Antimicrobial Drug Resistance in Fish Pathogens

The CRISPR/Cas system as an antimicrobial resistance strategy in aquatic ecosystems

With the growing population, demand for food has dramatically increased, and fisheries, including aquaculture, are expected to play an essential role in sustaining demand with adequate quantities of protein and essential vitamin supplements, employment generation, and GDP growth. Unfortunately, the incidence of emerging/re-emerging AMR pathogens annually occurs because of anthropogenic activities and the frequent use of antibiotics in aquaculture. These AMR pathogens include the WHO's top 6 prioritized ESKAPE pathogens (nosocomial pathogens: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.), extended-spectrum beta lactases (ESBLs) and carbapenemase-producing E. coli, which pose major challenges to the biomagnification of both nonnative and native antibiotic-resistant bacteria in capture and cultured fishes. Although implementing the rational use of antibiotics represents a promising mitigation measure, this approach is practically impossible due to the lack of awareness among farmers about the interplay between antimicrobial use and the emergence of antimicrobial resistance (AMR). Nevertheless, to eradicate these 'superbugs,' CRISPR/Cas (clustered regularly interspersed short palindromic repeats/CRISPR associate protein) has turned out to be a novel approach owing to its ability to perform precise site-directed targeting/knockdown/reversal of specific antimicrobial resistance genes in vitro and to distinguish AMR-resistant bacteria from a plethora of commensal aquatic bacteria. Along with highlighting the importance of virulent multidrug resistance genes in bacteria, this article aims to provide a holistic picture of CRISPR/Cas9-mediated genome editing for combating antimicrobial-resistant bacteria isolated from various aquaculture and marine systems, as well as insights into different types of CRISPR/Cas systems, delivery methods, and challenges associated with developing CRISPR/Cas9 antimicrobial agents.

Magnolia Officinalis Alcohol Extract Alleviates the Intestinal Injury Induced by Polygala Tenuifolia Through Regulating the PI3K/AKT/NF-κB Signaling Pathway and Intestinal Flora

Purpose

Polygala tenuifolia Willd. (PT), a traditional Chinese medicinal plant extensively employed in managing Alzheimer's disease, exhibits notable gastrointestinal side effects as highlighted by prior investigations. In contrast, Magnolia officinalis Rehd. et Wils (MO), a traditional remedy for gastrointestinal ailments, shows promising potential for ameliorating this adverse effect of PT. The objective of this study is to examine the underlying mechanism of MO in alleviating the side effects of PT.

Methods

Hematoxylin-eosin (H&E) staining was used to observe the structural damage of zebrafish intestine, and enzyme-linked immunosorbent assay (ELISA) was used to detect the levels of inflammatory factors and oxidative stress. The integrity of the intestinal tight junctions was examined using transmission electron microscope (TEM). Moreover, the expression of intestinal barrier genes and PI3K/AKT/NF-κB signaling pathway-related genes was determined through quantitative real-time PCR. The changes in intestinal microbial composition were analyzed using 16S rRNA and metagenomic techniques.

Results

MO effectively ameliorated intestinal pathological damage and barrier gene expression, and significantly alleviated intestinal injury by reducing the expression of inflammatory cytokines IL-1β, IL-6, TNF-α, and inhibiting the activation of PI3K/AKT/NF-κB pathway. Furthermore, MO could significantly increase the relative abundance of beneficial microorganisms (Lactobacillus, Blautia and Saccharomyces cerevisiae), and reduce the relative abundance of pathogenic bacteria (Plesiomonas and Aeromonas).

Conclusion

MO alleviated PT-induced intestinal injury, and its mechanism may be related to the inhibition of PI3K/AKT/NF-κB pathway activation and regulation of intestinal flora.

Functional Proteomics Analysis of Norfloxacin-Resistant Edwardsiella tarda

Multidrug-resistant Edwardsiella tarda threatens both sustainable aquaculture and human health, but the control measure is still lacking. In this study, we adopted functional proteomics to investigate the molecular mechanism underlying norfloxacin (NOR) resistance in E. tarda. We found that E. tarda had a global proteomic shift upon acquisition of NOR resistance, featured with increased expression of siderophore biosynthesis and Fe3+-hydroxamate transport. Thus, either inhibition of siderophore biosynthesis with salicyl-AMS or treatment with another antibiotic, kitasamycin (Kit), which was uptake through Fe3+-hydroxamate transport, enhanced NOR killing of NOR-resistant E. tarda both in vivo and in vitro. Moreover, the combination of NOR, salicyl-AMS, and Kit had the highest efficacy in promoting the killing effects of NOR than any drug alone. Such synergistic effect not only confirmed in vitro and in vivo bacterial killing assays but also applicable to other clinic E. tarda isolates. Thus, our data suggest a proteomic-based approach to identify potential targets to enhance antibiotic killing and propose an alternative way to control infection of multidrug-resistant E. tarda.

In Vitro Antimicrobial Activity of Nymphaea pubescens (Pink Water Lily) Leaf Extracts

This research comparatively investigates the in vitro antimicrobial activity of extracts from Nymphaea pubescens (pink water lily) leaves against pathogenic bacteria. The experimental extracts are aqueous, acetonic, and 95% ethanolic N. pubescens extracts; and the pathogenic bacteria being studied include Aeromonas hydrophila, Vibrio parahaemolyticus, Vibrio vulnificus, and Vibrio harveyi, which are commonly found in freshwater fish and brackish aquatic animals. The ethanolic N. pubescens extract achieves the highest bacterial inhibitory effects against V. parahaemolyticus and V. vulnificus. The minimum inhibitory concentrations of the ethanolic extract against A. hydrophila and V. harveyi are 10 mg/mL; and 2.5 mg/mL against V. parahaemolyticus and V. vulnificus. The ethanolic N. pubescens extract is effective against V. parahaemolyticus. The high-performance liquid chromatography results show that, in the phenolic acids group, gallic acid is the most dominant (0.600-3.21% w/w), followed by sinapic acid (0.37-0.83% w/w). In the flavonoids group, catechin is the most dominant (0.02-1.08% w/w), followed by rutin (0.002-0.03% w/w). Essentially, the ethanolic N. pubescens extract can potentially be used as a natural antibiotic agent to treat bacterial infections in fish and aquatic animals.

Misunderstandings and misinterpretations: Antimicrobial use and resistance in salmon aquaculture

The exponential growth of aquaculture over the past 30 years has been accompanied by a parallel increase in the use of antimicrobials. This widespread use has had negative effects on animal, human and environmental health and affected the biodiversity of the environments where aquaculture takes place. Results showing these harmful effects have been resisted and made light of by the aquaculture industry and their scientific supporters through introduction of misunderstandings and misinterpretations of concepts developed in the evolution, genetics, and molecular epidemiology of antimicrobial resistance. We focus on a few of the most obvious scientific shortcomings and biases of two recent attempts to minimise the negative impacts of excessive antimicrobial use in Chilean salmon aquaculture on human and piscine health and on the environment. Such open debate is critical to timely implementation of effective regulation of antimicrobial usage in salmon aquaculture in Chile, if the negative local and worldwide impacts of this usage are to be avoided.

