Elevated biosynthesis of palmitic acid is required for zebrafish against Edwardsiella tarda infection

Involvement of mitochondrial fatty acid β-oxidation in the antiviral innate immune response in head kidney macrophages of large yellow croaker (Larimichthys crocea)

As a vital pathway for cellular energy production, mitochondrial fatty acid β-oxidation (FAO) is essential in regulating immune responses to bacterial pathogens and maintaining intracellular homeostasis in vertebrates. However, the specific role of FAO in antiviral innate immune response in macrophages remains insufficiently understood. In this study, virus infection simulated by poly(I:C) inhibited FAO, as indicated by the reduced expression of FAO-related genes and proteins in the head kidney of large yellow croaker, with similar results observed in poly(I:C)-stimulated macrophages. Then, inhibition of FAO by supplementary mildronate in vivo and etomoxir treatment in vitro revealed varying increases in the mRNA expression of antiviral innate immune response genes after stimulated by poly(I:C) in the head kidney and macrophages. Notably, etomoxir significantly facilitated the transcriptional up-regulation of the IFNh promoter by IRF3. Moreover, inhibiting FAO by knockdown of cpt1b promoted antiviral innate immune response triggered by poly(I:C) in macrophages. Conversely, activating FAO through overexpression of cpt1b or cpt2 significantly reduced the mRNA levels of antiviral response genes in macrophages stimulated by poly(I:C). Unlike etomoxir, cpt1b overexpression inhibited the transcriptional up-regulation of the IFNh promoter by IRF3. Furthermore, in vivo dietary palm oil feeding and in vitro exposure to palmitic acid inhibited the antiviral innate immune response triggered by poly(I:C) in the head kidney and macrophages, respectively. These effects were partly associated with FAO activation, as evidenced by etomoxir. In summary, this study elucidates FAO's critical role in regulating antiviral innate immune response in head kidney macrophages. These findings not only deepen insights into the interaction between metabolic remodeling and host immune responses, but also offer valuable guidance for developing nutritional strategies to improve antiviral immunity in aquaculture.

Aspartate metabolic flux promotes nitric oxide to eliminate both antibiotic-sensitive and -resistant Edwardsiella tarda in zebrafish

Introduction

Metabolic reprogramming potentiates host protection against antibiotic-sensitive or -resistant bacteria. However, it remains unclear whether a single reprogramming metabolite is effective enough to combat both antibiotic-sensitive and -resistant bacteria. This knowledge is key for implementing an antibiotic-free approach.

Methods

The reprogramming metabolome approach was adopted to characterize the metabolic state of zebrafish infected with tetracycline-sensitive and -resistant Edwardsiella tarda and to identify overlapping depressed metabolite in dying zebrafish as a reprogramming metabolite.

Results

Aspartate was identify overlapping depressed metabolite in dying zebrafish as a reprogramming metabolite. Exogenous aspartate protects zebrafish against infection caused by tetracycline-sensitive and -resistant E. tarda. Mechanistically, exogenous aspartate promotes nitric oxide (NO) biosynthesis. NO is a well-documented factor of promoting innate immunity against bacteria, but whether it can play a role in eliminating both tetracycline-sensitive and -resistant E. tarda is unknown. Thus, in this study, aspartate was replaced with sodium nitroprusside to provide NO, which led to similar aspartate-induced protection against tetracycline-sensitive and -resistant E. tarda.

Discussion

These findings support the conclusion that aspartate plays an important protective role through NO against both types of E. tarda. Importantly, we found that tetracycline-sensitive and -resistant E. tarda are sensitive to NO. Therefore, aspartate is an effective reprogramming metabolite that allows implementation of an antibiotic-free approach against bacterial pathogens.

