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

Redox profile of silver catfish challenged with Aeromonas hydrophila and treated with hexane extract of Hesperozygis ringens (Benth.) Epling through immersion bath

The growing increase in the fish farming sector has favored the establishment of bacterial outbreaks caused by Aeromonas hydrophila in several species. The hexane extract of Hesperozygis ringens (HEHR) (Lamiaceae) leaves increased the survival rate of silver catfish (Rhamdia quelen) experimentally infected by A. hydrophila. However, it is noteworthy that no reports have been found on the possible mechanisms of action of this extract in infected fish. This study aimed to evaluate the effect of the HEHR, administered through single immersion bath, on lipid peroxidation and antioxidant defenses in muscle and liver tissue of silver catfish challenged with A. hydrophila. The results showed that the oxidative status of silver catfish was altered, although oxidative stress was not triggered during the experiment. HEHR at 30 mg/L (HEHR30) was not characterized as a pro-oxidant agent in the presence of infection, unlike florfenicol and HEHR at 15 mg/L treatments in some cases. In short, HEHR30 provided an important increase in hepatic catalase activity, characterizing one of the possible mechanisms involved in the greater survival of fish experimentally infected by A. hydrophila. Additionally, HEHR30 did not induce lipid peroxidation, nor reduced antioxidant defenses of silver catfish infected or not by A. hydrophila.

Chicken egg lysozyme enhanced the growth performance, feed utilization, upregulated immune-related genes, and mitigated the impacts of Aeromonas hydrophila infection in Nile tilapia (Oreochromisniloticus)

Functional supplements, including lysozyme, are highly approved as immunostimulant and antibacterial agents with a high potential for use in aquaculture. In this regard, Nile tilapia was treated with lysozyme at 0, 0.5, 1, 1.5, and 3 g/kg for 60 days, then challenged with Aeromonas hydrophila. Fish were stocked in 15 glass aquaria (70 L each) with an equal initial weight of 10.72 ± 0.71 g per fish and 15 fish per aquarium. The regression analysis revealed that dietary lysozyme supplementation at 1.83-2 g/kg enhanced the growth performance, protein efficiency ratio, and protein productive value while reducing the feed conversion ratio of tilapia. Markedly, tilapia treated with lysozyme had a low mortality rate (30-50 %) compared to the control, which recorded a 70 % mortality rate after 15 days of challenge with A. hydrophila. The regression analysis also revealed that the highest lysozyme activity of tilapia-fed lysozyme for 60 days is achieved by 2.05 g/kg lysozyme. The expression of Nf-κb, IL-1β, and IL-8 genes is upregulated in tilapia-fed lysozyme at 0.5, 1, 1.5, and 3 g/kg for 60 days before and after A. hydrophila infection. The expression of GPX and CAT genes was higher in tilapia-fed lysozyme at 0.5, 1, 1.5, and 3 g/kg for 60 days before and after A. hydrophila infection. Before infection, the relative transcription of the lysozyme and C3 was upregulated in tilapia-fed lysozyme at 0.5, 1, 1.5, and 3 g/kg. However, lysozyme gene expression in tilapia treated with 0.5 g/kg lysozyme had no significant differences from those fed 0 g/kg lysozyme. After infection, the relative transcription of the lysozyme gene was upregulated in tilapia fed 1 and 1.5 g/kg, while tilapia fed 1 g/kg lysozyme had the highest C3 gene transcription. After infection, the hepatocytes in the livers of fish fed 0 g/kg lysozyme exhibited a noticeable fatty alteration, along with congestion, a light infiltration of inflammatory cells, and the start of necrosed cell regeneration. However, the livers of fish that received lysozyme were normal except for infiltrations of perivascular and interstitial mononuclear cells, depending on the supplementation dose. In conclusion, dietary lysozyme is recommended at 1.83-2.05 g/kg to gain high growth performance, immune response, and high resistance to A. hydrophila in Nile tilapia.