Bacillus subtilis KM0 Impacts gut Microbiota Profile and Transcription of Genes Related to Transcellular Transport in Zebrafish (Danio rerio)

It is known that probiotic microorganisms play important roles in the composition of the intestinal microbiota. Also, probiotics can affect the paracellular and transcellular transport mechanisms performed by intestinal cells. The aim of this work was to evaluate the effect of the potential probiotic Bacillus subtilis KM0 on the profile of the gut microbiota and transcription of genes related to intestinal transport of zebrafish (Danio rerio). Zebrafish was exposed by immersion to B. subtilis KM0 for 48 h, and the intestines were collected for metataxonomic analysis and transcription of genes related to transcellular and paracellular transports. Although exposure to B. subtilis changed the intestinal microbiota profile of zebrafish, the diversity indices were not altered. A decrease in the number of genera of potentially pathogenic bacteria (Flavobacterium, Plesiomonas, and Pseudomonas) and downregulation in transcription of transcellular transport genes (cubn and amn) were observed. B. subtilis KM0 strain had the expected probiotic effect, by interfering with the proliferation of potentially pathogenic bacteria and decreasing the transcription of genes codifying for signals involved with a mechanism that can be used for invasion by pathogens. The present study demonstrated that, even with a short-term exposure, a bacterium with probiotic potential such as the KM0 strain of B. subtilis can modify the profile of the host's intestinal microbiota, with an impact on the regulation of intestinal genes related to mechanisms that can be used for invasion by pathogenic bacteria.

Fish Probiotics: Cell Surface Properties of Fish Intestinal Lactobacilli and Escherichia coli

The properties of intestinal bacteria/probiotics, such as cell surface hydrophobicity (CSH), auto-aggregation, and biofilm formation ability, play an important role in shaping the relationship between the bacteria and the host. The current study aimed to investigate the cell surface properties of fish intestinal bacteria and probiotics. Microbial adhesion to hydrocarbons was tested according to Kos and coauthors. The aggregation abilities of the investigated strains were studied as described by Collado and coauthors. The ability of bacterial isolates to form a biofilm was determined by performing a qualitative analysis using crystal violet staining based on the attachment of bacteria to polystyrene. These studies prove that bacterial cell surface hydrophobicity (CSH) is associated with the growth medium, and the effect of the growth medium on CSH is species-specific and likely also strain-specific. Isolates of intestinal lactobacilli from fish (Salmo ischchan) differed from isolates of non-fish/shrimp origin in the relationship between auto-aggregation and biofilm formation. Average CSH levels for fish lactobacilli and E. coli might were lower compared to those of non-fish origin, which may affect the efficiency of non-fish probiotics use in fisheries due to the peculiarities of the hosts’ aquatic lifestyles.

Enrofloxacin-induced transfer of multiple-antibiotic resistance genes and emergence of novel resistant bacteria in red swamp crayfish guts and pond sediments

Antibiotic resistance genes (ARGs) can be transferred from environmental microbes to human pathogens, thus leading to bacterial infection treatment failures. The aquaculture polluted by over-used antibiotics is considered as a notorious reservoir of ARGs. However, the origin, diachronic changes, and mobility of ARGs under antibiotic exposure in aquaculture systems remain elusive. Our findings showed that enrofloxacin application also increased the relative abundance of various ARGs in addition to quinolone-resistance genes and induced ARG dissemination in crayfish gut and sediment bacteria. Further investigation indicated that the transposase-mediated recombination was the major driver of horizontal gene transfer (HGT) of ARGs under antibiotic stress. Notably, enrofloxacin application also induced the generation of some metagenome-assembled genomes (MAGs) carrying multiple ARGs, which were identified as novel species. Additionally, Enterobacteriaceae constituted a mobile ARG pool in aquaculture. Therefore, aquaculture provides potential wide environmental pathways for generation and spread of antibiotic resistance. Our findings of ARG temporal variations and dissemination pattern in aquaculture with artificial use of antibiotics are critical to the management of antibiotic resistance, which is of great ecosystem and health implications.

A scoping review of the distribution and frequency of extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae in shrimp and salmon

Antimicrobial-resistant (AMR) bacteria are a threat to public health as they can resist treatment and pass along genetic material that allows other bacteria to become drug-resistant. To assess foodborne AMR risk, the Codex Guidelines for Risk Analysis of Foodborne AMR provide a framework for risk profiles and risk assessments. Several elements of a risk profile may benefit from a scoping review (ScR). To contribute to a larger risk profile structured according to the Codex Guidelines, our objective was to conduct a ScR of the current state of knowledge on the distribution, frequency and concentrations of extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae in salmon and shrimp. Articles were identified via a comprehensive search of five bibliographic databases. Two reviewers screened titles and abstracts for relevance and characterised full-text articles with screening forms developed a priori. Sixteen relevant studies were identified. This review found that there is a lack of Canadian data regarding ESBL-producing Enterobacteriaceae in salmon and shrimp. However, ESBL- producing Escherichia coli, Klebsiella pneumoniae and other Enterobacteriaceae have been isolated in multiple regions with a history of exporting seafood to Canada. The literature described herein will support future decision-making on this issue as research/surveillance and subsequent assessments are currently lacking.

Astragalus polysaccharides protect against inactivated Vibrio alginolyticus-induced inflammatory injury in macrophages of large yellow croaker

As an effective immunostimulant, Astragalus polysaccharides (APS) have been widely used in fish aquaculture, however, their action mechanisms remain poorly understood. In the present paper, the inflammatory macrophage model of large yellow croaker (Larimichthys crocea) was constructed by using formalin-inactivated Vibrio alginolyticus. Inactivated V. alginolyticus could cause cellular damage of primary head kidney macrophages (PKM) by decreasing cell activity and inducing reactive oxygen species (ROS) production and cell apoptosis. When PKM were pretreated with APS, the depressed cell activity induced by inactivated V. alginolyticus was significantly improved, and ROS overproduction and cell apoptosis were inhibited. Then the protection mechanism of APS was investigated by transcriptome analysis. After treated with inactivated V. alginolyticus, the expression of immune-related genes (TLR5s, TLR13, Clec4e, IKK, IκB, BCL-3, NF-κB2, REL, IL-1β, and IL-6) and pyroptosis-related genes (caspase-1, NLRP3, and NLRC3) in PKM were significantly up-regulated. However, APS pretreatment reversed the up-regulation of most of the above-mentioned genes, where TLR5s, BCL-3, REL, caspase-1, NLRP12, IL-1β, and IL-6 were significantly down-regulated compared with inactivated V. alginolyticus-treated group. These results suggested that APS could protect large yellow croaker PKM against inactivated V. alginolyticus-induced inflammatory injury, and may exert their protection effects by activating NF-κB and pyroptosis signaling pathways. These findings therefore advance our understanding of the immune regulation mechanism of APS in fish, and facilitate the application of APS in prevention and control of fish bacteriosis.

