotic potential. It is reported to cause considerable losses in world aquaculture. The current study has successfully demonstrated the induction of histopathological lesions in experimentally infected common carp. In the current study, the lethal concentration (LD₅₀-96 h) of typical A. Salmonicida for common carp was found to be 1.5 × 10⁷CFU mL^-1. About 40% and 60% fish mortalities occurred after 72 h in the groups inoculated with 10⁷ and 10⁸ CFU mL^-1 bacterial suspension, respectively. The fish challenged with A. salmonicida showed symptoms like abnormal swimming behaviour, lethargy, intra-abdominal fluid, haemorrhages on the ventral side of the body, vent and fins. The signs proceeded with the death of fish. In the histological sections, severe pathological alterations were reported in the tissue sections of internal organs. The microscopic observation showed sinusoidal and large blood vessel congestion in the liver, profuse haemorrhage, necrosis and infiltration of blood cells in the internal organs. The tubular architecture was lost with the infiltration of leucocytes in the kidney. In gills, more intense and prominent lamellar fusion was observed with leucocytic infiltration, telangiectasia and hyperplasia of lamellar epithelial cells. In summary, we have experimentally induced the typical A. salmonicida infection in common carp. The study will provide a research foundation for further studies on the host-pathogen interaction, therapeutics and epidemiology of A. salmonicida.

Draft Genome Sequence of Aeromonas popoffii ID682, Isolated from a Natural Water Source in Idaho

A draft genome sequence of Aeromonas popoffii ID682, isolated from a natural water source in Idaho, is presented here. A. popoffii is a relatively understudied species within a diverse, expanding freshwater genus of bacteria. Here, we describe only the second genome published for this species to date.

Novel atypical Aeromonas salmonicida bath challenge model for juvenile ballan wrasse (Labrus bergylta, Ascanius)

Atypical Aeromonas salmonicida (aAs) is currently one of the most routinely recovered bacterial pathogens isolated during disease outbreaks in farmed cleaner fish, ballan wrasse (Labrus bergylta, Ascanius). Vibrionaceae family bacteria have also been isolated from ballan wrasse in Scotland. This study determined the infectivity, pathogenicity and virulence of aAs and Vibrionaceae isolates in juvenile farmed ballan wrasse (n = 50; approx. 2 g) using a bath challenge, and fish were monitored for a period of 16 days. Atypical As caused significant mortalities in contrast to Vibrionaceae isolates. Notably, differential virulence was observed between two aAs vapA type V strains at similar challenge doses. Diseased fish exhibited a systemic infection where aAs was detected in all analysed tissues (liver, spleen and kidney) by PCR and qPCR. Macroscopically, moribund and survivor fish exhibited hepatomegaly and splenomegaly. In moribund and surviving fish, histopathology showed granulomatous hepatitis with eosinophilic granular cells surrounding bacterial colonies and endocarditis along with splenic histiocytosis. This is the first report of a successful aAs bath challenge model for juvenile ballan wrasse which provides an important tool for future studies on vaccine efficacy and immunocompetence.

Identification and characterization of caspases genes in rainbow trout (Oncorhynchus mykiss) and their expression profiles after Aeromonas salmonicida and Vibrio anguillarum infection

