The effects of essential oils and their major compounds on fish bacterial pathogens – a review

Lippia sidoides essential oil at concentration of 0.25% provided improvements in microbiota and intestine integrity of Danio rerio

The study evaluated the effects of dietary supplementation with Lippia sidoides essential oil on the microbiota and intestinal morphology of Danio rerio. For this, 448 fish were randomly distributed in 28 tanks divided into a control group fed a commercial diet without supplementation, a group fed a commercial diet containing grain alcohol and five groups fed a commercial diet containing essential oil of L. sidoides (LSEO) at concentrations of 0.25%, 0.50%, 0.75%, 1.00% and 1.25%. After the period of dietary supplementation, biological materials were collected for microbiological and histological analyses. There were no significant differences regarding the microbiological count between the groups. Diversity of the microbiome was higher in 0.25% group than in control group. LSEO inhibited the growth of potentially pathogenic bacteria. Fish fed LSEO0.25% showed greater intestinal histomorphometric indices. The inclusion of LSEO at 0.25% in the diet of D. rerio provided improvements in fish microbiota and intestine integrity.

Effect of edible coatings and films enriched with plant extracts and essential oils on the preservation of animal-derived foods

Edible coatings and films for food preservation are becoming more popular thanks to their environmentally friendly properties and active ingredient-carrying ability. Their application can be effective in contrasting quality decay by limiting oxidation and deterioration of foods. Many reviews analyze the different compounds with which films and coatings can be created, their characteristics, and the effect when applied to food. However, the possibility of adding plant extracts and essential oils in edible coatings and films to preserve processed animal-derived products has been not exhaustively explored. The aim of this review is to summarize how edible coatings and films enriched with plant extracts (EXs) and essential oils (EOs) influence the physico-chemical and sensory features as well as the shelf-life of cheese, and processed meat and fish. Different studies showed that various EXs and EOs limited both oxidation and microbial growth after processing and during food preservation. Moreover, encapsulation has been found to be a valid technology to improve the solubility and stability of EOs and EXs, limiting strong flavor, controlling the release of bioactive compounds, and maintaining their stability during storage. Overall, the incorporation of EXs and EOs in edible coating and film to preserve processed foods can offer benefits for improving the shelf-life, limiting food losses, and creating a food sustainable chain.

Growth Performance and Disease Resistance against Vibrio parahaemolyticus of Whiteleg Shrimp (Litopenaeus vannamei) Fed Essential Oil Blend (Phyto AquaBiotic)

Acute Hepatopancreatic Necrosis Disease (AHPND) is a serious and emerging disease caused by a group of strains of Vibrio parahaemolyticus and affects farmed shrimp, particularly whiteleg shrimps (Liptopenaeus vannamei). The objective of this study is to assess the effect of dietary supplementation with two dosages of an essential oil mixture (Phyto AquaBiotic, abbreviated as PAB) on growth performance and mortality reduction after challenge against V. parahaemolyticus. PAB was mixed with basal diets at rates of 0, 1 and 2 g/kg and fed for 42 days. Each tank was stocked with 100 individuals with experimentation performed in triplicate. The results showed an improvement in growth performance in a dose-dependent manner, specifically regarding daily weight gain, specific growth rate and total biomass, which were significantly improved compared to control (p < 0.05). Further, PAB significantly reduced mortalities when challenged against Vibrio parahaemolyticus (p < 0.05) and decreased Vibrio spp. count in the hepatopancreas of infected shrimp. Overall, PAB was efficient in reducing mortalities in cases of disease outbreaks at a rate of 2 g/kg.

Dietary coriander (Coriandrum sativum L) oil improves antioxidant and anti-inflammatory activity, innate immune responses and resistance to Aeromonas hydrophila in Nile tilapia (Oreochromis niloticus)

The use of essential oils (EOs) as a natural alternative to antibiotics for disease prevention strategies is gaining much interest in recent decade. Coriander (Coriandrum sativum L.) essential oil is rich in bioactive compounds like linalool and geranyl acetate which have antioxidant, anti-inflammatory and antimicrobial activities. The present work was proposed to evaluate the inclusion levels of coriander oil in tilapia feed to enhance tilapia health and resistance to bacterial infection. Five iso-nitrogenous and iso-lipidic feeds were prepared with graded levels of coriander oil (0, 0.5, 1, 1.5 and 2%). The fish were then fed with the five experimental diet twice daily for a period of 60 days in triplicate. Haemoglobin, mean corpuscular volume, mean cell haemoglobin increased significantly in the coriander oil treated groups. The thrombocyte count was more in 2% inclusion level. The superoxide dismutase activity increased significantly in all the treated groups. The feeds with 1.5 and 2% coriander oil showed increased respiratory burst and myeloperoxidase activities while lysozyme and antiprotease activities were significantly higher in 1, 1.5 and 2% dietary treatments compared to control. The survival increased in dose dependent manner post challenge with an intraperitoneal injection of Aeromonas hydrophila at a LD₅₀ dose of 5 × 10⁶ cfu mL^-1. The feed containing 1, 1.5 and 2% of coriander oil showed 89, 100 and 100% survival respectively compared to 39% in control diet. The expression level of IgM and IL-8 increased significantly post challenge with A. hydrophila in coriander oil fed groups. The expressions of TNFα, IL-1β, TGFβ and HSP 70 genes, however, decreased significantly in the treated groups compared to control. Histopathological examination of spleen showed large melano-macrophage centers in control and 0.5% coriander fed group with signs of necrosis and vacuolation post A. hydrophila infection, whereas 1, 1.5 and 2% treated groups showed normal architecture of spleen. From the above observations it can be concluded that coriander oil with 1% incorporation in feed improves tilapia health and resistance to bacterial infection.