Antibiotic resistant bacteria and genes in wastewater treatment plants: From occurrence to treatment strategies

This study aims to discuss the following: (1) occurrence and proliferation of antibiotic resistance in wastewater treatment plants (WWTPs); (2) factors influencing antibiotic resistance bacteria and genes in WWTPs; (3) tools to assess antibiotic resistance in WWTPs; (4) environmental contamination of antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) from WWTPs; (5) effects of ARB and ARGs from WWTPs on human health; and (6) treatment strategies. In general, resistant and multi-resistant bacteria, including Enterobacteriaceae, Pseudomonas aeruginosa, and Escherichia coli, exist in various processes of WWTPs. The existence of ARB and ARGs results from the high concentration of antibiotics in wastewater, which promote selective pressures on the local bacteria present in WWTPs. Thus, improving wastewater treatment technology and avoiding the misuse of antibiotics is critical to overcoming the threat of proliferation of ARBs and ARGs. Numerous factors can affect the development of ARB and ARGs in WWTPs. Abiotic factors can affect the bacterial community dynamics, thereby, affecting the applicability of ARB during the wastewater treatment process. Furthermore, the organic loads and other nutrients influence bacterial survival and growth. Specifically, molecular methods for the rapid characterization and detection of ARBs or their genes comprise DNA sequencing, real-time PCR, simple and multiplex PCR, and hybridization-based technologies, including micro- and macro-arrays. The reuse of effluent from WWTPs for irrigation is an efficient method to overcome water scarcity. However, there are also some potential environmental risks associated with this practice, such as increase in the levels of antibiotic resistance in the soil microbiome. Human mortality rates may significantly increase, as ARB can lead to resistance among several types of antibiotics or longer treatment times. Some treatment technologies, such as anaerobic and aerobic treatment, coagulation, membrane bioreactors, and disinfection processes, are considered potential techniques to restrict antibiotic resistance in the environment.

Environmentally relevant concentrations of triclosan exposure promote the horizontal transfer of antibiotic resistance genes mediated by Edwardsiella piscicida

Aquaculture pathogen and antibiotic resistance genes (ARGs) co-occur in the aquatic environment. Accumulated evidence suggests that aquaculture pathogens can facilitate the horizontal transfer of plasmid-mediated ARGs. However, the role of Edwardsiella piscicida (E. piscicida) in ARG dissemination is still not fully understood. In addition, the potential impact of triclosan (TCS) on the spread of ARGs mediated by E. piscicida is still unknown, so a mating model system was established to investigate the transfer process of ARGs. The results showed that E. piscicida disseminated ARGs on RP4 by horizontal gene transfer (HGT). Furthermore, TCS exposure promoted this process. The conjugative transfer frequencies were enhanced approximately 1.2-1.4-fold by TCS at concentrations from 2 to 20 μg/L, when compared with the control. TCS promoted the HGT of ARGs by stimulating reactive oxygen species (ROS) production, increasing cell membrane permeability, and altering expressions of conjugative transfer-associated genes. Together, the results suggested that aquaculture pathogens spread ARGs and that the emerging contaminant TCS enhanced the transfer of ARGs between bacteria.

Pharmacokinetics of cefquinome in rainbow trout (Oncorhynchus mykiss) after intravascular, intraperitoneal, and oral administrations

This study aimed to determine the pharmacokinetics and bioavailability of cefquinome in rainbow trout (Oncorhynchus mykiss) following intravascular (IV), intraperitoneal (IP), and oral (PO) administrations at 14 ± 1°C. In this study, three hundred and six clinically healthy rainbow trout (110-140 g) were used. The fish received single IV, IP, and PO injections of cefquinome at 10 mg/kg dose. The plasma concentrations of cefquinome were measured using HPLC-UV and were evaluated using non-compartmental analysis. Cefquinome was measured up to 96 h for PO route and 144 h for IV and IP routes in plasma. Following IV administration, t_1/2ʎz , Cl_T , and V_dss were 18.85 h, 0.037 L/h/kg, and 0.84 L/kg, respectively. The C_max of IP and PO routes was 9.75 and 1.64 μg/ml, respectively. The bioavailability following IP and PO administrations was 59.46% and 12.33%, respectively. Cefquinome at 10 mg/kg dose may maintain T > MIC above 40% at 72 and 96 h intervals, respectively, following the IP and IV routes for bacteria with MIC values of ≤2 μg/ml and at 24 h intervals following the PO route for bacteria with MIC value of ≤0.75 μg/ml. However, further studies are needed to determine in vitro and in vivo antibacterial efficacy and multiple dosage regimens of cefquinome against pathogens isolated from rainbow trout.

Edwardsiella piscicida YccA: A novel virulence factor essential to membrane integrity, mobility, host infection, and host immune response

YccA is a hydrophobic protein with seven transmembrane domains. The function of YccA is largely unknown in pathogenic bacteria. Edwardsiella piscicide (formerly known as E. tarda) is an aquatic pathogen that can infect various economically important fish, including flounder (Paralichthys olivaceus) and tilapia (Oreochromis niloticus). In this study, we investigated the role of YccA in E. piscicida by the construction of a mar kerless yccA in-frame mutant strain, TX01ΔyccA. We found that (i) in comparison to the wild type TX01, TX01ΔyccA exhibited markedly compromised tolerance to high temperature and tobramycin; (ii) deletion of yccA significantly impaired the integrity of the cell membrane and retarded bacterial biofilm formation and mobility; (iii) deficiency of yccA reduced bacterial adhesion and invasion of fish cells and immune tissues, while the introduction of a trans-expressed yccA gene restored the lost virulence of TX01ΔyccA; and (iv) host immune responses induced by TX01 and TX01ΔyccA were different in terms of reactive oxygen species (ROS) levels and expression levels of cytokines. Taken together, the results of our study indicate that YccA is a novel virulence factor of E. piscicida, and YccA is essential for bacterial pathogenicity through evasion of the host's innate immune functions.