Exploring the roles of phytobiotics in relieving the impacts of Edwardsiella tarda infection on fish: a mini-review

Edwardsiellosis caused by Edwardsiella tarda resulted in significant economic losses in aquaculture operations worldwide. This disease could infect a wide range of hosts, including freshwater, brackish water, and marine aquatic animals. Currently, antibiotics and vaccines are being used as prophylactic agents to overcome Edwardsiellosis in aquaculture. However, application of antibiotics has led to antibiotic resistance among pathogenic bacteria, and the antibiotic residues pose a threat to public health. Meanwhile, the use of vaccines to combat Edwardsiellosis requires intensive labor work and high costs. Thus, phytobiotics were attempted to be used as antimicrobial agents to minimize the impact of Edwardsiellosis in aquaculture. These phytobiotics may also provide farmers with new options to manage aquaculture species' health. The impact of Edwardsiellosis in aquaculture worldwide was elaborated on and highlighted in this review study, as well as the recent application of phytobiotics in aquaculture and the status of vaccines to combat Edwardsiellosis. This review also focuses on the potential of phytobiotics in improving aquatic animal growth performance, enhancing immune system function, and stimulating disease resistance.

Fructose potentiates the protective efficiency of live Edwardsiella tarda cell vaccine

Vaccination is an effective measure to prevent infection by pathogens. Live vaccines have higher protective efficacy than inactivated vaccines. However, how live vaccines interact with the host from a metabolic perspective is unknown. The present study aimed to explore whether a live Edwardsiella tarda vaccine regulates host metabolism and whether this regulation is related to the protective efficacy of the vaccine. Therefore, a gas chromatography mass spectrometry (GC-MS)-based metabolomics approach was used to investigate the metabolomic profile of mice serum after vaccination with live E. tarda vaccine. Fructose was identified as a key biomarker that contributes to the immune protection induced by the live vaccine. Moreover, co-administration of exogenous fructose and the live vaccine synergistically promoted survival of mice and fish after bacterial challenge. These results indicate that metabolites, especially fructose, can potentiate the live E. tarda vaccine to increase its protective efficiency.

Functional proteomics identify mannitol metabolism in serum resistance and therapeutic implications in Vibrio alginolyticus

Serum resistance is recognized as one of the most important pathogenic traits of bacterial pathogens, and no control measure is available. Based on our previous discovery that pathogenic Escherichia coli represses glycine, serine, and threonine metabolism to confer serum resistance and that the reactivation of this pathway by exogenous glycine could restore serum sensitivity, we further investigate the mechanism underlying the action of glycine in Vibrio alginolyticus. Thus, V. alginolyticus is treated with glycine, and the proteomic change is profiled with tandem mass tag-based quantitative proteomics. Compared to the control group, glycine treatment influences the expression of a total of 291 proteins. Among them, a trap-type mannitol/chloroaromatic compound transport system with periplasmic component, encoded by N646_0992, is the most significantly increased protein. In combination with the pathway enrichment analysis showing the altered fructose and mannitol metabolism, mannitol has emerged as a possible metabolite in enhancing the serum killing activity. To demonstrate this, exogenous mannitol reduces bacterial viability. This synergistic effect is further confirmed in a V. alginolyticus-Danio rerio infection model. Furthermore, the mechanism underlying mannitol-enabled serum killing is dependent on glycolysis and the pyruvate cycle that increases the deposition of complement components C3b and C5b-9 on the bacterial surface, whereas inhibiting glycolysis or the pyruvate cycle significantly weakened the synergistic effects and complement deposition. These data together suggest that mannitol is a potent metabolite in reversing the serum resistance of V. alginolyticus and has promising use in aquaculture.