The Use of Cinnamon Essential Oils in Aquaculture: Antibacterial, Anesthetic, Growth-Promoting, and Antioxidant Effects

Cinnamon essential oils (EOs) are widely known for their pharmaceutical properties; however, studies investigating the use of these EOs in aquaculture are scarce. The aims of this study were to evaluate the anesthetic effect of bathing silver catfish (Rhamdia quelen) with Cinnamomum cassia EO (CCEO) and its nanoemulsion (NCCEO); the growth-promoting and antioxidant effects of dietary supplementation with CCEO in silver catfish; and the in vitro antibacterial effect of CCEO, NCCEO, and Cinnamomum zeylanicum EO (CZEO) against bacteria isolated from diseased silver catfish. The two cinnamon EOs showed promising antibacterial activity, which was potentiated by the nanoemulsion. CCEO showed satisfactory anesthetic activity in silver catfish, and its nanoemulsion intensified the sedative activity. Supplementation of 1.0 mL CCEO per kg of diet for 60 days increased weight, length, and weight gain when compared to the control group, evidencing the growth-promoting activity of this EO. Dietary supplementation of CCEO for 30 and 60 days also showed an antioxidant effect, as it decreased levels of thiobarbituric acid reactive species and increased the superoxide dismutase activity in the liver of silver catfish. Therefore, cinnamon EOs have a promising use in aquaculture.

Nature-identical compounds as feed additives in aquaculture

Aquaculture sustainable development is necessary since it is categorized as the most important source of aquatic products for human consumption and it is expected to keep growing shortly. For this reason, the addition of natural immunostimulants to fish diet to improve fish health and to preserve the environment have great importance in aquaculture. In this sense, new biotechnological tools as nature-identical compounds are now being used as feed additives to strengthen and stimulate the fish immune system to prevent and/or control diseases due to their lesser cost and higher availability than plant compounds. This review aims to present the most recent studies in which nature-identical compounds have been used in the fish diet to establish their possible use in aquaculture. Nature-identical compounds can be considered a promising alternative to be added to fish diets to promote growth performance, manipulate the gut microbiota, and improve the immune and oxidative status of fish as wells as control bacterial infections in this important aquatic industry.

Effect of Essential Oils on the Oxyntopeptic Cells and Somatostatin and Ghrelin Immunoreactive Cells in the European Sea Bass (Dicentrarchus labrax) Gastric Mucosa

The current work was designed to assess the effect of feed supplemented with essential oils (EOs) on the histological features in sea bass's gastric mucosa. Fish were fed three diets: control diet (CTR), HERBAL MIX® made with natural EOs (N-EOs), or HERBAL MIX® made with artificial EOs obtained by synthesis (S-EOs) during a 117-day feeding trial. Thereafter, the oxyntopeptic cells (OPs) and the ghrelin (GHR) and somatostatin (SOM) enteroendocrine cells (EECs) in the gastric mucosa were evaluated. The Na+K+-ATPase antibody was used to label OPs, while, for the EECs, anti-SOM and anti-GHR antibody were used. The highest density of OP immunoreactive (IR) area was in the CTR group (0.66 mm2 ± 0.1). The OP-IR area was reduced in the N-EO diet group (0.22 mm2 ± 1; CTR vs. N-EOs, p < 0.005), while in the S-EO diet group (0.39 mm2 ± 1) a trend was observed. We observed an increase of the number of SOM-IR cells in the N-EO diet (15.6 ± 4.2) compared to that in the CTR (11.8 ± 3.7) (N-EOs vs. CTR; p < 0.05), but not in the S-EOs diet. These observations will provide a basis to advance current knowledge on the anatomy and digestive physiology of this species in relation to pro-heath feeds.

Antibacterial and antibiofilm activities and synergism with florfenicol from the essential oils of Lippia sidoides and Cymbopogon citratus against Aeromonas hydrophila

AIMS

Aeromonas hydrophila is an opportunistic bacterium, with a high capacity for biofilm production, which can cause severe damage in aquaculture. The objective of this study was to identify the chemical compounds of the essential oils of Lippia sidoides (EOLS) and Cymbopogon citratus (EOCC), and to evaluate the biocidal, antibiofilm and synergistic action with the antimicrobial florfenicol of these essential oils (EOs) against A. hydrophila.

METHODS AND RESULTS

The antibacterial activity of EOLS and EOCC was verified by the minimum bactericidal concentration and by the action of these EOs against both forming and consolidated biofilms. The synergistic activity of EOs with florfenicol was performed using the checkerboard technique. The main component of EOLS and EOCC was carvacrol (44.50%) and α-citral (73.56%), respectively. Both EOs showed weak inhibitory activity (≥3125.00 µg ml^-1 ). Two bacterial isolates were able to produce biofilm, and EOLS and EOCC acted upon the bacterial isolates to prevent biofilm formation. A bactericidal effect was verified for EOLS in the previously consolidated biofilm for both isolates and for EOCC in only one of the isolates. In general, EOLS had a synergistic effect with florfenicol, while EOCF had an additive effect.