High Genetic Diversity and Antimicrobial Resistance in Escherichia coli Highlight Arapaima gigas (Pisces: Arapaimidae) as a Reservoir of Quinolone-Resistant Strains in Brazilian Amazon Rivers

The increasing prevalence of multi-drug resistant (MDR) Escherichia coli in distinct ecological niches, comprising water sources and food-producing animals, such as fish species, has been widely reported. In the present study, quinolone-resistant E. coli isolates from Arapirama gigas, a major fish species in the Brazilian Amazon rivers and fish farms, were characterized regarding their antimicrobial susceptibility, virulence, and genetic diversity. A total of forty (40) specimens of A. gigas, including 20 farmed and 20 wild fish, were included. Thirty-four quinolone-resistant E. coli isolates were phenotypically tested by broth microdilution, while resistance and virulence genes were detected by PCR. Molecular epidemiology and genetic relatedness were analyzed by MLST and PFGE typing. The majority of isolates were classified as MDR and detected harboring bla_CTX-M, qnrA and qnrB genes. Enterotoxigenic E. coli pathotype (ETEC) isolates were presented in low prevalence among farmed animals. MLST and PFGE genotyping revealed a wide genetic background, including the detection of internationally spread clones. The obtained data point out A. gigas as a reservoir in Brazilian Amazon aquatic ecosystems and warns of the interference of AMR strains in wildlife and environmental matrices.

Pre-treatment of Nile tilapia (Oreochromis niloticus) with ozone nanobubbles improve efficacy of heat-killed Streptococcus agalactiae immersion vaccine

Nanobubble technology has shown appealing technical benefits and potential applications in aquaculture. We recently found that treatment with ozone nanobubbles (NB-O₃) activated expression of several immune-related genes leading to effective response to subsequent exposure to fish pathogens. In this study, we investigated whether pre-treatment of Nile tilapia (Oreochromis niloticus) with NB-O₃ can enhance specific immune responses and improve efficacy of immersion vaccination against Streptococcus agalactiae. Spleen and head kidney of fish in the vaccinated groups showed a substantial upregulation in expression levels of pro-inflammatory cytokine genes (IL-1β, TNF-α, IL-6) and immunoglobulin classes (IgM, IgD, IgT) compared with the unvaccinated control groups. The mRNA transcript of pro-inflammatory cytokine genes was greatest (approx. 2.8-3.3 folds) on day 7 post-vaccination, whereas the relative expression of immunoglobulin genes was greatest (approx. 3.2-4.1 folds) on day 21 post-immunization. Both systemic and mucosal IgM antibodies were elicited in vaccinated groups. As the result, the cumulative survival rate of the vaccinated groups was found to be higher than that of the unvaccinated groups, with a relative percent survival (RPS) ranging from 52.9 to 70.5%. However, fish in the vaccinated groups that received pre-treatment with NB-O₃, bacterial antigen uptakes, expression levels of IL-1β, TNF-α, IL-6,IgM, IgD, and IgT, as well as the specific-IgM antibody levels and percent survival, were all slightly or significantly higher than that of the vaccinated group without pre-treatment with NB-O₃. Taken together, our findings suggest that utilizing pre-treatment with NB-O₃ may improve the immune response and efficacy of immersion vaccination in Nile tilapia.

The Dynamics of the Antimicrobial Resistance Mobilome of Salmonella enterica and Related Enteric Bacteria

The foodborne pathogen Salmonella enterica is considered a global public health risk. Salmonella enterica isolates can develop resistance to several antimicrobial drugs due to the rapid spread of antimicrobial resistance (AMR) genes, thus increasing the impact on hospitalization and treatment costs, as well as the healthcare system. Mobile genetic elements (MGEs) play key roles in the dissemination of AMR genes in S. enterica isolates. Multiple phenotypic and molecular techniques have been utilized to better understand the biology and epidemiology of plasmids including DNA sequence analyses, whole genome sequencing (WGS), incompatibility typing, and conjugation studies of plasmids from S. enterica and related species. Focusing on the dynamics of AMR genes is critical for identification and verification of emerging multidrug resistance. The aim of this review is to highlight the updated knowledge of AMR genes in the mobilome of Salmonella and related enteric bacteria. The mobilome is a term defined as all MGEs, including plasmids, transposons, insertion sequences (ISs), gene cassettes, integrons, and resistance islands, that contribute to the potential spread of genes in an organism, including S. enterica isolates and related species, which are the focus of this review.

Colonization of Lactobacillus rhamnosus GG in Cirrhinus molitorella (Mud Carp) Fingerling: Evidence for Improving Disease Resistance and Growth Performance

The use of probiotic bacteria can not only enhance the nutritional utilization of fish feeds to produce more biomass but can also provide a practically “safer” alternative to the fish farming industry to reduce the abuse of antibiotics and drugs. This study investigated the possibility of colonizing Lactobacillus rhamnosus strain GG (LGG) to the intestine of Cirrhinus molitorella (mud carp) fingerling. Colonization of LGG was observed in gut tissue after 14 days of administration with a diet supplemented with 1 × 108 CFU/mL LGG. Moreover, growth performance parameters of the LGG-supplemented diet group, including relative weight gain, feed conversion ratio and feed efficiency, were found about two-fold higher than the control group after 60 days. In addition, fish fed with an LGG-supplemented diet for 60 days showed substantial resistance against the infection of pathogenic bacterial Aeromonas hydrophila, with a relative survival rate of up to 57% compared to the control group. In summary, the results indicated that LGG as dietary supplement for mud carp fingerling can enhance nutrition utilization and better protect fish against the infection of Aeromonas hydrophila. The results provide an insight to the fish farming industry, encouraging a reduction in the use of antibiotics and drugs and the production of “safer” mud carp for the market at a manageable cost.

A case study on the distribution of the environmental resistome in Korean shrimp farms

Hundreds of tons of antibiotics are widely used in aquaculture to prevent microbial infections and promote fish growth. However, the overuse of antibiotics and chemical products can lead to the selection and spreading of antibiotic-resistant bacteria (ARB) and antimicrobial resistance genes (ARGs), which are of great concern considering the threat to public health worldwide. Here, in-depth metagenome sequencing was performed to explore the environmental resistome and ARB distribution across farming stages in shrimp farms and examine anthropogenic effects in nearby coastal waters. A genome-centric analysis using a metagenome binning approach allowed us to accurately investigate the distribution of pathogens and ARG hosts in shrimp farms. The diversity of resistomes was higher in shrimp farms than in coastal waters, and the distribution of resistomes was dependent on the farming stage. In particular, the tetracycline resistance gene was found mainly at the early post-larval stage regardless of the farm. The metagenome-assembled genomes of Vibrio spp. were dominant at this stage and harbored tet34, which is known to confer resistance to oxytetracycline. In addition, opportunistic pathogens such as Francisella, Mycoplasma, Photobacterium, and Vibrio were found in abundance in shrimp farms, which had multiple virulence factors. This study highlights the increased resistance diversity and environmental selection of pathogens in shrimp farms. The use of environmental pollutants on farms may cause an increase in resistome diversity/abundance and the transmission of pathogens to the surrounding environment, which may pose future risks to public health and aquatic organisms.