Caspases are highly conserved cysteine-dependent aspartyl-specific proteases that play an important role in regulating cell death and inflammation. However, the caspase genes have not been systematically studied in rainbow trout (Oncorhynchus mykiss). Rainbow trout experienced 4 rounds (4R) of genome duplication in the evolutionary history. Thereby an increased numbers of paralogs are observed in trout, probably with more complicated gene functions. We identified 18 caspase genes in rainbow trout, including two inflammatory caspases (casp1a, casp1b), six apoptosis executioner caspases (casp3, casp3a1, casp3a2, casp3b, casp6, and casp7), nine apoptosis initiator caspases (casp2a, casp2b, casp8, casp9a, casp9b, casp10a, casp10b, casp20a, and casp20b) and one uncategorized caspase gene (casp17). To investigate the potentially physiological functions of caspase genes, we challenged the rainbow trout with Aeromonas salmonicida (A. salmonicida) and Vibrio anguillarum (V. anguillarum). Results showed that the CASP3-regulated intrinsic apoptosis was activated after A. salmonicida infection, while the CASP8 and CASP6-regulated extrinsic apoptosis exerted the greatest effect on trout challenged with V. anguillarum. In response to V. anguillarum infection, the data of RNA-Seq further showed the casp8 was tightly integrated with the significantly enriched Gene Ontology terms and functional pathways, including apoptosis regulation, pathogen detection and immunomodulation. Our study provides a foundation for the physiological functions and regulatory network of the caspase genes in teleosts.

Isolation and Characterization of Fish-Gut Bacillus spp. as Source of Natural Antimicrobial Compounds to Fight Aquaculture Bacterial Diseases

Aquaculture is responsible for more than 50% of global seafood consumption. Bacterial diseases are a major constraint to this sector and associated with misuse of antibiotics, pose serious threats to public health. Fish-symbionts, co-inhabitants of fish pathogens, might be a promising source of natural antimicrobial compounds (NACs) alternative to antibiotics, limiting bacterial diseases occurrence in aquafarms. In particular, sporeforming Bacillus spp. are known for their probiotic potential and production of NACs antagonistic of bacterial pathogens and are abundant in aquaculture fish guts. Harnessing the fish-gut microbial community potential, 172 sporeforming strains producing NACs were isolated from economically important aquaculture fish species, namely European seabass, gilthead seabream, and white seabream. We demonstrated that they possess anti-growth, anti-biofilm, or anti-quorum-sensing activities, to control bacterial infections and 52% of these isolates effectively antagonized important fish pathogens, including Aeromonas hydrophila, A. salmonicida, A. bivalvium, A. veronii, Vibrio anguillarum, V. harveyi, V. parahaemolyticus, V. vulnificus, Photobacterium damselae, Tenacibaculum maritimum, Edwardsiela tarda, and Shigella sonnei. By in vitro quantification of sporeformers' capacity to suppress growth and biofilm formation of fish pathogens, and by assessing their potential to interfere with pathogens communication, we identified three promising candidates to become probiotics or source of bioactive molecules to be used in aquaculture against bacterial aquaculture diseases.

Drosophila as a Model for Infectious Diseases

The fruit fly, Drosophila melanogaster, has been used to understand fundamental principles of genetics and biology for over a century. Drosophila is now also considered an essential tool to study mechanisms underlying numerous human genetic diseases. In this review, we will discuss how flies can be used to deepen our knowledge of infectious disease mechanisms in vivo. Flies make effective and applicable models for studying host-pathogen interactions thanks to their highly conserved innate immune systems and cellular processes commonly hijacked by pathogens. Drosophila researchers also possess the most powerful, rapid, and versatile tools for genetic manipulation in multicellular organisms. This allows for robust experiments in which specific pathogenic proteins can be expressed either one at a time or in conjunction with each other to dissect the molecular functions of each virulent factor in a cell-type-specific manner. Well documented phenotypes allow large genetic and pharmacological screens to be performed with relative ease using huge collections of mutant and transgenic strains that are publicly available. These factors combine to make Drosophila a powerful tool for dissecting out host-pathogen interactions as well as a tool to better understand how we can treat infectious diseases that pose risks to public health, including COVID-19, caused by SARS-CoV-2.