CONCLUSIONS

Both EOs were able to interfere with biofilm formation and did not have an antagonistic effect in combination with florfenicol. The best results were found for EOLS, which showed a synergistic effect with florfenicol and the ability to interfere in the formation of consolidated biofilm.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights the potential of EOLS and EOCC to interfere in biofilm and act in synergy with florfenicol to reduce the occurrence of A. hydrophila. Development of these compounds may contribute to the development of herbal medicines in aquaculture.

Effects of sodium butyrate and Lippia origanoides essential oil blend on growth, intestinal microbiota, histology, and haemato-immunological response of Nile tilapia

This study aimed to verify the effects of dietary supplementation with sodium butyrate and Lippia origanoides, combined and isolated, on the health and zootechnical performance of Nile tilapia juveniles Oreochromis niloticus. A total of 120 fish (5.38 ± 0.65 g) were randomly distributed in 12 experimental units and fed different experimental diets for 30 days, namely: commercial diet without supplementation (Unsupplemented); commercial diet supplemented with 0.5% sodium butyrate (Butyrate); commercial diet supplemented with 0.125% L. origanoides (Lippia) and commercial diet supplemented with a mixture of 0.5% sodium butyrate and 0.125% L. origanoides (Butyrate + Lippia). After preparing the experimental diets there was an increase in the pH of diet Butyrate when compared to the other diets. After 30 days the fish supplemented with Butyrate + Lippia showed reduction significate in the mean corpuscular haemoglobin, concentration of total heterotrophic bacteria in the intestine, and lymphocyte infiltrates in the liver. Besides that, the supplementation with Butyrate + Lippia promoted an increased number of intestinal villi compared to the fish Unsupplemented ones. Additionally, fish fed a diet containing only Lippia presented an increase in the villus perimeter in the posterior region of the gut and in the red blood cell number. Animals supplemented only with sodium butyrate demonstrated increased lactic acid bacterium in the gut and macrosteatosis in the liver, besides decreased melanomacrophages in the spleen. The use of sodium butyrate associated with essential oil had positive effects on the intestinal microbiota, intestinal structure, liver, and spleen integrity, suggesting a greater efficiency of the compounds when used together in the nutrition of Nile tilapia juveniles.

In Vitro Antimicrobial and Antibiofilm Activity of S-(-)-Limonene and R-(+)-Limonene against Fish Bacteria

Brazilian fish farming goes together with the emergence of numerous bacterial diseases, with Aeromonas hydrophila being the main bacterial pathogen. As a consequence, antimicrobials are excessively used. Considering that antimicrobials are relatively stable and nonbiodegradable, medicinal plants and their phytochemicals have been used as alternative treatments of bacteriosis in fish farming. Limonene is a monoterpene available in two enantiomers: R-(+)-limonene and S-(-)-limonene. This study analyzed the antibacterial activity of the phytochemicals S-(-)-limonene and R-(+)-limonene against some bacteria isolated from silver catfish (Rhamdia quelen). Furthermore, by means of spectrophotometry and atomic force microscopy, we also investigated the combination therapy of phytochemicals with antimicrobials and their activity in terms of inhibiting biofilm formation. Six clinical isolates and a standard strain were selected for antimicrobial activity testing. Biofilm formation was tested in 96-well plates and nylon cubes. The most sensitive of the strains tested was the A. hydrophila strain (MF 372510). S-(-)-limonene and R-(+)-limonene had high minimum inhibitory concentrations; however, they strongly inhibited A. hydrophila biofilm formation. R-(+)-limonene and S-(-)-limonene had an additive effect when combined with florfenicol and an antagonistic effect with oxytetracycline. In general, the phytochemicals tested showed strong antibiofilm activity against A. hydrophila, and when in combination therapy with florfenicol, they showed an additive effect against the treatment of A. hydrophila.

Exploring the garlic (Allium sativum) properties for fish aquaculture

The aquaculture industry's rapid growth to meet commercial demand can trigger an outbreak of infectious diseases due to high-density farming. Antibiotic overuse and misuse in fish farming and its global health consequences have led to searching for more natural alternatives such as medicinal plants. In this sense, garlic (Allium sativum) has different bioactive compounds with biological properties for animal health. Among them are the ajoene, alliin, and allicin, which confer biological properties such as growth promotion, antimicrobial, antiviral, antioxidant, and antiparasitic. Ways to use garlic in aquaculture include oil, fresh mash, aqueous extract, and garlic powder. The powder presentation is the most used in aquaculture; it is generally applied by oral administration, adding to the feed, and the dose used ranges from 0.05 to 40 g/kg of feed. Garlic has been used in the aquaculture of different species such as rainbow trout (Oncorhynchus mykiss), spotted grouper (Epinephelus coioides), catfish (Clarias gariepinus), tilapia (Oreochromis niloticus), guppy fish (Poecilia reticulata), goldfish (Carassius auratus), and barramundi (Lates calcarifer). In addition to its properties, garlic's usage became popular, thanks to its low cost, easy incorporation into food, and little environmental impact. Therefore, its application can be an effective solution to combat diseases, improve organisms' health using natural supplies, and as an alternative to antibiotics. This review reports and discusses plant-derived products' beneficial properties, emphasizing garlic and its usages in fish aquaculture.