Exogenous glycine promotes oxidation of glutathione and restores sensitivity of bacterial pathogens to serum-induced cell death

Pathogenic strains of bacteria are often highly adept at evading serum-induced cell death, which is an essential complement-mediated component of the innate immune response. This phenomenon, known as serum-resistance, is poorly understood, and as a result, no effective clinical tools are available to restore serum-sensitivity to pathogenic bacteria. Here, we provide evidence that exogenous glycine reverses defects in glycine, serine and threonine metabolism associated with serum resistance, restores susceptibility to serum-induced cell death, and alters redox balance and glutathione (GSH) metabolism. More specifically, in Vibrio alginolyticus and Escherichia coli, exogenous glycine promotes oxidation of GSH to GSH disulfide (GSSG), disrupts redox balance, increases oxidative stress and reduces membrane integrity, leading to increased binding of complement. Antioxidant or ROS scavenging agents abrogate this effect and agents that generate or potentiate oxidation stimulate serum-mediated cell death. Analysis of several clinical isolates of E. coli demonstrates that glutathione metabolism is repressed in serum-resistant bacteria. These data suggest a novel mechanism underlying serum-resistance in pathogenic bacteria, characterized by an induced shift in the GSH/GSSG ratio impacting redox balance. The results could potentially lead to novel approaches to manage infections caused by serum-resistant bacteria both in aquaculture and human health.

Temperature induces metabolic reprogramming in fish during bacterial infection

Water temperature elevation as a consequence of global warming results in increased incidence of bacterial disease, such as edwardsiellosis, in fish farming. Edwardsiellosis is caused by the bacterial pathogen Edwardsiella tarda and affects many farmed fish including flounder (Paralichthys olivaceus). Currently, the effect of temperature on the metabolic response of flounder to E. tarda infection is unclear. In this study, we found that compared to low temperature (15°C), high temperature (23°C) enhanced E. tarda dissemination in flounder tissues. To examine the impact of temperature on the metabolism of flounder induced by E. tarda, comparative metabolomics were performed, which identified a large number of metabolites responsive to E. tarda invasion and temperature alteration. During E. tarda infection, the metabolic profile induced by elevated temperature was mainly featured by extensively decreased amino acids and TCA intermediates such as succinate, a proven immune regulator. Further, 38 potential metabolite markers of temperature effect (MMTE) in association with bacterial infection were identified. When used as exogenous supplements, two of the MMTE, i.e., L-methionine and UDP-glucose, effectively upregulated the expression of pro-inflammatory cytokines and suppressed E. tarda infection in flounder leukocytes. Taken together, the results of this study indicate an important influence of temperature on the metabolism of flounder during bacterial infection, which eventually affects the survivability of the fish.

Fructose Promotes Crucian Carp Survival Against Aeromonas hydrophila Infection

Aquatic food is becoming an important food source that provides micronutrients to human beings. The decline of wild aquatic animals makes aquaculture become increasingly important to play this role. However, infectious diseases, especially bacterial infection, represent severe threat to aquaculture, which causes huge economic loss. Meanwhile, strategies in managing bacterial infection in an antibiotic-independent way are still lacking. In this study, we monitor the metabolomic shift of crucian carp upon Aeromonas hydrophila infection. We find that the metabolism of the fish that died of infection is distinct from the ones that survived. By multivariate analysis, we identify fructose as a crucial biomarker whose abundance is significantly different from the dying and surviving groups where the surviving group has a higher content of fructose than the dying group. Exogenous supplementation of fructose increases fish survival rate by 27.2%. Quantitative gene expression analysis demonstrated that fructose enhances the expression of lysozyme and complement 3 expression, which is also confirmed in the serum level. Furthermore, the augmented lysozyme and C3 levels enhance serum cell lytic activity which contribute to the reduced bacterial load in vivo. Thus, our study demonstrates a metabolism-based approach to manage bacterial infection through modulating immune response to clear bacterial infection.