Characterization and biological activities of carotenoids produced by three haloarchaeal strains isolated from Algerian salt lakes

Halophilic archaea represent a promising natural source of carotenoids. However, little information is available about these archaeal metabolites and their biological effects. In the present work, carotenoids of strains Haloferax sp. ME16, Halogeometricum sp. ME3 and Haloarcula sp. BT9, isolated from Algerian salt lakes, were produced, extracted and identified by high-performance liquid chromatography-diode array detector and liquid chromatography-mass spectrometry. Analytical results revealed a variation in the composition depending on the strain with a predominance of bacterioruberin. The evaluation of antioxidant capacity using ABTS [(2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)] and DPPH (2,2-diphenyl-1-picrylhydrazyl) assays showed that these extracts have a strong antioxidant potential, in particular those of Haloferax sp. ME16 which displayed antioxidant power significantly higher than that of ascorbic acid used as standard. Antibacterial activity of carotenoid extracts against four human-pathogenic strains and four fish-pathogenic strains was evaluated by agar disk diffusion method. The results showed a good antibacterial activity. These findings suggest that the C₅₀ carotenoids from the studied strains offer promising prospects for biotechnological applications.

Rosmarinic Acid and Sodium Citrate Have a Synergistic Bacteriostatic Effect against Vibrio Species by Inhibiting Iron Uptake

BACKGROUND

New strategies are needed to combat multidrug-resistant bacteria. The restriction of iron uptake by bacteria is a promising way to inhibit their growth. We aimed to suppress the growth of Vibrio bacterial species by inhibiting their ferric ion-binding protein (FbpA) using food components.

METHODS

Twenty spices were selected for the screening of FbpA inhibitors. The candidate was applied to antibacterial tests, and the mechanism was further studied.

RESULTS

An active compound, rosmarinic acid (RA), was screened out. RA binds competitively and more tightly than Fe3+ to VmFbpA, the FbpA from V. metschnikovii, with apparent KD values of 8 μM vs. 17 μM. Moreover, RA can inhibit the growth of V. metschnikovii to one-third of the control at 1000 μM. Interestingly, sodium citrate (SC) enhances the growth inhibition effect of RA, although SC only does not inhibit the growth. The combination of RA/SC completely inhibits the growth of not only V. metschnikovii at 100/100 μM but also the vibriosis-causative pathogens V. vulnificus and V. parahaemolyticus, at 100/100 and 1000/100 μM, respectively. However, RA/SC does not affect the growth of Escherichia coli.

CONCLUSIONS

RA/SC is a potential bacteriostatic agent against Vibrio species while causing little damage to indigenous gastrointestinal bacteria.

Characterization and Antimicrobial Resistance of Environmental and Clinical Aeromonas Species Isolated from Fresh Water Ornamental Fish and Associated Farming Environment in Sri Lanka

The aims of this study were to characterize and investigate antimicrobial susceptibility and presence of integrons in 161 Aeromonas spp. isolated from ornamental freshwater fish farming environment, apparently healthy and diseased fish. Phylogenetic analyses of the gyrB gene sequences identified Aeromonas veronii as the most abundant species (75.8%) followed by Aeromonashydrophila (9.3%), Aeromonas caviae (5%), Aeromonas jandaei (4.3%), Aeromonas dhakensis (3.7%), Aeromonas sobria (0.6%), Aeromonas media (0.6%), and Aeromonas popoffii (0.6%). Susceptibility to thirteen antimicrobials was determined and antimicrobial resistance frequencies were: amoxicillin (92.5%), enrofloxacin (67.1%), nalidixic acid (63.4%), erythromycin (26.1%), tetracycline (23.6%), imipenem (18%), trimethoprim-sulfamethoxazole (16.8%), and gentamicin (16.8%). Multi-drug resistance (MDR) was widespread among the isolates (51.6%, 83/161) with 51.6% (63/122) A. veronii isolates being MDR. In addition, 68.3% of isolates had multiple antibiotic resistance (MAR) indexes higher than 0.2, suggesting that they originated from a high-risk source of contamination where antimicrobials are often used. In all, 21.7% isolates carried class 1 integrons, with 97.1% having gene cassettes, while there were 12 isolates carrying class 2 integron gene cassettes. Our findings highlight that the aquatic environment and ornamental fish act as reservoirs of multidrug resistant Aeromonas spp. and underline the need for a judicious use of antimicrobials and timely surveillance of antimicrobial resistance (AMR) in aquaculture.

Inflammatory reactions in rainbow trout fins and gills exposed to biocides

Several biocides are widely used in rainbow trout aquaculture against various ectoparasites and ectobionts, but the inflammation induced in treated fish is less well described. Dose-response studies were conducted to elucidate the effects on rainbow trout (gills and fins) induced by a series of biocides including formalin, hydrogen peroxide (H2O2), peracetic acid (PAA) and the surfactant SPH6, which was isolated from the bacterium Pseudomonas H6. The compounds have documented antiparasitic effects, but the specific effects on fish needs further documentation. This study was performed over 24 h, and inflammatory reactions were evaluated in gills and fins. A dose-dependent effect was noted for expression of immune genes encoding for IL-1β, TNFα, IFNγ, IL-10, IL-8, lysozyme, serum amyloid A (SAA), hepcidin, precerebellin and complement factor C3. PAA induced the strongest upregulation of cytokine and acute phase reactant genes followed by H2O2 and formalin. SPH6 showed a lower effect, and in several cases the compound induced downregulation of several genes. Gills showed a stronger response compared to fins. The mucous cell density in fins showed a range of changes which varied by compound. PAA, and to a lesser degree H2O2 and formalin, initially induced mucous cell hyperplasia, whereas SPH6 immediately decreased the number of cells containing mucus.

Detection of Antimicrobial and Heavy-Metal Resistance Genes in Aeromonas spp. Isolated from Hard-Shelled Mussel (Mytilus Coruscus)

Hard-shelled mussel (Mytilus coruscus) is a popular seafood in South Korea because of its delicacy and high nutritional value. Our study aimed to identify antimicrobial and heavy-metal resistance determinants in Aeromonas isolates from marketed hard-shelled mussel in South Korea. A total of 33 Aeromonas species were isolated, and antimicrobial disk diffusion test was done to observe antimicrobial resistance patterns. In addition, broth microdilution test was performed to determine resistance to heavy-metals. PCR amplification was done to detect resistance genes. High resistance to amoxicillin (100.0%), ampicillin (93.9%), rifampicin (78.8%), and cephalothin (48.5%) was observed where least resistance to other antimicrobials was also detected. In addition, the isolates showed high resistance to cadmium (Cd) (57.6%), and 42.4% and 27.3% were resistant to chromium (Cr) and copper (Cu). The occurrence of antimicrobial resistance genes, such as blaTEM, blaSHV, blaCTX-M, tetB, tetE, and intI1 genes, was observed in 9 (27.3%), 8 (24.2%), 8 (24.2%), 6 (18.2%), 5 (15.2%), and 9 (27.3%) isolates, respectively. Also, heavy-metal resistance genes, czcA, copA, and merA were detected in 17 (51.5%), 11 (33.3%), and 7 (21.2%) of the isolates, respectively. The results suggest that mussels are a reservoir of multidrug and heavy-metal-resistant Aeromonas spp.