Maclura tinctoria Extracts: In Vitro Antibacterial Activity against Aeromonas hydrophila and Sedative Effect in Rhamdia quelen

Maclura tinctoria is a tree species native from Brazil and rich in phenolic compounds. Since plant antibacterial activity is highly associated with phenolic compound concentration, we aim to evaluate the in vitro antimicrobial activity of different extracts against fish pathogenic bacteria. In addition, some phenolic compounds have central depressant effects and can be useful in aquaculture due to possible sedative and/or anesthetic effects. Four M. tinctoria extracts were extracted separately with ethanol; leaves (LE), bark (BE), heartwood (HE), and the sapwood (SE). In vitro antimicrobial activity was tested against Aeromonas strains at concentrations of 6400 to 3.125 μg/mL. The sedative effect was evaluated for 24 h with 30 and 100 mg/L concentrations. Chemical composition was analyzed by HPLC-DAD-MS. The HE extract had the best MIC (400 µg/mL) and MBC (800 µg/mL) compared to the LE, BE, and SE extracts. LE extract induced deep sedation and the BE, SE, and HE extracts induced light sedation. Additionally, BE, SE, and HE induced a normal behavior without side effects. Polyphenolic compounds with antimicrobial activity and sedative effects were identified mainly in HE. Thus, HE extract is safe and can be used as a sedative for silver catfish.

Antimicrobial Activity and Chemical Composition of Essential Oils against Pathogenic Microorganisms of Freshwater Fish

Antimicrobials are widely applied in aquaculture for treatment of infectious diseases in fish. The increased antimicrobial resistance of fish pathogens to conventional antimicrobial treatment highlights the need for research on the antibacterial properties of natural products—in this case, essential oils (EOs). The aim of the present study was to detect the antimicrobial activity of the essential oils on pathogenic microorganisms found in freshwater fish. Freshwater fish isolates of Aerococcus spp., Aeromonas spp., Enterococcus spp., Escherichia spp., Pseudomonas spp., Shewanella spp., Yersinia spp., and Vagococcus spp. were tested for antimicrobial resistance and antimicrobial activity against 14 commercially available essential oils. Antimicrobial resistance was identified in Pseudomonas spp. isolates against cefepime and ciprofloxacin; while all Aeromonas, Enterococcus, and Yersinia isolates were fully susceptible. All tested EOs revealed antimicrobial activity against the tested freshwater fish isolates at different extents. Cinnamomum camphora exhibited strong antimicrobial activity against Aeromonas spp. (3.12 μL/mL), Enterococcus spp. (0.78–1.56 μL/mL), and Pseudomonas spp. with the MIC method. EOs of Gaultheria procumbens and Litsea cubeba showed strong antibacterial activity against Yersinia spp. and Vacococcus spp. (6.25 μL/mL). The study shows the antimicrobial activity of EOs against the most relevant freshwater fish pathogens and indicates the application opportunities in aquaculture.

Combined Effect of the Essential Oil and Collagen Film on the Quality of Pacific Mackerel (Pneumatophorus japonicus) Fillet During Cold Storage

Essential oils (EOs) and collagen have received recent attention in the seafood industry due to their abilities of antibacterial and seafood preservation individually. However, to the authors' best knowledge, very few publications address the issue of the combined effect of EOs and collagen on seafood preservation. Pacific mackerel is one of the most economically valuable fish species in China and easy to deteriorate during storage. Therefore, present study investigated the effect of combined EOs (cinnamon, oregano, and clove) and collagen on the quality of Pacific mackerel during cold storage. A suite of microbiological, physical, and chemical properties that are indicative of quality was measured. From the results, mackerel fillets treated with an EO-collagen film had a smaller increase in microbial counts compared with control. Furthermore, total volatile basic nitrogen (TVB-N), thiobarbituric acid related substance, and pH of mackerel fillet were lower when treated with an EO-collagen film and somewhat lower when treated with collagen alone. According to texture measurements of muscle, samples treated with EO-collagen film began to deteriorate in 8 d, versus only 4 d for control samples. EOs likely contributed to antibacterial and antioxidative activity, and the collagen film isolated muscle from air, which in turn reduced oxidation and retained the quality. Consequently, combination of EOs and collagen film efficiently extends shelf-life of Pacific mackerel during storage.

Dietary cinnamaldehyde nanoemulsion boosts growth and transcriptomes of antioxidant and immune related genes to fight Streptococcus agalactiae infection in Nile tilapia (Oreochromis niloticus)

The present study was conducted to investigate the effects of dietary cinnamaldehyde nanoemulsion (CNE) on growth, digestive activities, antioxidant and immune responses and resistance against Streptococcus agalactiae (S. agalactiae) in Nile tilapia. Four experimental diets were formulated containing CNE at levels of 0, 100, 200 and 300 mg/kg diet for 12 weeks. At the end of the experiment, all fish were challenged by S. agalactiae. The results showed that the final body weight was increased in fish groups fed 200 and 300 mg CNE/kg diet by 18.4 and 17.2% with respect to the control group. Moreover, feed conversion ratio and digestive enzymes' activities were improved in groups fed 200 and 300 then 100 mg of dietary CNE/kg diet. Groups fed CNE exhibited a significant increase in serum immune-related parameters when compared with control group. Additionally, the hypocholesterolemic effects was achieved after CNE feeding unlike the control group in a dose dependent manner. With increasing dietary CNE levels, genes expression of cytokines and antioxidant enzymes were upregulated. Less severe adverse clinical symptoms and respectable cumulative mortalities associated with S. agalactiae infection were observed in fish fed CNE. To our knowledge, this study was the first offering a protective effect of CNE against S. agalactiae infection in Nile tilapia with a maximum down-regulation of cylE and hylB virulence genes expression noticed in group fed 300 mg of CNE/kg diet (up to 0.10 and 0.19- fold, respectively). Therefore, the present study recommended that an incorporation of CNE at level of 300 mg/kg diet for Nile tilapia could promote their growth, enhance their immunity and antioxidant status and provide protection against virulent S. agalactiae.

