Effects of compound antimicrobial peptides on the growth performance, antioxidant and immune responses and disease resistance of grass carp (Ctenopharyngodon idellus)

Development strategies and application of antimicrobial peptides as future alternatives to in-feed antibiotics

The use of in-feed antibiotics has been widely restricted due to the significant environmental pollution and food safety concerns they have caused. Antimicrobial peptides (AMPs) have attracted widespread attention as potential future alternatives to in-feed antibiotics owing to their demonstrated antimicrobial activity and environment friendly characteristics. However, the challenges of weak bioactivity, immature stability, and low production yields of natural AMPs impede practical application in the feed industry. To address these problems, efforts have been made to develop strategies for approaching the AMPs with enhanced properties. Herein, we summarize approaches to improving the properties of AMPs as potential alternatives to in-feed antibiotics, mainly including optimization of structural parameters, sequence modification, selection of microbial hosts, fusion expression, and industrially fermentation control. Additionally, the potential for application of AMPs in animal husbandry is discussed. This comprehensive review lays a strong theoretical foundation for the development of in-feed AMPs to achieve the public health globally.

Effects of antimicrobial peptides from dietary Hermetia illucens larvae on the growth, immunity, gene expression, intestinal microbiota and resistance to Aeromonas hydrophila of juvenile red claw crayfish (Cherax quadricarinatus)

Antimicrobial peptides (AMPs), which are widely present in animals and plants, have a broad distribution, strong broad-spectrum antibacterial activity, low likelihood of developing drug resistance, high thermal stability and antiviral properties. The present study investigated the effects of adding AMPs from Hermetia illucens larvae on the growth performance, muscle composition, antioxidant capacity, immune response, gene expression, antibacterial ability and intestinal microbiota of Cherax quadricarinatus (red claw crayfish). Five experimental diets were prepared by adding 50 (M1), 100 (M2), 150 (M3) and 200 (M4) mg/kg of crude AMP extract from H. illucens larvae to the basal diet feed, which was also used as the control (M0). After an eight-week feeding experiment, it was discovered that the addition of 100-150 mg/kg of H. illucens larvae AMPs to the feed significantly improved the weight gain rate and specific growth rate of C. quadricarinatus. Furthermore, the addition of H. illucens larvae AMPs to the feed had no significant effect on the moisture content, crude protein, crude fat and ash content of the C. quadricarinatus muscle. The addition of 100-150 mg/kg of H. illucens larvae AMPs in the feed also increased the antioxidant capacity, nonspecific immune enzyme activity and related gene expression levels in C. quadricarinatus, thereby enhancing their antioxidant capacity and immune function. The H. illucens larvae AMPs improved the structure and composition of the intestinal microbiota of C. quadricarinatus, increasing the microbial community diversity of the crayfish gut. Finally, the addition of 100-150 mg/kg of H. illucens larvae AMPs in the feed enhanced the resistance of C. quadricarinatus against Aeromonas hydrophila, improving the survival rate of the crayfish. Based on the aforementioned findings, it is recommended that H. illucens larvae AMPs be incorporated into the C. quadricarinatus feed at a concentration of 100-150 mg/kg.

Effects of Antimicrobial Peptides on Antioxidant Properties, Non-specific Immune Response and Gut Microbes of Tsinling Lenok Trout (Brachymystax lenok tsinlingensis)

Antimicrobial peptides (AMPs) are an important part of non-specific immunity and play a key role in the cellular host defense against pathogens and tissue injury infections. We investigated the effects of AMP supplementation on the antioxidant capacity, non-specific immunity, and gut microbiota of tsinling lenok trout. 240 fish were fed diets (CT, A120, A240 and A480) containing different amounts of AMP peptides (0, 120 mg kg-1, 240 mg kg-1, 480 mg kg-1) for 8 weeks. Our results showed that the activity of total antioxidant capacity (T-SOD) and glutathione peroxidase (GSH-Px), lysozyme (LZM), catalase (CAT) and acid phosphatase (ACP) in the A240 and A480 group were higher than that in the CT group (P < 0.05). The content of malondialdehyde (MDA) in AMP group was significantly lower than that in CT group (P < 0.05). Furthermore, we harvested the mid-gut and applied next-generation sequencing of 16S rDNA. The results showed that the abundance of Halomonas in AMP group was significantly lower than that in CT group. Functional analysis showed that the abundance of chloroalkane and chloroalkene degradation pathway increased significantly in AMP group. In conclusion, AMP enhanced the antioxidant capacity, non-specific immunity, and intestinal health of tsinling lenok trout.