Activation of the TCA Cycle to Provide Immune Protection in Zebrafish Immunized by High Magnesium-Prepared Vibrio alginolyticus Vaccine

Vaccines are safe and efficient in controlling bacterial diseases in the aquaculture industry and are in line with green farming. The present study develops a previously unreported approach to prepare a live-attenuated V. alginolyticus vaccine by culturing bacteria in a high concentration of magnesium to attenuate bacterial virulence. Furthermore, metabolomes of zebrafish immunized with the live-attenuated vaccines were compared with those of survival and dying zebrafish infected by V. alginolyticus. The enhanced TCA cycle and increased fumarate were identified as the most key metabolic pathways and the crucial biomarker of vaccine-mediated and survival fish, respectively. Exogenous fumarate promoted expression of il1β, il8, il21, nf-κb, and lysozyme in a dose-dependent manner. Among the five innate immune genes, the elevated il1β, il8, and lysozyme are overlapped in the vaccine-immunized zebrafish and the survival from the infection. These findings highlight a way in development of vaccines and exploration of the underlying mechanisms.

Serine Metabolism Tunes Immune Responses To Promote Oreochromis niloticus Survival upon Edwardsiella tarda Infection

Overactive immune response is a critical factor triggering host death upon bacterial infection. However, the mechanism behind the regulation of excessive immune responses is still largely unknown, and the corresponding control and preventive measures are still to be explored. In this study, we find that Nile tilapia, Oreochromis niloticus, that died from Edwardsiella tarda infection had higher levels of immune responses than those that survived. Such immune responses are strongly associated with metabolism that was altered at 6 h postinfection. By gas chromatography-mass spectrometry-based metabolome profiling, we identify glycine, serine, and threonine metabolism as the top three of the most impacted pathways, which were not properly activated in the fish that died. Serine is one of the crucial biomarkers. Exogenous serine can promote O. niloticus survival both as a prophylactic and therapeutic upon E. tarda infection. Our further analysis revealed exogenous serine flux into the glycine, serine, and threonine metabolism and, more importantly, the glutathione metabolism via glycine. The increased glutathione synthesis could downregulate reactive oxygen species. Therefore, these data together suggest that metabolic modulation of immune responses is a potential preventive strategy to control overactive immune responses. IMPORTANCE Bacterial virulence factors are not the only factors responsible for host death. Overactive immune responses, such as cytokine storm, contribute to tissue injury that results in organ failure and ultimately the death of the host. Despite the recent development of anti-inflammation strategies, the way to tune immune responses to an appropriate level is still lacking. We propose that metabolic modulation is a promising approach in tuning immune responses. We find that the metabolomic shift at as early as 6 h postinfection can be predictive of the consequences of infection. Serine is a crucial biomarker whose administration can promote host survival upon bacterial infection either in a prophylactic or therapeutic way. Further analysis demonstrated that exogenous serine promotes the synthesis of glutathione, which downregulates reactive oxygen species to dampen immune responses. Our study exemplifies that the metabolite(s) is a potential therapeutic reagent for overactive immune response during bacterial infection.

Maltose promotes crucian carp survival against Aeromonas sobrial infection at high temperature

Temperature influences fish’s susceptibility to infectious disease through an immune response. However, the mechanism underlying this regulation is yet to be elucidated. In this study, we compared the susceptibility of crucian carp that were grown at 18°C and 33°C, respectively, to Aeromonas sobrial infection and found that crucian carp was more susceptible when grown at 33°C. These distinct susceptibilities of fish at different temperatures to infection may partially be explained by their differences in the metabolism as revealed by comparative metabolomics profiling: crucian carp demonstrated enhanced TCA cycle but reduced fatty acid biosynthesis; Our study also found that maltose was the most suppressed metabolite in fish grown at 33°C. Importantly, exogenous injection of maltose enhances crucian carp survival grown at 33°C by 30%. Further study showed that exogenous maltose downregulated the production of several cytokines but enhanced the lysozyme (lyz) and complement component c3, which involves the humoral innate immunity. Our results suggest that maltose promotes the survival of crucian carp likely through fine tuning the immune gene expression, and this finding provides a novel approach to manage bacterial infection.