Short- and long-term probiotic effects of Enterococcus hirae isolated from fermented vegetable wastes on the growth, immune responses, and disease resistance of hybrid catfish (Clarias gariepinus × Clarias macrocephalus)

This study evaluated the short- and long-term effects of dietary supplementation with Enterococcus hirae strain UPM02 on the growth performance, immunity, and disease resistance of hybrid catfish (Clarias gariepinus × Clarias macrocephalus) against Aeromonas hydrophila infection. In the long-term trial, fingerling fish were fed diets containing 0 (control), 2 × 10⁵, or 2 × 10⁷ CFU/g E. hirae UPM02 for 120 days. Administration of E. hirae UPM02 had significant effects on the specific growth rate (SGR), feed utilization efficiency, body indices (P < 0.05), and gut villus physiology of the catfish. E. hirae UPM02 application also significantly increased the complete blood cell counts, phagocytic activity, respiratory burst, lysozyme activity, and alternative complement pathway hemolytic (ACH₅₀) activity in tested catfish throughout the experimental periods (P < 0.05). Dietary E. hirae UPM02 at both concentrations significantly increased the expression levels of the alpha-2-macroglobulin (α2M), CC chemokines, CXC chemokines, lysozyme c (LYZC), myeloperoxidase (MYE), NF-kappa-B1 p105 subunit (NF-K), and bactericidal permeability-increasing protein (BPIP) genes in the head kidney, liver, and spleen (P < 0.05) at days 80, 100 and 120 after application. However, heat shock protein 70 (HSP70) gene expression was slightly downregulated in these organs. Interestingly, fish fed the diets containing 2 × 10⁵ and 2 × 10⁷ CFU/g E. hirae UPM02 exhibited a significantly lower (P < 0.05) postchallenge mortality rates (32% and 30%, respectively) after 14 days of A. hydrophila challenge than the control fish (58%). In short-term (28 days) application to juvenile catfish, the two concentrations of E. hirae did not affect all growth parameters. Nevertheless, these concentrations markedly elevated all tested immune parameters, similarly to long-term application. Immune-related gene expression was significantly upregulated at day 28 in the head kidney, at day 14 in the liver, and at day 7 in the spleen in fish treated with the two concentrations of the probiotics (P < 0.05). Mortality at 14 days after challenge with A. hydrophila in the groups receiving the two concentrations of the probiotic was significantly lower than that in the control group, at 28, 24, and 48%, respectively (P < 0.05). These results collectively suggest that dietary supplementation with E. hirae UPM02 at 2 × 10⁵ and 2 × 10⁷ CFU/g effectively influenced immune responses, enhanced disease protection, and stimulated immunity-related gene expression in hybrid catfish under both short- and long-term application. However, growth enhancement was significantly evidenced with long-term application only.

Role played by the environment in the emergence and spread of antimicrobial resistance (AMR) through the food chain

The role of food-producing environments in the emergence and spread of antimicrobial resistance (AMR) in EU plant-based food production, terrestrial animals (poultry, cattle and pigs) and aquaculture was assessed. Among the various sources and transmission routes identified, fertilisers of faecal origin, irrigation and surface water for plant-based food and water for aquaculture were considered of major importance. For terrestrial animal production, potential sources consist of feed, humans, water, air/dust, soil, wildlife, rodents, arthropods and equipment. Among those, evidence was found for introduction with feed and humans, for the other sources, the importance could not be assessed. Several ARB of highest priority for public health, such as carbapenem or extended-spectrum cephalosporin and/or fluoroquinolone-resistant Enterobacterales (including Salmonella enterica), fluoroquinolone-resistant Campylobacter spp., methicillin-resistant Staphylococcus aureus and glycopeptide-resistant Enterococcus faecium and E. faecalis were identified. Among highest priority ARGs bla CTX -M, bla VIM, bla NDM, bla OXA -48-like, bla OXA -23, mcr, armA, vanA, cfr and optrA were reported. These highest priority bacteria and genes were identified in different sources, at primary and post-harvest level, particularly faeces/manure, soil and water. For all sectors, reducing the occurrence of faecal microbial contamination of fertilisers, water, feed and the production environment and minimising persistence/recycling of ARB within animal production facilities is a priority. Proper implementation of good hygiene practices, biosecurity and food safety management systems is very important. Potential AMR-specific interventions are in the early stages of development. Many data gaps relating to sources and relevance of transmission routes, diversity of ARB and ARGs, effectiveness of mitigation measures were identified. Representative epidemiological and attribution studies on AMR and its effective control in food production environments at EU level, linked to One Health and environmental initiatives, are urgently required.

Global trends in antimicrobial use in aquaculture

Globally aquaculture contributes 8% of animal protein intake to the human diet, and per capita consumption is increasing faster than meat and dairy consumption. Reports have documented antimicrobial use in the rapidly expanding aquaculture industry, which may contribute to the rise of antimicrobial resistance, carrying potential consequences for animal-, human-, and ecosystem-health. However, quantitative antimicrobial use across a highly diversified aquaculture industry is not well characterized. Here, we estimate global trends in antimicrobial use in aquaculture in 2017 and 2030 to help target future surveillance efforts and antimicrobial stewardship policies. We estimate antimicrobial use intensity (mg kg⁻¹) for six species groups though a systematic review of point prevalence surveys, which identified 146 species-specific antimicrobial use rates. We project antimicrobial use in each country by combining mean antimicrobial use coefficients per species group with OECD/FAO Agricultural Outlook and FAO FishStat production volumes. We estimate global antimicrobial consumption in 2017 at 10,259 tons (95% uncertainty interval [UI] 3163–44,727 tons), increasing 33% to 13,600 tons in 2030 (UI 4193–59,295). The Asia–Pacific region represents the largest share (93.8%) of global consumption, with China alone contributing 57.9% of global consumption in 2017. Antimicrobial consumption intensity per species group was: catfish, 157 mg kg⁻¹ (UI 9–2751); trout, 103 mg kg⁻¹ (UI 5–1951); tilapia, 59 mg kg⁻¹ (UI 21–169); shrimp, 46 mg kg⁻¹ (UI 10–224); salmon, 27 mg kg⁻¹ (UI 17–41) and a pooled species group, 208 mg kg⁻¹, (UI 70–622). All antimicrobial classes identified in the review are classified as medically important. We estimate aggregate global human, terrestrial and aquatic food animal antimicrobial use in 2030 at 236,757 tons (95% UI 145,525–421,426), of which aquaculture constitutes 5.7% but carries the highest use intensity per kilogram of biomass (164.8 mg kg⁻¹). This analysis calls for a substantial scale-up of surveillance capacities to monitor global trends in antimicrobial use. Current evidence, while subject to considerable uncertainties, suggests that for some species groups antimicrobial use intensity surpasses consumption levels in terrestrial animals and humans. Acknowledging the fast-growing nature of aquaculture as an important source of animal nutrition globally, our findings highlight the urgent need for enhanced antimicrobial stewardship in a high-growth industry with broad links to water and ecosystem health.