Antibacterial Effects of Essential Oils of Seven Medicinal-Aromatic Plants Against the Fish Pathogen Aeromonas veronii bv. sobria: To Blend or Not to Blend?

Despite progress achieved, there is limited available information about the antibacterial activity of constituents of essential oils (EOs) from different medicinal-aromatic plants (MAPs) against fish pathogens and the complex interactions of blended EOs thereof. The present study aimed to investigate possible synergistic antimicrobial effects of EOs from seven Greek MAPs with strong potential against Aeromonas veronii bv. sobria, a fish pathogen associated with aquaculture disease outbreaks. The main objective was to evaluate whether blending of these EOs can lead to increased antimicrobial activity against the specific microorganism. A total of 127 combinations of EOs were prepared and their effect on A. veronii bv. sobria growth was tested in vitro. We examined both the inhibitory and bactericidal activities of the individual EOs and compared them to those of the blended EOs. The vast majority of the investigated combinations exhibited significant synergistic and additive effects, while antagonistic effects were evident only in a few cases, such as the mixtures containing EOs from rosemary, lemon balm and pennyroyal. The combination of EOs from Greek oregano and wild carrot, as well as the combinations of those two with Spanish oregano or savoury were the most promising ones. Overall, Greek oregano, savoury and Spanish oregano EOs were the most effective ones when applied either in pure form or blended with other EOs.

Carvacrol, Thymol, and Garlic Essential Oil Promote Skin Innate Immunity in Gilthead Seabream (Sparus aurata) Through the Multifactorial Modulation of the Secretory Pathway and Enhancement of Mucus Protective Capacity

One of the main targets for the use of phytogenics in aquafeeds is the mucosal tissues as they constitute a physical and biochemical shield against environmental and pathogenic threats, comprising elements from both the innate and acquired immunity. In the present study, the modulation of the skin transcriptional immune response, the bacterial growth capacity in skin mucus, and the overall health condition of gilthead seabream (Sparus aurata) juveniles fed a dietary supplementation of garlic essential oil, carvacrol, and thymol were assessed. The enrichment analysis of the skin transcriptional profile of fish fed the phytogenic-supplemented diet revealed the regulation of genes associated to cellular components involved in the secretory pathway, suggesting the stimulation, and recruitment of phagocytic cells. Genes recognized by their involvement in non-specific immune response were also identified in the analysis. The promotion of the secretion of non-specific immune molecules into the skin mucus was proposed to be involved in the in vitro decreased growth capacity of pathogenic bacteria in the mucus of fish fed the phytogenic-supplemented diet. Although the mucus antioxidant capacity was not affected by the phytogenics supplementation, the regulation of genes coding for oxidative stress enzymes suggested the reduction of the skin oxidative stress. Additionally, the decreased levels of cortisol in mucus indicated a reduction in the fish allostatic load due to the properties of the tested additive. Altogether, the dietary garlic, carvacrol, and thymol appear to promote the gilthead seabream skin innate immunity and the mucus protective capacity, decreasing its susceptibility to be colonized by pathogenic bacteria.

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.

The microbial safety of fish and fish products: Recent advances in understanding its significance, contamination sources, and control strategies

Microorganisms play a crucial and unique role in fish and fish product safety. The presence of human pathogens and the formation of histamine caused by spoilage bacteria make the control of both pathogenic and spoilage microorganisms critical for fish product safety. To provide a comprehensive and updated overview of the involvement of microorganisms in fish and fish product safety, this paper reviewed outbreak and recall surveillance data obtained from government agencies from 1998 to 2018 and identified major safety concerns associated with both domestic and imported fish products. The review also summarized all available literature about the prevalence of major and emerging microbial safety concerns, including Salmonella spp., Listeria monocytogenes, and Aeromonas hydrophila, in different fish and fish products and the survival of these pathogens under different storage conditions. The prevalence of antibiotic-resistant bacteria (ARB) and antibiotic-resistant genes (ARGs), two emerging food safety concerns, is also reviewed. Pathogenic and spoilage microorganisms as well as ARB and ARGs can be introduced into fish and fish products in both preharvest and postharvest stages. Many novel intervention strategies have been proposed and tested for the control of different microorganisms on fish and fish products. One key question that needs to be considered when developing and implementing novel control measures is how to ensure that the measures are cost and environment friendly as well as sustainable. Over the years, regulations have been established to provide guidance documents for good farming and processing practices. To be more prepared for the globalization of the food chain, harmonization of regulations is still needed.