Effect of kelp powder on the resistance of Aeromonas hydrophila in the gut of hybrid snakeheads (Channa maculata ♀ × Channa argus ♂)

To assess the level of oxidative stress, expression of immune-related genes, histomorphology, and changes in the intestinal tract of hybrid snakeheads(Channa maculata ♀ × Channa argus ♂) under stress from kelp powder in place of flour against Aeromonas hydrophila. We set up experimental diets: a control (C) diet of 20% flour, an experimental (MR) diet of 10% kelp powder and 10% flour, and an experimental (FR) diet of 0% starch and 15% kelp powder. The experimental fish in each group were infected with Aeromonas hydrophila after 60 days of feeding. For this experiment, some of the experimental fish in group C were injected with PBS as a negative control group (PBS). The results showed that the C group had significantly higher SOD, CAT, and T-AOC activity and expression of TAK1, IKKβ, IL-1β, and TNF-α genes in the MyD88 pathway than the PBS group. CAT activity and the expression of TAK1, IL-1β and TNF-α genes in the MyD88 pathway were significantly lower in the MR group than in the C group. Furthermore, the number of goblet cells in the MR group was significantly higher than in the C group. Furthermore, microorganisms such as Bacteroidota and Actinobacteriota were significantly lower in the C group than in the PBS and FR groups, as were beneficial bacteria such as Clostridium_sensu_stricto_1 and Sphingomonas. Replacing flour with kelp powder increases hybrid snakehead gut resistance to Aeromonas hydrophila.

Oral administration of hepcidin and chitosan benefits growth, immunity, and gut microbiota in grass carp (Ctenopharyngodon idella)

Intensive high-density culture patterns are causing an increasing number of bacterial diseases in fish. Hepcidin links iron metabolism with innate immunity in the process of resisting bacterial infection. In this study, the antibacterial effect of the combination of hepcidin (Cihep) and chitosan (CS) against Flavobacterium columnare was investigated. The dosing regimen was also optimized by adopting a feeding schedule of every three days and every seven days. After 56 days of feeding experiment, grass carp growth, immunity, and gut microbiota were tested. In vitro experiments, Cihep and CS can regulate iron metabolism and antibacterial activity, and that the combination of Cihep and CS had the best protective effect. In vivo experiments, Cihep and CS can improve the growth index of grass carp. After challenge with Flavobacterium columnare, the highest survival rate was observed in the Cihep+CS-3d group. By serum biochemical indicators assay and Prussian blue staining, Cihep and CS can increase iron accumulation and decrease serum iron levels. The contents of lysozyme and superoxide dismutase in Cihep+CS-3d group increased significantly. Meanwhile, Cihep and CS can significantly reduce the pathological damage of gill tissue. The 16S rRNA sequencing results showed that Cihep and CS can significantly increase the abundance and diversity of grass carp gut microbiota. These results indicated that the protective effect of consecutive 3-day feeding followed by a 3-day interval was better than that of consecutive 7-day feeding followed by a 7-day interval, and that the protective effect of Cihep in combination with chitosan was better than that of Cihep alone. Our findings optimize the feeding pattern for better oral administration of Cihep in aquaculture.

Functionality of dietary antimicrobial peptides in aquatic animal health: Multiple meta-analyses