White Paper: Bridging the gap between surveillance data and antimicrobial stewardship in the animal sector-practical guidance from the JPIAMR ARCH and COMBACTE-MAGNET EPI-Net networks

BACKGROUND

The JPIAMR ARCH and COMBACTE-MAGNET EPI-Net networks have joined efforts to formulate a set of target actions to link the surveillance of antimicrobial usage (AMU) and antimicrobial resistance (AMR) with antimicrobial stewardship (AMS) activities in four different settings. This White Paper focuses on the veterinary setting and embraces the One Health approach.

METHODS

A review of the literature was carried out addressing research questions in three areas: AMS leadership and accountability; AMU surveillance and AMS; and AMR surveillance and AMS. Consensus on target actions was reached through a RAND-modified Delphi process involving over 40 experts in infectious diseases, clinical microbiology, AMS, veterinary medicine and public health, from 18 countries.

RESULTS/DISCUSSION

Forty-six target actions were developed and qualified as essential or desirable. Essential actions included the setup of AMS teams in all veterinary settings, building government-supported AMS programmes and following specific requirements on the production, collection and communication of AMU and AMR data. Activities of AMS teams should be tailored to the local situation and capacities, and be linked to local or national surveillance systems and infection control programmes. Several research priorities were also identified, such as the need to develop more clinical breakpoints in veterinary medicine.

CONCLUSIONS

This White Paper offers a practical tool to veterinary practitioners and policy makers to improve AMS in the One Health approach, thanks to surveillance data generated in the veterinary setting. This work may also be useful to medical doctors wishing to better understand the specificities of the veterinary setting and facilitate cross-sectoral collaborations.

Passive Immunization Delays Disease Outcome in Gilthead Sea Bream Infected With Enteromyxum leei (Myxozoa), Despite the Moderate Changes in IgM and IgT Repertoire

Passive immunization constitutes an emerging field of interest in aquaculture, particularly with the restrictions for antibiotic use. Enteromyxum leei is a myxozoan intestinal parasite that invades the paracellular space of the intestinal epithelium, producing a slow-progressing disease, leading to anorexia, cachexia and mortalities. We have previously demonstrated that gilthead sea bream (GSB, Sparus aurata) that survive E. leei infection become resistant upon re-exposure, and this resistance is directly related to the presence of high levels of specific IgM in serum. Thus, the current work was aimed to determine if passive immunization could help to prevent enteromyxosis in GSB and to study in detail the nature of these protective antibodies. Serum from a pool of resistant (SUR) or naïve (NAI) animals was intracoelomically injected 24 h prior to the E. leei-effluent challenge and at 9 days post-challenge (dpc). Effluent challenge lasted for 23 days, and then the injected groups were allocated in separate tanks with clean water. A non-lethal parasite diagnosis was performed at 56 dpc. At the final sampling (100 dpc), blood, serum and tissues were collected for histology, molecular diagnosis and the detection of circulating antibodies. In parallel, we performed an immunoglobulin repertoire analysis of the fish generating SUR and NAI sera. The results showed that, fish injected with parasite-specific antibodies (spAbs) became infected with the parasite, but showed lower disease signs and intensity of infection than the other groups, indicating a later establishment of the parasite. Repertoire analysis revealed that E. leei induced a polyclonal expansion of diverse IgM and IgT subsets that could be in part an evasion strategy of the parasite. Nonetheless, GSB was able to produce sufficient levels of parasite-spAbs to avoid re-infection of surviving animals and confer certain degree of protection upon passive transfer of antibodies. These results highlight the crucial role of spAb responses against E. leei and set the basis for the development of effective treatment or prophylactic methods for aquaculture.

Antimicrobial Resistance analysis of Pathogenic Bacteria Isolated from Freshwater Nile Tilapia (Oreochromis niloticus) Cultured in Kerala, India

Aquaculture of popular freshwater species, Nile tilapia (Oreochromis niloticus), accounts for around 71% of the total global tilapia production. Frequent use of antibiotics for treating bacterial infections in tilapia leads to the emergence of antimicrobial resistance. To mitigate the issue, proper evaluation methods and control strategies have to be implemented. This study was aimed to analyze the antimicrobial resistance of bacterial isolates from the infected Nile tilapia cultured in freshwater. The recovered isolates were identified as Pseudomonas entomophila, Edwardsiella tarda, Comamonas sp, Delftia tsuruhatensis, Aeromonas dhakensis, A. sobria, A. hydrophila, A. lacus, Plesiomonas shigelloides and Vogesella perlucida through phenotypic and genotypic analyses. Using Primer-E software, Shannon Wiener diversity index of the isolates was determined as H' (loge) = 2.58. Antibiotic susceptibility test of the recovered strains through disk diffusion using 47 antibiotics, showed an elevated resistance pattern for Aeromonas hydrophila, Pseudomonas entomophila and Comamonas with higher multiple antibiotic resistance indexes (MAR index > 0.3). The minimum inhibitory concentration of antibiotics was > 256 mcg/ml for most of the resistant isolates. Meanwhile, all the recovered isolates were susceptible to amikacin, aztreonam, kanamycin, cefalexin, cefotaxime, levofloxacin, norfloxacin, piperacillin, and polymyxin-B.

Zebrafish as a Model for Fish Diseases in Aquaculture

The use of zebrafish as a model for human conditions is widely recognized. Within the last couple of decades, the zebrafish has furthermore increasingly been utilized as a model for diseases in aquacultured fish species. The unique tools available in zebrafish present advantages compared to other animal models and unprecedented in vivo imaging and the use of transgenic zebrafish lines have contributed with novel knowledge to this field. In this review, investigations conducted in zebrafish on economically important diseases in aquacultured fish species are included. Studies are summarized on bacterial, viral and parasitic diseases and described in relation to prophylactic approaches, immunology and infection biology. Considerable attention has been assigned to innate and adaptive immunological responses. Finally, advantages and drawbacks of using the zebrafish as a model for aquacultured fish species are discussed.

Pharmacokinetics and tissue disposition of enrofloxacin in rainbow trout after different routes of administration

Plasma pharmacokinetics (PK) and tissue disposition of enrofloxacin (EFX) was studied in rainbow trout (Oncorhynchus mykiss) after a single oral administration of 10 mg/kg, and by immersion baths of 20 ppm during 2.5 h and 100 ppm during 0.5 h, at water temperature of 16.3 ± 0.3 °C.Concentrations of EFX in plasma and tissues (skin, muscle, liver, kidney and gut) were determined using high performance liquid chromatography (HPLC) with fluorescence detection.Pharmacokinetic parameters were analyzed with a non-compartmental model. After oral administration, t½β, AUC and AUC_tissues/AUC_plasma ratio were 42.98 h, 21.80μg-h/ml and ≤ 18.63, respectively.After immersion baths of 20 ppm during 2.5 h and 100 ppm during 0.5 h, the t½β, AUC and AUC_tissues/AUC_plasma were 42.77 and 44.67, 9.83 and 12.83 μg-h/ml and ≤ 9.81 and ≤ 7.13, respectively.Therefore, oral (10 mg/kg) and bath administration in rainbow trout can provide AUC/MIC of ≥125 and Cmax/MIC of ≥10 to treat diseases caused by susceptible bacteria with MIC ≤ 0.04 μg/ml. This information can be helpful for the right use of EFX in rainbow trout. Also, this is the first study that determines the antibiotic tissue disposition in rainbow trout by using different administration routes.