Fish infections associated with the genus Aeromonas: a review of the effects on oxidative status

The aim of this review was to summarize the current knowledge regarding the effects of aeromonosis on fish oxidative status. The bibliographic survey was carried out on the research platforms: Scopus and Science Direct. The keywords 'Aeromonas', 'fish' and 'oxidative status' (or 'oxidative stress', 'oxidative damage' and similar terms) were used. Scientific papers and short communications were considered. Studies involving fish aeromonosis and enzymatic or non-enzymatic markers of oxidative status were selected. The results of antioxidant enzymes activities/expressions after infection lack consistency, suggesting that these findings should be interpreted with caution. Most of the analysed studies pointed to an increase in reactive oxygen species, malondialdehyde and protein carbonylation levels, indicating possible oxidative damage caused by the infection. Thus, these three biomarkers are excellent indicators of oxidative stress during infection. Regarding respiratory burst activity, several studies have indicated increased activity, but other studies have indicated unchanged activity after infection. Nitric oxide levels also increased after infection in most studies. Therefore, it is suggested that the fish's immune system tries to fight a bacterial infection by releasing reactive oxygen and nitrogen species.

Antibiotics modulate biofilm formation in fish pathogenic isolates of atypical Aeromonas salmonicida

Atypical Aeromonas salmonicida causes furunculosis infections of non-salmonid fish, which requires antibiotic therapy. However, antibiotics may induce biofilm in some bacteria, which protects them against hostile conditions while allowing them to persist on surfaces, thus forming a reservoir for infection. The aim of this study was to determine whether atypical isolates of A. salmonicida increased biofilm in the presence of two antibiotics, florfenicol and oxytetracycline. A microtitre plate assay was used to quantify biofilm in the presence and absence of each antibiotic. Fifteen of 28 isolates formed biofilms under control conditions, while 23 of 28 isolates increased biofilm formation in the presence of at least one concentration of at least one antibiotic. For oxytetracycline, the most effective concentration causing biofilm to increase was one-quarter of that preventing visible bacterial growth, whereas for florfenicol it was one-half of this value. This is the first study to demonstrate that a bacterial pathogen of fish increases biofilm in response to antibiotics. Biofilm formation may increase the risk of re-infection in culture systems and this lifestyle favours the transmission of genetic material, which has implications for the dissemination of antibiotic-resistance genes and demonstrates the need for enhanced disease prevention measures against atypical A. salmonicida.

Recent Insights into Aeromonas salmonicida and Its Bacteriophages in Aquaculture: A Comprehensive Review

The emergence and spread of antimicrobial resistance in pathogenic bacteria of fish and shellfish have caused serious concerns in the aquaculture industry, owing to the potential health risks to humans and animals. Among these bacteria, Aeromonas salmonicida, which is one of the most important primary pathogens in salmonids, is responsible for significant economic losses in the global aquaculture industry, especially in salmonid farming because of its severe infectivity and acquisition of antimicrobial resistance. Therefore, interest in the use of alternative approaches to prevent and control A. salmonicida infections has increased in recent years, and several applications of bacteriophages (phages) have provided promising results. For several decades, A. salmonicida and phages infecting this fish pathogen have been thoroughly investigated in various research areas including aquaculture. The general overview of phage usage to control bacterial diseases in aquaculture, including the general advantages of this strategy, has been clearly described in previous reviews. Therefore, this review specifically focuses on providing insights into the phages infecting A. salmonicida, from basic research to biotechnological application in aquaculture, as well as recent advances in the study of A. salmonicida.

Aeromonas salmonicida activates rainbow trout IgM+ B cells signalling through Toll like receptors

As B cells are singularly equipped with a B cell receptor (BCR) and a range of innate receptors, they are able to integrate both antigen-specific and innate signals, with the latter being essential to reach an adequate level of activation. Whether teleost B cells sense pathogens through innate mechanisms has not yet been explored, despite the fact that fish B cells display a wider array of innate receptors than many mammalian B cell subsets. Hence, in the current study, we have investigated the effects of inactivated Aeromonas salmonicida, a Gram negative rainbow trout pathogen, on trout splenic IgM⁺ B cells in vitro in the presence or absence of different inhibitors of Toll-like receptor (TLR) signalling, to establish to what degree innate signals are contributing to the activation of B cells in teleosts. Our results demonstrate that most of the effects that A. salmonicida exerts on trout IgM⁺ B cells are significantly blocked in the presence of inhibitors of MyD88 and TRIF, important nodes in TLR signal pathways. Thus, the data presented demonstrates that, also in teleost, TLR signalling is essential for the activation of IgM⁺ B cells. These results will be useful for the future optimization of novel vaccines and adjuvants.