Malate enhances survival of zebrafish against Vibrio alginolyticus infection in the same manner as taurine

Development of low-cost and eco-friendly approaches to fight bacterial pathogens is especially needed in aquaculture. We previously showed that exogenous malate reprograms zebrafish’s metabolome to potentiate zebrafish survival against Vibrio alginolyticus infection. However, the underlying mechanism is unknown. Here, we use GC-MS based metabolomics to identify the malate-triggered metabolic shift. An activated TCA cycle and elevated taurine are identified as the key metabolic pathways and the most crucial biomarker of the reprogrammed metabolome, respectively. Taurine elevation is attributed to the activated TCA cycle, which is further supported by the increased expression of genes in the metabolic pathway of taurine biosynthesis from the isocitrate of the TCA cycle to taurine. Exogenous taurine increases the survival of zebrafish against V. alginolyticus infection as malate did. Moreover, exogenous taurine and malate regulate the expression of innate immunity genes and promote the generation of reactive oxygen species and nitrogen oxide in a similar way. The two metabolites can alleviate the excessive immune response to bacterial challenge, which protects fish from bacterial infection. These results indicate that malate enhances the survival of zebrafish to V. alginolyticus infection via taurine. Thus, our study highlights a metabolic approach to enhance a host’s ability to fight bacterial infection.

Metabolic mechanism of colistin resistance and its reverting in Vibrio alginolyticus

Colistin is a last-line antibiotic against Gram-negative multidrug-resistant bacteria, but the increased resistance poses a huge challenge to this drug. However, the mechanisms underlying such resistance are largely unexplored. The present study first identified the mutations of two genes encoding AceF subunit of pyruvate dehydrogenase (PDH) and TetR family transcriptional regulator in colistin-resistant Vibrio alginolyticus (VA-R_CT ) through genome sequencing. Then, gas chromatography-mass spectroscopy-based metabolomics was adopted to investigate metabolic responses since PDH plays a role in central carbon metabolism. Colistin resistance was associated with the reduction of the central carbon metabolism and energy metabolism, featuring the alteration of the pyruvate cycle, a recently characterized energy-producing cycle. Metabolites in the pyruvate cycle reprogramed colistin-resistant metabolome to colistin-sensitive metabolome, resulting in increased gene expression, enzyme activity or protein abundance of the cycle and sodium-translocating nicotinamide adenine dinucleotide-ubiquinone oxidoreductase. This reprogramming promoted the production of the proton motive force that enhances the binding between colistin and lipid A in lipopolysaccharide. Moreover, this metabolic approach was effective against VA-R_CT in vitro and in vivo as well as other clinical isolates. These findings reveal a previously unknown mechanism of colistin resistance and develop a metabolome-reprogramming approach to promote colistin efficiency to combat with colistin-resistant bacteria.

Palmitic Acid Promotes Virus Replication in Fish Cell by Modulating Autophagy Flux and TBK1-IRF3/7 Pathway

Palmitic acid is the most common saturated fatty acid in animals, plants, and microorganisms. Studies highlighted that palmitic acid plays a significant role in diverse cellular processes and viral infections. Accumulation of palmitic acid was observed in fish cells (grouper spleen, GS) infected with Singapore grouper iridovirus (SGIV). The fluctuated content levels after viral infection suggested that palmitic acid was functional in virus-cell interactions. In order to investigate the roles of palmitic acid in SGIV infection, the effects of palmitic acid on SGIV induced cytopathic effect, expression levels of viral genes, viral proteins, as well as virus production were evaluated. The infection and replication of SGIV were increased after exogenous addition of palmitic acid but suppressed after knockdown of fatty acid synthase (FASN), of which the primary function was to catalyze palmitate synthesis. Besides, the promotion of virus replication was associated with the down-regulating of interferon-related molecules, and the reduction of IFN1 and ISRE promotor activities by palmitic acid. We also discovered that palmitic acid restricted TBK1, but not MDA5-induced interferon immune responses. On the other hand, palmitic acid decreased autophagy flux in GS cells via suppressing autophagic degradation, and subsequently enhanced viral replication. Together, our findings indicate that palmitic acid is not only a negative regulator of TBK1-IRF3/7 pathway, but also a suppressor of autophagic flux. Finally, palmitic acid promotes the replication of SGIV in fish cells.