Edwardsiella piscicida: A versatile emerging pathogen of fish

Edwardsiella piscicida is an Enterobacteriaceae that is abundant in water and causes food and waterborne infections in fish, animals, and humans. The bacterium causes Edwardsiellosis in farmed fish and can lead to severe economic losses in aquaculture worldwide. E. piscicida is an intracellular pathogen that can also cause systemic infection. Type III and type VI secretion systems are the bacterium’s most lethal weapons against host defenses. It also possesses multi-antibiotic resistant genes and is selected and enriched in the environment due to the overuse of antibiotics. Therefore, the bacterium has great potential to contribute to the evolution of the resistome. All these properties have made this bacterium a perfect model to study bacteria virulence mechanisms and the spread of antimicrobial genes in the environment. We summarize recent advance in E. piscicida biology and provide insights into future research in virulence mechanisms, vaccine development and novel therapeutics.

Dietary supplementation with probiotics regulates gut microbiota structure and function in Nile tilapia exposed to aluminum

BACKGROUNDS AND AIMS

Aluminum contamination of water is becoming increasingly serious and threatens the health status of fish. Lactobacillus plantarum CCFM639 was previously shown to be a potential probiotic for alleviation aluminum toxicity in Nile tilapia. Considering the significant role of the gut microbiota on fish health, it seems appropriate to explore the relationships among aluminum exposure, probiotic supplementation, and the gut microbiota in Nile tilapia and to determine whether regulation of the gut microbiota is related to alleviation of aluminum toxicity by a probiotic in Nile tilapia.

METHODS AND RESULTS

The tilapia were assigned into four groups, control, CCFM639 only, aluminum only, and aluminum + CCFM639 groups for an experimental period of 4 weeks. The tilapia in the aluminum only group were grown in water with an aluminum ion concentration of 2.73 mg/L. The final concentration of CCFM639 in the diet was 108 CFU/g. The results show that environmental aluminum exposure reduced the numbers of L. plantarum in tilapia feces and altered the gut microbiota. As the predominant bacterial phyla in the gut, the abundances of Bacteroidetes and Proteobacteria in aluminum-exposed fish were significantly elevated and lowered, respectively. At the genus level, fish exposed to aluminum had a significantly lower abundance of Deefgea, Plesiomonas, and Pseudomonas and a greater abundance of Flavobacterium, Enterovibrio, Porphyromonadaceae uncultured, and Comamonadaceae. When tilapia were exposed to aluminum, the administration of a probiotic promoted aluminum excretion through the feces and led to a decrease in the abundance of Comamonadaceae, Enterovibrio and Porphyromonadaceae. Notably, supplementation with a probiotic only greatly decreased the abundance of Aeromonas and Pseudomonas.

CONCLUSION

Aluminum exposure altered the diversity of the gut microbiota in Nile tilapia, and probiotic supplementation allowed the recovery of some of the diversity. Therefore, regulation of gut microbiota with a probiotic is a possible mechanism for the alleviation of aluminum toxicity in Nile tilapia.

Entry Exclusion of Conjugative Plasmids of the IncA, IncC, and Related Untyped Incompatibility Groups

Conjugative plasmids of incompatibility group C (IncC), formerly known as A/C₂, disseminate antibiotic resistance genes globally in diverse pathogenic species of Gammaproteobacteria. Salmonella genomic island 1 (SGI1) can be mobilized by IncC plasmids and was recently shown to reshape the conjugative type IV secretion system (T4SS) encoded by these plasmids to evade entry exclusion. Entry exclusion blocks DNA translocation between cells containing identical or highly similar plasmids. Here, we report that the protein encoded by the entry exclusion gene of IncC plasmids (eexC) mediates entry exclusion in recipient cells through recognition of the IncC-encoded TraG_C protein in donor cells. Phylogenetic analyses based on EexC and TraG_C homologs predicted the existence of at least three different exclusion groups among IncC-related conjugative plasmids. Mating assays using Eex proteins encoded by representative IncC and IncA (former A/C₁) and related untyped plasmids confirmed these predictions and showed that the IncC and IncA plasmids belong to the C exclusion group, thereby explaining their apparent incompatibility despite their compatible replicons. Representatives of the two other exclusion groups (D and E) are untyped conjugative plasmids found in Aeromonas sp. Finally, we determined through domain swapping that the carboxyl terminus of the EexC and EexE proteins controls the specificity of these exclusion groups. Together, these results unravel the role of entry exclusion in the apparent incompatibility between IncA and IncC plasmids while shedding light on the importance of the TraG subunit substitution used by SGI1 to evade entry exclusion.IMPORTANCE IncA and IncC conjugative plasmids drive antibiotic resistance dissemination among several pathogenic species of Gammaproteobacteria due to the diversity of drug resistance genes that they carry and their ability to mobilize antibiotic resistance-conferring genomic islands such as SGI1 of Salmonella enterica While historically grouped as "IncA/C," IncA and IncC replicons were recently confirmed to be compatible and to abolish each other's entry into the cell in which they reside during conjugative transfer. The significance of our study is in identifying an entry exclusion system that is shared by IncA and IncC plasmids. It impedes DNA transfer to recipient cells bearing a plasmid of either incompatibility group. The entry exclusion protein of this system is unrelated to any other known entry exclusion proteins.

In Vitro Antibacterial and Synergistic Effect of Chitosan-Phytochemical Conjugates Against Antibiotic Resistant Fish Pathogenic Bacteria

Chitosan-phytochemical conjugates exhibited significant antibacterial effect with minimum inhibitory concentration (MIC) ranging from 128 to 2048 µg/ml against antibiotic-resistant fish pathogenic bacteria such as Edwardseilla tarda, Vibrio harveyi and Photobacterium damselaewhich were isolated from Korean cultured fish. Furthermore, the MIC values of old-fashioned antibiotics such as erythromycin and oxytertacycline drastically reduced in combination with chitosan-phytochemical conjugates against the fish pathogenic bacteria. The combination of conjugates with erythromycin and oxytetracycline gave median ∑FIC results ranging from 0.281 to 0.625 and 0.312 to 0.625, respectively. This result indicates the synergistic antibacterial effects and an increased susceptibility against the antibiotics.