Effects of antimicrobial peptides (AMP) added to diets on aquatic animal health and body function are influenced by multiple factors such as animal species, initial body weight, the dosage of AMP and feeding duration. However, there is limited knowledge on the relationship between these factors and the body function of aquatic animals. Here, we aimed to perform multiple meta-analyses to investigate the effects of dietary AMP on growth performance (feed conversion ratio [FCR], specific growth rate [SGR]), enzyme activity (superoxide dismutase activity [SOD], lysozyme activity [LSA]), disease resistance (cumulative survival rate [CSR], the expression of immune-related genes [GENE]) and the abundance of gut microbiota (MICRO) from a pool of empirical studies. Additionally, the dose-effect model was applied to determine the optimal AMP dose, initial body weight and feeding duration to maximize body function. To conduct the meta-analyses, we included 34 publications that estimated 705 effect sizes across 21 fish, 2 shrimp and 2 shellfish species. The results confirmed that the inclusion of AMP in the diet can significantly improve SGR, SOD, LSA, CSR and GENE and decrease FCR for aquatic animals. Interestingly, our findings implied a slight positive effect of AMP on MICRO albeit with a limited number of studies available on fish gut microbial communities. Although no significant linear or quadratic relationship was predicted by meta-regression, the dose-effect indicated that the optimal AMP doses for FCR, SGR, SOD and LSA were 707.5, 750.0, 1,050.0 and 937.5 mg/kg, respectively. Taken together, fish with an initial body weight of 30 g could be fed with a dose of 600 to 800 mg/kg for 2 mo when AMP-supplemented diets were applied in aquaculture, which can effectively improve body function and health while lowering aquafeed costs. In addition, more studies should address fish gut microbiota to delimitate the influence of dietary AMP on MICRO in the future.

Effect of Tributyrin on Growth Performance and Pathway by which Tributyrin Regulates Oligopeptide Transporter 1 in Juvenile Grass Carp (Ctenopharyngodon idellus)

Simple Summary

Oligopeptide transporter 1 (PepT1) plays a role in the transportation and absorption of oligopeptides, which is an important part of protein nutrition and affects the growth of animals. Tributyrin (TB), the precursor of butyrate, exhibits similar functions to those of the butyrate in intestinal nutrients absorption. The analysis of TB on the growth of grass carp and its regulation pathway on PepT1 may help us to better understand the functions of TB and oligopeptide transportation via PepT1, which can be modulated by diet. In this study, we demonstrated that an appropriate level of tributyrin supplementation in the diet promoted the growth of juvenile grass carp and elevated the expressions of caudal type homeobox 2 (CDX2), specificity protein 1 (SP1), and PepT1 in the grass carp intestine and primary intestine cell. In addition, CDX2 and SP1 regulating the expression of PepT1 was investigated. Finally, CDX2/SP1-mediating tributyrin regulation on PepT1 was elucidated. This study verified the effect of tributyrin on the growth of juvenile grass carp and clarified the tributyrin regulation pathway on CDX2/SP1-PepT1.

Abstract

The nutritional functions of tributyrin (TB) have been extensively studied, but questions remain regarding its influence on the growth of juvenile grass carp (Ctenopharyngodon idellus) and the regulation pathway to PepT1 in the intestine of grass carp. To answer the remaining questions, feeding trials, cell trials, and peritoneal injection trials were conducted in this study. The results showed that an appropriate level of TB (0.5 g/kg and 1.0 g/kg) supplementation in feed significantly promoted the growth performance of juvenile grass carp. The expressions of intestine genes (CDX2, SP1 and PepT1) related to oligopeptide transportation increased in the 0.5 g/kg TB group of feeding trials and both the 5 mM and 10 mM TB groups of the intestine cell trials, respectively. Subsequently, the injection trials of inhibitors CDX2 and SP1 demonstrated that the inhibition of CDX2 or SP1 decreased the mRNA expression of PepT1. Finally, the results of independent or combined treatments of TB and the inhibitors suggested that CDX2/SP1 mediated TB regulation on PepT1. These findings may help us to better understand the functions of TB on growth and PepT1 oligopeptide transportation, which could be modulated by dietary TB through the CDX2/SP1-PepT1 pathway in juvenile grass carp.