Carvacrol essential oil stimulates growth performance, immune response, and tolerance of Nile tilapia to Cryptococcus uniguttulatus infection

This study investigated the antifungal activity of 5 essential oils (EOs) towards yeasts recovered from diseased fishes; and focused on the efficacy of one EO (carvacrol) on growth performance, non-specific immunity, and disease resistance of Nile tilapia Oreochromis niloticus against Cryptococcus uniguttulatus challenge. Thymoquinone, thymol, carvacrol, eugenol, and cinnamon were first tested in vitro against 20 clinical yeast strains in comparison with antifungal drugs (fluconazole, ketoconazole, itraconazole, amphotericin B, nystatin, and clotrimazole) using disc diffusion and broth microdilution methods. For the in vivo challenge, fish (n = 150) were divided into 5 groups (carvacrol prophylaxis, carvacrol treatment, itraconazole treatment, unchallenged control, and positive control; 30 fish group-1) with 3 replicates. Phagocytic activity, reactive oxygen species production, reactive nitrogen species production, myeloperoxidase, lysozyme activity, and total immunoglobulins were tested before and after challenge. Relative percent survival (RPS) and mortality percent were determined as indicators for functional immunity. EOs displayed divergent degrees of antifungal activity, and carvacrol was the most effective against the tested yeasts. The dietary additive of carvacrol significantly enhanced growth performance, all immunological parameters, and the RPS values (90%) compared to other treatments. This unique experimental model indicates that carvacrol seems promising not only for enhancing immunity and promoting fish growth, but also for controlling emerging fungal infections. Future studies should investigate different concentrations of carvacrol as well as its antifungal activity in different fish species.

Cymbopogon flexuosus essential oil as an additive improves growth, biochemical and physiological responses and survival against Aeromonas hydrophila infection in Nile tilapia

The objective of the present study was to evaluate growth, biochemical, hematological and intestinal enzymes responses and survival of Nile tilapia juveniles fed a diet containing the essential oil of lemongrass Cymbopogum flexuosus (EOCF) and infected by Aeromonas hydrophila. Five diets were evaluated (in quadruplicate) with increasing levels of EOCF (0.0 - control; 0.25; 0.50; 1.0 and 2.0 mL kg diet-1). On day 45, eight fish per treatment were sampled and blood, liver and intestine samples were taken. Others eight fish per treatment were infected with A. hydrophila followed by a 15-day period of observation. Citral is the main constituent of EOCF. The inclusion of 2.0 mL EOCF kg diet-1 increased specific growth rate and survival after A. hydrophila infection and decreased feed conversion ratio of Nile tilapia. In general, higher concentrations of EOCF in the diet reduced plasma glucose and triglycerides levels, and increased plasma amino acids, alanine aminotransferase (ALT) and hepatic ALT levels, hematological parameters, and the activity of intestinal enzymes. It was concluded that the inclusion of 2.0 mL EOCF kg diet-1 improved growth performance, biochemical and physiological responses and decreased mortality of Nile tilapia after A. hydrophila infection.

Zein Nanoparticles Impregnated with Eugenol and Garlic Essential Oils for Treating Fish Pathogens

The supply of food derived from aquaculture has increased significantly in recent years. The aim of this industrial sector is to produce sustainable products to meet the needs of consumers, providing food security and nutritional benefits. The development of aquaculture has faced challenges including disease outbreaks that can cause substantial economic losses. These diseases can be controlled using chemicals such as antibiotics. However, the indiscriminate use of these substances can have major negative impacts on human health and the environment with the additional risk of the emergence of resistant organisms. The present manuscript describes the use of phytotherapy in association with nanotechnology in order to obtain a more effective and less harmful system for the control of bacterial diseases in fish. Zein nanoparticles associated with eugenol and garlic essential oil were prepared through antisolvent precipitation and characterized. Zein nanoparticles are promising carrier systems as zein proteins are biodegradable and biocompatible and, in this way, good candidates for encapsulation of active ingredients. The system presented good physicochemical properties with an average particle diameter of approximately 150 nm, a polydispersity index lower than 0.2, and a zeta potential of approximately 30 mV. High encapsulation efficiency was obtained for the active compounds with values higher than 90%, and the compounds were protected against degradation during storage (90 days). The nanoparticle formulations containing the botanical compounds also showed less toxicity in the tests performed with a biomarker (Artemia salina). In addition, the systems showed bactericidal activity against the important fish pathogenic bacteria Aeromonas hydrophila, Edwardsiella tarda, and Streptococcus iniae in vitro. The present study opens new perspectives for the use of botanical compounds in combination with nanotechnology to treat fish diseases caused by bacteria, contributing to a more sustainable fish chain production.

V, Izquierdo M, Terova G. Assessment of dietary supplementation with galactomannan oligosaccharides and phytogenics on gut microbiota of European sea bass (Dicentrarchus Labrax) fed low fishmeal and fish oil based diet