The role of Aeromonas genotyping in virulence for Dicentrarchus labrax

Aeromonas septicemia still represents a serious challenge facing the global aquaculture sector. In the present study, Aeromonas caviae and A. veronii were isolated from four diseased European seabass (Dicentrarchus labrax) farms experiencing a high mortality rate. Diseased fish showed haemorrhages on the external body surface with exophthalmia, cataracts, scale desquamation, skin ulcers and fin erosions. The most common post-mortem findings were congested internal organs, particularly the liver and posterior kidney. Twenty-eight A. Veronii and 11 A. caviae isolates were identified biochemically by the Vitek 2 system and then confirmed by PCR and phylogenetic analysis. Hemolysin (hlyA) and aerolysin (aer) were the most abundant virulence genes in the recovered isolates, followed by cytotoxic enterotoxin (act) and heat-stable enterotoxin (ast). A. caviae was more virulent than A. veronii for D. labrax fingerlings as LD50 ranging between (>1 × 108 -6.2 × 107 ) for A. veronii and (2.9 × 107 -8.3 × 107 ) for A. caviae. The sensitivity test indicated the effectiveness of norfloxacin, doxycycline and oxytetracycline against the tested isolates. Serum cortisol significantly increased in the infected groups, while catalase and glutathione peroxidase activities significantly decreased at 2 days post-infection (DPI) and then increased at 6 DPI. The presence of virulence genes was associated with bacterial pathogenicity expressed in fish mortality rate. Virulence genes also drastically affect cortisol levels more than catalase and glutathione peroxidase levels.

Nelumbo nucifera synthesized selenium nanoparticles modulate the immune-antioxidants, biochemical indices, and pro/anti-inflammatory cytokines pathways in Oreochromis niloticus infected with Aeromonas veronii

Bacterial infection is considered one of the major issues in fish culturing that results in economic losses. Metal nanoparticles are a cutting-edge and effective disease management and preventive strategy because of their antibacterial ability. In this investigation, the selenium nanoparticles were prepared by a biological method using Nelumbo nucifera leaves extract. The in-vitro antibacterial activity of N. nucifera synthesized selenium nanoparticles (NN-SeNPs) was tested against Aeromonas veronii. A treatment assay was conducted on 210 Oreochromis niloticus (average body weight: 27 ± 2.00 g). A preliminary approach was conducted on 90 fish for determination of the therapeutic concentration of NN-SeNPs which was found to be 4 mg/L. Fish (n = 120) were categorized into four groups for 10 days; G1 (control) and G2 (NN-SeNPs) were non-challenged and treated with 0 and 4 mg/L NN-SeNPs, respectively. While, G3 and G4 were infected with 2 × 106 CFU/mL of A. veronii and treated with 0 and 4 mg/L NN-SeNPs, respectively. NN-SeNPs exhibited an inhibition zone against A. veronii with a diameter of 16 ± 1.25 mm. The A. veronii infection increased the hepato-renal biomarkers (alanine and aspartate aminotransferases and creatinine) than the control group. An oxidative stress was the consequence of A. veronii infection (higher malondialdehyde and hydrogen peroxide levels with lower glutathione peroxidase superoxide, dismutase, and catalase activity). A. veronii infection resulted in lower immunological biomarker values (immunoglobulin M, lysozyme, and complement 3) with higher expression of the inflammatory cytokines (interleukin-1β and tumor necrosis factor-ɑ) as well as lower expression of the anti-inflammatory cytokines (interleukin-10 and transforming growth factor-β). Therapeutic application with 4 mg/L NN-SeNPs prevented the disease progression; and modulated the hepato-renal function disruptions, oxidant-immune dysfunction, as well as the pro/anti-inflammatory cytokines pathway in the A. veronii-infected fish. These findings suggest that NN-SeNPs, employed as a water therapy, can safeguard fish from the harmful effects of A. veronii and serve as a promising antibacterial agent for sustainable aquaculture.

Vitamin D ameliorates Aeromonas hydrophila-induced iron-dependent oxidative damage of grass carp splenic macrophages by manipulating Nrf2-mediated antioxidant pathway

Aeromonas hydrophila (A. hydrophila) is one of major pathogenic bacteria in aquaculture and potentially virulent to grass carp (Ctenopharyngodon idella). As an essential nutrient for fish, vitamin D3 (VD3) has been reported to play a role against oxidative stress, but the exact mechanism remains to be elusive. In this study, we found that A. hydrophila induced ferrugination and macrophage aggregation in the spleen of grass carp. Along this line, using the splenic macrophages as the model, the effects of VD3 on A. hydrophila-caused iron deposition and subsequent injuries were determined. In the context, 1,25D3 (the active form of VD3) significantly reduced cellular free Fe2+, lipid peroxidation and lactic dehydrogenase (LDH) release induced by A. hydrophila in the splenic macrophages, indicating the protective effects of VD3 on A. hydrophila-led to ferroptosis-related injuries. In support of this notion, 1,25D3 was effective in hindering ferroptosis inducers-stimulated LDH release in the same cells. Mechanically, 1,25D3 enhanced iron export protein (ferroportin1) and glutathione peroxidase 4 (GPX4) protein levels, and glutathione (GSH) contents via vitamin D receptor (VDR). Moreover, NF-E2-related factor 2 (Nrf2) pathway mediated the regulation of 1,25D3 on GPX4 protein expression and GSH synthesis. Meanwhile, 1,25D3 maintained the stability of Nrf2 proteins possibly by attenuating its ubiquitination degradation. Furthermore, in vivo experiments showed that 1,25D3 injection could not only improve the survival of fish infected by A. hydrophila, but also enhance GSH amounts and decrease malonaldehyde (MDA) contents and iron deposition in the spleen. In summary, our data for the first time suggest that VD3 is a potential antioxidant in fish to fight against A. hydrophila induced-ferroptotic damages.