Exogenous maltose enhances Zebrafish immunity to levofloxacin-resistant Vibrio alginolyticus

Understanding the interplay between bacterial fitness, antibiotic resistance, host immunity and host metabolism could guide treatment and improve immunity against antibiotic-resistant pathogens. The acquisition of levofloxacin (Lev) resistance affects the fitness of Vibrio alginolyticus in vitro and in vivo. Lev-resistant (Lev-R) V. alginolyticus exhibits slow growth, reduced pathogenicity and greater resistance to killing by the host, Danio rerio (zebrafish), than Lev-sensitive (Lev-S) V. alginolyticus, suggesting that Lev-R V. alginolyticus triggers a weaker innate immune response in D. rerio than Lev-S V. alginolyticus. Differences were detected in the metabolome of D. rerio infected with Lev-S or Lev-R V. alginolyticus. Maltose, a crucial metabolite, is significantly downregulated in D. rerio infected with Lev-R V. alginolyticus, and exogenous maltose enhances the immune response of D. rerio to Lev-R V. alginolyticus, leading to better clearance of the infection. Furthermore, we demonstrate that exogenous maltose stimulates the host production of lysozyme and its binding to Lev-R V. alginolyticus, which depends on bacterial membrane potential. We suggest that exogenous exposure to crucial metabolites could be an effective strategy for treating and/or managing infections with antibiotic-resistant bacteria.

Reduced ROS-mediated antibiotic resistance and its reverting by glucose in Vibrio alginolyticus

Antibiotic-resistant Vibrio alginolyticus poses a big challenge to human health and food safety. It is urgently needed to understand the mechanisms underlying antibiotic resistance to develop effective approaches for the control. Here we explored the metabolic difference between gentamicin-resistant V. alginolyticus (VA-R_GEN ) and gentamicin-sensitive V. alginolyticus (VA-S), and found that the reactive oxygen species (ROS) generation was altered. Compared with VA-S, the ROS content in VA-R_GEN was reduced due to the decreased generation and increased breakdown of ROS. The decreased production of ROS was attributed to the decreased central carbon metabolism, which is associated with the resistance to gentamicin. As such a mechanism, we exogenously administrated VA-R_GEN with the glucose that activated the central carbon metabolism and promoted the generation of ROS, but decreased the breakdown of ROS in VA-R_GEN . The gentamicin-mediated killing was increased with the elevation of the ROS level by a synergistic effect between gentamicin and exogenous glucose. The synergistic effect was inhibited by thiourea, a scavenger of ROS. These results reveal a reduced ROS-mediated antibiotic resistance mechanism and its reversal by exogenous glucose.

Identification and innate immunity mechanism of protective immunogens from extracellular proteins of Edwardsiella tarda

One of the most important emerging pathogens in the aquaculture industry is Edwardsiella tarda, and it causes extensive losses in farmed fish globally. The identification of protective immunogens against E. tarda is increasingly valued. We previously investigated 20 recombinant proteins of 38 E. tarda extracellular secretory proteins and identified 10 as protective immunogens in a zebrafish model. Here, we clone 10 of the remaining 18 genes, and the resulting recombinant proteins are used for evaluation of immune protection. ETAE_2147 (FliK), ETAE_0654 (PpdD), and ETAE_3259 (DamX) are identified as protective immunogens. Furthermore, their protection mechanism is explored by the detection of innate immunity genes encoding IL-1b, IL-6, IL-8, C3b, and NF-κB. The three protective immunogens stimulate zebrafish to produce higher and more lasting expression of the five immunity genes than non-protective immunogens during the first 48 h of infection. In addition, these protective immunogens are prone to be regulated by host products, which is helpful for cross-talk between host and pathogen, and thus they become vaccine candidates. These results highlight the way to understand the working mechanisms of protective immunogens.