Effects of dietary antimicrobial peptides on intestinal morphology, antioxidant status, immune responses, microbiota and pathogen disease resistance in grass carp Ctenopharyngodon idellus

This study aims to highlight the effects (8 weeks) of dietary antimicrobial peptides (AMPs, a compound of 6 kDa and 5 kDa from intestine) on intestinal morphological functions and health status in grass carp (Ctenopharyngodon idellus). Fish were supplemented with various gradient concentrations of AMPs, including M0 (0 mg/kg), M1 (100 mg/kg), M2 (200 mg/kg), M3 (400 mg/kg), M4 (800 mg/kg) and M5 (1600 mg/kg). Our results showed that amylase, lipase, chymotrypsin enzymatic levels, and total antioxidant capacity (T-AOC) were significantly increased (p < 0.05), while malondialdehyde (MDA) content was significantly decreased in the intestines of the AMP treated groups compared to the M0. Histological analysis revealed villus height and crypt depth of foregut and midgut in the M4 group were significantly different (p < 0.05) compared to the M0. In the M3 group, the gene expression levels of IL-1β were significantly up-regulated, while levels of IL10 and TGF-β were significantly down-regulated than other treated and control groups. The abundance of Firmicutes was significantly increased (p < 0.05), while the Planctomycetes abundance was decreased at phylum level in M1-M5 groups. Subsequent to the AMP treatment, fish were injected with Aeromonas. hydrophila to assess disease resistant potential. In A. hydrophila injected M3-group, the gene expressions of IL-1β, IL8, and TNF-α were significantly down-regulated while that of TGF-β was significantly up-regulated, and IL10 showed no significant difference compared to the control. Further, AMPs also increased the abundance of the Acidobacteria, Proteobacteria, and Patescibacteria, and decreased the abundance of the Fusobacteria and Firmicutes. Therefore, dietary AMPs (400-800 mg/kg) boosted intestinal health by promoting intestinal morphology, digestive and antioxidant capacities, immunity, and intestinal microbiota in C. idellus.

Effects of Antimicrobial Peptides Gal-13 on the Growth Performance, Intestinal Microbiota, Digestive Enzyme Activities, Intestinal Morphology, Antioxidative Activities, and Immunity of Broilers

To evaluate the application effect of antimicrobial peptides Gal-13 (AMP Gal-13) instead of antibiotic feed additives, 90 7-day-old Ross 308 broilers were randomly divided into 3 groups. Group A was fed a basic diet as the control, and Groups B and C were supplemented with AMP Gal-13 (100 mg/kg and 200 mg/kg, respectively). After a 35-day feeding experiment, the weight and average daily gain (ADG) of the broilers in Group B were significantly higher than those of the broilers in Group A. The Enterococcus sp. and Escherichia coli counts in the ileum and cecum in Group A were significantly higher than those in Groups B and C, while the Lactic acid bacteria (LAB) and Bifidobacterium sp. counts were significantly lower. The amylase activity of the jejunum in Group B was significantly higher than that in Group A. The villus length (VL): crypt depth (CD) ratios of the jejunum and ileum in Group B were significantly higher than those in Group A. The glutathione peroxidase (GSH-Px) activities in the liver and serum in Groups B and C were significantly higher than those in Group A, while the malondialdehyde (MDA) activity was significantly lower. The titers of Newcastle disease virus (NDV)-specific antibodies were elevated significantly in Group B at the age of 42 days. Additionally, the weights of the spleen and thymus were significantly increased. The expression levels of Il-2, Il-6, Tgf-β4, Tnf-α, and Mif in the spleen in Groups B and C were significantly downregulated to different degrees; Il-4 expression in Group B was significantly upregulated, while Ifn-γ expression in Group C was significantly upregulated. The results suggested that adding AMP Gal-13 to the diet could improve intestinal digestion, the antioxidant capacity, and immune function, ultimately promoting the growth of broilers.

Effects of Nisin, Cecropin, and Penthorum chinense Pursh on the Intestinal Microbiome of Common Carp (Cyprinus carpio)

Following a ban on antibiotic use in the feed industry, trials on the effects of various immunostimulants (prebiotics, probiotics, antimicrobial peptides [AMPs], and herbs) on the survival, growth, immunity, and disease control of farmed fish in aquaculture are being rapidly conducted. The wide variety of microbes with roles in nutrition, metabolism, and immunity in the fish intestine is the primary factor affecting the fermentability and functionality of dietary immunostimulants. For this reason, the dynamic interactions between immunostimulants and the intestinal microbiome may influence fish health. In this study, the effects of two agriculturally important AMPs (nisin and cecropin) and one herb (Penthorum chinense) on the gut microbiome of common carp were investigated, using 16S rDNA high-throughput sequencing. The results suggest that all three substances can alter the richness, diversity, and composition of the intestinal microbiota of common carp. P. chinense had a similar effect on the gut microbiota of common carp to that of nisin, and both promoted more striking changes in the gut microbiota community than did cecropin. The relative abundance of Proteobacteria was lower in the nisin and P. chinense groups than in the control and cecropin groups. The relative abundance of Bacteroidetes in the nisin, cecropin, and P. chinense groups was markedly increased, compared with that of the control group. Additionally, nisin, cecropin, and P. chinense showed obvious anti-inflammatory effects on the fish intestine, which was reflected by significantly increasing the expression levels of two anti-inflammatory cytokines IL-10 and TGF-β. Some digestive enzyme activities in the fish intestine were also significantly enhanced by supplementing these three substances in feeds.