In-Depth In Silico Search for Cuttlefish (Sepia officinalis) Antimicrobial Peptides Following Bacterial Challenge of Haemocytes

Cuttlefish (Sepia officinalis) haemocytes are potential sources of antimicrobial peptides (AMPs). To study the immune response to Vibrio splendidus and identify new AMPs, an original approach was developed based on a differential transcriptomic study and an in-depth in silico analysis using multiple tools. Two de novo transcriptomes were retrieved from cuttlefish haemocytes following challenge by V. splendidus or not. A first analysis of the annotated transcripts revealed the presence of Toll/NF-κB pathway members, including newly identified factors such as So-TLR-h, So-IKK-h and So-Rel/NF-κB-h. Out of the eight Toll/NF-κB pathway members, seven were found up-regulated following V. splendidus challenge. Besides, immune factors involved in the immune response were also identified and up-regulated. However, no AMP was identified based on annotation or conserved pattern searches. We therefore performed an in-depth in silico analysis of unannotated transcripts based on differential expression and sequence characteristics, using several tools available like PepTraq, a homemade software program. Finally, five AMP candidates were synthesized. Among them, NF19, AV19 and GK28 displayed antibacterial activity against Gram-negative bacteria. Each peptide had a different spectrum of activity, notably against Vibrio species. GK28—the most active peptide—was not haemolytic, whereas NF19 and AV19 were haemolytic at concentrations between 50 and 100 µM, 5 to 10 times higher than their minimum inhibitory concentration.

Investigation of the Correlation between the Use of Antibiotics in Aquaculture Systems and Their Detection in Aquatic Environments: A Case Study of the Nera River Aquafarms in Italy

Antimicrobial resistance (AMR) arises mostly because of the use and abuse of antibiotics in both the human and veterinary sectors. Furthermore, antibiotic residues can be discharged into the environment, promoting the spread of AMR as well as becoming a potential ecotoxicological risk. Aquaculture is one such source of environmental pollution. In the present work, we evaluated the correlation between antibiotic consumption in 11 selected aquafarms located along the Nera River, and the detection of their residues in surface water and sediment samples gathered monthly from the same stream. Antibiotic consumption was calculated using the ESVAC defined daily doses for animals (DDDvet) method, and a chemical analysis was conducted using a multiclass method. Therefore, the ecological risk of antibiotics being detected in surface waters was established based on the risk quotient (RQ). A strong positive correlation was identified between the concentrations detected in surface waters and the consumption of flumequine (r = 72%, p-value = 0.0085) and florfenicol (r = 83%, p-value = 0.0008). The RQ, however, was low for all the detected antibiotics in surface waters. Therefore, we proved that the antibiotics used in aquaculture can reach the near aquatic environment, but, in this work, they did not represent a toxicological risk to the surrounding ecosystem.

Detection and characterization of Aeromonas salmonicida subsp. salmonicida infection in crucian carp Carassius auratus

Aeromonas salmonicida is one of the most important pathogens in salmonids and non-salmonids species. Nevertheless, very little was reported in cyprinids about A. salmonicida infection. Hence, a pathogenic A. salmonicida subsp. salmonicida, namely isolate GCA-518, was isolated from diseased crucian carp Carassius auratus. Its optimal growth conditions were at 28 °C, pH 7.0 and 1.5% NaCl. Furthermore, the quantitative real-time PCR (qPCR) targeting serine protease (aspA) gene was established for rapid detection of the lowest limit of 5.6 × 10² copies per reaction. The pathogenicity was confirmed in crucian carp by intraperitoneal infection. Histopathologic examination displayed multifocal necrosis and infiltration of inflammatory cells in gill, liver, kidney and intestine. This is the first report on typical A. salmonicida infection in cultured crucian carp.