Dietary Supplementation with R-(+)-Limonene Improves Growth, Metabolism, Stress, and Antioxidant Responses of Silver Catfish Uninfected and Infected with Aeromonas hydrophila

R-(+)-limonene is a monoterpene from plants of the genus Citrus with diverse biological properties. This research evaluated the effects of dietary supplementation with R-(+)-limonene on growth, metabolic parameters in plasma and liver, and the antioxidant and stress responses in silver catfish, Rhamdia quelen, challenged or not with Aeromonas hydrophila. Fish were fed for 67 days with different doses of R-(+)-limonene in the diet (control 0.0, L0.5, L1.0, and L2.0 mL/kg of diet). On the 60th day, a challenge with A. hydrophila was performed. R-(+)-limonene in the diet potentiated the productive performance of the fish. The metabolic and antioxidant responses indicate that R-(+)-limonene did not harm the health of the animals and made them more resistant to the bacterial challenge. Histological findings showed the hepatoprotective effect of dietary R-(+)-limonene against A. hydrophila. Igf1 mRNA levels were upregulated in the liver of fish fed with an L2.0 diet but downregulated with bacterial challenge. The expression levels of crh mRNA were higher in the brains of fish fed with the L2.0 diet. However, the L2.0 diet downregulated crh and hspa12a mRNA expression in the brains of infected fish. In conclusion, the results indicated that R-(+)-limonene can be considered a good dietary supplement for silver catfish.

Enteric Aeromonas Infection: a Common Enteric Bacterial Infection with a Novel Infection Pattern Detected in an Australian Population with Gastroenteritis

Aeromonas species are emerging human enteric pathogens. However, they are currently not routinely detected in many diagnostic laboratories, and information regarding Aeromonas enteric infections detected using molecular methods is lacking. Here, we investigated the detection of Aeromonas species and four other enteric bacterial pathogens in 341,330 fecal samples from patients with gastroenteritis processed in a large Australian diagnostic laboratory between 2015 and 2019. These enteric pathogens were detected using quantitative real-time PCR (qPCR) methods. Furthermore, we compared the qPCR cycle threshold (CT) values obtained from fecal samples that tested positive for Aeromonas only by molecular detection with those of samples that tested positive by both molecular detection and bacterial isolation methods. Aeromonas species were found to be the second most common bacterial enteric pathogens among patients with gastroenteritis. We observed a unique pattern of three infection peaks for Aeromonas, which correlated with the age of the patients. Aeromonas species were the most common enteric bacterial pathogens in children younger than 18 months. Fecal samples that tested positive for Aeromonas only by molecular detection had significantly higher CT values than fecal samples that tested positive by both molecular detection and bacterial culture. In conclusion, our findings reveal that Aeromonas enteric pathogens exhibit an age-related three-peak infection pattern, distinguishing them from other enteric bacterial pathogens. Moreover, the high rate of Aeromonas enteric infection discovered in this study suggests that Aeromonas species should be routinely tested in diagnostic laboratories. Our data also show that combining qPCR with bacterial culture can enhance the detection of enteric pathogens. IMPORTANCE Aeromonas species are emerging human enteric pathogens. However, these species are currently not routinely detected in many diagnostic laboratories, and no studies have reported the detection of Aeromonas enteric infection using molecular methods. We investigated the presence of Aeromonas species and four other enteric bacterial pathogens in 341,330 fecal samples from patients with gastroenteritis using quantitative real-time PCR (qPCR) methods. Interestingly, we discovered that Aeromonas species were the second most common bacterial enteric pathogens in patients with gastroenteritis, exhibiting a novel infection pattern compared to those of other enteric pathogens. Furthermore, we found that Aeromonas species were the most prevalent enteric bacterial pathogens in children aged 6 to 18 months. Our data also revealed that qPCR methods exhibit higher sensitivity in detecting enteric pathogens compared to that of bacterial culture alone. Moreover, combining qPCR with bacterial culture enhances the detection of enteric pathogens. These findings emphasize the importance of Aeromonas species in public health.