Effects of mixed antimicrobial peptide on the growth performance, antioxidant and immune responses and disease resistance of Pengze crucian carp (Carassius auratus var. Pengze)

Antimicrobial peptides have broad-spectrum antibacterial properties and low drug resistance, and they demonstrate great potential as antibiotic substitutes. In this study, five dietary mixed antimicrobial peptide supplement groups were set and fed to Pengze crucian carp for 10 weeks. The 6 groups were G0 (control group) and 5 additional groups: G1 (100 mg/kg), G2 (200 mg/kg), G3 (400 mg/kg), G4 (800 mg/kg) and G5 (1600 mg/kg). The results showed that the final body weight (FBW), weight gain rate (WGR) and specific growth rate (SGR) of fish in G1 and G2 were higher than those of fish in the control group, and G1 was significantly higher than G0 (P < 0.05). In addition, the FBW, WGR, and SGR of the G3 group were significantly lower than those of the G0 group. The chymotrypsin, lipase and amylase activities of G1 and G2 were significantly upregulated compared with G0 and reached peak values in G1. The activity of T-AOC and SOD in the addition group was higher (except G2 and G4) than that in the control groups, and significantly increased in G3 compared to the control group. The activity of MDA in the addition group was lower than that in the control group (p > 0.05). The expression levels of TLR-4, MYD88 and TNF-α in the three organs of the addition group were higher than those in G0 and reached the peak value in G3 (p < 0.05). Furthermore, the expression levels of TLR-4, MYD88 and TNF-α in the three organs of G3 were significantly lower than those in G0 and lower than those in the other supplemented groups. The expression levels of IL-10 and IL-11 tended to be upregulated after A. hydrophila challenge, and G3 in different organs was significantly higher than that in other supplemented groups and G0. The results of this study show that an appropriate amount of mixed antimicrobial peptides can improve the growth performance and antioxidant and immune capabilities of Pengze crucian carp and can also play a positive role in the treatment of A. hydrophila infection.

Isolation of Antimicrobial Genes from Oryza rufipogon Griff by Using a Bacillus subtilis Expression System with Potential Antimicrobial Activities

Antimicrobial genes are distributed in all forms of life and provide a primary defensive shield due to their unique broad-spectrum resistance activities. To better isolate these genes, we used the Bacillus subtilis expression system as the host cells to build Oryza rufipogon Griff cDNA libraries and screen potential candidate genes from the library at higher flux using built-in indicator bacteria. We observed that the antimicrobial peptides OrR214 and OrR935 have strong antimicrobial activity against a variety of Gram-positive and Gram-negative bacteria, as well as several fungal pathogens. Owing to their high thermal and enzymatic stabilities, these two peptides can also be used as field biocontrol agents. Furthermore, we also found that the peptide OrR214 (MIC 7.7-10.7 μM) can strongly inhibit bacterial growth compared to polymyxin B (MIC 5-25 μM) and OrR935 (MIC 33-44 μM). The cell flow analysis, reactive oxygen burst, and electron microscopy (scanning and transmission electron microscopy) observations showed that the cell membranes were targeted by peptides OrR214 and OrR935, which revealed the mode of action of bacteriostasis. Moreover, the hemolytic activity, toxicity, and salt sensitivity experiments demonstrated that these two peptides might have the potential to be used for clinical applications. Overall, OrR214 and OrR935 antimicrobial peptides have a high-throughput bacteriostatic activity that acts as a new form of antimicrobial agent and can be used as a raw material in the field of drug development.