There is an increasing interest from the aquafeed industry in functional feeds containing selected additives that improve fish growth performance and health status. Functional feed additives include probiotics, prebiotics, organic acids, and phytogenics (substances derived from plants and their extracts). This study evaluated the effects of dietary inclusion of a mucilage extract rich in galactomannan oligosaccharides (GMOS), a mixture of garlic and labiatae-plants oils (PHYTO), and a combination of them (GMOSPHYTO), on gut microbiota composition of European sea bass (Dicentrarchus labrax) fed with a low fishmeal (FM) and fish oil (FO) diet. Three experimental diets and a control diet (plant-based formulation with 10% FM and 6% FO) were tested in a 63-days feeding trial. To analyze the microbiota associated to feeds and the intestinal autochthonous (mucosa-adhered) and allochthonous (transient) microbial communities, the Illumina MiSeq platform for sequencing of 16S rRNA gene and QIIME2 pipeline were used. Metabarcoding analysis of feed-associated bacteria showed that the microbial communities of control (CTRL) feed deeply differed from those of experimental diets. The number of reads was significantly lower in CTRL feed than in other feeds. The OTU (operational taxonomic unit) number was instead similar between the feeds, ranging from 42 to 50 OTUs. The variation of resident gut microbiota induced by diet was lower than the variation of transient intestinal microbiota, because feedstuffs are a major source of allochthonous bacteria, which can temporarily integrate into the gut transient microbiome. However, the composition of transient bacterial communities was not simply a mirror of feed-borne bacteria. Indeed, the microbial profile of feeds was different from both faecal and mucosa profiles. Our findings suggest that the dietary inclusion of GMOS (0.5%) and PHYTO (0.02%) in a low FM and FO diet induces changes in gut microbiota composition of European sea bass. However, if on allochthonous microbiota the combined inclusion of GMOS and PHYTO showed an antagonistic effect on bactericidal activity against Vibrionales, at mucosa level, only GMOSPHYTO diet increased the relative abundance of Bacteroidales, Lactobacillales, and Clostridiales resident bacterial orders. The main beneficial effects of GMOS and PHYTO on gut microbiota are the reduction of coliforms and Vibrionales bacteria, which include several potentially pathogenic species for fish, and the enrichment of gut microbiota composition with butyrate producer taxa. Therefore, these functional ingredients have a great potential to be used as health-promoting agents in the farming of European sea bass and other marine fish.

Benefits of nanotechnology: Dietary supplementation with nerolidol-loaded nanospheres increases survival rates, reduces bacterial loads and prevents oxidative damage in brains of Nile tilapia experimentally infected by Streptococcus agalactiae

Rampant and uncontrolled use of antibiotics is a major concern for aquaculture; the practice foments the emergence of resistant strains of Streptococcus agalactiae, among other negative impacts. Constituents of plant essential oils such as nerolidol are being considered as replacements for synthetic drugs to support fish nutrition and health. There is evidence to suggest that nanotechnology may enhance the efficacy of natural bioactive compounds; this is a substantial advance for the development and sustainability of aquaculture. Against the backdrop of this evidence, we aimed determine whether dietary supplementation with free nerolidol and nerolidol-loaded nanospheres would exert bactericidal effects against S. agalactiae, as well as prevent S. agalactiae-induced brain oxidative damage. In Experiment I, we measured the antimicrobial properties of dietary supplementation of nerolidol and nerolidol nanosphere in terms of mortality, longevity and relative percent survival. Fish infected with S. agalactiae fed 0.5 and 1.0 mL nerolidol nanospheres kg/diet demonstrated lower mortality and higher relative percent survival than the control group, while longevity was higher in all infected plus supplementation groups. Experiment II showed significantly lower microbial loads in brains of fish infected with S. agalactiae that were fed 1.0 mL nerolidol nanospheres kg/diet than in the control group. Brain nerolidol levels were significantly higher in uninfected as well as infected fish supplemented with nerolidol nanospheres than in fish supplemented with free nerolidol. Finally, brain reactive oxygen species and lipid peroxidation levels were higher in infected fish supplemented with basal diet compared to uninfected fish and supplemented with basal diet, and the supplementation with 1.0 mL/kg nerolidol nanospheres prevented this augmentation caused by infection. These data suggest that dietary supplementation with nerolidol nanospheres (1.0 mL/kg diet) has potent bactericidal effects in terms of augmentation of fish longevity and survival, and reduction of brain microbial loads. Also, S. agalactiae-induced brain oxidative damage that contributed to disease pathogenesis, and the dietary supplementation with nerolidol nanospheres (1.0 mL/kg diet) prevented this alteration. In summary, nanotechnology is a compelling approach to enhancing the efficacy of nerolidol, giving rise to reduction of S. agalactiae loads in fish brains.

Comparative Evaluation of Essential Oils from Medicinal-Aromatic Plants of Greece: Chemical Composition, Antioxidant Capacity and Antimicrobial Activity against Bacterial Fish Pathogens

The administration of antibiotics in aquaculture has raised concern about the impact of their overuse in marine ecosystems, seafood safety and consumers' health. This "green consumerism" has forced researchers to find new alternatives against fish pathogens. The present study focused on 12 Mediterranean medicinal-aromatic plants as potential antimicrobials and antioxidant agents that could be used in fish aquaculture. In vitro assays showed that the essential oils (EOs) from all studied plants had anti-bacterial and antioxidant properties, with their efficacy being dependent on their chemical composition. More specifically, EOs rich in carvacrol, p-cymene and γ-terpinene exhibited not only the strongest inhibitory activity against the growth of bacterial pathogens (inhibitory concentration: 26-88 μg mL-1), but also the greatest total antioxidant capacity (ABTS: 2591-5879 μmole mL-1; CUPRAC: 931-2733 μmole mL-1). These compounds were mainly found in the EOs from Greek oregano (Origanum vulgare subsp. hirtum), Spanish oregano (Thymbra capitata) and savoury (Satureja thymbra) collected from cultivations in Greece. The specific EOs stand out as promising candidates for the treatment of bacterial diseases and oxidative stress in farmed fish. Further in vivo experiments are needed to fully understand the effects of EO dietary supplementation on fish farming processes.