Review of infectious agent occurrence in wild salmonids in British Columbia, Canada

Wild Pacific salmonids (WPS) are economically and culturally important to the Pacific North region. Most recently, some populations of WPS have been in decline. Of hypothesized factors contributing to the decline, infectious agents have been postulated to increase the risk of mortality in Pacific salmon. We present a literature review of both published journal and unpublished data to describe the distribution of infectious agents reported in wild Pacific salmonid populations in British Columbia (BC), Canada. We targeted 10 infectious agents, considered to potentially cause severe economic losses in Atlantic salmon or be of conservation concern for wild salmon in BC. The findings indicated a low frequency of infectious hematopoietic necrosis virus, piscine orthoreovirus, viral haemorrhagic septicaemia virus, Aeromonas salmonicida, Renibacterium salmoninarum, Piscirickettsia salmonis and other Rickettsia-like organisms, Yersinia ruckeri, Tenacibaculum maritimum and Moritella viscosa. No positive results were reported for infestations with Paramoeba perurans in peer-reviewed papers and the DFO Fish Pathology Program database. This review synthesizes existing information, as well as gaps therein, that can support the design and implementation of a long-term surveillance programme of infectious agents in wild salmonids in BC.

Comparative transcriptomics and histopathological analysis of crucian carp infection by atypical Aeromonas salmonicida

Aeromonas salmonicida is a ubiquitous fish pathogen known to cause furunculosis. With the emergence of new subtypes and the expansion of the host range, it has threatened the health of a variety of marine and freshwater fish, particularly the non-salmonids, manifesting differently from the classical furunculosis. Although there have been reports of infection by atypical strains on the crucian carp, the pathogenesis and tissue pathology remain unclear. In this study, transcriptomics and histopathology were used to analyze the immune response and lesions of crucian carp infected with A. salmonicida. Comparative analysis showed 6579 differentially expressed genes (DEGs) (3428 down-regulated and 3151 up-regulated) were identified on day 5 post-infection (5 dpi). Further annotation and analysis revealed that the DEGs were enriched in enzyme regulator activity, response to oxidative stress, iron ion homeostasis and other functions, and mitogen-activated protein kinase (MAPK), nuclear factor-κB (NF-κB), toll-like receptor (TLR), and nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) etc., and immune-related signaling pathways. Meanwhile, the four C-type lysozyme genes found in all DEGs were significantly up-regulated after infection. In addition, there was severe bleeding on the body of the infected fish. Also, the intestine, liver, spleen, and kidney showed varying degrees of inflammatory damage, especially the goblet cell hyperplasia of intestinal mucosa epithelium and degeneration and necrosis of renal tubular epithelium cells. Additionally, with the increase in pathogen concentration, the cumulative mortality increased, the severity of lesions in the hindgut and head-kidney tissues increased. The relative expression levels of four immune-related genes (TNF-α, IL-1β, IL-11, C-lysozyme) were also significantly upregulated, compared with the control (P < 0.05). In conclusion, this study provides a scientific basis for further study on the immune response, pathological diagnosis, and prevention of crucian carp infection caused by atypical A. salmonicida.