Bullfrogs (Lithobates catesbeianus) as a Potential Source of Foodborne Disease

In Mexico, bullfrogs (Lithobates catesbeianus) are produced as gourmet food. However, bullfrogs can be carriers of pathogens because the frogs' preferred living conditions occur in stagnant water. The present study aimed to identify bacteria that cause foodborne diseases or are associated with human diseases. For molecular identification, based on the sequential analysis by 16S rRNA or rpoD was conducted on all isolates obtained from bullfrog. A total of 91 bacterial isolates were obtained from bullfrogs; 14 genera and 23 species were identified, including Acinetobacter johnsonii 16.5%; Aeromonas media 14.3%; Aeromonas veronii 13.2%; Providencia rettgeri 7.7%; Citrobacter freundii 6.6%; Aeromonas caviae 4.4%; Aeromonas hydrophila and Elizabethkingia ursingii 3.3%; Pseudomonas stutzeri, Raoultella ornithinolytica, and Shewanella putrefaciens 2.2%; Acinetobacter guillouiae, Acinetobacter pseudolwoffii, Citrobacter portucalensis, Citrobacter werkmanii, Edwardsiella anguillarum, Klebsiella michiganensis, Kluyvera intermedia, Kocuria rosea, Myroides odoratimimus, Myroides odoratus, Proteus sp., and Proteus hauseri 1.1%. In this study, 49.4% of the isolates obtained cause foodborne disease, 19.8% are bacteria that play an important role in the spoilage of food, 5.5% of isolates have nosocomial significance, 13.2% of bacteria are considered to be pollutants of the ecosystem, and in the case of A. salmonicida and Edwardsiella anguillarum (12.1%) to have a negative impact on aquaculture. Acinetobacter pseudolwoffii and Citrobacter portucalensis have not been reported to cause disease. Lastly of these isolates, 97.8% (89/91) can cause disease by food consumption or by direct contact for immunocompromised persons. The presence of these bacteria in bullfrogs represents a significant problem for human health. There is evidence that these microorganisms are pathogenic and frogs may also be reservoirs.

The variation of intestinal autochthonous bacteria in cultured tiger pufferfish Takifugu rubripes

Intestinal autochthonous bacteria play important roles in the maintenance of the physiological homeostasis of animals, especially contributing to the host immune system. In the present study, the variation of autochthonous bacterial community in the intestinal tract of 2-7 months-old tiger pufferfish Takifugu rubripes and bacterial communities in the seawater of recirculating aquaculture system (RAS) and the following offshore sea cage aquaculture system (OSCS) were analyzed during the aquaculture period from May to October 2021. Proteobacteria was found to be the most dominant phyla in both intestinal and seawater bacterial communities, which accounted for 68.82% and 65.65% of the total bacterial abundance, respectively. Arcobacter was the most core bacterial taxon in the intestinal bacterial community, with the most dominant abundance (42.89%) at the genus level and dominant positions in co-occurrence relationships with other bacterial taxa (node-betweenness value of 150). Enterococcaceae was specifically enriched in the intestinal bacterial community of pufferfishes from RAS, while Vibrionaceae was enriched in the intestinal bacterial community from OSCS. The F-values of beta diversity analysis between intestinal and seawater bacterial communities generally increased from May (6.69) to October (32.32), indicating the increasing differences between the intestinal and seawater bacterial communities along with the aquaculture process. Four bacterial taxa of Weissella sp., Akkermansia muciniphila, Dietzia sp. and Psychrobacter pacificensis had significant correlations with immune response parameters, and they were suggested to be the indicators for immune status and pathological process of pufferfish. The knowledge about the specific core bacteria, potentially pathogenic bacteria and the change of bacterial community in the intestinal tract of cultured pufferfish is of great scientific significance and will contribute to the understanding of intestinal bacterial homeostasis and biosecurity practice in pufferfish aquaculture.