Dietary phytogenics and galactomannan oligosaccharides in low fish meal and fish oil-based diets for European sea bass (Dicentrarchus labrax) juveniles: Effects on gut health and implications on in vivo gut bacterial translocation

European sea bass were fed four low FM/FO (10%/6%) diets containing galactomannan oligosaccharides (GMOS), a mixture of garlic oil and labiatae plants oils (PHYTO), or a combination of both functional products (GMOSPHYTO) for 63 days before exposing the fish to an intestinal Vibrio anguillarum infection combined with crowding stress. In order to evaluate functional diets efficacy in terms of gut health maintenance, structural, cellular, and immune intestinal status were evaluated by optical and electron microscopy and gene expression analyses. A semi-automated software was adapted to determine variations in goblet cell area and mucosal mucus coverage during the challenge test. Feeding with functional diets did not affect growth performance; however, PHYTO and GMOS dietary inclusion reduced European sea bass susceptibility to V. anguillarum after 7 days of challenge testing. Rectum (post-ileorectal valve) showed longer (p = 0.001) folds than posterior gut (pre-ileorectal valve), whereas posterior gut had thicker submucosa (p = 0.001) and higher mucus coverage as a result of an increased cell density than rectum. Functional diets did not affect mucosal fold length or the grade of granulocytes and lymphocytes infiltration in either intestinal segment. However, the posterior gut fold area covered by goblet cells was smaller in fish fed GMOS (F = 14.53; p = 0.001) and PHYTO (F = 5.52; p = 0.019) than for the other diets. PHYTO (F = 3.95; p = 0.049) reduced posterior gut goblet cell size and increased rodlet cell density (F = 3.604; p = 0.068). Dietary GMOS reduced submucosal thickness (F = 51.31; p = 0.001) and increased rodlet cell density (F = 3.604; p = 0.068) in rectum. Structural TEM analyses revealed a normal intestinal morphological pattern, but the use of GMOS increased rectum microvilli length, whereas the use of PHYTO increased (p≤0.10) Ocln, N-Cad and Cad-17 posterior gut gene expression. After bacterial intestinal inoculation, posterior gut of fish fed PHYTO responded in a more controlled and belated way in terms of goblet cell size and mucus coverage in comparison to other treatments. For rectum, the pattern of response was similar for all dietary treatments, however fish fed GMOS maintained goblet cell size along the challenge test.

Essential Oils as Stress-Reducing Agents for Fish Aquaculture: A Review

In fish, stressful events initiate a hormone cascade along the hypothalamus-pituitary-interrenal and hypothalamus-sympathetic-chromaffin (HSC) axis to evoke several physiological reactions in order to orchestrate and maintain homeostasis. Several biotic and abiotic factors, as well as aquaculture procedures (handling, transport, or stocking density), activated stress system inducing negative effects on different physiological processes in fish (growth, reproduction, and immunity). In order to reduce these consequences, the use of essential oils (EOs) derived from plants has been the focus of aquaculture studies due to their diverse properties (e.g., anesthetic, antioxidant, and antimicrobial), which have been shown to reduce biochemical and endocrine alterations and, consequently, to improve the welfare status. Recently, several studies have shown that biogenic compounds isolated from different EOs present excellent biological activities, as well as the nanoencapsulated form of these EOs may potentiate their effects. Overall, EOs presented less side effects than synthetic compounds, but their stress-reducing efficacy is related to their chemical composition, concentration or chemotype used. In addition, their species-specific actions must be clearly established since they can act as stressors by themselves if their concentrations and chemotypes used are not suitable. For this reason, it is necessary to assess the effect of these natural compound mixtures in different fish species, from marine to freshwater, in order to find the ideal concentration range and the way for their administration to obtain the desired biological activity, without any undesired side effects. In this review, the main findings regarding the use of different EOs as stress reducers will be presented to highlight the most important issues related to their use to improve fish welfare in aquaculture.

Antimicrobial activity of trans-cinnamic acid and commonly used antibiotics against important fish pathogens and nonpathogenic isolates

AIMS

Antibiotics and several other chemicals have been used to prevent fish diseases. However, this situation results in economic loss for the companies in the aquaculture industry and most importantly it pollutes the environment. Cinnamic acid is a naturally occurring aromatic acid and is considered to be safe for human consumption. Therefore, in this study, the antibacterial activity of trans-cinnamic acid and commonly used antibiotics, namely chloramphenicol, vancomycin, streptomycin and erythromycin, were tested against 32 bacteria, including fish pathogens, nonpathogenic isolates and collection strains.

METHODS AND RESULTS

Trans-cinnamic acid was applied against the bacteria using the disc diffusion and microdilution method under in vitro conditions. Antibiotics were also tested under similar conditions against all tested bacteria using the disc diffusion method. The results show that among 32 bacterial strains trans-cinnamic acid exhibited potent inhibitory effect on the Gram-negative fish pathogen Aeromonas sobria. In addition, a moderate inhibition of trans-cinnamic acid of fish pathogens Aeromonas salmonicida, Vibrio (Listonella) anguillarum, Vibrio crassostreae and Yersinia ruckeri was also observed for trans-cinnamic acid in our study. On the contrary, the majority of nonpathogenic intestinal isolates were resistant to trans-cinnamic acid.

CONCLUSIONS

To the best of our knowledge, this is the first report on the antimicrobial activity of trans-cinnamic acid on 24 of the studied bacteria isolated from fish. In conclusion, trans-cinnamic acid can be used as an environmentally friendly alternative additive to prevent and control primarily A. sobria, as well as other pathogenic bacteria such as A. salmonicida, V. anguillarum, V. crassostreae and Y. ruckeri.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study indicated that trans-cinnamic acid may present an environmentally friendly alternative therapeutic agent against A. sobria infections in the aquaculture industry.