Comparative Study on A Novel Pathogen of European Seabass. Diversity of Aeromonas veronii in the Aegean Sea

Aeromonas veronii is an emerging pathogen causing severe pathology and mortalities in European seabass aquaculture in the Aegean Sea, Mediterranean. More than 50 strains of the pathogen were characterized biochemically and genetically in order to study the epidemiology of the disease, as well as the phylogeny and virulence of the bacterium. Based on the phenotypic characteristics, the isolates form three groups consisting of: (a) the West Aegean Sea, non-motile, non-pigment-producing strains, (b) the West Aegean Sea, motile, and pigment-producing strains and (c) the East Aegean Sea motile strains that produce minute amounts of pigment. All strains were highly similar at the genomic level; however, the pattern of West/East geographic origin was reflected in biochemical properties, in general genomic level comparison and in the putative virulent factors studied. Type VI secretion system was not detected in the western strains. The outer membrane protein (OMP) profile which contains proteins that are putative antigenic factors, was very similar between strains from the different areas. Although most of the OMPs were detected in all strains with great sequence similarity, diversification according to geographic origin was evident in known antigenic factors such as the maltoporin LamB. A systematic comparative analysis of the strains is presented and discussed in view of the emergence of A. veronii as a significant pathogen for the Mediterranean aquaculture.

Beyond the A-layer: adsorption of lipopolysaccharides and characterization of bacteriophage-insensitive mutants of Aeromonas salmonicida subsp. salmonicida

Aeromonas salmonicida subsp. salmonicida is a fish pathogen that causes furunculosis. Antibiotherapy used to treat furunculosis in fish has led to resistance. Virulent phages are increasingly seen as alternatives or complementary treatments against furunculosis in aquaculture environments. For phage therapy to be successful, it is essential to study the natural mechanisms of phage resistance in A. salmonicida subsp. salmonicida. Here, we generated bacteriophage-insensitive mutants (BIMs) of A. salmonicida subsp. salmonicida, using a myophage with broad host range and characterized them. Phage plaques were different depending on whether the A-layer surface array protein was expressed or not. The genome analysis of the BIMs helped to identify mutations in genes involved in the biogenesis of lipopolysaccharides (LPS) and on an uncharacterized gene (ASA_1998). The characterization of the LPS profile and gene complementation assays identified LPS as a phage receptor and confirmed the involvement of the uncharacterized protein ASA_1998 in phage infection. In addition, we confirmed that the presence of an A-layer at the bacterial surface could act as protection against phages. This study brings new elements into our understanding of the phage adsorption to A. salmonicida subsp. salmonicida cells.

Identification and expression of the hepcidin gene from brown trout (Salmo trutta) and functional analysis of its synthetic peptide

Hepcidin, a hepatic antimicrobial peptide, is a key player of the nonspecific immune system. The structure of hepcidin gene from brown trout (Bthepc) has been characterized at the molecular level. The 1158-bp mRNA generates a coding sequence (CDS) of 267 bp, which encodes an 88-amino acid protein. Molecular evolution analysis classified Bthepc to the family Salmonidae. Amino acid sequence homologies between Bthepc and hepcidin in other species such as Oncorhynchus mykiss, Salmo salar, and Hucho taimen were found to be 93.18%, 96.59%, and 92.05% respectively. The mature peptide and the signal peptide of Bthepc are made of 25 and 24 amino acids, respectively. Similar to the other species, eight conserved cysteines in the mature peptide of Bthepc are held together by four disulphide bonds. Expression profiling of Bthepc indicated its highest expression in the liver. Further, iron levels or inflammation did not induce the age-dependent expression of Bthepc. Bthepc mRNA expression analysis in six immune tissues (liver, gill, spleen, skin, head kidney and intestine) indicated different levels of increase when challenged with Aeromonas salmonicida and Aeromonas hydrophila. The antimicrobial activity of synthetic Bthepc to typical pathogens was verified in vitro. In addition, Bthepc showed moderate haemolytic activity to mammalian erythrocytes. The antimicrobial activity of Bthepc was attributed to the disruption of the bacterial outer membrane integrity, which was evident from our scanning electron microscopy results. In summary, hepcidin gene of brown trout was characterized, and its antimicrobial activity was verified on different levels.