ROS Induced by Streptococcus agalactiae Activate Inflammatory Responses via the TNF-α/NF-κB Signaling Pathway in Golden Pompano Trachinotus ovatus (Linnaeus, 1758)

Streptococcus agalactiae is common pathogenic bacteria in aquaculture and can cause mass mortality after fish infection. This study aimed to investigate the effects of S. agalactiae infection on the immune and antioxidant regulatory mechanisms of golden pompano (Trachinotus ovatus). Serum and liver samples were obtained at 0, 6, 12, 24, 48, 96, and 120 h after golden pompano infection with S. agalactiae for enzyme activity and gene expression analyses. After infection with S. agalactiae, the content of reactive oxygen species (ROS) in serum was significantly increased (p < 0.05). Serum levels of glucose (GLU), alanine aminotransferase (ALT), aspartate aminotransferase (AST), and malondialdehyde (MDA) increased and then decreased (p < 0.05), reaching a maximum at 6 h. Serum antioxidant enzyme (LZM) activity increased significantly (p < 0.05) and reached a maximum at 120 h. In addition, the mRNA expression levels of antioxidant genes (SOD, CAT, and GPx) in the liver increased and then decreased, reaching the maximum at 24 h, 48 h, and 24 h, respectively. During the experimental period, the mRNA expression levels of NF-κB-related genes of the inflammatory signaling pathway inhibitory κB (IκB) showed an overall decreasing trend (p < 0.05) and the lowest expression at 120 h, whereas the mRNA expression levels of tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), IκB kinase (IKK), and nuclear factor NF-κB increased significantly (p < 0.05) and the highest expression was at 120 h. In conclusion, these results showed that S. agalactiae could activate internal regulatory signaling in the liver of golden pompano to induce defense and immune responses. This study is expected to lay a foundation to develop the healthy aquaculture of golden pompano and promote a more comprehensive understanding of its disease resistance mechanisms.

Effects of probiotics on Zebrafish model infected with Aeromonas hydrophila: spatial distribution, antimicrobial, and histopathological investigation

Usage of “probiotics” for treatment of food-borne pathogens associated diseases, makes a significant reduction in transmission of resistant bacteria, and antimicrobial resistance genes from aquaculture environments to humans. In this research, the authors aim to evaluate the immunomodulatory, and histological effects of two probiotic strains on the Zebrafish model. Fish models were treated with Lactobacillus delbrueckii (G2), Lactobacillus acidophilus (G3) and both probiotics (G4) and compared with the control group (G1) (only infected by pathogen and receiving no probiotic). Biometric tests, height, weight, and mortality rate of the fishes were assessed. Afterward, RT-PCR was conducted for bacterial existence of probiotic strains, and quantitative assessment of alterations in targeted immune genes. Subsequently, histological sampling was done for investigation of spatial distribution, and villus length in proximal, middle, and distal sections of intestinal tissues. Based on the results, G4 showed the highest gene expression for Lactobacillus acidophilus after 28 days (P < 0.05). G4 also showed an increase in the number of goblet cells and villus length in the middle and distal sections of intestinal tissue after 56 days. Furthermore, after 56 days, the highest number of intraepithelial cells was observed in the proximal sections of intestinal tissue in G4. G2 and G3 showed significant differences in comparison with G1 (P < 0.05). After 60 days, the highest gene expression for Lactobacillus bulgaricus was found in group treated with only this probiotic bacteria. The highest expression level of IL-1β and TNF-α were found in G1. The highest survival rate was in the case of groups only treated with Lactobacillus bulgaricus (G2). To sum up, it seems that usage of probiotics for the improvement of public health and fisheries industries can be helpful.

Short-Term Immune Responses of Gilthead Seabream (Sparus aurata) Juveniles against Photobacterium damselae subsp. piscicida

Photobacteriosis is a septicaemic bacterial disease affecting several marine species around the globe, resulting in significant economic losses. Although many studies have been performed related to the pathogen virulence and resistance factors, information regarding the host defence mechanisms activated once an infection takes place is still scarce. The present study was designed to understand innate immune responses of farmed juvenile gilthead seabream (Sparus aurata) after Photobacterium damselae subsp. piscicida (Phdp) infection. Therefore, two groups of seabream juveniles were intraperitoneally injected with 100 µL of PBS (placebo) or 100 µL of exponentially growing Phdp (1 × 10⁶ CFU/mL; infected). The blood, plasma, liver, and head kidney of six fish from each treatment were sampled immediately before infection and 3, 6, 9, 24 and 48 h after infection for the broad screening of fish immune and oxidative stress responses. Infected animals presented marked anaemia, neutrophilia and monocytosis, conditions that are correlated with an increased expression of genes related to inflammation and phagocytic activity. Similar studies with different fish species and bacteria can be useful for the definition of health biomarkers that might help fish farmers to prevent the occurrence of such diseases.

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.